Role of yarn tension in weaving 978-93-80308-27-2, 9380308272

Citation preview

Role of Yarn Tension in Weaving

Role of Yarn Tension in Weaving

Samir Kumar Neogi

WOODHEAD PUBLISHING INDIA PVT LTD New Delhi, India

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742

Woodhead Publishing India Pvt. Ltd. 303, Vardaan House, 7/28, Ansari Road Daryaganj, New Delhi – 110002, India

© 2016 by Woodhead Publishing India Pvt. Ltd. Exclusive worldwide distribution by CRC Press an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20151116 International Standard Book Number-13: 978-93-80308-27-2 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www. copyright.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com For information about WPI Publishing visit their website at http://www.woodheadpublishingindia.com

Contents

Preface

xi

Foreword 1

Fundamentals of yarn tension

1

1.1

Introduction

1

1.2

Definition of yarn tension

1

1.3

Necessity of tension in textile processing

2

1.3.1

Tension at weaving preparatory

2

1.3.2

Tension at weaving

3

1.4

Yarn tension and yarn breakage

4

1.5

Tensioning system

4

1.6

Tensioner (or Tension device)

10

1.6.1

Disc tensioner

10

1.6.2

Gate-type tensioner

11

1.6.3

Roller tensioner

11

1.6.4

Capstan tensioner

11

1.6.5

Tensioner with automatic control system

11

1.7 2

xiii

Tension measurement

12

Yarn tension at the weaving preparatory process

17

2.2

Weaving preparatory processes

18

2.3

Winding process

19

2.3.1

Unwinding

20

2.3.2

Tensioning (and clearing)

26

2.3.3

Winding

27

2.3.4

Pirn winding

28

2.4

Warping process

30

2.5

Slashing (or sizing) process

32

vi 3

4

5

Contents

Warp tension measurement

37

3.1

Introduction

37

3.2

Passage of warp yarns through the loom

37

3.3

Stretch and strain of warp yarns due to shedding

38

3.4

Warp tension control by let-off motion

42

3.5

Measurement techniques of warp tension

44

3.5.1

Tension measurement of the whole warp sheet

44

3.5.2

Tension measurement of the bunch of warp yarns 47

3.5.3

Tension measurement of single warp yarn

General form of warp tension variation

59

4.2

Warp tension variation

60

4.2.1

Tension variation of the whole warp sheet

60

4.2.2

Tension variation of the bunch of warp yarns

62

4.2.3

Tension variation of single warp yarn in shuttle loom 63

4.2.4

Tension variation of single warp yarn in shuttleless loom 69

Weft tension measurement

73

5.1

Introduction

73

5.2

Weft tension measurement in shuttle loom

74

5.2.1

Measurement during unwinding from pirn

74

5.2.2

Measurement during insertion of a pick

76

5.3 6

49

Weft tension measurement in shuttleless loom

79

General form of weft tension variation

81

6.1

Introduction

81

6.2

Weft tension variation in shuttle loom

82

6.2.1

Tension variation during unwinding from pirn

82

6.2.2

Tension variation during insertion of a pick

83

6.3

Weft tension variation in shuttleless looms

86

6.3.1

Tension variation in projectile picking

87

6.3.2

Tension variation in rapier picking

88

vii

Contents

7

6.3.3

Tension variation in air-jet picking

90

6.3.4

Comparison of weft tensions in different shuttleless picking systems 92

Effects of loom settings and other factors on warp tension

97

7.1

Introduction

97

7.2

Effects of warp beam and let-off motion

98

7.2.1

Warp tension control in negative let-off motion

99

7.2.2

Warp tension control in positive let-off motion

102

7.3

7.4

7.5

Effects of back-rest

105

7.3.1

Back-rest setting

105

7.3.2

Back-rest movement

108

7.3.3

Back-rest mounting

112

7.3.4

Back-rest type

118

Effects of warp stop motion setting and warp leasing pattern

119

7.4.1

Warp stop motion setting

119

7.4.2

Warp leasing pattern

120

Effects of shedding

123

7.5.1

Front shed geometry

123

7.5.2

Heald movement

124

7.5.3

Heald position

128

7.5.4

Warp drafting pattern

129

7.5.5

Shed timing

130

7.6

Formation of clean warp shed

132

7.7

Effects of warp denting pattern

138

7.8

Effects of beat-up

139

7.9

Effects of temple

148

7.10

Effects of looming-in lengths of warp and cloth

151

7.11

Effects of other factors

155

7.11.1 Weave

155

7.11.2 Loom speed

156

viii

8

9

10

Contents

7.11.3 Yarn join

157

7.11.4 Weft tension

160

Effects of loom settings and yarn characteristics on weft tension

167

8.1

Introduction

167

8.2

Shuttle picking

168

8.2.1

Effects of pirn and shuttle modifications

168

8.2.2

Effects of shed timing

171

8.2.3

Effects of shuttle checking

172

8.3

Shuttleless picking

172

8.4

Effects of weft package

173

8.5

Weft storage system (weft accumulator or weft feeder) 175 8.5.1

Forces acting on the yarn at weft accumulator 177

8.5.2

Effects of weft accumulator

180

8.5.3

Effects of type and setting of weft accumulator

184

8.6

Effects of other loom mechanisms

189

8.7

Effects of loom speed

193

8.8

Effects of weft yarn characteristics

197

Effects of yarn tensions on loom performance

207

9.1

Introduction

207

9.2

Loom performance

208

9.2.1

Warp break

209

9.2.2

Weft break

215

Effects of yarn tensions on fabric properties

221

10.1

Introduction

221

10.2

Mechanics of cloth formation in relation to yarn tensions 222

10.3

Beat-up force

225

10.4

Cloth properties

233

10.4.1 Structural and dimensional properties

233

10.4.2 Physical properties

242

10.4.3 Comfort properties

249

ix

Contents

Index

10.5

Weave prominence

250

10.6

Roll of yarn tensions on the remedy of some cloth defects 251 10.6.1 Starting mark or setting-on place

251

10.6.2 Repping

258

10.6.3 Reed mark or reediness

260

10.6.5 Selvedge distortion

268 273

Preface

In the process of weaving, warp and weft yarns are interlaced to produce a cloth and proper interlacement of these two sets of yarns are possible if both of them remain under some tensions. Again, to ensure continuity of the production of the cloth, the warp and weft yarns are needed to be fed from their respective packages and smooth feeding of the yarns can be assured if their packages are prepared under certain tensions of the yarns. Furthermore, the magnitude and nature of tension variations of both the warp and weft yarns are significantly influenced by the settings of different loom mechanisms. Working performances of the looms as well as many of the physical properties of the cloths woven are also determined by the tensions under which the warp and weft yarns are woven. Yarn tension, therefore, plays the most vital role in weaving and each and every person associated with this process ought to have a clear knowledge and perception about this. Hence, discussing weaving without referring to warp and weft tensions is akin to cooking without salt. Over the long years, warp and weft tensions at weaving have been the subject of many investigations and discussions by many researchers including myself and workers of weaving. The fruitful outcomes of these have improved and enriched, a great deal, the art of weaving. Unfortunately, many of these have not yet been collated and presented in the proper manner under one cover for the benefit of the readers. A sincere attempt has, therefore, been made to write a textbook titled Role of Yarn Tension in Weaving. As mentioned, the contents of the book written in 10 Chapters is primarily the compilation of different published work on yarn tensions at weaving preparatory, weaving and the related matters. With a view to presenting the topics in proper perspective and enabling the readers to understand the subject more clearly, the book starts with the fundamental of yarn tension which then follows with the important aspects of yarn tension at preparatory stages to weaving, the techniques of measurement of tension and the general form of tension variations of both the warp and weft yarns, effects of loom settings on warp and weft tensions, effects of

xii

Preface

warp and weft tensions on the loom performance and finally the influences of warp and weft tensions on the various cloth properties. For the purpose, fairly exhaustive review of literature has been made. I do not, however, claim that I have reviewed all the literatures published so far on this subject, but I have nonetheless made an honest and sincere effort to study as many number of published papers and books on this subject as possible. The text in each chapter has been substantiated by numerous photographs, diagrams, graphs and tables for understanding the subject better. However, if any reader kindly makes any valuable suggestion on further reference on this topic or on necessary correction or modification for upgrading the quality of the book, these will definitely be considered with due importance in future publication. In epilogue, I wish to express my sincere gratitude to the Director, Indian Jute Industries’ Research Association, Kolkata; the Director, Ahmedabad Textile Industries’ Research Association, Ahmedabad; and the Principal, Institute of Jute Technology (presently Department of Jute and Fibre Technology, University of Calcutta), Kolkata for providing me with their library services for survey of the literatures during the course of writing the book. Dr. Samir Kumar Neogi

Foreword

Amongst the various methods commercially accepted and adopted all over the world for producing a cloth, weaving is not only the oldest but also the most popular one till today. Weaving means interlacement of two sets of yarns, warp, and weft. For the purpose of carrying out the weaving process successfully, the factor that plays the most important role is the tension of the yarns. No cloth can be woven without desired and adequate tensions of its constituent warp and weft yarns. Despite the fact that the importance of yarn tensions has been recognized ever since the advent of the weaving process and hence, numerous useful studies have been carried out on this over the years, these are very scattered and each of these elucidates on some particular aspects only and not on the entire gamut of weaving process. For understanding the process of weaving in proper perspective, clear knowledge on the various aspects of yarn tension in weaving as well as the effects of yarn tensions on cloth characteristics is very important. Dr. Samir Kumar Neogi, basically a textile technologist and the former Deputy Director of the Indian Jute Industries’ Research Association, Kolkata, has made a laudable effort to write a text book titled Role of yarn tension in weaving. As I understand, the book contains the findings of different published work on yarn tensions at preparatory to weaving, weaving, cloth formation and the related matters pertaining to both the shuttle and shuttleless looms. In the undergraduate and post graduate courses all over the world, yarn tension in weaving is taught with great importance and I firmly believe that the book will therefore, be found very useful to the students, teachers and also to the researchers working on the related subjects. I appreciate the efforts of Dr. Neogi for writing such a book and Woodhead Publishing India Pvt. Ltd. Delhi for publishing the same. Dr. Prabir Roy Director Indian Jute Industries’ Research Association Kolkata

1 Fundamentals of yarn tension

Abstract: Yarn tension plays an important role in producing a cloth in weaving. Proper tension of the yarn is necessary to produce suitable warp packages, the warp beams and the weft packages, the pirns or cones, to be used on the looms for weaving cloth. Again, tension on the yarn imposes strain on it and makes it liable to break. Judicious application of tension on the yarn is, therefore, very important. Various suitable tensioning systems are adopted for imparting tension on the yarns. Irrespective of the type of tensioning system, the output tension of a yarn is determined to a large extent on the frictional characteristic of the surface over which the yarn passes. Yarn tension is measured by a tension measuring instrument which generally operates on a three-pulley system. The yarn passes through the pulley system and the variation in tension of the moving yarn causes deflection of the middle pulley to indicate the magnitude of tension of the yarn. Keywords: yarn tension; tensioner; active tensioning system; passive tensioning system; additive tensioning system; multiplicative tensioning system; tensometer

1.1

Introduction

In the processing of any textile material, tension poses to be one of the most important parameters particularly from spinning onward. In the process of weaving, tension of the yarn is an essential and inherent phenomenon in forming the cloth, technically called the fabric, and achieving the desired efficiency of the weaving machine and the required quality characteristics of the fabric produced. Yarn is therefore more concerned with and affected by the tension than the fiber or fabric. Proper interlacement between the warp and weft yarns, and thereby; weaving of the fabric becomes possible, provided that the yarns are held under certain tensions during the process.

1.2

Definition of yarn tension

According to the Oxford dictionary, the term “tension” is defined as an effect produced by forces pulling against each other as indicated in Fig 1.1. It is, thus,

2

Role of yarn tension in weaving

a uniaxial force tending to cause the extension of a body or the balancing force within that body resisting the extension. In simple textile language, tension of a yarn means the tensile stress developed within the yarn when it is subjected to an external tensile strain. Therefore, the magnitude of yarn tension depends upon the external strain. Tension is expressed in gram or Newton.

Figure 1.1 Tension produced in a yarn.

1.3 Necessity of tension in textile processing We know that any type of textile fiber can be converted into yarn at spinning stage and the spun yarn is wound on a package. The yarn packages produced at spinning are again converted into different types of packages suitable for subsequent processing. For the purpose of weaving, the yarn packages are finally made into (a) warp packages, and (b) weft packages for the production of fabrics on the looms. Irrespective of the type of looms, the warp package is made in the form of a large bobbin like warp beam prepared in warping and sizing. The weft package is made in the form of pirn (or cop) for shuttle loom and in the form of cross wound cheese or cone (the latter is more popular) for shuttlleless loom. While the cops are used for weaving jute yarns, the pirns are used for cotton and others.

1.3.1 Tension at weaving preparatory At the weaving preparatory stage, tension of the yarn is necessary for various reasons. In spinning, the yarn spun is wound to form a package of desired shape and size with required compactness. Such a package can be produced if certain amount of tension is applied on the yarn during the process of winding at spinning stage. Proper shape and stability of the yarn package allow the yarn to readily unwind in the next process and enable the package to withstand handling during transportation without

yarn sloughing off. In preparatory to weaving, the yarns for either warp or weft are first unwound from one type of package, and then rewound to make another package twice or thrice before the ultimate packages are

Fundamentals of yarn tension

3

made for the looms. During unwinding, the yarn is subjected to some amount of tension due to inertia, when the package from which the yarn is withdrawn is rotated or due to ballooning, when the yarn is withdrawn from the stationary package. Again during subsequent winding, the yarns have to be wound under certain tension in order to build the yarn packages of required shape, size, and stability. Besides this, it is almost customary that some amount of tension is imposed on the yarn during the process of winding at the preparatory stages to weaving, so that some of the faults such as weak places in the yarn are eliminated. Depending on the count and breaking strength of the yarn, the magnitude of tension is so adjusted that it causes the yarn to break at the weak places but does not strain it so much as to annihilate its elastic property. Characteristics of unwinding tension from a yarn package depend on the factors such as yarn withdrawal speed, package size, balloon height, yarn count, type of tensioning device used, and so on.

1.3.2 Tension at weaving Among the different processing stages from spinning to weaving, yarn tension plays the most vital role in weaving. For proper weaving operation, tension of the warp yarns is essential for forming clear shed for insertion of weft. Warp tension is also necessary for holding the fell of the cloth in the correct preset position for obtaining the predetermined pick spacing of the cloth. Cloth with high warp sett may require more while the loosely sett cloth may require low warp tension for satisfactory weaving. Tension of the weft yarn helps to remove the slackness and kink in the yarn during insertion and the desired length of a pick is inserted in the warp shed. As the warp and weft yarns are interlaced to form the cloth, tension of the yarns determine the magnitude of crimp imposed in the two sets of yarns due to interlacement. This, in turn, governs the various properties of the cloth. Thus, tension is one of the most important parameters in the various processing stages of textile. Yarn tension at any processing stage has considerable effects on the performance and efficiency of the processing machine and on the qualities of the product made. During processing, it is always tried to keep the yarn tension as low as suitable for the particular purpose and to maintain it throughout. It is also desired that the yarn tension should remain as uniform as possible throughout the processing. High tension or high variation in tension of a yarn strains the yarn unduly and may even cause it to break resulting in loss in machine productivity and deterioration in the qualities of the product.

4

Role of yarn tension in weaving

1.4 Yarn tension and yarn breakage Maintenance of constant yarn tension all through the processing is very difficult to achieve. It is also extremely difficult to eliminate the fluctuation of tension, whatever the range may be, and the extent of fluctuation depends on the processing parameters the yarn is subjected to. For example, the extent of variation in yarn tension during shed formation in weaving is naturally much greater than that during formation of yarn package in winding. Usually, the tension at which a yarn is processed is below the average strength of the yarn so that the yarn does not break. Nevertheless, the weak places in the yarn will be unable to withstand the processing tension and eventually break. The applied tension on the running yarn should be so adjusted as to keep the elastic property of the yarn unaffected. There is a relationship between the number of breaks per given length of a yarn and its running tension [3]. This is Tn = u − Ks

(1.1)

where, Tn is tension required to produce n breaks per 1000 yards,

u is mean single thread strength at a given test length, σ is the standard deviation of the single-thread strength results, and K is a factor which varies with n and is also governed by the test length used in the single-thread test. For cotton yarns with the test length of 20 inches (50.8 cm) and 8 breaks/1000 yards K is 3.3. Above equation (Eq. 1.1) can be written as Tn = u − 3.3s

(1.2)

The tension of a yarn increases with the strain imposed on it. So when a yarn is processed through a machine, the chances of its breakage due to machine-associated factors can be reduced by reducing the strain on it.

1.5 Tensioning system As pointed out above, the yarn should be subjected to some amount of tension during processing. The magnitude of this tension depends on the count of the yarn and the type of processing so that the desired performances and productivities of the processing machines and the quality characteristics of the products can be achieved. The required amount of tension of the yarn is obtained by imparting strain on the yarn and for the purpose suitable devices

Fundamentals of yarn tension

5

are employed on the running yarn. Although this increases the over all tension of the yarn, it helps to minimize the tension variation during the processing of the yarn. Tensioning of the yarn also serves as a detector for excessively weak places in the yarn that break under the added tension induced by the device. The device which applies and controls the tension is called the tension device or tensioner. Different systems adopted for imparting tension on the yarn can be classified as shown in Fig 1.2.

Figure 1.2 Classification of yarn tensioning system. Active tensioning system requires auxiliary pneumatic or electrical energy for applying tension on the yarn [4]. The pneumatic tensioner that imparts tension on the yarn by air current is shown in Fig. 1.3. The pneumatic tensioner can keep the yarn under tension even when the yarn is stationary. The demerit of the system is that the yarn structure is susceptible to be damaged due to its excessive exposure to the air stream. In case of passive tensioning system, tension on the yarn is created by the movement of the yarn against the frictional contact of a body and it therefore, cannot impart tension on the yarn when the yarn is at rest, as generally happens during stoppage of the machine.

Figure 1.3 Yarn tension by pneumatic system. The simplest way of imparting tension to a yarn is to rub it against a surface while running it over. The rubbing or friction of the yarn adds tension to the

6

Role of yarn tension in weaving

yarn, the magnitude of which depends on the force with which the yarn is rubbed against the surface while passing over it and also on the coefficient of friction between the yarn and the surface. The coefficient of friction is the ratio of the force applied to the force acting normal to the surface.

Figure 1.4 Friction between two sliding bodies. Suppose a solid material (Fig. 1.4) is being pulled with the force F over a surface and N is the force normal to the material, then the coefficient of friction μ between the two sliding materials is given by m=F/N

(1.3)

If the solid material in Fig. 1.4 is replaced by a yarn and the yarn is simply dragged over the surface in a straight path along its length, it will hardly be subjected to any strain. Strain or tension will be imposed on the yarn if it is pressed between the surfaces of two solid materials, as shown in Fig. 1.5, and the force or tension F of the yarn is F = m N + m N = 2m N

(1.4)

Figure 1.5 Additive tensioning system by discs. The coefficient of friction depends on the surface properties or frictional characteristics of the two materials rubbing against each other. Thus, if a given yarn is rubbed against a glass and a metal surface and the same force N is applied normal to the yarn, the resultant force F of the yarn will be different in two cases because of the difference in μ of the two rubbing surfaces.

7

Fundamentals of yarn tension

As indicated in Fig. 1.2, there are basically two methods by which tension in passive system can be applied on the yarn, additive and multiplicative. In the additive system of tensioner, the running yarn is passed in a straight path between two metal discs with dead weight or spring applying force on one of them (Fig. 1.5). If the normal force applied on the yarn by the loaded disc is N, the same force will be applied on the yarn by the other disc and the total force applied on the yarn is 2N. With this additive tensioning system if a yarn is fed to the tensioner with the input tension Ts, its output tension Tt is T t = Ts + 2 μ N

(1.5)

Thus, the frictional resistance developed is added to the existing that is, the input tension under which the yarn enters the tensioning system or tensioner. With the given type of discs and force applied to it, the output tension Tt is obtained by simply adding the constant (μ N) to the input tension Ts and hence the name, additive. With this tensioning system, the output tension Tt of a yarn fed with the same input tension Ts can be changed by changing the weight or spring pressure that is, — the force N applied through the disc and/ or the surface characteristic that is μ of the discs. The additive tensioner does not need to have a certain initial tension and so it is suitable for tensioning the weft yarn in shuttleless weaving machine. A common example of this tensioning system has been shown in Fig. 1.6 where the weft yarn in a rapier loom is passing between two leaf springs of the tensioner and the pressure exerted by the springs is adding tension to the yarn. Narrow leaf spring of low mass and high rigidity made of carbon fiber composite has been found to be very suitable for the purpose [4].

Figure 1.6 Weft yarn passing through leaf tensioner in rapier loom. In multiplicative system of tensioner, the yarn passes around a curved surface as shown in Fig. 1.7 and thus changes its direction of path while

8

Role of yarn tension in weaving

passing round the surface. The multiplicative tensioning is therefore, the capstan like tensioning system. The output tension of the yarn is now determined by Amontons Law and depends on the angle of wrap in addition to the coefficient of friction between the yarn and the curved surface. If the input tension of the yarn with which it is fed to the tensioner is Ts, the output tension Tt is Tt = Ts e mq

(1.6)

Where, e is the base of the Napierian logarithm,

μ is the coefficient of friction of the yarn on the capstan surface, and θ is the angle of wrap.

Figure 1.7 Multiplicative tensioning system. With the frictional coefficient μ of the curved surface, the angle θ made by the yarn while passing around the surface and e constant, the output tension of the yarn is the constant multiple of the input tension and hence the name, multiplicative. With this tensioning system,, the output tension Tt of the yarn fed with the same input tension Ts can be changed by changing the angle of wrap θ and/or the surface characteristic of the capstan that is μ. It is to be noted that, unlike with the additive system, the required output tension with multiplicative tensioner can be obtained if there is certain input tension. For this reason this type of tensioner is not suitable for tensioning the weft yarn in the weaving machine [4]. A simple example of multiplicative tensioning system is the warp yarns passing over the backrest of the loom, Fig. 1.8.

9

Fundamentals of yarn tension

Back-rest

Warp yarns

Figure. 1.8 Warp yarns passing over the backrest. It is very often observed that the yarn passes around a post and between a pair of discs placed over the post, as shown in Fig. 1.9. Dead weight or spring is usually attached with one of the discs for obtaining the desired tension of the yarn. In this combined system of tensioning, the yarn is subjected to additive tensioning as it passes between the pair of discs as well as to multiplicative tensioning as it passes around the post. The output tension of the yarn is now the function of normal force N and the angle of wrap θ besides the coefficient of friction μ. If the input tension of the yarn is Ts, the output tension Tt is Tt = (Ts + 2m N ) + Ts emq = Ts (1 + emq ) + 2m N

(1.7)

Figure 1.9 Combination of additive and multiplicative tensioning systems.

10

Role of yarn tension in weaving

Yarn tensioning system at the warp creel of the warping machine, shown in Fig. 1.10, is an example of the combined system of additive and multiplicative tensioning. The yarn from the package, usually the cone, mounted on the creel is withdrawn over the end of the package, passed through the tensioner consisting of discs and the post, and turned almost right angle before been taken to the warping or sizing machine.

Tensioner

Figure 1.10 Warp yarn passing through the combined tensioning system in the warp creel.

1.6 Tensioner (or Tension device) In order to suit the different types of yarn, speed of operation and requirements and different types of tensioner or tension devices are employed. Irrespective of the type, the tensioner should be reliable in operation, cleaned and could be adjusted easily, so designed that the yarn can be threaded easily through it and should not affect the twist of the yarn. The tensioner should also not induce or increase tension fluctuations of the yarn passing through it.

1.6.1 Disc tensioner The most popular type is disc tensioner as shown in Fig. 1.5. Tension on the passing yarn is imparted by placing dead weight as shown in the said figure or

Fundamentals of yarn tension

11

by spring on one of the discs. The disc type tensioner is often used along with a capstan. The disc tensioner is suitable for almost all types of yarn of wide range of count and high speed operation.

1.6.2 Gate-type tensioner The gate-type tensioner is a very simple device which consists of a number of pins arranged in a series. The yarn is passed through these pins in crisscross manner with small angle of wrap at each pin, as shown in Fig. 1.11. The output tension of the yarn passing through the tensioner is the cumulative effect of multiplicative tensioning system.

Figure 1.11 Gate-type tensioner.

1.6.3 Roller tensioner Another type is roller tensioner [1]. The device consists of a fixed rubber roller and a movable rubber roller which remains pressed on the fixed roller by the action of a spring to put pressure on the yarn passing between them. As the yarn passes through the nip of the rollers, it causes them to rotate. This is suitable for all types of staple yarn as well as continuous filaments with or without twist.

1.6.4 Capstan tensioner In this system, the yarn guidance element is a capstan roller that is rotated by unwinding yarn wrapped round it [1]. Tension on the yarn is imposed by a mobile rubber-coated roller pressing against the axis of the capstan roller. The main merit of this system is that it does not wear the yarn during processing and hence, it is suitable for delicate yarns.

1.6.5 Tensioner with automatic control system It may often be necessary to provide automatic control in the level of running tension of the yarn during its processing at winding. For this purpose, in

12

Role of yarn tension in weaving

the most common system as indicated in Fig. 1.12, the top disc of the disc tensioner is spring loaded and attached with a lever with a pin fitted at its free end for the yarn to pass over it.

Figure 1.12 Automatic tension control system. As the yarn passes over the pin, it acts on the pin; and depending on its output tension, alters the amount of load applied on it at the disc region through the lever and this in turn changes the magnitude of tension imposed. Thus, if the output tension of the yarn becomes too high, the pressure imposed by the discs is reduced to bring the tension level back to the proper level. This is known as negative feedback [5]. The ideal tensioner will be that which can take care of variations in input tension of a yarn and produce the exact and constant output tension of the desired magnitude. This is achieved by the tension control device based on gravity [1]. This consists of a plastic cylinder containing one or more plastic or steel balls inside. Yarn enters the device at the bottom, passes through the surface of the ball (or balls) and comes out from its top. As the yarn passes through the cylinder if its input tension is high, it has to lift the balls higher against gravity and in the process consumes its extra energy. This as a result compensates for the high input tension of the yarn.

1.7 Tension measurement From the above discussions it is observed that depending on the type of tensioner applied, the output tension of a yarn can be calculated with the help of Eq. 1.5 for additive tensioning, Eq. 1.6 for multiplicative tensioning, and Eq. 1.7 for the combined tensioning systems, provided of course, the input tension of the yarn is known. It is therefore essential to know the value of input tension of the yarn and for that, measurement of tension is necessary.

13

Fundamentals of yarn tension

The instrument with the help of which yarn tension is measured is called tensometer or tension meter. There are different types of tensometers, from the simple mechanical to the intricate electronic. Irrespective of the type, the tensometer should be reliable, easily adjustable, easily threaded, preferably inexpensive, easy to operate and maintain and not be affected by dirt, oil, humidity, etc. It should not affect the twist nor introduce undesired variations in tension of the yarn during operation. The most commonly used instrument works on the three-pulley system. Basic principle of the system is shown in Fig. 1.13.

Figure 1.13 Principle of tension measurement by three-pulley system. The tension head of the tensometer contains the three-pulley system. The pulleys A and B at two sides can rotate freely on their fixed centers and are used as guide pulleys. The middle pulley C also free to rotate on its centre is capable of movement at right angle to a line joining the axes of A and B. The yarn whose tension is to be measured is threaded through the pulleys of the tension head as shown in Fig. 1.13, and as the pulley C is at the centre of the line joining the axes of the pulleys A and B, the path of the yarn through them forms an isosceles triangle. Tension of the yarn imposes pressure on the centre pulley C to deflect it from its normal position. The pulley tries to assume an equilibrium position where the resultant R (Fig. 1.13) of two forces T of the yarn acting at an angle θ is balanced by a restraining device linked to the pulley C. The resultant force R is given by R = 2 T cos q / 2

(1.8)

The deflection of the centre pulley thus depends on the magnitude of the tension in the yarn.

14

Role of yarn tension in weaving

Figure 1.14 Simple handheld tensometers for indicating yarn tension In few simple instruments which work on analog system (Fig. 1.14A), the centre pulley moves against the pull of a spring and a pointer connected to the centre pulley moves over a scale to directly indicate the tension of the yarn. In the modern instrument, the value of yarn tension is indicated digitally as shown in Fig. 1.14B. Whether analog or digital system is used, these instruments which indicate the tension values only are generally held by hand and the yarn whose tension is to be measured is passed through the pulleys as shown in Fig. 1.14. The simple tension indicating systems, discussed above and displayed in Fig 1.14, only gives an idea about the level of static tension of the yarn and may be useful where tension fluctuation is low or where the instant information

Fundamentals of yarn tension

15

of the value of yarn tension is needed. If the tension measuring instrument indicates the tension in analog system and the fluctuations in tension are rapid, the system may not be very useful as the inertia of the moving parts of the instrument will be too great to allow the meter to follow them. Further, the oscillations of the pointer make the reading of the meter difficult. Moreover, it often becomes necessary to get more detailed information on the nature of variation of yarn tension during its processing as the mere knowledge of the value of tension does not serve the purpose. Again it has been observed while measuring the warp tension in a loom that although the static tension values measured at different positions of the crank of the loom follow the similar trend to those of the dynamic values, these are a little higher because on a running loom the warp let-off is actuated and tension equilibrium is attained quickly throughout the warp sheet [2]. For all these reasons, recording of the yarn tension is essential and for that more sophisticate instruments with recording system are necessary. These have been discussed in the succeeding Chapters. As mentioned in the Introduction of this chapter, proper tensions of both the warp and weft yarns are not only crucial in the formation of the cloth on the loom, but astute application and control of yarn tensions at weaving determine largely the performance of the loom and many of the physical properties also of the woven cloth. For the purpose, accurate information on warp and weft tensions is essential. Moreover, the working performances of the looms and also the properties of the cloths are greatly dependent on the qualities of the feed materials prepared at the preparatory processes. In the succeeding chapters, therefore, the important aspects of yarn tension at the weaving preparatory processes, various methods adopted for measuring the warp and weft tensions, natures of their tension variation, effects of different loom settings on the warp and weft tensions and their effects on loom performances and cloth properties have been discussed in details.

References 1.

Adanur S., (2001). Handbook of weaving, Technomic Publishing Company, Inc. USA pp. 66–69.

2.

Badve N.P. and Bhattacharya U., (1960). Study of warp tension variations on Roper and Sakamoto automatic warp let-off motions, Part II, Textile Trends, Volume 3, pp. 55–63.

3.

Booth J.E., (1968). Principles of Textile Testing, Butterworth Scientific, London, p. 414.

16

Role of yarn tension in weaving

4.

Lehnert F., Ballhausen U., and Wulfhorst B., (1990). Analysis of Weft Thread Tensioners for Weaving Machines, Melliand Textilberichte, Volume 71, pp. 257–262.

5.

Lord P.R. and Mohamed M.H., (1976). Weaving: conversion of yarn to fabric, Merrow Publishing Co. Ltd, England, p. 52.

2 Yarn tension at the weaving preparatory process

Abstract: In weaving preparatory processes, warp and weft packages of desired compactness and dimensions are prepared for trouble free operation of the looms and for that, yarn tension plays a very important role. Winding tension of a yarn in building a package and unwinding tension of the yarn at withdrawal from a package depend on many factors such as tensioning device, yarn winding speed, yarn withdrawal speed, package size, method of unwinding, balloon size, yarn count, etc. It is often observed that tension is used to eliminate some of the yarn faults such as weak places. This is not a very prudent idea. Indiscreet application of tension beyond what is necessary to give required compactness and stability to a package, is very likely to cause irrevocable damage to the extensibility of the yarn, particularly of those with low extensibility. In slashing or sizing process, the warp yarns are subjected to stretch at various zones in wet and dry states and the extensibility of most of the man-made fibers are adversely affected at high temperature. In preparing the final warp beam utmost care should, therefore, be taken to maintain correct and uniform tension of all the yarns but without overstraining them. Keywords: winding; unwinding; side withdrawal; over-end withdrawal; ballooning; yarn tensioning; tensioner; pirn winding; warping; creel; warper’s beam; slashing; sizing; yarn stretch; warp beam.

2.1

Introduction

The preparatory processes for weaving, which prepare the feed materials (warp and weft) for weaving, are the most important stages between spinning and weaving. In order to achieve the desired performances of the weaving machine, be it with shuttle or without, as well as the requisite physical characteristics of the fabrics produced, the yarn packages used as the feed materials must be built with proper compactness, stability, shapes, and sizes. Compact and stable yarn packages are also essential for trouble free operations of the various preparatory processes. Tension of the yarn is the key to build such a package of the required density and dimensions and yarn preparatory process is a valuable link between spinning and weaving. Discussions on the role of yarn tension in forming a fabric by weaving and in determining

18

Role of yarn tension in weaving

many of its physical characteristics in addition to achieving desired weaving performance cannot, therefore, be complete without discussing the tension of the yarn at the weaving preparatory processes. Considering the subject matter of the text, discussions in this chapter have been restricted to the various relevant factors pertaining to the yarn tension only, at the different processing stages prior to weaving.

2.2 Weaving preparatory processes As pointed out in the previous chapter, two sets of yarn are required in weaving a cloth. One is warp, which runs along the length of the cloth and the other is weft, which runs along the width of the cloth. Thanks to the technique of weaving, the warp yarns are subjected to much higher stresses because of the more rigorous treatments meted out to them than the weft yarns in a weaving machine. In order to make the warp yarns aptly prepared for that, they require extra and more elaborate preparation. The weft yarns in contrast, are subjected to far less stresses in any type of weaving system and are thus, easily prepared for the weaving process. Moreover, the feed package of weft yarn for weaving, pirn or cone, is made from the single yarn whereas, the warp beam is made from a few thousand warp ends. Therefore, the preparations of the warp and weft yarns for weaving are significantly different and the preparation of warp beam demands more precise and uniform control of tension of all the constituent warp yarns. Figure 2.1 indicates the process flow chart for preparation of the warp and weft yarns for weaving.

Figure 2.1 Process flow chart of yarn from spinning to weaving.

Yarn tension at the weaving preparatory process

19

With the advent of new methods of spinning viz. open-end, air-jet and friction spinning, the spun yarns are wound on large packages. In such cases the packages of weft yarns may not be required to be prepared further and are taken straight to the weaving process with shuttleless looms.

2.3 Winding process The first stage of weaving preparatory process is winding, where the yarn, warp or weft spun in the spinning process is wound to make a suitable package for the next processing. The yarn from the spinner’s package called ring tube or bobbin, produced in the ring spinning, is transferred into a cheese, cone or spool depending on the requirement at the next stage of processing. Winding is, thus, primarily the transfer of yarn from one form of package to another but, under proper tension and in proper manner, as illustrated in Fig. 2.2.

Figure 2.2 Winding process.

20

Role of yarn tension in weaving

The fundamental difference between weft winding for shuttle loom and warp winding is the relative sizes of the feed and delivered packages. In case of weft winding a fairly large feed package like cheese or cone is used to produce a comparatively small delivered package like pirn, while in case of the other, the feed package, ring tube or bobbin is very much smaller than the delivered package, cheese or cone. However, if the delivered package of weft is made for shuttleless looms, then the large package like cone is used straightway.

2.3.1 Unwinding Transferring of yarn from one package to another naturally calls for first, unwinding of the yarn from a feed or supply package and then, winding it on to form the delivered package imparting requisite tension on the yarn in the process for achieving desired stability of the wound package. Unwinding or withdrawal of yarn from a package may again take place (i) sideways, or (ii) over the end.

2.3.1.1 Side withdrawal In this method of yarn withdrawal, the yarn is withdrawn from the package along its side that is across its axis, as shown in Fig. 2.3. The package is so mounted that as the yarn is withdrawn, the package rotates under the withdrawal tension of the yarn but the yarn does not rotate.

Figure 2.3 Side withdrawal.

Yarn tension at the weaving preparatory process

21

As the yarn has to rotate the package, the yarn is subjected to high variations in tension. At the start of unwinding, high tension is imposed on the yarn to overcome the friction between the package and its holder and inertial effect of the stationary package and these depend on the weight of the package. Then, once the package starts rotating, the tension reduces. Again, when the rate of unwinding reduces or yarn withdrawal is stopped suddenly for some reasons, overrunning of the package causes significant fall in yarn tension. This system is, therefore, not very suitable for highspeed operation.

Figure 2.4 Tension on the yarn to rotate the package. From Fig. 2.4, suppose the radius of the package at any stage at which the yarn is withdrawn is r and the tension of the yarn at that stage is t. Then, if all other forces are ignored torque — T = t × r or, t=T/r

(2.1)

Since the radius diminishes as unwinding proceeds, the tension increases and there is an inverse relationship between tension and radius. To annul this problem, the package is generally provided with suitable braking system to impose some control on package rotation. The system should so act that as the diameter of the package with the yarn wound varies the tension in the yarn being withdrawn remains fairly the same all along and prevent over-run when the yarn withdrawal is reduced so as to maintain the unwinding tension of the yarn, as far as possible, constant. The main merit of the side withdrawal system is that, since the yarn does not rotate during withdrawal, the twist of the yarn is not affected.

22

Role of yarn tension in weaving

2.3.1.2 Over-end withdrawal In this method of yarn withdrawal, the yarn is withdrawn from the package along its axis that is over its end and passes through a guide located at a suitable distance from the yarn package on the package axis as shown in Fig. 2.5. The package remains stationary and the yarn rotates around the package while unwinding. As the yarn is withdrawn and unwound from the package at high speeds, the centrifugal force causes the yarn to follow a balloon shape path and the ballooning causes uneven tensions in the yarn.

Figure 2.5 Over-end withdrawal. This method of yarn withdrawal (over-end withdrawal) is suitable for high speed operation as the package does not rotate but every time a yarn coil is unwound from the package, the twist in the yarn is disturbed by one turn in the unwound length. Comparison of unwinding tensions of yarns of nearly the same count from two positions of a package has shown that, irrespective of the unwinding speed, the tension is much lower with the vertical position (over-end withdrawal) than with the horizontal position (side withdrawal) of the package [3].

Yarn tension at the weaving preparatory process

23

2.3.1.3 Ballooning Because of balloon formation, any given section of the unwinding yarn experiences two motions, one along the length of the yarn and, the other in a circular fashion with a certain angular velocity. The configuration of the balloon depends on the distance of the thread guide from the yarn package. Therefore, the position of the guide has important effects on withdrawal tension of the yarn. As the yarn is unwound from the package, the take-off point, A, of the yarn (Fig. 2.6) on the package surface changes continuously and the resultant changes in the balloon height, H directly affect the size and shape of the balloon and thus, the yarn unwinding tension.

Figure 2.6 Effect of ballooning on yarn tension. From Fig. 2.6 let us consider the point where an element of yarn is just leaving the surface of the package and analyze the effects of different forces acting at that point [6]. Owing to ballooning, the yarn at that point will be under tension. This tension may be resolved into three mutually perpendicular

24

Role of yarn tension in weaving

components (i) P which acts parallel to the package axis, (ii) T which acts tangential to the package surface, and (iii) R which acts radial at that point. The parallel component P must be opposed by frictional and cohesive forces between the yarn element under consideration and the neighbouring material or the package surface. The tangential component T will be balanced by a component of the tension in the yarn just about to be removed and the radial component R will be balanced by the tension t of the yarn plus the frictional and cohesive forces which act in that direction. The angle of yarn departure will automatically adjust itself until these conditions are met. The radial cohesive forces will tend to lift the coil of yarn below the departing element and loosen it, while the parallel cohesive force will try to move that underlying coil laterally from its proper position. If the tension in the underlying coil is sufficiently high, the actions of these forces will not have any adverse effect but, if the tension is low, a whole coil can be displaced and eventually sloughoff may occur. This is especially true if the package is nearly parallel, wound with tapered ends. If the package is cross-wound such as the cheese, spool or cone, the helical coils produce greater stability to the package. Now, the parallel force P is opposed by a component of the yarn tension as it exists on the package. The cohesive forces acting are no longer concentrated locally and extend over a much larger surface of the package with the result that many more coils are now involved in contributing to the stability of the package. More importantly, the crossing of the yarns between the successive layers gives an interlocking effect to ensure far greater package stability. Grishin [5] had studied the unwinding tension of the yarn from a cone and derived that the vertical component of yarn tension Tx due to balloon is Tx = mv 2 {2 + k ( H / r ) 2 sin 2 b }

(2.2)

where, m is mass per unit length, v is linear velocity, H is balloon height, r is winding-off radius, β is coil angle, and

k is co-efficient which depends on unwinding conditions mainly on the drag of the yarn on the package. Padfield [8] had derived the unwinding tension of the ballooning yarn from the cylindrical package such as spool or cheese. According to him, unwinding tension T is

Yarn tension at the weaving preparatory process T = mv 2 / g{ A + B( H / r ) 2 }

25 (2.3)

where, g is 981cm/s2, A is a constant depending on the value of air resistance, B is a constant depending upon the unwinding angle and the rest are the same as above. Here, the winding angle is assumed to be positive when the balloon is increasing and negative when the balloon is decreasing with unwinding of the yarn from the package. From the equations 2.2 and 2.3, it may be observed that irrespective of the type of package, unwinding tension of the yarn varies: i.

with m, the mass per unit length that is, coarseness of the yarn,

ii.

as the square of the unwinding speed v that is, with machine speed, and

iii. with the ratio H/r that is, when the balloon height between the guide and the yarn winding-off point becomes large in comparison with the winding-off radius of the package. These two equations are, however, valid when a single balloon exists as shown in Fig. 2.6. Depending on the distance of first thread guide from the yarn package, type of package and winding-off position on the package, multiple balloons are formed. Height of each balloon is then reduced and the yarn tension decreases with practically no variations. Suppose, there are n number of balloons from the thread guide to the winding-off point and the height of each balloon is h, then h = H/n and a straight substitution of h for H in the two equations gives an approximate value for the unwinding tension of the yarn with multiple balloons. It has been found in practice that undesirable high tension is always experienced with unwinding at the base of the supply package, when balloon height is considerably increased and a few number of balloon loop exists. In order to reduce this and level the tension within the reasonable value, unwinding accelerator or balloon breaker is employed. This is in the form of an obstruction placed in the way of the balloon to cause an artificial collapse of the balloon from a low to a high number of loops. It has been found that the resulting lower yarn tension permits the winding speed to be increased without an increase in the yarn breakage rate. Between the two types of package, the unwinding tension is greater from a cylindrical than from a conical yarn package. The tension increases sharply when unwinding takes place from the large end of the conical package that is, from the base [4]. Other factors which affect yarn tension to some extent

26

Role of yarn tension in weaving

are coil angle and conditions of unwinding such as drag of the yarn with air or the package, which are rather difficult to define quantitatively. To avoid undue strain on the yarn it is necessary not only to control unavoidable fluctuations of tension within the allowable limit as unwinding proceeds but also to ensure proper formation of the balloon. As the shape and size of the balloon affect the unwinding tension of the yarn, correct alignment of the first thread guide with respect to the yarn package is very important. As mentioned above, not only the thread guide should be positioned at a suitable distance from the package but also the axis of the package should pass through the thread guide. There is an optimum guide distance from the package where the balloon height is a minimum and hence the unwinding tension is also minimum. If the guide distance is large, slight nonalignment of the guide may not be that much detrimental than when the guide distance is small.

2.3.2 Tensioning (and clearing) Following unwinding from the package, either side ways or over the end, the yarn passes through suitable tensioning system, as shown in Fig. 2.2. As the stability and compactness of the wound package are decided largely by the tension of the yarn during winding, these must be achieved by judicious application of tension. More than required tension will unduly stretch the yarn and destroys much of its extensibility while, less than required tension will produce soft and unstable package which will not unwind cleanly in the next process and slough-off may result. In view of these, running tension of the yarn should not exceed 10 - 12% of the breaking strength of the single yarn [9, 10] to preserve the extensibility of the yarn. Depending on the winding machine and the processing adopted, different types of tensioner, discussed in the Chapter 1, are used and even more than one tensioner can also be employed. While the disc tensioner has been found to lower the rate of increase in tension with cylindrical package that is, spool, with the yarn unwinding speed, it adds to the overall tension of the yarn [3]. Whatever the type or number of tensioner is used, the output tension is always higher than the input tension and hence, some tension units incorporate a compensating feature, as shown in Fig. 1.11 (Chapter 1). It is sometimes advocated that one of the important criteria for imparting tension on the yarn is to break the yarn at its weak places to eliminate that fault, as has been mentioned in the Chapter 1. This does not; however, appear to be an astute idea. Application of tension always tends to stretch the yarn and any tension excess than what is necessary to produce the desired stability to the wound package is very likely to have deleterious effect on the

Yarn tension at the weaving preparatory process

27

extensibility of the yarn. Clearing of any fault of the yarn should better be carried out by the yarn clearer incorporated in the process for the purpose (see Fig. 2.2). Moreover, clearing of every yarn fault is associated with joining of the yarn. So, the aim should be to produce the yarn as much free from fault as possible at the spinning process so that not much clearing is necessary at the succeeding stages.

2.3.3 Winding In the winding process, the yarn is ultimately wound in the desired manner to build a suitable package for further processing. Here, the flangeless package is generally built by laying the yarn cross wise in the successive layers (that is, cross wound package). The package may be cylindrical or conical in shape with the latter being more suitable for over-end withdrawal. Yarn content of a package naturally increases with the length of yarn traverse and wound diameter of the package. Long traverse and large package diameter however, tend to increase yarn tension and in case of cone, the effect of the diameter can be countered by increasing the package conicity. The basic requirement of building a package is to wind the yarn under uniform and constant tension all through for consistent winding as well as smooth unwinding of the yarn from it in the next process. As the package increases in diameter with winding, the winding tension of the yarn normally increases, but it is desired that the yarn tension remains constant. The tension determined by the tensioner will remain constant provided the winding speed of the yarn is constant and the yarn speed will remain constant if the surface speed of the package remains constant during winding.

Figure 2.7 Package drive.

28

Role of yarn tension in weaving

Suppose, at any instant of winding, the radius of the package with the yarn wound is R and the angular velocity of the package is ω, from Fig. 2.7 A. Then, the speed of the yarn V at that instant of winding on the package is V =wR

(2.4)

From equation 2.4, it is seen that as the package increases in radius with the progress of winding, the yarn speed V will increase proportionately if package velocity ω remains unaltered. Yarn tension is the function of yarn winding speed, and to keep it constant, the product ω R will have to be kept constant by reducing ω proportionately with the increase of R. This, in other words, indicates that the surface speed of the yarn package should remain constant throughout winding. If the yarn package is driven directly by mounting it on the rotating spindle, as shown in Fig. 2.7 A, the spindle is rotated through speed variable system to keep the surface speed of the package constant. The simplest way to achieve the constant surface speed of the yarn package, to maintain the winding tension of the yarn constant, which is desired, is to drive the package in surface contact of a rotating drum, as shown in Fig. 2.7 B. Now, at the point of contact between the package and the drum, the velocity of the yarn Vy will remain the same (assuming no slippage between the package and the drum) irrespective of the package radius. Since, the package is driven in surface contact of the drum with a given radius Rd rotating at a given constant angular velocity ωd, Vy = wd × Rd = constant

( 2.5)

For this reason, winding of yarn by simple friction drive of the package by the rotating drum is more popular than by the direct spindle drive with intricate variable speed system, particularly for staple yarns.

2.3.4 Pirn winding The process adopted for winding of a pirn, shown in Fig. 2.8, is different from the regular winding process, shown in Fig. 2.2 and discussed above because, the pirn is used inside the shuttle. For this reason, the yarn in building a pirn is wound crosswise in conical form like in normal cone winding but with a progressive conical traverse with short yarn traverse, more like in building of a bobbin on a ring spinning frame. This type of winding helps to reduce ballooning effects, maintain uniform tension, and reduce the possibility of yarn sloughing-off. In order to ensure sufficient running time of the shuttle in the loom, the pirn should contain the maximum possible amount of yarn in the space available within the shuttle. Also, the pirn should have sufficient stability so that, as the yarn is unwound from the pirn, there should not be any incidence of

Yarn tension at the weaving preparatory process

29

slough-off. Desired degree of stability of pirn is achieved by the progressive conical traverse of the yarn along with layer locking and application of proper winding tension. This type of wind also limits the tension variations induced in the original winding of the package.

Figure 2.8 Pirn winding process. At pirn winding, it is not possible to increase the compactness of the pirn by increasing the pressure between the drum and the package like in normal winding and there is no further need for removal of yarn faults which is dealt with, in the preceding process of winding. The desired compactness of the yarn on the pirn is obtained by tension only of the yarn during pirn winding. Hence, usually a higher level of tension than what is normally necessary for cone or cheese winding is applied in pirn winding. A tension level of 15% of the breaking strength of the single yarn can be considered adequate to produce the pirns of desired compactness and this much tension is not likely to have

30

Role of yarn tension in weaving

any adverse effects on the yarn and cause weft breaks [10, 9]. However, while tensioning, the yarn care should be taken so that it is not over tensioned. Over tensioning will damage the yarn, particularly the filament yarns, and produce defects in the cloth. It is, therefore, very important to control the yarn tension during winding within quite close limits. Tensions of around 75, 60, 40, and 30 g are suitable for coarse, medium, fine and super fine weft yarns [9]. Pirn winding machines are equipped with suitable tension control devices. Usually, disc or gate tensioners are used in conjunction with some form of compensator to even out tension fluctuations.

2.4 Warping process As indicated in Fig. 2.1, while the preparation of weft for shuttle looms ends with pirn winding after the normal winding, preparation of warp proceeds to warping following winding. From here onwards, many warp ends are processed together to produce a warp package in the form of a beam, which in warping is the warpers’ beam. Required number of cones of warp yarns is placed on the creel and the yarns are wound side-by-side under uniform tension to produce a large parallel wound beam. Each warpers’ beam is required to contain as much yarn as possible and hence, each beam has to be prepared hard and compact (except however, those for dyeing). Although any type of yarn package can be used on the creel of the warping machine, the cone is the most suitable for high speed operation because of the reasons stated earlier. It is necessary to control the tension of the yarn individually at the creel and the simple disc type tensioner, shown in Fig. 1.10 of Chapter 1, is usually preferred for the purpose. Each tensioner is placed on the creel close to the package. The average tension of a yarn applied depends on its count and it is usually sufficient to apply as much tension as to prevent snarling and entangling of the yarn. For this purpose, the average yarn tension should not exceed 5% of the single yarn strength. In practice, tension levels of 14 – 16 g for coarse, 10 – 12 g for medium, 8 – 9 g for fine, and 6 – 8 g for super fine counts of yarn may be considered appropriate [9]. In modern machines, yarn tension of 10 – 20 g/end is generally achieved, depending on the linear density of the yarn [7]. Ballooning of the yarn during unwinding from the package on the creel must be well controlled. So, the yarn guides are placed in the correct positions with respect to the packages and there is sufficient space between the packages. It is desired that all the yarns have equal tension as they reach the headstock of the warping machine, so that the beam is built with the same compactness all along and contains the yarns of the same required length. Otherwise, there will be high wastage of yarn at slashing (sizing) owing to non-uniform length

Yarn tension at the weaving preparatory process

31

of warp yarns. Properly prepared warpers’ beam with desired and uniform yarn tension can reduce the yarn wastage by 50 – 75% depending on the count of yarn [1]. The yarn packages are mounted at different positions on the creel along its row and column and this imparts variations in yarn tension. The tension variations show a characteristic pattern in the level of the yarn tension at the headstock and the tension is affected more by the position of the package along the column than by the position of the package along the row of the creel. In case of the former, the tension variation follows an extended “V” shaped pattern with the minimum value at the centre (Fig. 2.9 A) and in case of the latter, it shows a “saw tooth” pattern with teeth inclination in the opposite directions from the centre (Fig. 2.9 B) [9]. Such tension variations are difficult to control but amongst the various types of tensioning system, disc type tensioners are found to satisfy most of the requirements.

Figure 2.9 Yarn tension variations at the different creel positions. At the headstock of the warping machine, the warpers’ beam is built by driving it in surface contact of a drum or directly by the spindle. Irrespective of the type of drive, the beam must be rotated at the same circumferential speed so that all the yarns are wound under the same tension. Otherwise, the yarns wound with varying tensions will also be different in length on the beam. When such beams will be processed at the next stage, the rate at which the ends are unwound will vary and this will result in slack and tight ends. Between the two types of drive to the warpers’ beam, the drum drive causes greater tension variations amongst the ends and the effect is further augmented by the variations in yarn tension at the creel [7]. The ends coming from the back of the creel are under higher tension than the rest and the yarn at the middle of the beam may therefore, be longer in length than those adjacent to the flanges.

32

2.5

Role of yarn tension in weaving

Slashing (or sizing) process

Final preparation of the warp beam for weaving is carried out in slashing or sizing process. Depending on the total number of warp ends required for weaving, one warp beam is prepared from a number of warpers’ beam. Although many of the faults of the warp yarns are removed mainly at winding and perhaps to some extent at warping and the qualities of the yarns are thus much improved, the yarns are not yet good enough to withstand the rigorous abrasive actions at weaving. This is taken care of at slashing or sizing, where the abrasion resistance of the yarns in addition to their strength is increased and their hairiness is reduced by applying a protective coating that is, size, on them. Slashing process consists of five main working zones as indicated in simple diagrammatic form in Fig. 2.10.

Figure 2.10 Different zones in slashing. During the process of slashing, the warp yarns are always subjected to certain amounts of stretch which are unavoidable. Stretching affects the extensibility of the yarns and so it needs to be controlled very carefully so that it does not exceed 2% [9]. If the stretch is more, the extensibility of the sized yarn will be so badly affected that warp breakage will increase at weaving. Yarn stretching at slashing or sizing varies at different parts of the sizing machine. There are five tension zones in slashing (1) beam creel to feed roller of the sow box, (2) sow box feed to final size nip, (3) drying cylinder to guide roller at splitting, (4) in splitting rods, and (5) between draw roller and beam at the headstock. Depending on the type of yarn, different magnitudes of tension are required at these zones. At any stage of sizing higher than requisite tension is very harmful. It tends to cause permanent deformation of the yarn which, as a result, reduces its strength and increases its stiffness. Yarns with greater stiffness yield more easily under load and break. Again, at slashing or sizing, the dry warp yarns are first wetted by application of a size paste and then dried. Thus, the yarns pass through four phases, viz., unsized dry yarn, sized wet yarn, sized drying yarn and sized dry yarn. At each stage, the properties of the yarns are different [1] as they are subjected to stretching once when they are dry and then when they are wet. When the yarns are in wet state, they are generally very sensitive to tensions. The sensitivity differs in magnitude from yarn to yarn and for some types like viscose, for example, which has very low wet strength and

Yarn tension at the weaving preparatory process

33

high extensibility, is very sensitive even at low tension level. It is, therefore, very important to manipulate the yarn tensions very judiciously at each phase at sizing and if possible, maintain sufficiently low tension so that the yarns are never overstrained. Generally speaking, the stretch of yarn at the creel zone should not exceed 0.5% and at the drying and splitting zones about 1.5%, while at the winding zone at the headstock it should practically be nil [9]. Keeping this in mind, the tension of the warp sheet in both wet and dry zones is controlled to restrict the total stretch of the yarn to 1–1.5% [10]. Control of unwinding tension of yarn sheet should be maintained effectively from the creel of the sizing machine as any increase in tension in this zone is automatically transferred through the other zones up to the draw roller where the yarn tension is significantly higher than that at back beam. Yarn tension at the creel depends on the manner the warper’s beams are arranged on the creel [6]. Figure 2.11 shows the various arrangements.

Figure 2.11 Arrangements of warper’s beams on the sizing creel 20. If the warper’s beams are creeled horizontally one after another (Fig. 2.11 A), the yarns of all the sheets will be under equal tension but the space required will be high, which for accommodating more number of beams will rather be unusually large. If the beams are creeled in a zigzag manner (Fig. 2.11 B) then the space required will be much less but the uniformity of tension will be affected adversely. To tackle these problems, a compromise is therefore, made in creeling the beams, as indicated in Fig. 2.11C, where the beams are placed on inclined creel.

34

Role of yarn tension in weaving

The let-off system of the beams at the creel is generally negative but in some cases the beams are driven by positive means to exercise better control on yarn tension. Another important zone for tension control is the sow box. Yarn tension can also be affected because of shrinkage or extension of the yarns as they are at wet state, and it is therefore, necessary to make provision for this dimensional instability. Where comprehensive control on yarn tension during sizing is not provided, the first three cylinders are sometimes made to run freely on precision bearings to permit the yarns to adjust in length without tightening or going slack on the cylinders [7]. Maximum stretch on the yarn takes place between the last drying cylinder and the take-up roller. Again, yarn tension between the size roller and the first drying cylinder is very critical, as the yarns in this zone are wet and hence, even a small tension causes stretching of the yarns, which is stabilized during drying. It is reported that about 70% of the stretch occurs in this zone [1]. The headstock of the sizing machine, where the warp beam is finally built, is similar to that of the warping machine with added facilities for applying predetermined tension. Like in warping, here also the angular speed of the beam is retarded as the beam diameter increases to maintain the preset tension between the winding beam and draw roller. It is naturally required to accommodate as much yarn as possible on the warp beam. This is achieved by controlling the winding tension of the yarn and employing the press roll. While increasing the compactness of the beam to accommodate longer length of warp yarn, care should be taken that the beam is not built “rock-hard”. Warp yarns of such a beam will unnecessarily be strained and the beam will lose some of its resiliency which is not good for weaving. Study [2] on jute yarn has shown that proper adjustments of the yarn tensioner at the creel of the warping machine and the braking force of the warper’s beams at the creel of the sizing machine minimize the variation in tension amongst the warp yarns in a warp beam and thereby, the warp breakages at warping, sizing and weaving. For achieving high weaving efficiency, especially, in shuttleless weaving machines where the depth of shed is much less than that in shuttle loom or with multi-width applications, the warp yarns must not only be well sized, but every warp end must also be evenly tensioned with minimum stretch. If the tension amongst the yarns varies beyond acceptable limits, the taut ends would break repeatedly and the slack ends will cause the drop wires of the warp stop motion stop the loom. As regard the man-made fibers, most of them being highly extensible, especially, at high temperature, proper control of tension and temperature are very important at slashing of such yarns. Again, the effect of stretching is even more critical on the filament yarns than on the staple yarns of man-made fibers and adequate care should therefore, be taken while slashing such yarns [6].

Yarn tension at the weaving preparatory process

35

As discussed above, stretching takes place in many zones of sizing but, the maximum stretching occurs between the last drying cylinder and the take-up roller at the headstock. To reduce the extent of stretching, the sheet of filament yarns is split in the wet state and then partially dried in the separated condition rather than following the usual practice of splitting them after drying, as done with staple yarns. Weaving preparatory processes play major roles in achieving the desired performance at weaving. Ultimate packages of warp and weft yarns prepared at these stages are used as the feed materials for looms, where the yarns are unwound during the course of weaving. How best the yarns can be unwound from their respective packages depend on how best the packages have been prepared at the various preparatory processes. Requisite shape and size of a yarn package needed for a given type of loom can be prepared if the yarn is wound under optimum tension with as little variation as possible. Too low winding tension will result in soft packages and cause irregular withdrawal of the yarns while, too high winding tension will destroy much of the extensibility of the yarn and make the yarn vulnerable to breakage. The qualities of the feed materials for weaving have become even more stringent after the introduction of the high speed shuttleless weaving machines, which with relatively small depth of shed, demand greater uniformity in tension of all the warp yarns and smooth withdrawal of the weft yarn from its package for faultless insertion. It may therefore, be said that properly prepared warp beams and weft packages assure half the desired performance of a loom.

References 1.

Ajgaonkar D.B., Talukdar M.K., and Wadekar V.P., (1982). Sizing materials methods machines, Textile Trade Press. Ahmedabad, India, pp. 5–398.

2.

Bhattacharyya P.K. and Banerjee B.L., (1984). Control of jute yarn tension during preparatory processes, Indian Journal of Textile Research, Volume 9, pp. 123–129.

3.

Bhattacharyya P.K., Bhattacharyya A., and Banerjee B.L., (1983). Unwinding tension of jute yarn from a package, The Indian Textile Journal, Volume 93, pp. 79–84.

4.

Efremov R.D., (1972). The unwinding of a cross-wound cylindrical cheese, Technology of the Textile Industry U.S.S.R., No 2, pp. 56–58.

5.

Grishin P.F., (1934). Report of the Central Research Institute for the textile industry, Moscow.

36

Role of yarn tension in weaving

6.

Lord P.R. and Mohamed M.H., (1976). Weaving: conversion of yarn to fabric. Merrow Publishing Co. Ltd., England. pp. 56–118.

7.

Ormerod A., (1983). Modern preparation and weaving machinery, Butterworth & Co. (Publishers) Ltd., UK. pp. 43–101.

8.

Padfield D.G., (1956). A note on the fluctuations of tension during unwinding, The Journal of the Textile Institute, Volume 47, pp. T301– T308.

9.

Paliwal M.C. and Kimothi P.D., (1974). Process control in weaving, ATIRA, Ahmedabad. pp. 25–90.

10. Talukdar M.K., Sriramalu P.K., and Ajgaonkar D.B., (1998). Weaving machine mechanism management, Mahajan Publishers Pvt. Ltd. Ahmedabad. pp. 445–446.

3 Warp tension measurement

Abstract: Tension of warp yarns during weaving is essential for formation of clear shed (which although imposes appreciable strain on the yarns) and achieving required beat-up force for proper interlacement of the warp and weft yarns to form the cloth. Much needed information on the nature of variation in warp tension, which helps to achieve desired performance of the loom, can be obtained if precise and correct measurements as well as recording of warp tension during the course of weaving are made. For the purpose, various methods have been adopted and gadgets have been devised to measure the tension of the whole warp sheet, the bunch of warp yarns and the single warp yarn. All these methods including the sophisticated electronic yarn tenometer, presently used for measuring the yarn tension, have been discussed here. Keywords: warp tension; warp tension measurement; tensometer.

3.1

Introduction

Certain amount of tension of the warp yarns is essential first to obtain a clear shed and then to achieve required beat-up force for proper interlacement of the warp and weft yarns. The basic warp tension during weaving is controlled by the let-off motion of the loom. The amount of warp tension required for satisfactory weaving depends on various factors and a clear knowledge and information on this are crucial for ensuring satisfactory weaving. Warp tension is so important a parameter in the production of the woven fabric that, some weaving machine manufacturers supply their machines permanently fitted with tension measuring devices for continuous checking of warp tension during weaving. Requisite information on warp tension and the characteristics of its variation can be obtained if only the tension of the warp yarn is measured and recorded properly so that these can be utilized to take appropriate measures for adjusting the settings of the loom, reducing the undue tension of the yarn and consequently improving the efficiency at weaving.

3.2

Passage of warp yarns through the loom

Figure 3.1 illustrates the cross-sectional view of a loom with the passage of warp yarns through it. The sheet of warp yarns is fed from the warp beam and

38

Role of yarn tension in weaving

passes over the back rest, through the drop wires of warp stop motion (or the lease rods in some shuttle looms), the heald shafts (or the mail eyes of healds in case of jacquard) and the reed. During weaving, the warp sheet is separated into two layers by the heald shafts to form the shed in front of the reed for insertion of weft. The cloth woven as the result of interlacement between the warp and weft yarns, passes over the front rest, around the take-up roller and finally wound on the cloth roller.

Figure 3.1 Passage of warp yarns through the loom. If we consider the entire path of the warp sheet from the warp beam at the rear to the cloth roller at the front of the loom, we find that the portion of the sheet from the warp beam to the fell of the cloth in front of the reed consists of the individual yarns and that from the fell to the cloth roller is transformed into a fabric because of interlacement with the wefts. As a result, the tension of the warp yarn is concerned mainly at the yarn zone from the warp beam to the cloth fell while its effects propagate into the fabric at the fabric zone of the loom. The tension imposed on the warp yarns during the course of weaving are mainly owing to the formation of the shed by the reciprocating movements of the healds carrying them and the beating of the weft by the forward movement of the reed.

3.3

Stretch and strain of warp yarns due to shedding

Before discussing the various aspects of warp tension it would be more appropriate to discuss first the stretch and strain experienced by the warp yarns on the loom as, the tension imposed on the yarn is the result of these. During shedding, the warp yarns move alternately up and down to form the shed and this causes stretching and therefore, strains on the yarns. Strain on the warp yarns due to shedding increases with the speed at which they move

Warp tension measurement

39

to form the shed and with the square of the depth of shed. The magnitudes of stretch and strain on the yarns depend on the lifts and positions of the heald shafts with respect to the fell of the cloth. Let us now see how the stretch and strain of the warp yarns are related with the positions of the heald shafts. Figure 3.2 shows the geometry of a shed with two heald shafts.

Figure 3.2 Warp stretching due to shed formation. Suppose a is the distance of the first heald shaft from the fell of the cloth, b is the distance between the successive heald shafts, that is pitch, c is the distance of the second heald shaft from the drop wire of the warp stop motion,

α is the half of the shed angle that is the angle formed between the top or bottom shed line and the horizontal line AH, θ1 and θ2 are the angles made by the yarns of the first and second heald shafts, respectively, with the line AH at the back zone of the shed, and

S1 and S2 are the shed depth that is, the extent of movement of the warp yarns to form the shed, at the first and second heald shafts, respectively. From definition, Stretching of yarn = Increased length – Original length. Therefore, from triangle ABH, the stretch of the yarns of the first heald shaft while forming the top shed is (AB + BH) – AH = {a / cos α + (b + c) / cos θ1} – (a + b + c)

(3.1)

and from triangle AEH, the stretch of the yarns of the second heald shaft while forming the top shed is

40

Role of yarn tension in weaving

(AE + EH) – AH = {(a + b) / cos α + c / cos θ2} – (a + b + c)

(3.2)

From equations 3.1 and 3.2 it is observed that in case of the second heald shaft, the value of the factors in the left hand side bracket is higher than that of the same for the first heald shaft as θ2 > θ1, and therefore, the yarns of the second heald shaft are stretched more. Again from definition of strain, Strain on yarn = Increased length / Original length. Therefore, from the same triangle ABH, strain on the yarns of the first heald shaft while forming the top shed is (AB + BH) / AH = {a / cos α + (b + c ) / cos θ1} / (a + b + c)

(3.3)

and similarly from triangle AEH, strain on the yarns of the second heald shaft while forming the top shed is (AE + EH) / AH = {(a + b) / cos α + c / cos θ2 } / (a + b + c)

(3.4)

Here also, it is found that since the second heald shaft moves through a greater distance than the first heald shaft (S2 > S1) to maintain the same shed angle, it makes a greater angle with the central line AH at the back zone. This, as a result, imposes greater strain on the warp yarns controlled by the second heald shaft. Because of the greater movement or lift of the second heald shaft compared to that of the first heald shaft, the warp yarns of the second heald shaft are expected to experience higher magnitude of tension. Thus, farther a heald shaft is from the fell of the cloth higher will be its lift and therefore, greater will be the stretch and tension of the warp yarns controlled by it. The size of the front shed from the heald shaft to the cloth fell is smaller than that of the rear, from the heald shaft to the drop wire (see Fig. 3.1) and the yarns with the given lifts will be stretched more at the front shed. The deleterious effects of excessive stretching at the front shed will be abated to some extent if the free length of the warp is not too short at the rear shed. For this reason the free length of the warp yarns at the rear shed is much larger in the modern weaving machines than in the older types of loom.

It is, however, very difficult to measure the angles α, θ1 and θ2 in actual practice from the loom and the stretch and strain on the yarns can be calculated alternately from the length of the stretched yarn between the fell of the cloth and the drop wires, depth of shed and the distance AH. For example, considering the same triangles ABH and AEH for the first and the second heald shafts, respectively, stretch on the yarns of the first heald shaft =

{

}

a 2 + ( S1 / 2) 2 + (b + c) 2 + ( S1 / 2) 2 − (a + b + c)

(3.5)

41

Warp tension measurement

and that on the yarns of the second heald shaft =

{

}

(3.6)

}

(3.7)

}

(3.8)

(a + b) 2 + ( S 2 / 2) 2 + c 2 + ( S 2 / 2) 2 − (a + b + c)

Similarly, strain on the yarns of the first heald shaft =

{

a 2 + ( S1 / 2) 2 + (b + c) 2 + ( S1 / 2) 2 / (a + b + c)

and that on the yarns of the second heald shaft =

{

(a + b) 2 + ( S 2 / 2) 2 + c 2 + ( S 2 / 2) 2 / (a + b + c)

It may now be thought that in order to reduce the strain on the yarns the lift of the heald shaft should be small. Reduction in heald lift reduces the shed angle and the resultant smaller shed interferes with the passage of the weft insertion element, particularly if it is shuttle. In shuttleless weaving machines, considerably smaller weft insertion element like projectile or rapier or even air or water is used for inserting the weft and comparatively much smaller depths of shed are therefore, arranged there. Stretch and strain and thereby, tension of the warp yarns are also dependent on the type of weave, loom speed, time allotted for shed change, weft density, and as pointed out above, the free length of the yarn between A and H in Fig. 3.2. For instance, if the weft density is p per cm, length of yarn between A and H is y cm and loom speed is n picks per minute and then, to weave plain cloth the yarn AH is stretched py times for py/n minutes before it is made into the cloth. The movement of a warp yarn due to shedding is small at first and then, gradually, increases until it reaches the heald eye where the movement and thereby, the strain and tension are the greatest. The movement of the yarn then gradually decreases again as the yarn approaches the cloth fell. The warp yarns also experience a great deal of friction with different parts such as the back rest, drop wires, heald eyes, and reed wires of the loom. In addition, they are also exposed to interyarn friction particularly during shedding and also during interlacing with the weft yarns while forming the cloth. The cyclic stress causes deformation and affects the breaking strength of the yarn [9]. Breaking force of the warp yarns progressively reduces as the warp proceeds towards the fell of the cloth to form the fabric with weft. Deformation of the warp yarn caused by higher tension also depends on the type of weave. Compared to the twill and sateen weaves, the plain weave has the maximum number of interlacement with the weft in a given area and hence, the highest deformation and thereby, the maximum reduction in the breaking force of warp yarns occurs in case of plain weave [9].

42

Role of yarn tension in weaving

According to the shed geometry, shown in Fig. 3.2, the yarns of a given heald shaft move to the equal distance from the central position AH to form the top and bottom shed lines and hence stretch or strain on them is the same at both the shed lines. This, in other words, indicates that the tension of the warp yarns of the said heald shaft will vary to the same extent whether the yarns are forming the top shed or the bottom shed. In actuality, however, the heald shafts are generally made to move through a greater distance while forming the bottom shed and stretch and strain and therefore, the tension of the yarns are greater at the bottom shed line. Detailed discussion on this has been made in Section 7.3.1 of Chapter 7.

3.4 Warp tension control by let-off motion To ensure efficient working of the loom and desired qualities of the cloth produced, the warp yarns are required to be fed up to the required extent and under a certain optimum tension in each pick, determined by the type of cloth to be woven, throughout the weaving out of the warp beam. This is achieved by the let-off motion of the loom. Shedding and beating operations cause rapid fluctuations of warp tension, but the let-off motion maintains the preset average level of tension of the warp sheet by controlling the rate of delivery of the warp yarns. Under the normal weaving condition, the warp tension during the course of weaving is, therefore, the sum of static and dynamic tensions of the yarn. While the static component is determined by the warp let-off system of the loom, the dynamic component is determined by the shed geometry, shed timing and cloth structure. The let-off mechanism employed is fundamentally of two types, negative and positive or controlled.

Warp tension measurement

43

Figure 3.3 Warp let-off motions of the loom. In the negative let-off system, shown in line diagram in Fig. 3.3 A, the sheet of warp yarns from the warp beam passes over the back rest which is either fixed on the loom frames or in most cases, oscillated to a fixed amplitude by a cam and the warp beam is held tightly by chains at two sides of the loom. A chain is wrapped one and half turns round the beam ruffle and anchored with the loom frame at one end and attached with a weight lever with a dead weight placed on it at the other. The same system is employed on both sides of the warp beam. Total gripping force on the warp beam and thereby, the tension of the warp yarns is thus, determined by the amount of the dead weight and its distance from the fulcrum of the weight lever at both sides of the beam. As the rotation of the warp beam to deliver the warp yarns is effected by the pulling force exerted by the warp sheet on the beam, the position of the dead weight at each side is shifted manually towards the lever fulcrum so that the angular rotation of the warp beam gradually increases in accordance with the reduction in diameter of the beam. This is essential to maintain the tension of the warp yarns as far as possible constant throughout weaving. The negative let-off motion is generally employed in the orthodox, over-pick shuttle looms. In the positive let-off system, shown in simplified form in Fig. 3.3 B, the basic criterion is that the required angular rotation of the warp beam, whether intermittent or continuous, is effected by positive means, generally by gear drive, and the amount of rotation of the beam is determined by the back rest. It is however, a misnomer to call it the “positive let-off system”. In the true positive let-off mechanism, the warp sheet is fed through a pair of positively driven metal rollers [14]. Each roller driven by gears is fluted and covered with elastic oil resistant rubber sleeve. The rotations of the rollers are synchronized with the take-up motion of the loom and depending on the type of cloth being woven, a let-off change wheel, incorporated in the gearing

44

Role of yarn tension in weaving

system, controls the weaving tension of the warp yarns. This system has, however, not been adopted commercially (and hence, not shown here) and so, the more popular system, in which the angular rotation of the warp beam is controlled by a sensing back rest (Fig. 3.3 B), is generally referred to as the positive (or controlled or sometimes the semi-positive) let-off mechanism and the same has been followed in this text. In this system, the back rest is carried by brackets fitted on each side frame of the loom and attached to springs or dead weights. The back rest continuously senses the tension of the warp sheet passing over it and controls the rotation of the warp beam by a Positive Infinite Variable (PIV) system, as shown here, or by other means such as ratchet and pawl, friction clutch, etc. As the warp is woven out, the diameter of the warp beam gradually decreases, the overall tension of the warp sheet increases (because of the decrease in the length of the warp sheet from the beam to the fell of the cloth), the back rest is depressed and thereby, the angular rotation of the warp beam is increased proportionately to maintain the preset tension of warp constant by delivering nearly the same required amount of warp yarn in each pick cycle. Because of its positive control on the rotation of the warp beam and therefore, its ability to maintain the level of average warp tension constant, the positive let-off motion is invariably adopted in the modern shuttle and shuttleless looms.

3.5

Measurement techniques of warp tension

Over the period of time various methods have been used to understand the tension of the warp yarns during weaving. Some of these can merely provide a general idea about the overall tension of the whole or a part of the warp sheet, while the others can provide fairly accurate information of the exact nature of tension of the single warp end during the course of weaving. Generally, three methods are adopted for the measurement of warp tension in a loom. These are (i) the average tension of the whole warp sheet, (ii) the average tension of a bunch of warp yarns, and (iii) the detailed tension of the single warp yarn.

3.5.1 Tension measurement of the whole warp sheet In the positive let-off motion of the loom (Fig. 3.3 B), total pressure exerted on the back rest by the springs and/or the weight may indicate the total tension imposed on the warp sheet when the loom is at rest [17]. Figure 3.4 indicates such a system in simplified form where, the tension is applied on the warp sheet by the sensing back rest attached to a lever carrying the dead weight.

45

Warp tension measurement

Figure 3.4 Tension application on warp by weight-lever system. The force exerted on the warp sheet for obtaining the required warp tension depends on the weight-lever moment along with the weight of the rod connecting the lever with the back-rest bracket carrying the back-rest and the relevant dimensions of the bracket. The weight-lever moment is the product of (a) the weight of the dead weight A and that of the lever B itself, which carries the dead weight, and (b) the distance a, of the dead weight from the lever fulcrum C. If the force applied by the weights of the lever B and the connecting rod D is e, then according to Fig. 3.4, the total force F, exerted on the warp sheet, is F = (A × a/b × c/d) + e

(3.9)

When the loom is in operation, the extent of oscillation of the weightlever or the sensing back rest (because of fluctuation of warp tension), which mainly depends on the type of weave, may give an idea about the overall tension variations of the warp sheet. Back rest, as the means for measuring warp tension, has also been adopted to study the influence of lengths of the warp sheet and the cloth in the loom on warp tension and warp breakage [10]. Tensions of the warp sheet and the cloth are measured on the back rest and the breast-beam, respectively. For measuring warp tension, deflection of the back rest caused by change in aggregate tension of all warp yarns has been measured by a resistance strain gauge bonded to it and the result is then fed to a four-channel electronic amplifier and recorded on an oscillograph. Tension of the cloth has likewise been measured at the breast-beam which, for the

46

Role of yarn tension in weaving

purpose, has been replaced by a special roller. The results of cloth tension have been discussed in Section 7.10 of Chapter 7. The overall tension of the entire warp sheet in both, static and running, conditions of the loom can also be indicated by the lease rod gauge [8,16], which essentially works as a lease rod and replaces the back lease rod, as shown in Fig. 3.5. Because of very large cyclic fluctuation in warp tension during weaving (discussed in the next chapter), the device is damped so heavily that it indicates the mean warp tension only.

Figure 3.5 Lease rod-type warp tension measuring device. The device is composed of a long compressible thin rubber tube filled with air and connected to a manometer in the form of a glass “U” tube which contains coloured liquid and is marked on its surface or attached to a scale, as shown in Fig. 3.5. The rubber tube is placed between two thin and light metal sheets. The rubber tube, with the metal sheets, is placed between the two layers of warp sheet like the lease rod. During the course of weaving, the varying pressure of the warp sheet on the rubber tube causes the coloured liquid in the manometer to rise and fall and thus, indicates the tension of the warp sheet. The main merit of the system is that, as the device serves as the lease rod, it does not deflect the warp yarns from their normal path on the loom, as shown in Fig. 3.5, but the actual warp tension is likely to be influenced by the viscosity of the liquid. Moreover, since the warp tension is indicated by the vertical component of the warp tension acting on the gauge and this depends on the angle at which the warp yarns are situated when they pass over it, the location of the device in relation to the normal first lease rod and the back rest is very important.

Warp tension measurement

47

Weight-lever oscillation of the positive let-off motion can only give a qualitative idea of warp tension and not any accurate measurement, mainly because the resultant oscillation of the weight-lever as the consequence of movement of the sensing back-rest is conveyed through a number of movable links and levers which naturally introduce friction in the system. Similarly, the deflection of the back-rest caused by tension of the entire warp sheet will give an idea of the overall tension variation of the warp yarns in a pick cycle and not the difference in tension in the successive picks or between the yarns. On the other hand, the lease rod tension measuring system is particularly useful for indicating the static tension of the whole warp sheet which in turn may help proper adjustment of the let-off mechanism of the loom.

3.5.2 Tension measurement of the bunch of warp yarns In practice, it often becomes necessary to ascertain the general level of warp tension variation during weaving in a loom so as to correctly set and adjust the warp tension of a group of looms weaving the same type of fabric. In such cases, measurement of the tension of a group of warp yarns rather than of the single yarn may serve the purpose. Dynamic tension of a group of warp yarns of the warp sheet can be measured with the help of shell-type gauge, generally known as Wetzel gauge [5]. It consists of a rectangular shaped swallow channel A, of spring-brass plate as shown in Fig. 3.6. The channel has two guide rods, B and C, at the two ends and a deflecting rod D at the centre. The shell has two strain gauges, E and F, bonded at its centre, one at each side of the plate for thermal equilibrium. Two strain gauges form two limbs of Wheatstone bridge, the output voltage of which varies in proportion to the extent of bending experienced by the shell.

Figure 3.6 Shell-type gauge for warp tension measurement. To measure the warp tension, the shell is first placed over a selected group of warp yarns whose tension is to be measured and then, the deflecting

48

Role of yarn tension in weaving

roller D is attached to the shell with the said group of yarns passing over it, as demonstrated in Fig. 3.6. The gauge is conveniently placed in a region between the back rest and the warp stop motion of the loom. During weaving, the dynamic tension of the warp yarns causes deformation of the shell, which in turn results in variation in output voltage. This is recorded as variation of tension of the selected group of warp yarns. The method of threading of the warp yarns through the shell-type gauge, shown in Fig. 3.6, and the device itself may influence the accuracy of the measurement of warp tension to some extent. When the warp yarns are threaded through the gauge, they are diverted from their normal path and this may register higher tension owing to the extra tension imposed on them. Friction between the gauge and the yarns also influences the deformation of the shell under the tension variations undergone by the yarns. Moreover, the additional weight of the gauge may also affect the tension of the yarns. With a view to avoid these problems, Mallah [12] had considered measuring the force exerted by the heald shafts (controlling the warp yarns) during operation. For the purpose, the heald shaft, whose yarns are to be measured for tension, is connected with an electrical resistance strain gauge at the top where it is fastened with its reversing mechanism (negative shedding) in the loom. As indicated in Fig. 3.7, the vertical component of the force, F on the heald shaft can then be measured and converted into tension of the warp yarn having measured the angle, θ made by the warp sheet at the mail eyes of the heald shaft at that instance.

Figure 3.7 Warp tension measurement from the force exerted on the heald frame.

Warp tension measurement

49

Here also, the force exerted on the heald shaft under consideration is the cumulative effect of all the warp yarns controlled by it. Although this method is apt for measuring both the static and dynamic tensions of the group of warp yarns, its main problem is, when the heald shaft is at the closed shed position and the warp yarns are under the least tension, the vertical component of the force on the heald shaft is zero. During the period of shed change when a heald shaft moves from top to bottom or bottom to top, its warp yarns momentarily lose contact with the mail eyes, as the ends then change contact from one end of the eyes to the other (as the diameter of the mail eye is larger than the diameter of the warp yarn passing through it). As a result, even though the yarns are still under certain tension at this changeover stage, they cannot exert any force on the healds controlling them. Again, it is hardly likely that each yarn of a heald shaft will exert identical force at any given time. It is, therefore, not possible to obtain the actual tension of the single warp end. These problems have later been tried to be overcome by introducing single thread strain gauge technique [12]. A short length of fine wire is attached giving slightly twisted around the warp yarn whose tension is to be measured. This has been found to measure the tension of the single warp yarn, even at the closed shed position. But this technique too is not free from some shortcomings. Difference in extensibility of the strain gauge wire and the warp yarn and the twist of the yarn affect the tension values of the yarn. Lunenschloss & Schlichter [11] have also considered measuring the tension of a bunch of warp yarns as, according to them, it has higher degree of measuring sensitivity as compared with other measuring systems, particularly for recording the fall in warp tension when the loom remains at rest during a stoppage (see Section 10.6.1 of Chapter 10). They have found that the bunch containing 60 to 250 warp ends can be regarded as adequate for the purpose. Dynamic tension of the group of warp yarns can be measured and indicated by WIRA tension meter [2]. The measuring head of the instrument comprises two fixed guides and a centrally mounted cantilever tube attached to a silicon strain gauge. The meter measures the aggregate average tension within the range of 0–25 kg and 0–50 kg of the warp yarns in a strip of 12.7 cm width. Modern electronic tension measuring instrument, “Weave Master” [3], can also measure the tension of a single as well as of a group of 20 to 80 warp yarns by employing the suitable high-frequency yarn tension measuring head. The instrument can also measure the weft tension and the data of the warp and weft tensions can be processed on a computer and the desired information displayed on the monitor for taking corrective measures in the weaving machines.

3.5.3 Tension measurement of single warp yarn As mentioned earlier, the different systems discussed so far for indicating warp tension will only give a general idea about the trend of variation of

50

Role of yarn tension in weaving

tension of the warp sheet during the course of weaving from its preset level of tension but no details of warp tension characteristics. Moreover, as they measure the tension of either the whole warp sheet or the group of warp yarns, they cannot always distinguish the differences in the nature of warp tension variation owing to the different types of weave, constructional features of the fabrics and, the mechanical setting of the looms. In order to obtain the quantitative values of individual warp end and acquire more useful information, measurement of tension of single yarn is, therefore, essential as it is more likely to reveal the significant features which may otherwise remain concealed. As indicated in Section 1.7 of Chapter 1, in addition to measurement of the single yarn tension, tracing of tension is also needed for detailed practical investigation and research for studying and analysing the various aspects of settings and timings of different loom mechanisms and the dimensional and physical properties of the cloths. The deflections of the centre pulley of the three-pulley system of the tension measuring head, discussed in Chapter 1, as the result of variations in tension of the running yarn threaded through it causes the tension recorder attached to it to trace the variations in yarn tension against any selected time scale. While measuring the tension of the yarn during weaving, the yarn should not come out of the measuring unit. That becomes possible if such a region of the warp sheet on the loom is selected where the yarns only move along, and not across, their axes. The tension of the warp yarns is, therefore, conveniently measured in the region between the warp beam and the back-rest or between the back-rest and the drop wires of the warp stop motion (or the lease rods) of the loom. While measuring the tension in this region, it however is to be taken into account that, the actual value of the tension of the warp yarn under consideration will be higher in the weaving zone from the heald shafts to the fell of the cloth because of its comparatively shorter length (see Fig. 3.1) and the frictional effect of the mail eye of the heald through which the yarn passes. It is important that the tension measuring unit as a whole is supported independently of the loom so that the loom vibration is not transmitted to the recording system of the tensometer. Owen [13] was possibly the first to make a fairly detailed study on the tension of the single warp yarn during weaving with the help of an instrument designed for the purpose. The instrument, the main measuring unit of which is shown in Fig. 3.8, records the tension by photographic method. It consists of three pulleys made of duralumin. The middle pulley, A, is floating and senses the tension of the yarn and the other two, B are fixed guide pulleys. All the pulleys are mounted on jewel bearings. The warp yarn to be measured passes round the first guide pulley, the floating

Warp tension measurement

51

pulley, the second guide pulley and then over the back rest (not shown). The floating pulley is attached by a thread to a flat spring, C made of steel strip of 9.0 × 0.3 × 0.11 cm dimensions. Under proper conditions, the instantaneous tension in the yarn produces a proportional deflection of the spring, by which the tension may be measured. To ensure proper function of the device, the motion of the spring is critically damped so that it takes up its position immediately without any vibration. For this purpose the spring carries a damper D which is immersed in a damping medium, a very viscous liquid. Critical damping is secured by adjusting the temperature of the damping medium. The vibration of the spring, owing to variation in tension of the warp yarn under test sensed by the floating pulley A, is suitably traced to record the warp tension. A mirror (not shown) is attached to the spring and as the spring vibrates, the mirror deflects a beam of light across a strip of bromide paper moving inside a camera. The yarn tension is traced on the bromide paper. The bromide paper can be driven at the desired speed past the camera lens.

Figure 3.8 Tension measuring system with photographic recorder. Measurement of warp tension has also been made with the help of cathode ray oscillograph [7]. The general principle of this method is that a fine pencil of electrons, generated from a heated cathode and suitably focused by electrostatic fields, is made to impinge at the centre of a circular fluorescent screen forming one end of the tube, where it produces a small luminous spot. The electron beam can be deflected from its position of rest by means of direct or alternating voltages applied to two sets of deflecting plates situated within the tube. One of the plates produces deflection in the horizontal direction and the other one in the vertical direction. Thus, if two quantities which vary simultaneously can be translated into voltages varying in similar

52

Role of yarn tension in weaving

fashion and these two voltages are applied simultaneously to the two plates of the oscillograph, then the luminous spot on the screen will trace out the curve which relates the two variables to one another on Cartesian co-ordinate. The two variables, which are converted into voltages, are (i) the tension in a selected warp yarn and (ii) the angular displacement of the loom main shaft from some arbitrarily selected starting point. The yarn tension measuring system is shown in Fig. 3.9. The warp yarn, whose tension is to be measured, is threaded through the usual three-pulley arrangement, of which the middle one, A, senses tension of the warp yarn and the pulleys, B, at two sides are the guide pulleys. The pulley, A, is hung by means of an adjustable suspension from the centre of a flat steel spring, C, which is a gramophone motor spring of 4.45 × 1.9 × 0.15 cm dimensions. All the pulleys are mounted on needle-point bearings. The selected thread is picked up as close to the back rest as practicable. Variations in the tension of the selected warp thread cause corresponding minute deflections of the spring and hence, variations in the capacity of the condenser, which are subsequently converted into voltage variations by means of a suitable electric circuit.

Figure 3.9 Tension measuring system by cathode ray oscillograph. Almost similar method with three-pulley system has been used in measuring the tension of single warp end with the aim of studying the nature of variation of warp tension with two types of let-off mechanism in the shuttle looms

Warp tension measurement

53

[4]. One of the let-off motions is of friction type with auto control system and eccentric operated oscillating back-rest and the other is of positive type with gear drive to the warp beam and spring loaded back-rest for sensing the warp tension. The changes in tension of the warp yarn are converted into corresponding changes in capacity between two electrodes by a tensiometer which consists of a capacity plate fixed at the top and a bottom capacity plate supported by a gramophone spring of 6.0 × 1.9 × 0.22 cm dimensions and carrying a floating pulley. The changes in capacity are fed to a proximity meter and the corresponding voltage variations are fed to a cathode ray double beam oscilloscope. The changes in amplitude of the light beam are recorded photographically on a 35 mm film in the form of tension trace. Mechanical system has also been devised to measure the static tension of single warp yarn with a hand operated instrument [18]. The instrument, shown in Fig. 3.10, basically consists of three forks in a row of which the first, A, and the last one, B, are fixed. The yarn under measurement is threaded through the prongs of the three forks. Each prong is 1.3 mm in diameter and the distance between the two prongs of the fork is 2.54 mm. The middle fork C, is attached with a spring and turned manually to deviate the yarn from its straight path to measure its tension. The extent of deviation of C is measured as the tension of the yarn. The main drawback of this system is that the yarn, under test, tends to wrap round the forks and thus, influence the actual tension of the yarn.

Figure 3.10 Static warp tension measurement by hand operated instrument. All these systems discussed above are rather cumbersome and the deflection of the middle (or sensing) pulley either indicates that static warp tension [18] or the variation in dynamic warp tension is relayed to the recording system by mechanical means which naturally takes time and is also not very accurate. These shortcomings can be overcome by the use of modern electronic tension meter with three-pulley system [20]. In this, the centre pulley is mounted on a cantilever spring system and the deflections of the pulley cause the springs to move between the plates of a capacitor system. Variation in yarn tension

54

Role of yarn tension in weaving

differentially varies the capacity of the system. The transducer, that is the holder, which carries the pulleys, converts the yarn tension into electrical capacitance which can produce an electrical signal proportional to capacitance. The signal can subsequently be indicated and recorded by suitable devices. The modern electronic yarn tensometer with indicator apparatus and recorder is shown in Fig. 3.11. The holder, which carries the three-pulley system, is called the measuring head and it is the tensometer proper. The measuring head comes in direct contact with the yarn to be measured. It, generally, consists of a metal cylinder on the outside, from one end of which protrudes the centric measuring pulley (or in some cases rod) and the two yarn guides. At its other end, is connected the indicator apparatus. The measuring pulley is anchored to a torsion spring by which it is adjusted to its initial position and the measuring head is designed as a differential capacitor. A capacitor electrode in the form of measuring pulley or rod is so placed that it can be moved even very slightly by the running yarn. The resultant movement of the electrode causes the capacity variations.

Figure 3.11 Modern electronic tensometer and high-speed recorder. The measuring system shown in Fig. 3.11 is damped sub-critically so that about 70% of its resonant frequency can be exploited. Thus, the 100 g measuring head can follow up to 300 load variations per second with full amplitude accuracy. The measuring heads can cover a total measuring range

Warp tension measurement

55

of 1–200,000 g. As the yarn passes through the pulleys of the measuring head there are angle of wrap at each pulley and friction between the yarn and the pulley. These may influence the tension values measured. If this angle is high the indication of tension increases according to an angular function but the friction increases exponentially and that results in disproportional increase of the friction component of the indication. Hence, the angle of wrap at the measuring pulley or rod is usually maintained 300 [1]. As the measuring rod moves only 0.1 mm at the utmost, the angle of wrap of 300 remains practically always unaltered. The electrical tensometers operate on either the capacitance, induction or resistance principles and recently on the piezo-electric principle [19]. Recently developed Waweon measuring instrument [15] with yarn and warp sensors can measure dynamic tensions of two simultaneously or alternately picked wefts, warp and weft yarns simultaneously, two differentially weaved warp yarns and so on and the instrument can be connected to a computer for storing the results. The basic measuring range for the yarn sensors is from 300–1000 cN and that for the warp sensors is from 100–300 N. The software of the device offers five basic measuring modes for measuring fast changes in tension configuration of the yarn, long-term measurement, periodical measurement, statistical evaluation of the tension values and the measured and analyzed curves can be saved for facilitating setting of the other machines, accordingly. Warp tension value obtained in measurement, however, depends on the position on the loom where the measurement has been taken. As tension varies from yarn to yarn within a warp, tension measurements on adjacent 25 and 30 tex singles worsted wool yarns have been found to show large variation [6]. It is, therefore, recommended that the tension of up to ten adjacent yarns should be measured to obtain meaningful average tension values.

References 1.

Adanur S. and Qi J., (2008). Property analysis of denim fabrics made on air-jet weaving machine, Part I: Experimental system and tension measurements, Textile Research Journal, Volume 78, pp. 3–9.

2.

Anon (1969). Warp tension meter developed by WIRA, The Textile Manufacturer, Volume 95, p. 368.

3.

Anon (1991). Analysis system for measuring yarn tension in the weaving process, Melliand Textilberichte, Volume 72, p. 109.

4.

Badve N.P. and Bhattacharya U., (1960). Study of warp tension variations on Roper and Sakamoto automatic warp let-off motions, Part I, Textile Trends, Volume 3, pp. 31–37.

56

Role of yarn tension in weaving

5.

Bandra M.P.U. and Mirjalili S.A., (2001). Evaluation of the shelltype gauge for warp-tension measurement on a weaving machine, The Journal of the Textile Institute, Volume 92, pp. 222–234.

6.

Blanchonette I., (1996). Tension measurements in weaving of singles worsted wool yarns, Textile Research Journal, Volume 66, pp. 323– 328.

7.

Chamberlain N.H. and Snowden D.C., (1948). Loom study by means of the cathode ray oscillograph, The Journal of the Textile Institute, Volume 39, pp. T23–T43.

8.

Greenwood K., (1959). The lease rod as a tension indicator, Textile Recorder, Volume 76, pp. 64–67.

9.

Kovacevic S., Hajdarovic K., and Grancaric A.M., (2000). Influence of warp loading on weaving machines upon yarn deformation, Textile Research Journal, Volume 70, pp. 603–610.

10. Kulikova N.A., (1966). The influence of the relative warp sheet and fabric lengths on the loom on the warp tension and end breakage rate, Technology of the Textile Industry U.S.S.R., No 4, pp. 69–73. 11. Lunenschloss J. and Schlichter S., (1987). The development of new measuring elements for electronically controlled warp let-off units, International Textile Bulletin, Fabric forming, Volume 3, pp. 56–71. 12. Mallah Y.N., (1967). A new method of measuring warp tension, The Indian Textile Journal, Volume 77, pp. 465–469. 13. Owen A.E., (1928). The tension in a single warp thread during plain weaving, The Journal of the Textile Institute, Volume 19, pp. T365– T388. 14. Poole E.J., (1947). Wool yarn weaving, with particular reference to the controlling warp tension, The Journal of the Textile Institute, Volume 38, pp. P286 - P297. 15. Pustka M., Kloucek P., and Skop P., (2008). Waweon – instrument for dynamic measurement and analysis of yarn and warp tension forces, Melliand International, Volume 14, pp. 40–41. 16. Ramaswamy B.R. and Paliwal M.C., (1965). Warp tension in weaving: its effect on performance and quality, Proceedings, 6th Technological conference of ATIRA, BTRA & SITRA, pp. 87–95. 17. Snowden D.C., (1949). Some factors influencing the number of warp breakages in woolen and worsted weaving, The Journal of the Textile Institute, Volume 40, pp. 317–330.

Warp tension measurement

57

18. Snowden D.C., (1950). Some aspects of warp tension, The Journal of the Textile Institute, Volume 41, pp. 237–249. 19. Talavasek O. and Svaty V., (1981). Shuttleless Weaving Machines, Elsevier Scientific Publishing Company, Amsterdam, Oxford, New York, p. 512. 20. Technical literature of Electronic-Tensiometer R – 1192, Rothschild (1982), 8002 Zurich, Traubenstrasse 3, Switzerland.

4 General form of warp tension variation

Abstract: While the basic warp tension required for weaving is controlled by the let-off motion of the loom, the variation in tension in a pick cycle is attributed to shedding and beat-up. The variation of warp tension in a pick cycle consists of two peaks, one of fairly long duration for shed formation and the other of shorter duration but very sharp for beat-up and a trough of minimum tension for shed level. The heald shafts further away from the cloth fell generally register correspondingly higher tensions of their warp yarns. With asymmetric setting of the shed lines, generally maintained in practice, tension of the warp yarns of a heald shaft is considerably higher in one pick cycle than in the other and this nature of tension variation repeats on two pick cycles for plain weave construction. For other weaves where an end remains up or down for successive insertions of weft, the tensions of the changing yarns vary similarly to those for plain weave but the tensions of the other yarns vary differently. Warp tension in shuttleless loom varies in almost the similar manner like in shuttle loom but the basic warp tension maintained is much higher. Keywords: warp tension; hypothetical warp tension; shedding tension; beatup tension; shed level tension; actual warp tension.

4.1

Introduction

Formation of warp shed causes the warp yarns to move up and down and the yarns thereby, experience variations in tension from their normal levels set by the let-off motion of the loom. Beat-up thrust also causes sudden sharp rise in warp tension. These basic functions in weaving naturally result in variations in tension of the warp yarns from their preset values. Thus, while the let-off maintains the constant mean tension, shedding and beating are responsible for cyclic tension variations. As discussed in Chapter 3, a general idea of the variations in tension of the whole warp sheet can be obtained from the magnitudes of oscillation of the weight lever of the positive let-off motion of the loom, from the extent of deflection of the back rest under the pressure

60

Role of yarn tension in weaving

of the warp sheet or of the selected group of warp yarns from the shell-type measuring gadget. Since, however, the exact nature and magnitude of warp tensions are determined by many other factors such as, the type of weave, position of the heald shaft, type of shed line arranged, shed timing, etc., the results of single warp tension will provide more precise and useful information on these aspects.

4.2 Warp tension variation It has been discussed in Chapter 3 that the different methods generally adopted for measuring the tension of the warp yarns either provide general information on the trend of variation of warp tension often found suitable for making gross adjustments of warp tension in a number of looms weaving the same sort, or a detailed and specific information of the single warp yarn which is more useful for studying, analyzing, and consequently carrying out the necessary settings of the looms for achieving the desired weaving performance.

4.2.1 Tension variation of the whole warp sheet Figure 4.1 demonstrates the weight-lever oscillations during running of the loom. As can be perceived from the figure, this basically indicates the range of weaving tension variations of the whole warp sheet of a given construction of fabric.

Figure 4.1 Weight-lever oscillation. The magnitude of oscillation of the weight-lever is determined mainly by the type of weave, the warp density, and the weft density. Irrespective of the

General form of warp tension variation

61

type of weave, the warp parameters or the loom settings and timings, weightlever oscillation increases with pick density and/or coarseness of the weft yarn. The weight-lever oscillation can, therefore, be considered a measure of weaving difficulty that is, the degree of difficulty encountered in beating the weft picks into the cloth [4]. From this it can be assumed that, the pick densities that produce the same weight-lever oscillation with the same warp and weft counts, the same number of warp yarns and the same loom settings but with different warp densities experience similar weaving difficulty [9]. In plain weave all the warp yarns change position in every pick and so the normal range of weight-lever oscillation for plain weave is higher than those for others which have less number of interlacements in a weave repeat. Aggregate warp tension variation measured from the deflection of the back rest resulting from the pressure exerted on it by the warp sheet [6] has been shown in Fig. 4.2. Corresponding tension variation of the cloth measured on the breast-beam has also been exhibited in the same figure.

Figure 4.2 Warp and cloth tension variations. Results of warp tension variation clearly show the beat-up and shedding tensions. The sharp peak tension C indicates the beat-up tension while the shedding tension is represented from E to F which is high as well, but extends over a longer duration because of shed dwell. The low tension, A is the tension at shed change when all the yarns are at the same level. Since the method adopted for tension measurement indicates the overall tension of the entire warp sheet, the warp tension varies in exactly similar manner in successive picks. Similar results of warp tension variations are expected from the leaserod gauge also, which indicates the variations in mean warp tension of the entire warp sheet.

62

Role of yarn tension in weaving

It is also observed from Fig. 4.2 that the tension of the cloth varies in similar manner as that of warps at shedding and shed change but at beat-up the cloth tension falls sharply. At the instance of beat-up, the reed exerts force on the fell of the cloth to cause rise in warp tension as the yarns are then stretched and simultaneously fall in cloth tension as it is then momentarily relaxed. Detailed discussions on these and their roles in beat-up have been made in Sections 7.8 and 7.10 of Chapter 7.

4.2.2 Tension variation of the bunch of warp yarns Warp tension variations for plain weave observed with shell-type gauge [2] have been illustrated in Fig. 4.3.

Figure 4.3 Warp tension variations with shell-type gauge. The variations of tension of a group of warp yarns of a given heald shaft indicate that the tension varies in a definite manner and every alternate tracing is exactly similar in nature [2]. In other words, the tension variation repeats in two pick cycles because of plain construction. It is also observed that the magnitude of tension is greater in one pick cycle than the other. This is due to the asymmetric setting of warp shed line, discussed in Section 7.3.1 of Chapter 7.

General form of warp tension variation

63

4.2.3 Tension variation of single warp yarn in shuttle loom The variation in warp tension in the form of weight-lever oscillation (Fig. 4.1) only gives a rough idea about the weaving condition of a given type of fabric. It does not provide any clear information on the extent of tension experienced by the yarns at different instances in a pick cycle nor on the nature of variation in tension with different types of weave. Warp tensions measured from the deflection of the back rest (Fig. 4.2) or shell-type gauge (Fig. 4.3) although furnish more information on the tension variation than the other, information on the variations in tension amongst the different ends of even the same heald shaft still remain concealed. It does not also indicate if there exists any significant difference in warp tension along the entire width of the warp sheet or, if there is any need for more precise settings and adjustments of the different loom mechanisms or for improving the qualities of the warp beams produced. All these information essential for achieving desired productivity at weaving and the physical properties of the cloths produced can be obtained from the tension of the single warp yarn.

4.2.3.1 Hypothetical features of warp tension variation Nature of tension variation of single warp yarn with symmetric shed lines (see Fig. 3.2 of Chapter 3) and weaving plain weave with two heald shafts should ideally be as illustrated in Fig. 4.4. Here, the warp tension in two consecutive picks have been shown in relation to the angular movement of the main shaft that is, the crank shaft of the loom and the tension of the warp yarns of the front heald shafts has been indicated by the firm and that of the back heald shaft by the broken lines. As mentioned earlier and as Fig. 4.4 exhibits, the tension of the warp yarns of either heald shaft consists of two tension peaks and one tension trough in each pick cycle. One of the two peaks, A of fairly long duration is due to separation of warp yarns of two heald shafts for shed formation that is, shedding and the other, B of shorter duration but very sharp is due to forward movement of the cloth by the reed at beat-up that is, beating. The trough, C in the tension cycle indicates the closed or level position of the shed when the warp yarns of the two heald shafts are crossing and hence, at the same plane. At this stage, where the yarns are at the middle of their traverse, they are least strained and thus, have the minimum tension in the pick cycle. The minimum tension at this instance should, however, not be so low as to become zero. Identical nature of tension variation of the warp yarns exists for both the heald shafts and in both the consecutive picks cycles, but the magnitude of tension of the yarns of the back heald shafts is higher than that of the yarns of the front, except at the shed level position. This is because of higher lift of the

64

Role of yarn tension in weaving

back heald shaft. At shed level all the yarns lie at the same horizontal plane and are, therefore, tensioned equally.

Figure 4.4 Shedding and beating tensions with symmetric shed. The peak tension at shedding may vary in duration depending on the type of weave but that at beat-up is always a sharp and short one whose magnitude depends primarily on the count and density of the weft yarn of the cloth. The shedding tension for weaves with longer float of warp yarns may remain high for longer duration. It is also observed from the tension diagram in Fig. 4.4 that the warp tension of either the front or the back heald shaft varies to the same extent at a given instance whether the yarns form the top or the bottom shed, as shown in two successive pick cycles. This is because of the exact symmetry of the shed lines where the yarns of either shed line are stretched to the same extent, as shown in Fig. 3.2. It may also be assumed that the magnitude of tension of all the warp yarns of a given heald shaft will be the same, provided of course, all the warp yarns being unwound from the warp beam are under the same tension. Warp tension at shed level poison is generally the level of tension set by let-off motion of the loom. The magnitude of warp tension at shedding depends on the height of the shed and the location of the heald shafts with respect to the fell of the cloth. The tension of the warp yarns at the front shed that is, in the region from the heald shafts to the cloth fell, is greater than the tension of the yarns at the back shed that is, in the region from the heald shafts to the back-rest and the tension of the latter is again greater than the tension of the yarns from the back-rest to the warp beam. This is because of the frictional resistances at the heald eyes in the first case and at the back-rest in the second, as discussed in the previous chapter. Extent of variation in warp tension during shedding may, however, decrease with the increase in the lengths of warp sheet and the cloth on the loom as the increase in length of the yarn and/or the cloth will be more able to suppress the variation in tension [10].

General form of warp tension variation

65

The nature of variation of warp tension shown in Fig. 4.4 should rather be taken as more hypothetical than actual because, it has been assumed that (i) the formation of shed is absolutely symmetric and (ii) there is no variation in tension within and amongst the warp yarns of the warp beam. In practice, as a matter of fact, symmetric shed line is seldom arranged for various reasons (see Section 7.3.1 of Chapter 7) and all the warp yarns can hardly be wound on the warp beam under the identical condition and tension, and can therefore be unwound under the same tension. Nonetheless, the assumptions made do not invalidate the main characteristics of the nature of variation of weaving tension of the warp yarns and the discussions made on them, as can be ascertained from the results of actual variation in warp tension discussed below.

4.2.3.2 Actual feature of wrap tension variation Let us now see how the tension of the warp yarn actually varies during weaving with the type of shed line generally arranged in practice. Owen’s [8] study made on an orthodox over-pick shuttle loom fitted with tappet shedding mechanism, negative let-off mechanism with chain and dead weight, positive intermittent take-up mechanism, cam operated vibrating back rest and weaving plain weave reveals that the tension of the yarn fluctuates appreciably but in a very definite manner as shown in Fig. 4.5. The fluctuation repeats fairly uniformly at regular intervals of two pick cycles. In plain weave with 1 up – 1 down construction, the movement of a warp yarn completes in two pick cycles. A warp yarn moves from its top (or bottom) position to the bottom (or top) position and then returns to the top (or bottom) position in two consecutive pick cycles. Thus, the shedding cycle or the cycle of movement of a warp yarn consists of two pick cycles and hence the nature of warp tension variation repeats at every two picks. As indicated in Fig. 4.5, tension variation from A to B is for one pick and that from B to C is for the next and one complete cycle of tension variation from A to C repeats in two pick cycles. The tension of the warp yarn has one maximum, a and one minimum, b values in each pick cycle. The maximum tension is registered at the incident of beat-up when the sudden thrust of the reed at the fell of the cloth causes sharp pull and thereby, the rise in tension of the warp yarns. Similarly, the minimum tension is obtained when the shed is nearly closed and the yarns are at the relaxed state. It is also observed that immediately after beat-up the warp tension in each pick cycle, falls a little to c and then rises steadily. As the reed recedes from the cloth fell after beatup, the warp yarns are momentarily relaxed and the tension falls but soon after beat-up, the yarns are stretched again as they reach the shed dwells, d to form the shed and this causes the rise in tension. Figure 4.5 also exhibits that tension in one pick cycle is considerably higher than that in the other because of common practice of maintaining the asymmetric shed lines, pointed out

66

Role of yarn tension in weaving

above. With asymmetric shed line (see Fig. 7.4 of Chapter 7), the warp yarns are stretched more while forming the bottom shed than when they are at the top shed. Barring the difference in nature of warp tension in two successive pick cycles, the natures of warp tension variations shown in Figs. 4.4 and 4.5 are otherwise largely similar.

Figure 4.5 Tension variations of single warp yarn. Studies of warp tension have also been made with different constructions and weaves of the cloth [4]. The cloths were woven on an under-pick loom fitted with dobby, ratchet-type positive let-off with weighted lever for imparting tension on the warp sheet through the hanging back-rest and positive continuous take-up mechanisms. The loom was weaving two plain cloths with two different picks/ cm and one twill cloth with 2/2 construction. Tension was measured of the single warp yarn drawn through the front heald shaft. While some of the features of the variations of warp tension are common for both types of weave and different pick densities, there are some special features with the type of weave. There is a rise in tension associated with beat-up and fall in tension associated with shed closing and the warp yarns forming the bottom shed register higher tension than those forming the top shed, as observed earlier and shown in Fig. 4.5, because of asymmetric setting of the shed line. The maximum tension registered at beat-up is followed by a tension minimum and in general, the maximum and minimum tensions are related inversely. Since the peak tension at beat-up is followed by a tension minimum, the latter is also related to the weaving difficulty in the similar manner. As the warp tension increases during the process of beat-up, it initially rises slowly and then sharply. The latter part of the beat-up tension with greater rate of increase indicates that during this phase the reed makes contact with the weft and carries it forward between the crossed warp yarns to the fell of the cloth. The nature of warp tension variation in two pick cycles is fairly

General form of warp tension variation

67

similar in both the plain weave constructions with two pick densities except that the peak tension at beat-up is much higher in case of the cloth with higher picks/cm. With higher pick density, the fell of the cloth creeps more toward the reed and this causes higher peak tension at beat-up. The magnitude of beat-up tension indicates the extent of difficulty encountered in beating the picks into the cloth. Higher the picks/cm of the cloth greater is the difficulty in packing the picks at the cloth fell. In the case of 2/2 twill weave, there is one set of warp yarns which is changing positions from the top shed to the bottom shed and vice versa and there is another set of yarns which remains stationary either at the top or at the bottom shed in each pick cycle. There are, therefore, two distinctly different patterns of variation of warp tension in a pick cycle [4]. Tensions of the yarns changing positions differ significantly from those remaining raised or lowered both at beat-up and shed change. The tension variation of the changing yarns is similar to that observed in case of plain weave. For the yarns remaining up or down, there is double peak in the tension at the instance of beat-up. The first of the two peaks is higher and occurs slightly before the beat-up that is, the forward most position of the reed. The second of the two peaks is due to beat-up as usual, while the first one is due to the reed carrying the weft to the cloth fell with the changing yarns yet to reach their dwells (that is, the top and bottom shed lines) to take a reasonable share of the beating up strain. The yarns which remain stationary (that is, at dwell) at the raised or lowered position in the pick cycle also show rise in tension at the time of shed change. The magnitude of this rise in tension of these yarns is expected to vary according to the relative proportion of the stationary yarns to the changing yarns. The natures of variation of warp tension observed with plain and 2/2 twill weaves may, therefore, indicate that if the weave is such that the yarns remain raised or lowered and the consecutive picks are inserted in the same shed like in matt weave, the tension of the yarn with shed open will remain fairly constant throughout except at the time of beat-up when the tension peak will occur. As the let-off motion of the loom referred to above [4], operates with weighted lever and hanging back rest, the sudden sharp pull by the warp yarns on the back rest at beat-up and the inertial effects of the weighted lever and the back-rest also produce some additional tension peaks on the warp tension. Detailed discussion on this has been made in Section 7.3.2 of Chapter 7. If the tensions of the two warp yarns, one of the front and the other of the back heald shafts, are measured together some interesting observations, which differ significantly from the tension of the single warp end, can be made [7]. Tensions of the two warp ends (that is, double yarn tension) weaving plain weave with two heald shafts and passing through the same reed dent have

68

Role of yarn tension in weaving

been shown in Fig. 4.6. Now, higher warp tension as well as similar nature of tension variation are observed in all the pick cycles.

Figure 4.6 Tension variations of single and double warp yarns. When two single yarns of the two heald shafts pass together through the tension head of the recorder and the magnitudes of tension at the two shed lines, top and bottom, are different, the tension of that yarn higher in a pick cycle is only recorded. In this case, the tension of the yarn is higher at the bottom shed and so the tension of the double yarn is actually the tension of the yarns forming the bottom shed. This is evident from the fact that the nature of variation of double yarn tension is very similar to the corresponding nature of tension variation of the single yarn forming the bottom shed (Fig. 4.6). Quantitative values of tension of the double yarn are also higher than those of the single yarn excepting at shed level, where all the yarns occupy the same and the central position. When the two yarns of two heald shafts pass through the tension head and one is alternately at higher tension than the other, there is always an abrasion between the two as they move forward, and this registers higher level of tension. These observations also substantiate the fact mentioned earlier that the measurement of tension of the group of warp yarns merely indicates the general nature of variation of warp tension during weaving (see Fig. 4.2 for warp tension) and cannot reveal some important characteristics of the warp tension in a pick cycle, unless the measurement is made of single warp yarn. While the average tension of the warp yarns varies considerably depending on the type of weave and setting of the warp line from the back-rest to the fell of the cloth as observed above, there is further variation in tension within and

General form of warp tension variation

69

between the individual yarns of the given layer of the warp sheet or in other words for any given diameter of the warp beam, as pointed out earlier. Even with modern weaving preparatory systems adopted for shuttleless weaving machines variation in tension between the adjacent warp yarns has been observed to be as much as 25% [3]. This is due to the variations in tension of the individual yarns during warp preparation and due to the position of the heald shaft through which the yarn has been drawn [9, 1]. During preparation of the warp beam, the warp yarns of a given layer lie sometimes on top of the yarns of the previous layer or sometimes between the yarns of the previous layer. Again, the individual warp yarn can hardly be controlled very precisely and uniformly during the preparation of the entire warp beam. As a result, when the yarns are unwound from the beam at weaving neither the tension nor the length of the yarns delivered can be expected to remain uniform throughout and there is considerable difference in tension between the yarns of the warp sheet [9]. Evenness of the yarn, particularly for fine yarns, also has a large influence on the yarn tension. Two-fold yarn being more even shows much less variation in tension [3].

4.2.4 Tension variation of single warp yarn in shuttleless loom In a shuttleless loom, the weft insertion element is much smaller in dimensions and lighter in weight than the shuttle. As a result, the required depth of warp shed is much smaller but the basic tension of the warp yarns is much higher than the same in a shuttle loom, in order to ensure clear formation of the shed for weft insertion. For the same reasons, the shuttleless looms can operate at much higher speed and the period of time for which the shed is required to remain open for insertion of the weft is less.

Figure 4.7 Warp tension variation in a projectile loom.

70

Role of yarn tension in weaving

Figure 4.7 shows the nature of tension variation of a single warp yarn in a projectile weaving machine [5]. The average warp tension at shedding is considerably higher (compare with Fig. 4.5) and the extent of increase in beatup tension over the open-shed tension is much less than the respective tensions observed in a shuttle loom. This is due to the higher level of basic tension of the warp yarns maintained in the shuttleless loom. The effect of warp letoff motion (positive type) on the warp tension during shedding can also be observed in Fig. 4.7. Following the beat-up there is very prominent transient vibration in the warp tension. Such vibrations are the natural response of the spring system contained in the warp let-off mechanism of the loom along with the after effect of the beat-up action on the warp yarns.

References 1.

Badve N.P. and Bhattacharya U., (1960). Study of warp tension variations on Roper and Sakamoto automatic warp let-off motions, Part II, Textile Trends, Volume 3, pp. 55–63.

2.

Bandra M.P.U. and Mirjalili S.A., (2001). Evaluation of the shelltype gauge for warp-tension measurement on a weaving machine, The Journal of the Textile Institute, Volume 92, pp. 222–234.

3.

Blanchonette I., (1996). Tension measurements in weaving of singles worsted wool yarns, Textile Research Journal, Volume 66, pp. 323– 328.

4.

Chamberlain N.H. and Snowden D.C., (1948). Loom study by means of the cathode ray oscillograph, The Journal of the Textile Institute, Volume 39, pp. T23–T43.

5.

Holcombe B.V., Griffith R.E. and Postle R., (1980). A study of weaving systems by means of dynamic warp and weft tension measurement, Indian Journal of Textile Research, Volume 5, pp. 1–5.

6.

Kulikova N.A., (1966). The influence of the relative warp sheet and fabric lengths on the loom on the warp tension and end breakage rate, Technology of the Textile Industry U.S.S.R, No 4, pp. 69–73.

7.

Neogi S.K. and Mukherjee B., (1978). How to control tension variation of warp yarn, The Indian Textile Journal, Volume 88, pp. 149–155.

8.

Owen A.E., (1928). The tension in a single warp thread during plain weaving, The Journal of the Textile Institute, Volume 19, pp. T365– T388.

General form of warp tension variation

9.

71

Snowden D.C., (1950). Some aspects of warp tension, The Journal of the Textile Institute, Volume 41, pp. 237–249.

10. Zuet N.N. (1969). The influence of the relative length of the warp sheet and cloth in a loom on warp deformation and tension during shedding, Technology of the Textile Industry U.S.S.R, pp. 55–59.

5 Weft tension measurement

Abstract: Tension of weft yarns during weaving is essential for proper interlacement with warp yarns for formation of the cloth. Like in case of warp, information on weft tension and its nature of variation during insertion of a pick through the warp shed are essential in order to achieve the desired performances of the looms, both shuttle and shuttleless, and also the quality characteristics of the fabrics produced. In any shuttleless weaving machine, taking measurement of weft tension is rather fairly simple and the conventional three-pulley system can be used, as the weft is inserted from the stationary yarn package located well outside the weaving area. In shuttle loom on the contrary, different methods suitable for the purpose have been devised for measuring the unwinding tension of weft during insertion of a pick and also during exhaustion of the pirn. Keywords: weft tension; weft tension measurement; pirn unwinding tension; single pick tension; shuttleless picking tension measurement.

5.1

Introduction

Like in case of the warp yarns, any idea of the weaving tension of the weft yarn, whether from the pirn of a running shuttle in a shuttle loom or from the stationary weft package in a shuttleless loom, can be obtained provided the tension of the yarn is measured properly and accurately. Measurement of withdrawal tension of the running weft from the stationary package is rather fairly simple as it can be made by interposing the tension head of the tension recorder at a suitable location before the weft insertion mechanism. In case of the pirn on the contrary, such a simple procedure can not be followed as the pirn is contained inside the shuttle. Moreover, the unwinding tension of weft from a pirn is subjected to long time variation owing to exhaustion of the pirn and short term variation owing to insertion of a pick and so, the same method can hardly be adopted for both the purposes. Different methods have, therefore, been devised for measuring the weft tension from the pirn.

74

Role of yarn tension in weaving

5.2 Weft tension measurement in shuttle loom In shuttle picking system, the shuttle, with a full pirn in it, moves to and fro from one shuttle box to the other to insert the picks and the pirn is exhausted much quicker than the large weft package used in any shuttleless picking system. As weaving proceeds, the unwinding tensions of weft from the pirn are, therefore, subjected to two distinctly different characteristics; one, owing to the exhaustion of the pirn and the other during the insertion of a single pick. Unwinding tension of weft from a full to empty pirn can conveniently be measured from a stationary shuttle (not necessarily to be placed in the loom) with the pirn inside, but a more complex but appropriate method is necessary for measuring the tension of the weft during its insertion through the warp shed in a running loom.

5.2.1 Measurement during unwinding from pirn Possibly the first fruitful efforts to measure the unwinding tension of weft yarn from a full to empty pirn in a shuttle have been made by Townsend [8] and later by others [7, 2] adopting basically the similar method. On the running loom, the shuttle runs from one shuttle box to the other during the course of picking and the weft is unwound from the pirn carried by it. Conversely, if the shuttle is kept stationary and the weft is withdrawn from it at a linear speed similar to that of the running shuttle the unwinding tension of the weft will remain practically the same. Based on this assumption unwinding tension of weft from the pirn has been measured with help of a test rig shown in Fig. 5.1.

Figure 5.1 Measurement of weft tension from a pirn in a stationary shuttle. The weft is withdrawn from a stationary shuttle and wound on the surface of a rotating drum whose speed is so maintained that it withdraws the yarn at a speed of about 13.7 m/s, almost similar to the speed of the shuttle on a running loom. The tension of the weft is measured with the help of threepulley measuring system, described earlier (Chapter 3). The measuring unit is interposed between the shuttle and the rotating drum. As shown in Fig. 5.1, the weft from the shuttle is led around two fixed guide pulleys and over

Weft tension measurement

75

the centre pulley attached to a small steel cantilever with a free period of about 50 cycles/sec [7]. The cantilever also carries a small mirror at its free end. A beam of light from a light source is reflected from the mirror on to the bromide photographic paper of a recording camera. As the yarn runs through the measuring unit, the variations in tension of the running yarn cause proportionate variations in the deflection of the free end of the cantilever and thereby, of the mirror and the magnitude of this deflection is recorded on the bromide paper. Again, when the weft is ejected from the shuttle, it imparts a drag on the shuttle, the magnitude of which can be taken as the measure of unwinding tension of the weft from the shuttle. Based on this notion another method has been employed where the weft unwinding tension from the shuttle is measured by “Shirley weft unwinding tension meter” [7], shown in simplified form in Fig. 5.2.

Figure 5.2 Shirley weft unwinding tension meter. The shuttle, with a pirn in it, is placed on a light suspended carriage, which can swing freely under the drag caused by the unwinding of the weft from the shuttle. The weft yarn from the shuttle is taken through a glass guide and wound round an aluminium drum fitted on the shaft of an electric motor. The diameter of the drum is such as to give an unwinding speed of 10 m/s, the speed of the shuttle while running from one shuttle box to the other at the loom speed of 180 picks/minute. As the motor is switched on, the yarn is unwound from the pirn in the shuttle and wound on the drum. The tension of the yarn exerts a force on the carriage causing it to swing against the action of gravity towards the winding unit. The resultant horizontal displacement of the shuttle carriage indicates the tension in the yarn which can be noted from a scale fitted on a fixed place besides the carriage. So that the shuttle carriage cannot swing backward, it is fitted with a pawl and ratchet arrangement and the carriage can swing forwards only and not backwards. When the weft is fully unwound from the pirn, the carriage remains locked in its deflected position by the ratchet and pawl arrangement. The position of the carriage relative to the scale is then taken as the maximum tension of the weft. Unlike

76

Role of yarn tension in weaving

the previous method however, this can only indicate the increase in the mean unwinding tension of the weft from the shuttle. It can neither give any idea of the fluctuation of tension nor can it record graphically the nature of variation of weft tension.

5.2.2 Measurement during insertion of a pick Tension of weft yarn, during insertion of a pick on the running loom, was measured first by Townsend [8]. He had measured the tension with the help of a small cantilever unit carrying a pair of wire resistance strain gauges. The measuring unit has been placed in proximity to the selvedge at the fell of the cloth on the loom and the weft yarn from the shuttle is fastened to a small wire stirrup on the cantilever. For taking the measurement, the shuttle is boxed at the same side of the loom as the measuring unit and the loom is started from such a position that the first pick is given from the other side of the loom where there is no shuttle. The shuttle is actually picked at the next pick. The false pick from the side where there is no shuttle ensures that when the shuttle is picked at the next pick cycle, the loom runs at full speed. The tension of the weft during insertion of the pick from the measuring side creates proportional strains in the cantilever and the resultant changes of resistance of the strain gauges are recorded with the help of cathode ray oscillograph. It can be assumed that the unwinding or withdrawal tension of the weft yarn during the flight of the shuttle through the warp shed is of little consequence to the formation of the cloth. The tension of the weft at the time the shed just closes and traps the weft is of great importance as the weft then, starts crimping and becomes part of the cloth. In view of this, Townsend, although measured the single pick tension during the entire period of weft insertion, gave more importance on the tension during the checking phase of the shuttle. As a consequence, specific attentions were paid on the tensions at the instants when (i) the shuttle came in contact with the swell, (ii) it was stopped at the back of the shuttle box and, (iii) the warp shed closed on the inserted pick. Redozubova [6] had measured the weft tension on the running loom at both the sides without stopping the loom. The weft tension has been measured with the help of resistance strain gauge, an electronic amplifier and an oscillograph for recording the tension. The strain gauge wires were bonded to special flat cantilever springs which were rigidly attached to the temple brackets at both sides (Fig. 5.3) at a distance of 50 mm from the selvedges of the cloth. Each spring was provided with an eye through which the weft was threaded for measurement of its tension. As the reed came forward to beat-up, it placed the weft yarn whose tension was to be measured into the eye of the spring. At the same time, the reed also removed the other weft yarn, whose tension had been measured at the previous pick from the eye of the other spring. As the shuttle

Weft tension measurement

77

passes from one box to the other, the weft threaded through the spring flexes it and the resultant deformation of the spring is eventually recorded by the oscillograph to indicate the dynamic tension of the yarn. The spring attached to the left temple bracket measures the tension of the weft when the shuttle flies to the right-hand shuttle box and vice versa, as indicated in Fig. 5.3.

Figure 5.3 Weft tension measurement from both sides of the loom. Greenwood and Vaughan [3] are the pioneers in giving detailed accounts on the nature of weft tension during the flight of the shuttle from one shuttle box to the other on an over-pick shuttle loom. The principal device used for measuring the weft tension has been shown in Fig. 5.4. It consists of a thin square plate of Rochelle Salt crystal of 12.25 mm2 area and 0.8 mm thickness. The plate is mounted at three corners on a solid brass block and at the fourth corner; a thin metal strip is attached at right angles to the plane of the crystal. To this strip, is cemented the weft yarn (as shown in Fig. 5.4) whose tension is to be measured. The procedure followed for measuring the tension is almost similar to that described above. As the shuttle runs from one box to the other, the forces applied to the yarn bend the crystal and the electric charges are formed on the two faces of the crystal. The resulting voltage is amplified and eventually recorded photographically by means of a camera attached to the measuring instrument. The tension of the weft yarn is recorded during one passage of the shuttle from one shuttle box to the other till the shed is closed. Here also, like in the earlier case [8], the tension of the weft yarn has been measured on the second pick after the first pick is false to ensure full running speed of the loom. Tests have also been conducted to examine if there exists any significant difference in the tension of the weft being unwound from the nose and the shoulder of the pirn.

78

Role of yarn tension in weaving

Figure 5.4 Pick tension measuring system with the weft attached to the crystal block. In all these methods discussed on the measurement of weft tension during insertion of the pick, the tension has been measured by passing the yarn from the shuttle through a transducer or a tensiometer located in the region of the selvedge of the cloth. They do not, therefore, allow the continuity of the tension signal and also interfere with the weaving cycle. The yarn under measurement is not held at the cloth fell by the selvedge during beat up and considerably more yarn is withdrawn from the shuttle than would be expected during normal picking. Moreover, the angle of contact between the transducer and shuttle is not constant during the course of measurement. All these do not occur during the actual weaving. Besides these, no information on the tension of the pick after the shuttle is boxed and beat-up takes place is available. These shortcomings can be evaded if a system can be devised by which the tension of the weft can be measured continuously during the entire period of picking of the shuttle from one shuttle-box to the other and also after the shuttle is at rest in the box after checking, without interfering with the normal operation of the loom. Taking these into consideration, a radio-telemetry system has been devised for continuous monitoring and direct measurement of weft tension within the shuttle during normal running of the loom [4]. The system has been fitted within the shuttle for direct measurement of weft tension. The radiotelemetry system consists of a transducer, transmitter and amplifier, power supply, switch gear and, aerial. All these have been embedded within the body of the shuttle after removing materials at suitable locations. The transducer is fitted on the existing shuttle eye unit but the normal entry and exit of the yarn from the shuttle eye along with normal facility for tension adjustment have been retained. However, to accommodate the cantilever of the transducer, the path of the weft has been changed. The empty shuttle with all these accessories

Weft tension measurement

79

weighs about 5% more but this has no significant effects on the shuttle flight. The measurement of weft tension has been made on Picanol President and Northrop looms under the operating conditions [5]. The variation in weft tension is relayed to an external receiver and recorded continuously by cathode-ray oscilloscope. Since the tension measuring devices are imbedded within the shuttle, the system can provide information on the tension of the weft yarn even after the shuttle is at rest after checking, as pointed out above.

5.3 Weft tension measurement in shuttleless loom The shuttleless looms which have gained enormous popularity work with projectile, rapier or fluid jet (air and water) picking systems. In projectile loom, a thin and light metal projectile is employed to insert the picks. It grips the weft by its tip and traverses unidirectional through the warp shed to insert the pick. In commercially more successful rapier loom working on tip transfer system, rigid rod or flexible tape fitted with gripper at one end grips the weft by its tip and drags it to the centre of the warp shed to transfer it to the other rapier which moving out of the shed completes the insertion of the pick. In fluid jet loom, the pick of weft is inserted in the warp shed by jetting of either air in air jet loom or water in water jet loom and the weft is inserted by frictional force.

Figure 5.5 Different weft insertion systems in shuttleless looms. Different systems of weft insertion in shuttleless weaving machines along with the path of weft normally followed from the weft package to the respective weft insertion element have been indicated in Fig. 5.5. Although not shown in detail in the figure, the weft yarn from the stationary package, generally crosswound cone, passes through a weft accumulator (or weft feeder), a suitable tensioning system, weft monitoring system, the yarn presenter and finally to the weft insertion element; projectile, rapier, or fluid jet as the case is. As mentioned earlier, it is comparatively easy to measure the weft tensions in any type of shuttleless loom than in shuttle loom. In a shuttleless loom, the three-pulley tension measuring head can be conveniently placed at a suitable position before the picking system but as near to the pick insertion element

80

Role of yarn tension in weaving

as possible [1], as indicted in Fig. 5.5. Here, the yarn is not subjected to any transverse movement and the dynamic tension of the weft can be measured continuously, like measuring the warp tension.

References 1.

Chahal V. and Mohamed M.H., (1986). Measuring filling yarn tension and its influence on fabric woven on a projectile weaving machine, Textile Research Journal, Volume 56, pp. 324–333.

2.

Foster R., (1959). Tension variations occurring during the unwinding of cops and pirns, The Journal of the Textile Institute, Volume 50, pp. P7–P35.

3.

Greenwood K. and Vaughan G.N., (1958). Weft tension during weaving, The Journal of the Textile Institute, Volume 49, pp. T247– T264.

4.

Holcombe B.V., Griffith R.E., and Postle R., (1976). The application of radio-telemetry to the measurement of weft tension during the weaving cycle, The Journal of the Textile Institute, Volume 67, pp. 434–439.

5.

Holcombe B.V., Griffith R.E., and Postle R., (1980). A study of weaving systems by means of dynamic warp and weft tension measurement, Indian Journal of Textile Research, Volume 5, pp. 1–5.

6.

Redozubova E.P., (1961). Weft tension during weaving, Technology of Textile Industry U. S. S. R., No 2, 84–91.

7.

Thomas I.H., (1957). Behaviour of weft during unwinding from a shuttle, The Textile Manufacturer, April, pp. 163–166.

8.

Townsend M.W.H., (1955). Weft tension in weaving, The Journal of the Textile Institute, Volume 46, pp. 699–712.

6 General form of weft tension variation

Abstract: Unwinding tension of weft depends on the weft insertion system. In shuttle loom, there are two types of weft unwinding tension; one during the exhaustion of the pirn in the shuttle and the other during the insertion of the single pick through the warp shed. For pirn body with straight tube, weft tension initially remains low and fairly even but increases appreciably as the pirn empties. During insertion of a pick, there is an initial sudden sharp rise in tension when the weft starts unwinding from the pirn, then a fairly constant unwinding tension of relatively low value during the flight of the shuttle and finally a gradual decay in tension till the shed is closed. Radiotelemetry system embedded in the shuttle body provides more information on weft tension. In shuttleless looms, the nature of weft tension variation depends on the type of weft insertion. In projectile picking, there is a maximum tension peak during the initial acceleration and high level of tension during braking of the projectile, in rapier picking (tip transfer system), the maximum tension occurs during the initial acceleration only while in air jet picking, the maximum tension occurs during braking of the inserted pick. Keywords: weft tension; pirn unwinding tension; pick tension with shuttle; weft tension in projectile; weft tension in rapier; weft tension in air-jet.

6.1

Introduction

Compared to warp, the weft in shuttle loom undergoes much less variation in tension as it is only unwound from the pirn (or cop). As the shuttle traverses from one shuttle box to the other to insert the pick, the weft is unwound from the pirn and the pirn is gradually depleted. Thus, as mentioned in the preceding chapter, weft in the shuttle loom is subjected to two types of tension variation; one for the insertion of the pick and the other for gradual exhaustion of the pirn. In shuttleless loom, on the contrary, the running out of the weft package has hardly any effect on the weft tension as the weft is fed, more often than not, through the weft accumulator (or weft feeder) to ensure uniform withdrawal tension but, since the shuttleless looms are employed with different weft insertion techniques, tension of the weft yarn during insertion through the warp shed by the different picking methods is therefore, the matter of concern.

82

Role of yarn tension in weaving

Again, in shuttle picking, the weft is simply laid in the warp shed and the yarn does not have any axial movement, while in shuttleless picking, the weft is dragged through the shed. As a result, the weft tension is affected by the loom speed more in shuttleless loom than in shuttle loom.

6.2 Weft tension variation in shuttle loom Unwinding tension of weft from full to empty pirn in a shuttle is supposed to be, mainly, owing to the unwinding of each coil of weft. The nature of tension variation throughout the unwinding of the pirn is of long term. Tension variation during the insertion of a pick is of short term which starts from the moment the weft starts unwinding from the shuttle during picking to the moment the shuttle reaches the other box and comes to rest.

6.2.1 Tension variation during unwinding from pirn During the course of weaving, the weft is gradually consumed from a full pirn carried by the shuttle. The portion of the body of the pirn on which the weft is wound is normally made straight with small taper towards the tip for smooth unwinding of weft. As the weft is unwound from the full to the empty pirn, a striking change is observed in the characteristic of the unwinding tension of the weft. Weft tension, throughout the period of unwinding from the pirn with straight base, has been shown in Fig. 6.1. It is observed from Fig. 6.1 that, when the pirn is full and unwinding takes place from the pirn, the average weft tension remains reasonably constant with small rapid fluctuations because of unwinding of the weft from the nose and shoulder of the pirn. But, when the pirn becomes nearly exhausted and unwinding takes place from the base of the pirn, the tension increases steadily to a very high value with higher magnitude of fluctuation [15]. Let us find out why it so happens.

Figure 6.1 Weft unwinding tension from pirn with straight base. When the weft is unwound from the full pirn, profile of the pirn nose remains unchanged during unwinding and hence, the tension remains fairly constant. Again, when unwinding takes place from a full pirn, the yarn balloons away

General form of weft tension variation

83

without licking the body of the pirn. As the unwinding proceeds and the pirn is depleted, the balloon formed by the yarn while unwinding from the pirn gradually lengthens and there is licking of the yarn round the pirn body as the weft comes out from the shuttle. Eventually, when unwinding takes place from the base of the nearly exhausted pirn, the conical shape of the nose becomes shallower because of the straight-tubular base of the pirn (Fig. 6.1) and also the licking of weft extends over a greater length of the pirn body, as indicated in Fig. 6.2. As a result, the final tension of the weft increases appreciably. The extent of increase of the final tension depends on the count of the yarn and speed of unwinding or in other words on the speed of the loom.

Figure 6.2 Unwinding of weft from pirn end. It may now be noted that as the increase of weft tension at the base of the pirn is because of the presence of the pirn body on which the weft yarn is wound, the solid pirn or cop without a base at the core can obviate this problem and very uniform weft tension throughout the unwinding of the cop can be achieved. This type of cop is used in all types of shuttle loom employed for weaving jute fabrics [10].

6.2.2 Tension variation during insertion of a pick As discussed in the previous chapter, weaving tension of weft yarn has initially been recorded and studied during the insertion of the single pick from one side only of the loom. The tension has been measured from the moment the shuttle was picked from one shuttle box to the moment the warps closed on the weft after checking of the shuttle in the opposite shuttle box. Nature of variation of weft tension observed during the insertion of a pick through the warp shed [5] has been shown in Fig. 6.3. As the shuttle starts moving out of the shuttle box at about 80° of the crank position after beat-up, the length of weft from the selvedge to the eye of the shuttle, which has already been withdrawn from the shuttle during its traverse from the opposite box in the preceding pick, has first to be consumed before the weft starts unwinding from the pirn. So, during the course of picking, the weft tension initially remains at zero until this length of weft is taken up at A and then the tension rises sharply as the weft starts unwinding from the pirn and there is a sudden pull on the weft. This rise in tension B is the initial tension of the weft. Following the initial tension, the weft tension fluctuates fairly uniformly, indicated by C, about a mean value, as the weft is withdrawn from the running shuttle, until the shuttle comes to rest in the opposite box at

84

Role of yarn tension in weaving

about 250° of the crank position after beat-up. This tension C is the withdrawal tension of the weft and it is determined by the type of weft yarn, internal fittings of the shuttle, shuttle speed, and so on. Fluctuations in tension during the traverse of the shuttle are mainly due to coming off of the coils of weft from the pirn. Having reached the box, as the shuttle is decelerated and checked to come to rest, the weft tension starts falling until the fall is arrested by the closing of the shed. This weft tension D, at the closing of the shed, is the final tension. It is, thus, observed that after inserting the pick in the warp shed when the shuttle comes to the receiving box and is checked, the tension of the weft rapidly falls below the unwinding or withdrawal tension registered during the flight of the shuttle and reaches a low value. The retained or final tension D of the weft, when the shuttle has stopped, is, thus, substantially different from the withdrawal tension C, obtained when the shuttle is traveling across the shed. It has, however, been discussed later in Section 8.2.2 of Chapter 8, how the extent of difference between the withdrawal and final weft tensions can be minimized. Although it is quite natural that the initial tension peak B should be as low as possible, so as to avoid the risk of overstretching and the possibility of breakage of the weft yarn, even a pronounced tension peak has not been found to have any deleterious effect [5]. While the withdrawal tension may not have much influence on the fabric appearance, it should be kept as far as possible constant so that there is least variation in the final tension at trapping of the shed [14], which is desired.

Figure 6.3 Weft tension variation during single pick insertion. The weft yarn emerges from one end of the shuttle and not from the centre. So, the length of weft extended from the cloth selvedge to the eye of the shuttle boxed in the shuttle box is not the same at both sides of the cloth. The length is shorter at that side of the cloth where the eye is near to the box mouth than at the other. During weft insertion, the slackness of this shorter length of weft is, therefore, taken up earlier than the other albeit. The duration of shuttle traverse from one shuttle box to the other remains the same in each pick. This

General form of weft tension variation

85

causes difference in the duration of the withdrawal tension of the weft in the successive picks [12, 6], as shown in Fig. 6.4.

Figure 6.4 Weft tension traces from two sides of the loom. The weft tensions with the shuttle traversing to the right- and left-hand boxes have been indicated by R and L, respectively. The nature of weft tension differs significantly with the direction of shuttle traverse. When the shuttle flies to the right, the tension is much higher than when the shuttle flies to the left [12]. This is because the eye of the shuttle is located at the right end. As observed earlier in Fig. 6.3, the weft tension at the start remains zero till the already unwound length of the weft from the cloth selvedge to the shuttle eye is consumed. Soon after this, the yarn starts unwinding till the shuttle arrives in the opposite box. At the start of unwinding the tension rises sharply, indicated by A in Fig. 6.4. This sharp rise causes excess yarn to be unwound from the pirn and so the peak tensions A in both the picks follow the sharp drop indicated by B. Since the slackness of the weft is taken up earlier when the shuttle moves to the right (because of location of the shuttle eye at the right end of the shuttle), the peak tension at the start of unwinding has then been found to be 31.8 g when the shuttle speed is 11.5 m/s and when the shuttle moves to the left, it is 24.8 g with the shuttle speed of 10.5 m/s at that moment. These have been found with the loom speed of about 206 picks/minute. It is also observed in Fig. 6.4 that, when the shuttle enters the right-hand box, the weft tension increases, indicated by C. When the shuttle with the eye at its right end enters the right-hand box, the eye is located deep into the shuttle box towards the box end and the length of weft ejected from the shuttle eye is abraded between the shuttle front wall and the box front. This causes the rise in tension. Thus, depending upon the location of the shuttle eye with respect to the direction of the shuttle flight, the duration as well as the magnitude of withdrawal tension of the weft varies between the successive picks. If we now compare the results of weft tension shown in Fig. 6.3 and Fig. 6.4, we find that while the former gives a detailed account of the characteristic of weft tension variation during the single traverse of the shuttle from one

86

Role of yarn tension in weaving

shuttle box to the other, the latter provides further information on the effects of the location of the shuttle eye on the nature of weft tension during weft insertion. These two findings, however, do not still elucidate on what happens to the tension of the weft yarn after the shuttle remains at rest after checking and other incidents such as sley movement, beating, etc., take place. The radiotelemetry system [6], which is capable of providing the information on weft tension during the entire period of a pick cycle that is, even after the insertion of the pick is complete, shows that, in addition to what have been observed earlier and discussed above, the weft tension even after the shuttle remains at rest does not remain completely static between the picks and changes because of some incidents [7]. During beat-up, the reed forces the cloth fell to stretch to its full drafted width in the reed and as the reed retreats, the fell contracts a little. This action of the reed slightly pulls the weft yarn from the shuttle, which in turn causes the rise in weft tension. Again, as the sley moves backward after beat-up, the length of weft from the cloth selvedge to the shuttle eye increases to some extent and this may also cause increase in weft tension. If the loom is equipped with side weft-stop motion, this also shows its effect on the weft tension. When the sley with the shuttle boxed in the stop motion side comes forward to beat-up, the weft forceinteracts with the weft and causes small rise in weft tension. From these observations of weft tension traces, it is, thus, evident that the radio-telemetry system [6] not only produces continuous tracing of weft tension during weaving, but also provides some important information on weft tension even after the shuttle comes to rest in the box after checking, which are not possible to be obtained with the other weft tension measuring systems discussed earlier in Chapter 5.

6.3 Weft tension variation in shuttleless looms As discussed in Section 5.3 of Chapter 5, weft insertion in shuttleess weaving machines is executed by projectile, rapier or fluid jet (mainly air) and the type of control exercised by the weft insertion elements on the weft yarns during insertion is therefore vastly different from one to the other. Hence, the natures of weft tension variation during picking in shuttleless looms depend on how the weft is carried and controlled by its insertion element. Unlike in shuttle loom, the weft yarn in any shuttleless loom is inserted by dragging it either physically (rapier or projectile) or indirectly by friction (fluid jet). As a consequence, the weft in shuttleless picking system is subjected to mass acceleration forces and frictional forces. The inertia forces are created by the acceleration of the yarn and by ballooning during unwinding of the weft from the package. The frictional forces occur in withdrawal from the package, at thread guide, at tensioner, etc., and all these have their effects on weft tension. During the entire course of weft insertion, the tension characteristic of the weft yarn is determined by three phases; (i) the acceleration phase, when the weft insertion element starts

General form of weft tension variation

87

picking, (ii) the insertion phase, when the weft insertion element passes through the warp shed, and (iii) the deceleration phase, when the weft insertion element is braked at the completion of picking. Although this is true for both, the shuttle and shuttleless weft insertion systems, it is of greater importance in the latter. Firstly, the weft yarn is dragged (and not laid like in shuttle picking) and the peak forces at acceleration and deceleration increase with the square of the yarn speed and secondly, the yarn is subjected to friction at various deflection points and the friction increases with yarn speed. The weft tension can be deduced from the insertion speed and elastic modulus of the weft yarn [2]. If V is the insertion speed and E is the elastic modulus of the yarn then,

Yarn tension, T = V × E 10 −2

(6.1)

The tension of weft yarn increases with increase in insertion speed, in other words, the speed of the machine and decrease in modulus of elasticity of the yarn.

6.3.1 Tension variation in projectile picking For measuring the tension of the weft yarn in projectile loom, the transducer for measuring the tension is inserted on the yarn generally just behind the weft presenter [7]. The nature of weft tension variation during the course of picking by projectile has been shown in Fig. 6.5.

Figure 6.5 Weft tension during picking by projectile. In the preparation phase for picking by the projectile, the weft tension compensator starts to move downwards to release the weft and the weft tension falls to zero at A (Fig. 6.5). After the projectile is picked at B, the tension momentarily falls to zero as the slackness in the yarn released by the compensator is absorbed. The tension then rises rapidly to a peak C soon after which it falls as it is damped by the compensator. The weft tension then fluctuates around a fairly constant mean value D as the yarn is unwound from the weft package during the flight of the projectile. After the insertion of the

88

Role of yarn tension in weaving

weft through the warp shed, the projectile is braked in the receiving box and at the same time, the compensator starts to move upward for storage of the weft yarn. These lead to a substantial rise in weft tension E at checking. The checking is followed immediately by a sharp fall in weft tension F, probably due to differences between the dynamic and static frictions of the projectile brake and other yarn contact points as well as a slight over delivery of the yarn. But, as the compensator moves up, the tension increases, G. After checking the projectile is moved back a little and the weft gripped by it is released. During this phase the weft tension falls again H.

6.3.2 Tension variation in rapier picking Over a long period of time, different methods of weft insertion by rapier have been developed and commercialized but the tip (or end) transfer method by double rapier is now universally adopted all over the world. The nature of variation of weft tension during picking, shown in Fig. 6.6, therefore, represents for tip transfer system by double rapier. In case of rapier picking system, the convenient place for interposing the tension measuring unit is between the weft presence monitor and the weft presenter [9]. For the convenience of correlating the nature of weft velocity with that of weft tension, the weft velocity diagram has also been exhibited in Fig. 6.6.

Figure 6.6 Weft tension during picking by rapier. From Fig. 6.6, it is observed that at the commencement of picking, the donor (or giver) rapier picks up the weft at A and a high tension peak B occurs which incidentally is the highest during the entire course of insertion of a pick. This follows sudden fall to a very low value at C because of the overrunning of the yarn from the weft accumulator (or weft package if the accumulator is not used) due to acceleration jolt. With increasing rapier

General form of weft tension variation

89

speed, the tension increases, D, and then decreases as the rapier slows down for transfer of the weft. At the instant of transfer from the donor to the receiver (or taker) rapier at E at the centre of the warp shed, the weft tension falls down appreciably at F. Following the transfer, as the receiver rapier starts receding, the weft tension again first rises, G, and then falls in accordance with the nature of movement of the receiver rapier. Finally, when the weft is released by the receiver rapier at H at the completion of weft insertion, the weft tension decreases and remains nearly constant at J until the next weft insertion takes place. It should be noted here that there is no weft tension peak following the transfer, like that observed during picking of the weft by the donor rapier (B). This is because in most of the modern machines, the weft is not standstill at the instant of weft transfer and, therefore, does not need to be re-accelerated, as the transfer occurs when the receiver rapier just reverses its moving direction. It is also observed from Fig. 6.6 that the nature of weft velocity (upper diagram) and that of weft tension (lower diagram) during picking by rapiers in tip transfer method are very similar in appearance and the velocity and tension cycles for one complete insertion phase of a pick consists of two distinct cyclic variations in both velocity and tension; one for insertion of the rapiers and the other for the retreat of the rapiers. Again, each cycle of variation of either the weft velocity or the weft tension is also fairly similar with the other except the peak tension of the weft at the pick-up by the donor rapier. This, thus, indicates that the rapier is the only weft insertion system where the rapier as well as the weft always remains under positive control throughout the course of weft insertion. It is, however, very unlikely to obtain nearly the same weft tension from the start to the end of weft insertion in tip transfer system. The movements of the rapiers are, basically, simple harmonic. So, at the end of the insertion phase of the donor rapier and at the start of the withdrawal phase of the receiver rapier, their velocities will have to be zero at the moment of transfer when the two rapiers are momentarily at rest at their extreme inward positions in the warp shed. This results in the fluctuations of weft velocity and thereby of weft tension. It is naturally desirable to have the weft insertion tension as low as practicable but, precise insertion and transfer of the weft by the rapiers cannot be possible at very low weft tension. For example, for cotton weft yarn of 70 Nm, acceleration peak tension of about 2.5 to 2.8 cN/tex has been found to be very favourable [13]. In case of the loop transfer system, the weft is withdrawn at double the speed of the giver rapier at the first half of the weft insertion phase and in case of loop insertion system with single rapier [10], the weft is withdrawn at double the rapier speed throughout the rapier insertion phase in each pick. In such cases the weft tension during each pick insertion first rises rapidly to a very high peak and then abruptly falls to zero. This, thus, imparts high strain on the weft during picking. These insertion systems are, therefore, not suitable for many types of yarns owing to which, the loop transfer system is

90

Role of yarn tension in weaving

now obsolete and the loop insertion system is restricted to the production of jute sacking cloth only.

6.3.3 Tension variation in air-jet picking In jet picking, the weft is inserted through the warp shed solely by the drag force of the weft insertion element, air or water, and hence, it is subjected to force control only. This implies that the physical and structural characteristics of the weft yarns play much more important roles in jet picking than in others. Again, compared to water, the flow of air in air-jet picking is more complex. The flow of air is turbulent (as it passes through the partially open guides), unsteady (as the air nozzle is put off and on at high frequency, particularly in the modern high speed machines) and can be compressible or incompressible depending on its insertion velocity. All these have the cumulative effects on the characteristics of weft tension during insertion of the pick. The nature of weft tension variation during weft insertion in air-jet picking has been shown in Fig. 6.7. The tension measuring unit can be inserted just before the main nozzle of the loom [3] as indicated in Fig. 5.5 of Chapter 5.

Figure.6.7 Weft tension during picking by air-jet. As the weft is dragged through the warp shed by air, there is relatively gentle rise in weft tension A and the tension remains fairly uniform in this insertion phase (Fig. 6.7). There may also be sudden rise in tension at the start of weft movement as the protruding length of the weft yarn is accelerated [4]. The tension during insertion is mainly owing to the friction between the yarn carrying medium, air and the yarn. Soon after insertion, substantially high tension B occurs during braking of the weft. Although the weft brake is used

General form of weft tension variation

91

to reduce the tension peak occurring at the action of the stopper of the weft accumulator at the end of weft insertion, the peak tension is reached when the yarn is braked directly to a standstill from its high insertion velocity at the start of the braking process. Following the insertion of the pick as the weft tip is under the influence of suction nozzle at the off side the weft tension rises, C. Because of the nature of air-jet picking where the yarn is not controlled positively, the tensions of the single picks may exhibit higher tension peaks occurring at different times and with greater fluctuations, but the high tension peaks are evened out and the fluctuations decrease if the average of a large number of picks is considered [1]. From Fig. 6.7 we can thus observe that, in air-jet picking system, the maximum tension is experienced at the moment of stopping the yarn. At this time, the entire kinetic energy of the weft yarn is being transformed. A large amount of this kinetic energy causes elongation of the yarn, the magnitude of which depends on the type of material the weft yarn is made of [3]. The elongation of the yarn can be calculated as

1 b l n2 = 1 c( Δl ) 2 + U 2 2

(6.2)

where, β is mass of weft, l is length of weft, v is speed of weft prior to stopping, c is extensibility of the weft yarn, ∆l is weft elongation after stopping, and U is friction energy etc. The resulting tension on the weft can be determined by the yarn elongation as

T = c( Δl )

(6.3)

Where, T is weft tension and the maximum tension applied to the weft yarn during the stopping phase is

T = b × Δn × a

(6.4)

where ∆v is speed difference of the weft prior to and after stopping action and a is transmission speed of longitudinal waves in a yarn. If the sudden speed drop during the braking phase, which causes considerable rise in tension and thereby, the loading of the yarn, can be

92

Role of yarn tension in weaving

diminished, the weft braking problem as well as the problem of weft stretching can be reduced substantially. A diminishing of the speed drop can be obtained with a controlled weft brake. This has been discussed in Chapter 8. El Messiry and Mito [4] had deduced that the maximum tension Tmax experienced by the weft yarn at the time of braking is

Tmax = KW 3 N 2C

(6.5)

where, K is the constant with the value of 1.4, W is width of reed, N is loom speed, and C is weft count in tex. From equation 6.5, the maximum tension of the weft at brake varies as the cube of reed width and square of the loom speed, so the peak load on the weft yarn in air-jet picking depends mainly on the width and the speed of the air-jet weaving machine.

6.3.4 Comparison of weft tensions in different shuttleless picking systems From the nature of weft tension, variations observed in three different systems of weft insertion employed in shuttleless weaving machines viz, projectile (Fig. 6.5), rapier (Fig. 6.6) and air-jet (Fig. 6.7) the following observations can be made in regard to their effects on the weft yarns:1.

In case of projectile picking, the weft tension increases considerably at the start of picking as the projectile with the weft is picked and also during checking of the projectile C and E, respectively in Fig. 6.5. Although the peak tension due to picking and the increase in tension during checking are quite similar in magnitude, the latter extends over a longer period. The peak tension at the start of picking imparts strains on a short length of weft yarn and so, no much deleterious effects may be expected. But, the high tension at checking imparts strain on a much longer length of the weft. This as a result, makes the yarn more vulnerable to breakage [9]. Moreover, since the number of weak places will be more on the longer yarn, high tension during checking will have consequently greater adverse effects on it. In view of the prolonged high weft tension at checking, the setting of weft brake is very critical in controlling the weft tension in projectile weaving machine.

General form of weft tension variation

93

2.

In case of rapier picking with tip transfer system, the weft tension variation is the result of (i) acceleration forces due to movement of rapier and (ii) frictional and withdrawal forces owing to ballooning of the yarn and friction at the different points and at the weft tensioner. While the increase in speed of the weaving machine increases both the forces, reduction in insertion time increases mainly the force due to acceleration of the yarn. Any tensioning of the weft during the acceleration phase should, therefore, be avoided as much as possible as this causes unnecessary strain on the yarn. Optimum level of weft tension for each phase of weft insertion can be achieved by a tensioner incorporating time-phased control [8]. It is seen from Fig. 6.6 that the tension peak B occurs only at the start of picking and not after the transfer of the weft. As stated above, the peak tension at the start of insertion is not likely to have much damaging effects, as the length of the weft subjected to this high tension is short. It may be noted here that in some cases another tension peak may also occur as the receiver rapier starts moving back following the transfer of weft, although the second peak is usually not as high as the initial one [17]. This may happen in some cases where at the instant of weft transfer the weft is momentarily at rest and hence, as the receiver rapier starts moving backward following the transfer, the peak tension occurs. However, as discussed in Section 6.3.2, the weft insertion system whose weft velocity and insertion tension are shown in Fig. 6.6, is such that the weft is transferred when it is still in motion and so there is no sudden jerk on the weft at that moment to cause a tension peak.

3.

In case of air-jet picking, the weft tension determines to a large extent the desired velocity of the weft yarn during its insertion through the warp shed. This is because the weft is not dragged by a solid insertion element like projectile or rapier. In air-jet picking, the maximum tension is experienced at the moment of braking the weft yarn that is B in Fig. 6.7, and the peak tension at that instant may be as high as 30% of the tensile strength of the yarn [4]. As a long length of the pick is affected by it, this peak tension imposes fairly high strain on the yarn and weak and delicate weft yarns become more susceptible to break. Again, high strain at braking means a longer length of weft is exposed to this, like in case of projectile picking and hence, higher the length of the pick greater is the chance of breakage because of correspondingly increased number of weak places in it. This is also one of the reasons why weft yarns of high qualities are required for efficient operation of the air-jet weaving machine. The problem may, however, be abated if the weft braking system can be made to so act that the magnitude of

94

Role of yarn tension in weaving

peak tension is reduced reasonably. This aspect has been dealt with in Chapter 8. From the nature of weft tension variation observed in the different weft insertion systems employed in the modern shuttleless weaving machines, it may be concluded that irrespective of the type of picking system, there is wide variation in weft tension during the insertion of the pick. For rapier picking, the maximum tension changes of the weft yarn occur during the initial acceleration only. For projectile picking, the maximum tension changes of the weft yarn occur during the initial acceleration as well as the braking of the projectile, while for air jet picking, the maximum tension changes occur during braking of the inserted pick. However, as pointed out by Weissenberger and Frick [16], while comparing the weaving tensions imposed on the weft yarns by the different picking systems, it should better be taken into consideration that the quantitative comparison may not be very useful even at identical loom performance and weft yarns, since the extend and duration of loading on the yarns can be influenced by other factors like, the machine settings, resistances and driving forces of the weft feeding or guiding systems incorporated. With increasing speed of the shuttleless weaving machines as the outcome of development of the weft insertion system, the weft insertion rate has been increasing significantly, with consequent reduction in weft insertion time. This naturally imposes higher loading on the weft yarns and increases the propensity of the yarn to break. Therefore, in order to achieve high productivity at high speed weaving, the maximum tension of weft yarn should be as low as possible and the length of weft subjected to the maximum tension should be as short as possible during the insertion of weft at high speed. This will reduce the chances of weft breakage. To withstand the strain caused by considerable change in tension, the weft yarns for shuttleless looms should have adequate tensile strength and minimum unevenness. This is rather more pertinent for air-jet picking where the weft is inserted by drag force of air. In view of the facts that the rapier picking system exercises positive and precise control and generally imparts less tension on the weft yarns during insertion (particularly tip transfer system), this is more suitable than others for handling even the difficult yarns. For the weft yarn with given properties and for a given insertion speed, there is an optimum weaving width at which the maximum weft insertion rate can be reached. Based on these, the optimum loom speed can be determined for different types of weft yarn [11]. From the comparisons of weft tensions associated with shuttle and shuttleless picking systems discussed above, it may be worth noting that in shuttle looms, the weft tension during insertion of a pick is obtained not as soon as the shuttle starts traversing for picking but only after the weft starts unwinding from the pirn, when a tension peak is observed. The weft unwinding tension then remains fairly uniform till the pirn is nearly exhausted. When

General form of weft tension variation

95

the pirn is about to be consumed, the weft tension rises considerably but gradually if the normal pirn with straight base is used. In shuttleless looms in contrast, the weft tension is obtained as soon as picking commences and the tension changes rapidly with high peaks occurring at the different phases of weft insertion determined by the type of weft insertion employed. There may not, however, be much difference in the tension behaviour during exhaustion of the weft packages. Again, in shuttle picking, as each inserted pick lies inclined in the warp shed, the sley movement shows its effect on the weft tension while, in shuttleless picking, such effect is not expected as the pick is inserted parallel to the fell of the cloth.

References 1.

Adanur S. and Mohamed M.H., (1991). Analysis of yarn tension in air-jet filling insertion, Textile Research Journal, Volume 61, pp. 259–266.

2.

Adanur S., (2001). Handbook of weaving, Techno Publishing Company. Inc., U.S.A., p. 289.

3.

Anon, (1991). Reduction of the maximum weft yarn tension during weft insertion on air-jet weaving machines, Melliand Textilberichte, Volume 72, pp. 732–734

4.

El Messiry M.A. and Mito A., (1994). Dynamic analysis of weft yarn tension on air jet weaving machine, The Indian Textile Journal, October, pp. 14–17.

5.

Greenwood K and Vaughan GN (1958). Weft tension during weaving, The Journal of the Textile Institute, Volume 49, pp. T247–T264.

6.

Holcombe B.V., Griffith R.E., and Postle R., (1976). The application of radio-telemetry to the measurement of weft tension during the weaving cycle, The Journal of the Textile Institute, Volume 67, pp. 434–439.

7.

Holcombe B.V., Griffith R.E., and Postle R., (1980). A study of weaving systems by means of dynamic warp and weft tension measurement, Indian Journal of Textile Research, Volume 5, pp. 1–5.

8.

Lehnert F., Ballhausen U., and Wulfhorst B., (1990). Analysis of weft thread tensioners for weaving machines, Melliand Textilberichte, Volume 71, pp. 257–262.

9.

Lunenschloss J., and Schlichter S., (1987). Stress on weft and warp yarns in terms of the weft insertion speed and other weaving parameters, Melliand Textilberichte, Volume 68, pp. 93–98.

96

Role of yarn tension in weaving

10. Neogi S.K., (2009). Jute weaving, Dipa Neogi, 268, N. S. C. Bose Road Kolkata– 40, pp. 13,165. 11. Popov G., (1992). Method of determining optimum weaving machine speed relative to weft break frequency, International Textile Bulletin, Volume 4, pp. 20–24. 12. Redozubova E.P., (1961). Weft tension during weaving, Technology of Textile Industry U.S.S.R., No 2, pp. 84–91. 13. Schlegel W. and Glawe A., (1995). Thread tension analysis in weaving under mill conditions, Melliand Textilberichte, Volume 76, pp. 1077– 1081. 14. Thomas I.H., (1957). Behaviour of weft during unwinding from a shuttle, The Textile Manufacturer, April, pp. 163–166. 15. Townsend M.W.H. (1955). Weft tension in weaving, The Journal of the Textile Institute, Volume 46, pp. P699–P712. 16. Weissenberger W. and Frick E., (1989). Yarn stress and high performance weaving, Textile Month, December, pp. 60–63. 17. Zschenderlein D., Oschatz H., Hansch F., and Gries T., (2004). Woven fabric characteristics analysis relative to the weft insertion process, International Textile Bulletin, Volume 5, pp. 60–65.

7 Effects of loom settings and other factors on warp tension

Abstract: During the course of weaving, the warp yarns undergo considerable fluctuations of tension. Back rest setting along with shed timing determines the magnitude as well as the nature of variation of warp tension. While the shedding tension is affected by the heald lifts, heald positions, type of heald movement, etc., beat-up tension, governed by the beat-up force, is determined by the structure of the fabric. The resultant effects of the various loom settings on warp tension affect significantly the nature of tension variation. Besides these, various other factors which control the warp yarn and the cloth on the loom during weaving as well as the weave, yarn join, weft tension, etc., also affect the warp tension during the process of weaving. In comparison to the shuttle looms, the shuttleless looms often require slightly higher tension of the warp yarns to avoid short picks resulting from interference with the warp yarns. But, the over tensioning should not be so much that it affects the weaving performance. Keywords: warp tension; let-off motion; back-rest; warp stop motion; warp leasing pattern; shed geometry; yarn movement; heald lift; warp drafting; shed timing; clean warp shed; denting pattern; beat-up force; temple; warp yarn length; cloth length; loom speed; weave; yarn knot; weft tension.

7.1

Introduction

The warp yarns pass over or through different loom parts (see Fig. 3.1 of Chapter 3) whose movements and settings have significant effects on the dynamic tension and abrasion of the yarns. As weaving proceeds, the warp beam decreases in diameter, the back rest, which controls the shed line, vibrates with the change in warp tension, the heald shafts move for shedding, the reed reciprocates for beat-up, the temples stretch the cloth at the edges to restrict contraction, take-up mechanism draws the cloth forward and so on. Movements and settings of all these, naturally, influence the tension characteristics of the warp yarns. Moreover, the way the warp yarns are passed through the warp stop motion, lease rods, reed dents as well as the

98

Role of yarn tension in weaving

loom speed, type of weave, etc., are also supposed to show their effects on the weaving tension of the warp yarns. Warp tension, suitable for weaving, is determined by the types of weave, yarn, thread densities, etc., and the let-off motion should maintain this throughout, once it is set properly. Shedding and beating naturally impose the maximum and rapid variations in warp tension and the timing of shedding motion has important influence on that. Settings of the back rest, which help in forming the proper configuration of the warp shed by maintaining the desired tensions of the warp yarns, determine the magnitudes as well as the nature of variation of warp tension.

7.2 Effects of warp beam and let-off motion The functions of the let-off motion of the loom are (i) to feed the warp yarns to the weaving zone at the rate commensurate with the rate of take-up of the cloth and (ii) to keep the warp tension fairly constant all through the weaving of the warp beam. As mentioned earlier, in order to ensure these, the warp beam is needed to rotate with increasing angular movement as it is woven down. Figure 7.1 illustrates the relation between the warp tension and the diameter of the warp beam [52].

Figure 7.1 Relation between warp tension and warp beam diameter. As the warp beam decreases in diameter, the warp tension increases and the increase is initially gradual and then quite rapid with the result that the relation between the increase in warp tension and the decrease in beam diameter is exponential in nature, as shown in Fig. 7.1. The unusual rise in

99

Effects of loom settings and other factors on warp tension

warp tension is avoided by controlling the angular rotation of the warp beam in each pick cycle to such an extent that the tension remains constant within the reasonable range of ΔT. If the angular rotation of the warp beam is not adjusted properly, the effects will naturally be the excessive increase in warp tension leading to the increase in warp break, decrease in pick spacing and reduction in cloth width. This is because of the gradual decrease in the amount of warp yarn let-off per pick with the decreasing circumference of the warp beam. In the negative let-off mechanism (Fig. 3.3A of Chapter 3), the angular rotation of the warp beam is controlled by manual adjustment of the gripping force of the beam whereas in the positive mechanism (Fig. 3.3B of Chapter 3), it is achieved through the sensing back rest. It is also noted from Fig. 7.1 that as the diameter of the warp beam decreases, the warp tension increase at a faster rate. So, in case of the negative let-off motion, the need for manual adjustment of the gripping force of the beam becomes more frequent at the lower diameters of the warp beam.

7.2.1 Warp tension control in negative let-off motion Referring back to Fig. 3.3A of Chapter 3, the average tension of the warp sheet is made to remain by and large constant by adjusting the position of the dead weight on the weight lever at each side of the warp beam. Suppose the average tension of warp is Tw then

Tw = ( F × r ) / R

(7.1)

Tw a F / R

(7.2)

or

where, F is the friction force acting on the beam ruffle by the chain, r is the radius of the beam ruffle and this is constant, and R is the radius of the warp beam with yarn, which decreases during weaving. Again, the friction force

F = Tt − Ts

(7.3)

where, Tt is the tension of the tight side of the chain, and Ts is the tension of the slack side of the chain Now, according to coil friction and from equation 1.6 from Chapter 1

100

Role of yarn tension in weaving

F = Tt (1 − 1 / e mq )

(7.4)

or F α Tt

(7.5)

(since, the term within the bracket is constant) Then, from equations 7.2 and 7.5 Tw a Tt / R

(7.6)

This indicates that as the warp beam weaves down, tension of the tight side of the chain is needed to be decreased proportionately to maintain the average tension Tw of warp yarns constant throughout weaving. This is done by gradually moving the dead weight towards the fulcrum of the weight lever for proportionately reducing the gripping force exerted by the chain on the beam ruffle. While doing this, care should be taken that both the weights are moved to the same extent on both sides of the warp beam As the warp beam in the negative let-off motion is held by the dead weight system through the chains wrapped round its ruffles, it is free to swing back and forth under the changing tension of the warp sheet during running of the loom. Figure 7.2 shows the nature of oscillation of the warp beam during weaving, as observed by Owen [42].

Figure 7.2 Warp beam oscillation with negative let-off motion. The oscillation of the warp beam gradually moves upward due to the action of the take-up motion of the loom. The cycle of oscillation repeats in two picks but the difference between the two picks is far less than between the two picks of the tension cycle for a single warp yarn. As shown in Fig. 4.5 of Chapter 4, the magnitudes of warp tension for the top and bottom sheds are different in a given pick cycle because of asymmetric shed and the variation in tension of a given yarn repeats in two picks for plain weave. But the resultant effect on the warp beam (Fig. 7.2) is the sum of these two and the motion of the beam, therefore, depends

Effects of loom settings and other factors on warp tension

101

upon the tension in all the warp yarns. As a matter of fact, the two-pick tension cycle of the single warp yarn depends upon the difference between the two shed lines whereas, the two-pick oscillation cycle of the warp beam depends upon the tension differences between the yarns in the two sets. If the setting of the shed is symmetric with the warp tensions in both the shed lines same as indicated in Fig. 4.4 (Chapter 4), the oscillatory movement of the warp beam would repeat in every pick. When the warp tension is high the beam moves forward to deliver the warp and soon after that as the yarns become slack, the beam moves backward to wind the yarns and arrest the fall in tension. Thus, the oscillatory movement of the warp beam in negative let-off motion has the equalizing effect upon the tension cycle of the warp sheet and the equalization of tension becomes progressively smaller as the beam empties. Again, because of oscillatory motion and inertia of the beam, the maximum tension of the warp sheet is not the tension at which slipping of the beam occurs. When the tension is rising rapidly, a higher value may be reached than would be possible if the beam responded instantaneously. The beam acquires momentum, which carries it forward, after the warp tension passes its maximum value. The form of tension cycle due to slipping of the beam against the friction in negative let-off mechanism is, thus, influenced by the weight and size of the beam and may vary as the beam empties. If the form of the tension cycle influences the quality of weaving, the weaving may not be equally good throughout a beam if oscillation of the beam is permitted. The beatup tension of the warp becomes more prominent in the tension cycle with less total weights on the beam. From the above discussions it is understood that there is a relation between the let-off motion and warp tension. Depending on the type of fabric to be woven, the let-off motion is set for a certain value of warp tension. This value is maintained as long as the loom runs satisfactorily and the correct warp tension remains constant from pick to pick. If this correct tension value is, for any reason, disturbed for example, when the loom is stopped and there is relaxation or unsatisfactory letting back, the warp tension will change from pick to pick until it attains the correct preset value [17]. This has been discussed in detail in Section 10.6.1 of Chapter 10. In case the general level of warp tension falls because of inadequate weighting of the let-off motion and the low tension level persists, the cloth fell will gradually move towards the reed until it corresponds to the new tension level. There is, however, a limit to the compensation for the fall in warp tension by change in cloth fell position. If it is too low, the desired pick spacing of the cloth cannot be obtained and the cloth produced will be of poor quality. Detailed discussions on these have been made later in Section 7.8.

102

Role of yarn tension in weaving

7.2.2 Warp tension control in positive let-off motion In case of positive let-off mechanism shown in Fig. 3.3B (Chapter 3), the average warp tension is supposed to remain constant from full to empty warp beam, as the required length of warp let-off per pick remains constant all through by automatic control of angular rotation of the beam by the sensing back rest. Badve and Bhattacharya’s [3] observations with both the negative and positive (or controlled) let-off motions with beam diameter feeler are that, with the exhaustion of the warp beams, the average warp tensions in both the cases tend to increase at the initial stages of reduction in beam diameter and then decrease which is more pronounced with the positive let-off motion. With further decrease in beam diameter, the tension again rises appreciably with both the systems and this, therefore, indicates the failure of the type of positive let-off mechanism considered by them [3] to keep the warp tension constant when the beam reaches the minimum diameter. The average warp tension throughout weaving down of the warp beams is higher with the positive let-off motion, but if the differences in warp tension between the top and bottom shed lines for all the diameters of the warp beams are averaged, this is higher for the negative let-off motion. In regard to the control of variation in warp tension by positive let-off motion, Ramaswamy and Paliwal [45], however, had made different observations. While measuring the tension of the warp sheet of plain cotton cloth by lease rod gauge, they had observed that the positive (or controlled) let-off motion with beam feeler exercises effective control of tension variations provided the let-off is properly adjusted for ensuring normal level of warp tension suitable for the type of cloth being woven. At higher or lower level of tension than the normal, the variation in warp tension is higher. In the fully positive let-off mechanism with rollers [43], free length of the warp yarn from the warp beam to the fell of the cloth is much less than that in the conventional positive let-off mechanism with sensing back-rest, but the front shed geometry from the heald shafts to the cloth fell is similar with both the systems. As a result, any changes in basic warp tension and beat-up tension are more critical with the fully positive roller let-off mechanism than with the other because of the consequent changes in the elastic constant of the warp yarns [6]. Various types of mechanically controlled positive let-off mechanism are very much in use in many modern shuttle and shuttleless weaving machines. Their response time is naturally more owing to the mechanical relay systems and this may pose problem to high speed operations. Many attempts have, therefore, been made to replace the mechanical system fully or partly by electrically or electronically controlled systems [44, 21, 32, 27] where the measuring signal is generally obtained by means of optical,

Effects of loom settings and other factors on warp tension

103

inductive or capacitive movement transmitters from the conversion of back-rest movement into an electrical voltage. A fully positive hybrid type let-off system with a series of five feed rollers has been designed (for both weaving and warp knitting) and the continuous rotational speeds of the rollers are controlled by a DC motor through two gear boxes [44]. The operating principle of the system is similar to that referred to earlier [43] which is, however, driven fully mechanically. The performances of this motor driven hybrid type let-off mechanism on a loom have shown that if the input tension of the warp yarns or the pick spacing is accidentally changed during operation of the loom, the corrective measures are taken accurately and fairly quickly. In comparison to the mechanical let-off mechanism, the electronically controlled [21] and recently developed fuzzy logic controlled type let-off systems [32] have been found to be very effective because of very quick and precise control of warp let-off. In any type of let-off system, the basic need is to detect the change related to weaving tension of the warp yarns and modify the angular rotation of the warp beam accordingly to maintain the warp tension, as far as possible, constant throughout. In electronically controlled warp let-off system, the measuring element for controlling the warp let-off can be fitted anywhere where warp or fabric tension affects the loom components. In another effort [27], various alternative methods as compared with the more popular back-rest roller sensing system have been evaluated and although some of them have been found to be suitable for improving the quality of sensing, these are either too expensive or for some others, the installation on the weaving machine is not flexible enough for the purpose. The performance of a mechanically controlled let-off motion (Hunt let-off motion) has been compared with an electronically controlled system in a double flexible rapier loom [21]. Similar to mechanical system, different forces cause the back-rest roller displacement in the electronic let-off system and the movement of the back-rest roller is continuously sensed and processed by a processor computer whose output signal is transmitted to an independent DC motor to control the angular rotation of the warp beam accordingly for compensating the variation in warp tension. The electronically operated let-off system responses more rapidly than the mechanical system to regulate the disturbances created by sudden change in warp tension. In comparison to the mechanical letoff system, warp tension variation in the electronically controlled letoff motion reduces by about two times. The conventional mechanical control system of the semi-positive let-off mechanism using spring loaded back rest and friction drive to the warp beam of an air-jet rapier weaving machine (ATPR) has been substituted by Proportional Integral Derivative (PID) system and fuzzy logic based control system [32]. For

104

Role of yarn tension in weaving

the last two systems that is, PID and fuzzy logic based control systems, the servomotor and driver have been installed to control the let-off motion for quick response even for small variation in warp tension. The results of warp tension variations in a pick cycle for the three let-off systems viz., the conventional semi-positive, PID based and fuzzy logic based have been shown in Fig. 7.3.

Figure 7.3 Warp tension variations with different let-off control systems. While the warp tension with the conventional let-off system varies from 23 - 53 cN, that with PID and fuzzy logic based control systems vary from 20 - 45 cN and 18 - 36 cN, respectively. Thus, the warp let-off motion controlled by PID and fuzzy logic based systems reduce the peak tension at beat-up by 15% and 32%, respectively and the latter is more effective in controlling the variation in warp tension. Performances of the conventional and fuzzy logic based controlled systems in the loom have shown better control of warp tension with consequent significant reduction in warp breakage by the latter. Reduction in average warp tension value from 34.6 cN obtained with the conventional let-off system to 25.23 cN obtained with fuzzy logic controlled system reduces the warp breakage by 19%. It is now worth noting that while discussing the positive (controlled) let-off motion in Section 3.4 it has been stated that the warp tension is kept constant as the warp beam weaves down. Critical analysis of the angle of warp sheet in relation to the back-rest and the back-rest mounting, however, indicate that in actuality, the warp tension does not always remain constant through out the course of weaving. Discussions in Section 7.3.3 will reveal that as weaving proceeds, the gradual reduction in the diameter of the warp beam increases the

Effects of loom settings and other factors on warp tension

105

angle wrap of the warp sheet on the back-rest and this, in some cases, changes the warp tension to some extent.

7.3 Effects of back-rest Figure 3.1 of Chapter 3 shows that the sheet of warp yarns from the warp beam passes over the back-rest and proceeds towards the fell of the cloth. The main function of the back-rest is to help formation of proper warp shed during weaving by maintaining the desired tension of the warp yarns. As the free length of the warp yarns from the back-rest to the cloth fell is subjected to formation of the shed, the positions of the back-rest with respect to that of the cloth fell in the horizontal direction and with respect to that of the warp beam in the vertical direction affect the shape of the rear shed and thereby, the weaving tension of the warp yarns. Moreover, the manner of mounting of the back-rest, particularly in positive let-off motion, has significant influences on the dynamic tension of the warp yarns and the nature of its variation. However, with proper setting of the back rest, it oscillates in accordance with the movement of the shedding motion to compensate the resultant variations in warp tension.

7.3.1 Back-rest setting In normal position of the back-rest, the warp sheet from the top of the backrest to the fell of the cloth lies in a straight line when the shed is in level position, as indicated by broken lines in Fig. 7.4. Consequently, when the shed is open, the lengths of warp yarns at both the top and bottom sheds are equal (also see Fig. 3.2 of Chapter 3) and hence, their tensions are also the same in each pick cycle (see Fig. 4.4 of Chapter 4). This normal setting of the back-rest is, however, hardly maintained in practice, as mentioned earlier.

Figure 7.4 Warp lines with different back-rest positions.

106

Role of yarn tension in weaving

In most cases, the back-rest is set at a little higher than its normal position, as indicated by firm lines in Fig. 7.4. With the back-rest set at normal position, the geometry of the rear shed for a given heald shaft is identical at either position of the shed line. But when the back-rest is raised or lowered, the configuration of the rear shed formed by a heald shaft changes distinctly at two shed lines albeit, that of the front shed remains the same. With the positions of the cloth fell and shed level unaltered, if the back-rest is raised or lowered the length of warp yarns forming the bottom shed is greater or smaller, respectively than that forming the top shed, resulting in correspondingly higher or lower tension of the yarns of the bottom shed. The general form of warp tension variations of two heald shafts observed with raised back-rest are shown in Fig. 7.5 for two consecutive picks for plain weave.

Figure 7.5 Warp tensions with raised back-rest. If we now compare the nature of warp tension variations shown in Fig. 4.4 of Chapter 4 with that shown in Fig. 7.5, we find that, although the general form of tension variation is fairly similar in both the cases, the magnitude of warp tension of a given heald shaft is higher in one pick cycle, as observed earlier in Fig. 4.5 of Chapter 4. The shedding tension of the warp yarns forming the bottom shed considerably exceeds that of the yarns forming the top shed. Likewise, the beat-up tension of the yarns moving down is higher than that of the yarns moving up. The situation reverses as the shed changes and each yarn becomes once taut and then slack during the course of weaving. The lowest warp tension in a pick cycle is at the closed position of the shed when the yarns are least strained. After this minimum value, the warp tension rises slowly at first and then very sharply as the next beat-up takes place. Bramma [11] had also observed with plain weave construction that, when the warp shed lines are symmetric, beat-up and shedding tensions are fairly the same in successive pick cycles but when the shed lines are asymmetric with raised back-rest, beat-up and shedding tensions of the warp yarns forming bottom shed are significantly higher than those of the yarns forming top shed. Between the two shed settings, one with the normal and the other with the raised positions of the back-rest, however, over all tensions of the warp yarns are less with asymmetric shed

Effects of loom settings and other factors on warp tension

107

lines. According to Strazd [50], when the length of the front shed from the heald shafts to the fell of the cloth is half the length of the shed from the cloth fell to the back rest, warp deformation due to shedding is the minimum. For the fabrics with high weft densities, higher tension is observed at beatup than at shedding and the beat-up tension of the falling yarns is the maximum in the entire tension cycle as observed in Fig. 4.5 (Chapter 4). The value of the maximum tension is about 2 to 3 times the value of the minimum tension which is observed when the yarns reach the closed shed position on their way to the top shed [42]. If the weft density is low, the beat-up tension is generally a little lower than shedding tension as shown in Fig. 7.5, particularly at the bottom shed, but the raised position of the back-rest causes higher tension of the warp yarns forming the bottom shed all the same. The continuous change in the magnitude in warp tension at two consecutive shed lines owing to the raised position of the back-rest helps to obtain improved cover and higher weft density of the cloth (discussed in Chapter 10).

Figure 7.6 Warp tensions of different heald shafts with different back-rest heights. Figure 7.6 shows the warp tensions of the first and the fourth heald shafts with the positions of the back-rest from normal that is 0 mm, to a height of 120 mm [57]. Warp tensions of the first and the fourth heald shafts have been indicated by the broken and firm lines, respectively. With the normal position of the back rest, the shed is symmetrical and consequently, the warp tensions at both the top and bottom shed are identical for either the first or the fourth heald shaft, as discussed earlier. As the back rest is raised, the shed becomes more asymmetric and the yarns forming the bottom are stretched more while those of the top are stretched less. Consequently, the tensions of the bottom

108

Role of yarn tension in weaving

shed of both the heald shafts gradually increase and those of the top gradually decrease although the mean warp tensions remain fairly uniform irrespective of the position of the back-rest. At any position of the back rest, the warp tensions (at top, bottom and mean) of the fourth heald shaft are a little higher than those of the first but the differences are not very great. Similar effects have been observed with 2/2 twill wool cloth [41] but the effect is more distinct on the top shed. While the beat-up tension of the warp at the top shed is considerably higher with the lower position of the back-rest, it falls almost to zero when the back-rest is raised. Too low tension at the top shed tends the warp yarns to sag and interfere with weft insertion. Both the excessive and very low warp tensions, therefore, tend to increase the warp break. The length of the back shed that is, the length of the warp sheet between the back rest and the heald shafts, is also very important from the point of view of strain imposed on the warp yarns. If the distance of the back rest from the back heald shaft is less, the warp yarns while forming the shed are subjected to greater strain than if the back rest is located farther away from the heald shafts. As shown in Fig. 3.2 (Chapter 3), the angles θ1 and θ2 made by the yarns of the first and second heald shafts, respectively with the horizontal line AH at the back zone of the shed will be greater with closer location of the back rest. Again, with the raised position of the back rest and the resultant asymmetric shed line, the strain will be even more on the yarns forming the bottom shed. Location of the back rest farther away from the heald shafts, as is generally observed in the shuttleless looms, enables the yarns to withstand the strain better during shed formation and this in turn helps reduce the chances of warp breakage. Greater distance of back rest from the heald shafts is also beneficial particularly for weaving filament rayon [53]. With long free length of warp, within reason of course, the difference in warp tension between two shed lines with asymmetric shed will be reduced and this will be less harmful to the easily stretchable rayon yarns.

7.3.2 Back-rest movement In the loom fitted with negative or friction let-off motion (Fig. 3.3A of Chapter 3) the back rest is generally given an oscillatory movement of fixed amplitude with the help of a single throw cam mounted on the crank shaft of the loom to minimize the extent of variation in warp tension during shedding. The movement of the back rest is so set that as the shed is formed it leans forward (towards the healds) to release the warps and as the shed is closed it moves backward (away from the healds) to take up the slackness in the warps. This type of movement of the back rest with fixed amplitude is suitable mainly for plain or matt weaves where all the warp yarns move and come to a closed position in each pick cycle. It has been observed that although this increases the tension of the top shed and reduces the tension of the bottom shed to some extent with asymmetric shed, without affecting the beat-up, as is desired, the effects are, however, not very pronounced [42].

Effects of loom settings and other factors on warp tension

109

The main problem often observed with this system of oscillating back rest of fixed amplitude is that the nature of movement imparted by the cam to the back rest does not exactly harmonize with the nature of movement of the heald shafts. The cam is not generally provided with a dwell, at least to the extent to suit the dwell period of the heald shafts. As a consequence, while the warp yarns are still at dwell the back-rest starts pulling the yarns. Warp tension variations with cam operated back-rests are shown by firm line in Fig. 7.7A for cotton loom [3] and in Fig. 7.7B for jute loom [33], both weaving plain cloth.

Figure 7.7 Warp tensions with fixed oscillatory movement of back-rest.

110

Role of yarn tension in weaving

Warp tension varies significantly in a pick cycle for both top and bottom sheds and also in both types of loom, Fig. 7.7. What is more important to note is that, the warp tension at shed dwell varies considerably. As should be expected the warp tension is fairly low at the start of the dwell as the oscillator releases the yarn to reduce the shedding tension and the tension should remain low during the entire period of shed dwell. Since however, the cams operating the back rests in both the looms are not provided with dwells, the tension increases appreciably during the end of the dwell which is not desirable. The desired result of reduced and nearly uniform tension of warp yarns during the period shed remains open can be achieved by employing the cam with the extent of dwell commensurate with those of the heald shafts. At the closed position of the shed, however, the warp tension is fairly high (Fig. 7.7B), which thus tends to minimize the extent of variation in warp tension, as is desired. From these observations it, therefore, follows that the contour of the cam is supposed to be such that the oscillatory movement of the backrest follows simple harmonic motion (SHM) as this the type of movements adopted for the heald shafts in cam or dobby shedding mechanisms and also because the cyclic warp elongation percentage curve owing to heald movement follows SHM [19]. With a view to studying the effect of the back-rest movement on the warp tension variation and beat-up strip width in weaving, Gu [19] had designed cams of different profiles and compared their performances on a loom weaving a plain cotton fabric. It has been observed that back-rest movement of the simple harmonic type reduces the warp tension fluctuation, but to a limited extent. Modification in the movement of the back-rest where the back-rest is raised during beat-up and thereby, increases the tension difference between the top and bottom shed lines at that instance is beneficial for beat-up as it reduces the beat-up strip width that is, the movement of the cloth fell during beat-up. Instead of the fixed oscillatory movement of the back rest if the back rest is attached with springs, the magnitude of its oscillation will be in accordance with the change in tension of the whole warp sheet and not of fixed amplitude and also the movement of the back rest will be independent of the type of weave. A study on jute loom [7] has shown that compared to the fixed back rest, which does not oscillate, the spring loaded back rest reduces the beat-up tension by 12 – 34% and the shedding tension by 17 – 48% and these in turn, cause significant reduction in warp break (see Section 9.2.1 of Chapter 9). Similar to this spring loaded back rest discussed above, the back rest in positive let-off motion is attached with springs or dead weights and presses against the warp sheet. The oscillation of the back-rest is therefore governed wholly by the variation in warp tension. The nature of warp tension variation

Effects of loom settings and other factors on warp tension

111

produced by the sensitive back-rest of the positive let-off motion [3] has been illustrated in Fig. 7.7A by broken lines. Unlike the back rest of fixed oscillatory movement (whose warp tension is indicated by firm line in the same figure), the sensitive back rest produces fairly same warp tension throughout the shed dwell at bottom shed. While let-off of warp yarns with negative let-off motion is instantaneous, because as soon as the pulling force of the warp sheet is sufficient to overcome the frictional resistance of the beam ruffles it rotates the beam immediately, that with positive type is comparatively gradual owing to the early action of the beam rotating mechanism and the delayed response of the different mechanical links incorporated in the mechanism. Chamberlain & Snowden [12] had observed that the back rest which exerts pressure on the warp sheet through weight placed on the weight lever attached to it (see Fig. 3.4, Chapter 3), produces secondary peaks shortly after the beat-up peaks followed by tension fluctuations with decreasing magnitudes as the warp tension gradually falls. Higher is the sharpness of the primary peak at beatup, greater is the sharpness of the secondary peak, and this is true for both the plain and twill weaves studied. The action of beat-up causes a sudden jerk on the warp sheet which in turn, exerts a sharp pull on the back rest. As an effect, the back rest along with the weight lever are jerked away from their normal positions and, owing to their inertia, cannot recover immediately to regain control on the warp tension. The back rest, therefore, releases the pressure on the warp sheet momentarily before moving back again under the action of the weight lever to press on the warp sheet. This action of the backrest continues till it settles down to control the warp tension. These cause the secondary peaks after the beat-up peaks followed by vibration of the warp tension with decreasing magnitudes. The failure of the back rest to respond immediately to the rise in warp tension during beat-up and also shedding because of inertia causes undue stretching of the warp yarns. For this reason, compensating movement of the back-rest should possibly be avoided in modern high speed weaving machine because the inertia of the back-rest does not allow it to respond immediately to the change in warp tension and the resultant extension of the warp yarn causes harm to the yarns [58]. In some of the modern shuttleless weaving machines, warp tension is controlled by warp tensioner consisting of a square torsion bar in tension tube in place of the normal backrest, particularly for weaving heavy fabrics [1]. Tension on the warp sheet is applied by applying torque to the torsion bar. Torsion bar presses the back rest against the warp ends and depending on the type of fabric being woven torsion bar is pre-tensioned to the required torsion thus, setting the warp tension. The low mass back rest oscillates with shedding and beat-up to equalize the tension and eliminate tension peaks. The back rests in the positive let-off motion may be so mounted on its swing levers that either it is fixed and cannot rotate or, as mostly observed

112

Role of yarn tension in weaving

in the modern weaving machines, it is free to rotate on its axis by frictional contact of the warp sheet passing over it. If the back rest is fixed, the movement of the warp sheet over the back rest causes sliding friction between the yarns and the back rest and as the warp beam decreases in diameter the contact angle of the warp sheet with the back rest and thereby, the tension of the warp sheet in front of the back-rest gradually increases. Although this does not cause a very serious effect, this too can be avoided by incorporating an extra rail before the backrest [15]. If the back rest can rotate, the movement of the warp sheet rotates the back rest against any friction occurring on its bearing in which it is housed. Again, the cyclic variations in warp tension, in particular with asymmetric shed, cause some reciprocating movements of the warp sheet along its longitudinal direction which may also have effects on the oscillatory movement of the back rest, though to a small extent, in addition to its forward rotation. This produces complexity in the movement of the back rest as it consists of oscillation about its axis and rocking motion with its brackets about the pivots [51].

7.3.3 Back-rest mounting It has been mentioned above that the back rest in positive let-off motion may be so mounted that it may either rotate or remain fixed and it has also been pointed out in Section 7.2.2 that, owing to the change in the angle wrap of the warp sheet on the back rest the warp tension does not remain constant as the warp beam weaves down. Let us now then examine the effects of rotating and non-rotating (that is, fixed) back rests on warp tension. Foster [15] had first shown the effects graphically considering different designs of the back-rest bracket, but detailed analysis of these had been made by Marks and Robinson [30] in their book on Principles of Weaving.

7.3.3.1 Rotating back-rest Figure 7.8 shows the tension of the warp yarns and the forces acting on the back rest of freely rotating type. For convenience, let us assume that the forces do not change in magnitude and direction as the warp beam weaves down and the frictional effects of the back rest are negligible. Let us also assume that, as the backrest rotates freely by the warp sheet passing over it, there is no friction between the warp sheet and the back rest which is largely true if the back rest can rotate smoothly in its bearings without any restraint. As can be seen later, these assumptions do not invalidate the discussions made and the inferences drawn thereof.

Effects of loom settings and other factors on warp tension

113

Figure 7.8 Forces on the freely rotating back rest. Consequent to the assumptions made, the warp tension T in Fig. 7.8 on each side of the back rest is the same. Tensions T have the resultant R which bisects the angle formed between the top warp sheet, from the back-rest to the heald shafts, and the back warp sheet, from the warp beam to the back-rest. The angle between T and R is α. Under stable condition R is balanced by an equal and opposite force R1 which is again the resultant of the force F acting on the warp sheet at right angle to the short arm D of the back-rest bracket and the stress E developed in that arm. The force F that acts on the warp sheet to maintain the desired warp tension is obtained by attaching weights or springs with the back-rest brackets (Fig 3.4, Chapter 3). The arm makes an angle θ with the vertical. From Fig. 7.8 then, the warp tension T is given by

T = R / 2cos a

(7.7)

R1 = F / cos( a − q)

(7.8)

Now angle AOB = α and angle AOC = θ so angle BOC = (α − θ) and

Since R = R1, combining equations 7.7 and 7.8 we get

T = F / 2cos a cos (a − q )

(7.9)

With the given values of F and θ, we can now calculate the warp tension T for different values of α resulting from the decreases in the diameter of the warp beam, with the help of equation 7.9. If θ is changed because of

114

Role of yarn tension in weaving

alteration in the design of the back-rest bracket, the values of T will also change differently. Let us consider two cases where θ is 0° and 90°. When θ = 0° equation 7.9 is written as T = F/2 cos2α

(7.10)

when θ = 90° T = F / 2 cos α sin α = F / sin 2α

(7.11)

and

Figure 7.9 Effect of mounting of freely rotating back-rest. If we now plot the values of T/F against the different values of angle θ in Fig. 7.9, we can see how the warp tension varies as the warp beam weaves down (and α decreases) for three selected values of θ. When θ is 0° warp tension decreases appreciably, but when θ is 90° the tension remains, by and large, constant which therefore is the best arrangement of back rest mounting for a freely rotating back-rest. With θ = 45°, which is maintained in many letoff systems the warp tension decreases but not to any great extent.

7.3.3.2 Non-rotating back-rest If the back rest is fixed and cannot rotate on its axis by the traverse of the warp sheet over it, the tension T2 of the top warp sheet is greater than the tension T1 of the back warp sheet because of coil friction. The resultant R now does not bisect the angle formed between the top and back warp sheets and hence, the angle α formed between the resultant R and T2 will now not be equal to the angle β between the resultant R and T1 as shown in Fig 7.10.

Effects of loom settings and other factors on warp tension

115

Figure 7.10 Forces on the non-rotating back rest. If the perpendicular OD is drawn on the side of the parallelogram parallel to T2 then OD = R sin α = T1 sin δ (where δ is the angle of wrap, shown in inset diagram in Fig. 7.10) or T1 = R sin α / sin δ Now from equation 7.8 R1 = F / cos (α – θ), and since R = R1, then T1 = F sin α / cos (α – θ) sin δ

(7.12)

From Fig. 7.10, (180 – δ) = (α + β) and we know that sin (180 – δ) = sin δ. So, sin δ = sin (α + β). Then, the equation 7.12 may be written as T1 = F sin α / cos (α – θ) sin (α + β)

(7.13)

We know that in any triangle, the length of a side is proportional to the sin of the opposite angle. So T1 / sin α = T2 / sin β or T2 = T1 sin β / sin α

(7.14)

Replacing T1 by T2 in Equation (7.13) we get T2 = {F sinα / cos (α – θ) sin (α + β)} × (sin β / sin α) or T2 = F sin β / cos (α – θ) sin (α + β)

(7.15)

116

Role of yarn tension in weaving

As the back-rest does not rotate the tension of the warp sheet passing over it will be subjected to coil friction and from equations 7.14 and 1.6 from Chapter 1

T2 / T1 = sin b / sin a = e md

(7.16)

From equations 7.13 and 7.15, we may now calculate the values of T1/F and T2 /F, respectively as the warp beam decreases in diameter with θ = 900 which has been found to cause the least variation in warp tension for the rotating back-rest. If we now select certain practical values of coefficient of friction µ, we can calculate T2 / F for different angles of wrap that is, for different diameters of the warp beam. The calculated results of T2 / F are plotted in Fig. 7.11, together with the curve for T1 / F which is the same as that for θ = 900 in Fig. 7.9. From Fig. 7.11, it is observed that if the frictional coefficient µ is small, the effect on warp tension will be negligible, but if it is high the effect will be considerable with the non-rotating back-rest. It is also noticed from Fig. 7.9 and Fig. 7.11 that the trends of variations of T/F with two types of back-rest mounting are opposite to each other and in case of non-rotating back-rest, T/F increases as the warp beam becomes empty. This, thus, suggests that, with non-rotating back-rest, the value of θ should preferably be between 450 and 900 to minimize the warp tension variation resulting from the weaving down of the warp beam.

Figure 7.11 Effect of forces as the function of µ of non-rotating back-rest.

Effects of loom settings and other factors on warp tension

117

The force F applied on the warp sheet by the back-rest and the resultant tension T of the warp yarns also depend on the diameter of the back rest and its location with respect to the position of the warp beam on the loom. The variations in the ratio, F/T during weaving can be minimized if the diameter of the back rest is increased and it is set at higher level from the warp beam and closer to the heald shafts, but there are mechanical limitations in designing of the weaving machines. It has been found that the back-rest diameter of 12 cm, height of the back-rest axis from the axis of the warp beam of 65 cm and the horizontal distance of the back-rest axis from the axis of the warp beam of 20 cm towards the inner side are the optimum values for the purpose [47]. Although, it has been assumed above for the convenience of discussion, that there is no friction between the warp sheet and the back-rest, in reality, however, the friction does exist between the two. As already discussed, the back rests in positive let-off motions are either fitted with bearings so that they can rotate freely on their brackets by the friction of the moving warp sheets or fixed on the brackets and cannot rotate. In either case the tension of the top sheet of the warp yarns will be greater than that of the back sheet. In case of rotating back rest, which is more in use, the warp sheet applies force to rotate it against the friction of its bearings. If the bearings are faulty or not properly lubricated the back rest can fail to rotate very smoothly in its bearings. The sticking of the back-rest in its every revolution will increase the warp tension and consequently decrease the pick spacing of the cloth. Moreover, the back rest may not be perfectly straight (which is not unlikely to be found in practice) and this will also have adverse effects on warp tension. In case of non-rotating backrest, effects due to coil friction will be present and the tension of the top sheet will gradually increase as the angle of wrap of the warp sheet round the backrest increases with weaving down of the warp beam (Fig. 7.11). In view of these, as pointed out earlier, Foster [15] was of the opinion that the ideal condition is a non-rotating backrest along with some means like the use of an extra rail for maintaining the contact angle of the warp sheet with the back-rest constant. Special care is needed to be taken to weave filament rayon as it can be easily spoiled by uncontrolled stretch or undue tension. To reduce friction, particularly with viscose rayon, fluted back-rest instead of the widely used plain roller is found to be more suitable as the chances of warp stripiness arising out of differences in tension in the warp ends is reduced [54]. To effect a change in the rotation of the warp beam, because of the change in warp length between the beam and the fell of the cloth, a number brackets and levers have to move and in doing so they have to overcome the frictional resistances at their pivots and joints. Forces due to these frictional resistances, which in some of the let-off systems may be quite substantial, affect further the tension of the warp yarns. Whenever the warp sheet presses the back-rest

118

Role of yarn tension in weaving

to compensate for the change in warp length, the frictional resistances cause an increase in warp tension before the beam driving system responds. This means that the oscillation of the back rest causes fluctuations in the warp tension about its preset value. These may of course, be minimized by proper maintenance, design and lubrication of these pivots and joints or recently by replacing many of the mechanical components by electrical or electronic counterparts.

7.3.4 Back-rest type With the advent of the shuttleless weaving machines, it is always aimed to achieve high weft insertion rate mainly by increasing the operating speeds of the machines. High machine speed naturally demands very quick response for compensating the rise in warp stresses due to shedding and beat-up. This is generally achieved by the oscillating back-rest attached to springs through its brackets and the back rest moves to compensate changes in warp tension. With this system, however, the back rest cannot respond quickly to compensate the changes of warp tension because of its inertia, as discussed above. Very recently a new system has been devised where the warp let-off mechanism is equipped with air spring back-rest system [16], shown in Fig. 7.12.

Figure 7.12 Air spring back-rest system. The system consists of a deflecting plate or roll and a pressure hose as damping element. Depending on the requirement, the thickness of the deflecting plate and the air pressure inside the hose can be varied and hence, the weaving machine can be set up in an optimal way for different warp materials and fabric constructions. The air spring back-rest system has been found to reduce significantly, the phase difference between the warp tension and the response by the back-rest in particular and the warp stresses in general.

Effects of loom settings and other factors on warp tension

119

7.4 Effects of warp stop motion setting and warp leasing pattern Before entering the heald eyes the warp yarns are generally taken through the drop wires of warp stop motion as shown in Fig. 3.1 (Chapter 3). In the orthodox shuttle looms where warp stop motion is not used, the warp yarns are passed through a pair of lease rods located at a suitable position between the backrest and the heald shafts with a certain gap between the two rods. The lease rods are generally wooden bars of elliptical shape whose lengths are the same but more than the width of the warp sheet through which they are inserted. As the warp yarns pass through the warp stop motion or the lease rods, the back shed is virtually formed from this point, even though the free length of the warp remains unaffected. The settings of the stop motion or the lease rods, therefore, affect the warp tension and its variation during weaving.

7.4.1 Warp stop motion setting It has been discussed above in Section 7.3.3 that the geometry of the warp shed is changed by the vertical as well as horizontal movements of the back rest and the settings of the back rest have pronounced influences on dynamic warp tensions. Since all the warp yarns pass through the drop wires of the warp stop motion, changes in the settings of the back rest alone may not produce the desired effects. The warp stop mechanism should also be adjusted correspondingly otherwise the effects can either be insignificant or even the opposite to what is desired under certain conditions. In fact, the change in the position of the dropper cradle of the warp stop mechanism in vertical or horizontal direction has greater effects on the warp tension than the similar change in the back rest position because of the resultant shorter length of the rear shed. The warp yarns are drawn through drop wires positioned at different rows. The shed geometry of individual warp end is, therefore, determined by the position of the heald shaft as well as the location of the row of the drop wire. As a result, the tension of a warp end is affected not only by the position of the heald shaft through which it is drawn but also by the row of the drop wire through which it is threaded. With the length of the front shed unchanged, tension of the warp yarns due to shedding also depends on the lift of the yarns in the drop wires [50]. If this is increased, the length of the warp sheet from the drop wires to the back-rest should also be increased to control the warp deformation. If the warp stop mechanism is raised, the strain on the yarns of the bottom shed increases and that of the top shed decreases and similarly, if the mechanism is moved horizontally towards the heald shafts, both the bottom and top shed tensions and the mean warp tension increase and consequently, the formation

120

Role of yarn tension in weaving

of the shed is more clean. There are of course limits to shortening the length of the rear shed, as it causes sharp increase in warp tension and thereby strain on the warp yarns [57]. Conversely, moving the dropper bar back reduces elongation and thus, the tension of the warp yarns during shedding [20]. Unfortunately, however, the correct adjustments of the warp stop motion are often overlooked. As Weinsdorfer, Wolfrum & Stark [56] have commented, this is possibly because either the settings of warp stop motion correctly are a little time consuming or the influence of the position of the warp stop motion on the warp tension is underestimated or perhaps not clearly understood. It is quite natural that, since the warp stop motion is located nearer to the shedding harness than the back rest, a certain change in the position of the warp stop motion in vertical direction has a greater influence on warp tension than the same amount of change in back-rest position.

7.4.2 Warp leasing pattern The lease rods are used for the following purposes: 1.

To assist separation of the warp yarns possibly sticking together by the size applied to them during sizing;

2.

To allow the formation of a clean warp shed;

3.

To enable the weaver draw the warp yarns correctly through the heald eyes, particularly after the repair of the broken warp; and

4

To improve the cover of the cloth.

Fig 7.13 Warp leasing patterns.

Effects of loom settings and other factors on warp tension

121

There are different patterns adopted for leasing of the warp yarns through the lease rods. These are one-by-one generally known as end-and-end leasing, two-by-two, etc. A study has been made on a jute loom weaving plain cloth to examine the effects of leasing pattern of warp yarns on warp tension [34]. As only two heald shafts are used for weaving jute cloth with plain weave, the different leasing patterns have been made involving the yarns of two heald shafts. Figure 7.13 illustrates the two leasing patterns, one-by-one and twoby-two, considered in the study. In one-by-one leasing (Fig. 7.13A) invariably adopted for weaving jute cloth of plain weave construction, all the warp yarns of the front heald shaft pass under the back lease rod and over the front lease rod and those of the back heald shaft pass in the reverse order that is, over the back lease rod and under the front lease rod. Two successive warp ends of the two heald shafts pass through the same dent of reed. In two-by-two leasing (Fig. 7.13B), the two successive warp yarns, one of the front and the other of the back heald shafts, pass through the same reed dent like in the above, but under the back-lease rod and over the front-lease rod and vice versa together in alternate manner. Twoby-two leasing are generally adopted for weaving plain cotton and other fabrics but there at least four heald shafts are used. Figure 7.14 shows the results of the study made on the jute loom where two heald shafts have been used for both the leasing patterns.

122

Role of yarn tension in weaving

Figure 7.14 Warp tension variations with two leasing patterns. As can be expected, one-by-one leasing produces only two magnitudes of warp tension variation in a pick cycle, one for the bottom shed and the other for the top shed, as shown in Fig. 7.14A, and these are reversed in the next pick cycle because of plain weave construction. So, with this leasing pattern the entire warp yarns of a given heald shaft are under the same tension in a given shed line. In comparison to this, the two-by-two leasing produces much greater variations in tension amongst the warp yarns, as shown in Fig. 7.14B, even for the same weave. Unlike in one-by-one leasing, the warp yarns in two-by-two leasing pass through the lease rods in different orders even though they are controlled by the same heald shaft. As a result, there are four different levels of tension of the warp yarns in a pick cycle. Half of the total warp yarns of a given heald shaft are at higher tension than the rest and the similar pattern of warp tension variations repeats in every alternate pick because of plain weave. It is observed from Fig. 7.14B that, when the warp yarns of either heald shaft form the bottom shed, those passing over the front lease rod are always under higher tension than the others passing under the front lease rod because the formers being diverted from the front lease rod located closer to the heald shafts are strained more than the others. For the same reason, when the yarns of either heald shaft form the top shed, those passing under the front lease rod are always under higher tension than the rest passing over the front lease rod. Moreover, the difference in tension is much more pronounced at the top shed for both the heald shafts. Irrespective of the type of leasing pattern,

Effects of loom settings and other factors on warp tension

123

however, the beat-up tension is higher than the shedding tension in most cases and owing to the asymmetric shed (because of raised backrest) the warp yarns forming the bottom shed are under higher tension than those forming the top shed in a pick cycle. The greater variation in tension amongst the warp yarns produced by two-by-two leasing improves the cloth cover, discussed in Section 10.6.3 of Chapter 10.

7.5 Effects of shedding The purpose of shedding, the first primary motion of a weaving machine, is to separate the warp sheet into two layers for insertion of weft. Extent of separation of the warp yarns must be such as to allow the weft insertion element pass through the shed unimpeded to insert the pick. After a pick is inserted, the shed closes and again opens to form the next shed. The repeated opening and closing of the warp shed impose strain on the warp yarns and the magnitude of the strain is determined by the position of the heald shaft with respect to the fell of the cloth, as discussed earlier. In view of the facts that shedding is mainly responsible for producing the maximum fluctuation in warp tension and the sudden thrust at beat-up aggravates the warp strain further, utmost cares should be taken in respect of the nature of warp movement and the timing of shedding motion so that no undue strain is imposed on the yarns.

7.5.1 Front shed geometry It is seen from Fig. 3.2 of Chapter 3 that the size of the front shed, that is the shed formed in front of the heald shafts, is smaller than that of the back shed from the heald shafts to the back rest. The portion of the front shed in front of the reed is again, the most important zone as this is the area where insertion of the pick takes place. The geometry of the front shed, therefore, depends primarily on the shape and size of the weft insertion element employed. As pointed out earlier in Chapter 6, let us now compare the paths taken by the wefts inserted by shuttle and other means viz., projectile, rapier or fluid-jet inside the warp sheds. In shuttle loom, each pick is connected with the preceding pick (except of course, at the instant of pirn change) and the continuity of the successive picks can be seen at the cloth selvedges. Hence, when the shuttle inserts a pick, it traverses tangential within the shed with its one end attached to the fell of the cloth at one selvedge and the other farther away from the fell and towards the reed where it is ejected from the eye of the shuttle boxed in the shuttle box. In any shuttleless weaving machine, each pick inserted is separate from the preceding ones, as it is severed from the weft package after completion of its insertion. This indicates that when a pick is inserted in the shed it lies parallel to the fell of the cloth and hence, the

124

Role of yarn tension in weaving

position of its insertion point can be varied with respect to the cloth fell. This is naturally more possible in case of fluid-jet picking system.

Figure 7.15 Pick position in warp shed. Figure 7.15A illustrates the position of a pick inside the shed formed horizontally. If the pick is inserted close to the cloth fell, the clearances between the pick and the top and bottom warp shed lines will be small. There is then the risk of the pick coming in contact with the warp ends, particularly if the warp yarns of the shed are not very taut. This may be avoided by increasing the tension of the warp yarns and/or increasing the shed angle but these correspondingly increase the strain on the yarns. It is, therefore, more advantageous to shift the pick towards the healds to obtain the permissible clearances a and b. Let us now consider the inclined warp lines arranged in “Maxbo” air-jet and “Eltex” air- and water-jet weaving machines as shown in Fig. 7.15B. With the warp lines inclined at an angle, the effective clearances a and b are now relatively much higher with the same distance df of the pick from the fell of the cloth. This implies that the satisfactory weft insertion can be achieved without increasing the warp tension or the shed angle if the warp line is set at a certain angle with the horizontal. From these we thus find that the distance of the weft insertion point from the cloth fell should be such as to allow unimpeded weft insertion without increasing unduly the tension of the warp yarns.

7.5.2 Heald movement Shedding operation produces unavoidable variation in warp tension and thus, imposes appreciable strain on the yarns. It is therefore, imperative that the yarns are not subjected to further undue strain, particularly due to malfunction of the shedding mechanism. Conventional shuttle looms are generally equipped with negative tappet shedding mechanism with compound acting (roller

Effects of loom settings and other factors on warp tension

125

type) heald reversing mechanism at the top. Although every care is taken to ensure smooth movement of the heald shafts, this type of shedding system is often found to be wanting in this respect and causes jerky movements of the yarns. To avoid this shortcoming, the conventional negative tappet shedding mechanism of a jute loom was replaced by a modified mechanism to exercise positive control on the heald shafts with the help of a single grooved cam for weaving plain cloth [35]. The cam was designed to produce the desired simple harmonic motion to the heald shafts. Comparative tension variations of the single warp yarn observed with the conventional and the modified shedding mechanisms [36] are shown in Fig. 7.16. Yarn tension was measured by an electronic tensometer shown in Fig. 3.11 (Chapter 3).

Figure 7.16 Warp tension variations with conventional and modified tappet shedding mechanisms. Important observations that can be made from Fig. 7.16 are the extents of variations in warp tension at shed dwells, which is supposed to be 1/3rd (1200) of a pick cycle. With the conventional shedding mechanism the tension does not remain static and vary considerably all along the dwells for both the top and bottom shed lines. Tension is the highest at shed full open, lowest at the middle of the dwell and rises again as the shed starts closing when the dwell ends indicating that the heald shaft failed to operate smoothly because of slackness in the mechanism of the negative type. Warp tension, with the modified shedding system in comparison, is far more uniform at the dwell periods at both the shed lines. This is owing to more correct profile of the grooved cam and fully positive control on the heald shafts. There is, of course, usual variation in warp tension in two consecutive pick cycles because of

126

Role of yarn tension in weaving

asymmetric shed line. Warp tension traces in Fig. 7.16 thus, clearly signifies the need for correct design of the shedding tappets and also the settings of the shedding mechanism for proper functioning of the mechanism. As discussed above, besides the need for the desired amount of shed dwell for uninterrupted passage of the weft insertion element through the warp shed, movement of each heald shaft should also be smooth through out to obviate undue strain on the warp yarns. Simple harmonic and cycloidal motions are found to be suitable for producing the desired movements of the heald shafts. Figure 7.17 shows the characteristics of the two motions.

Figure 7.17 Simple harmonic and cycloidal motions.

Effects of loom settings and other factors on warp tension

127

In both the motions, the velocity and acceleration curves vary smoothly, but the cycloidal motion has some distinct advantages over the other. In case of simple harmonic motion, the maximum acceleration is produced at both ends of the stroke, as shown in Fig. 7.17A. This causes radical change in acceleration if the motion starts from the static condition. With cycloidal motion in comparison, the acceleration starts from zero, Fig. 7.17B and hence, it is preferable for high speed shedding motion. For the given picking time and shed timing, available depth of shed is also greater with cycloidal cam than with the simple harmonic cam and at the lower speed ranges, the cycloidal cam without dwell is better able to reduce the shock and variation of acceleration than the harmonic cam with dwell. Moreover, during the period of shed opening the magnitudes of the lateral force resulted from the forces exerted by the shedding cams on the treadle bowls and responsible for wear and vibration of the shedding mechanism, are also comparatively much less with cycloidal motion. Because of these, the cycloidal cam produces slightly smoother variation in warp tension during shedding, as shown in Fig. 7.18, and hence, causes lower breakage of warp yarns than the simple harmonic cam [8].

Figure 7.18 Warp tension variations with simple harmonic and cycloidal cams. It should, however, be noted here that, many of these benefits of the cycloidal motion are greatly affected if the cycloidal cams are not fabricated with fair amount of perfection which naturally involves more cost. Moreover, as can be observed from Fig. 7.17, for the same displacement that is the lift, the maximum value of acceleration of the simple harmonic curve is less than that of the cycloidal curve. For these reasons, the movement of the heald shaft conforming to the simple harmonic motion is opted for most cases.

128

Role of yarn tension in weaving

7.5.3 Heald position It has been discussed earlier in Section 3.3 (Chapter 3) that in order to maintaining the shed angle constant for successful insertion of weft, as a heald shaft is positioned farther away from the fell of the cloth its lift is proportionately greater. Greater heald lift means correspondingly higher strain and thereby, higher tension of the warp yarns. With symmetry shed shown in Fig. 3.2 (Chapter 3), tension of warp of a given heald shaft is the same at either position (top or bottom), but that of the rear shaft is a little higher than that of the front because of its greater lift (see Fig. 4.4, Chapter 4). Therefore, the movements of the warp yarns and the magnitudes of their tension during shedding are position dependent. Farther a heald shaft is from the fell of the cloth, greater is its lift and higher should be the tension of the warp yarns controlled by it. This is indicated in Fig. 7.19, which shows the tensions of warp yarns of six heald shafts of a rapier weaving machine [28].

Figure 7.19 Warp tensions of different heald shafts. The tensions of the warp yarns forming the bottom shed (BST) increase towards the back healds on account of their greater lifts. Similarly, the tensions of the warp yarns forming the top shed should also increase for the same reason but, because of the type of shed maintained in the rapier loom, referred to [28], the warp tensions of the top shed (TST) have been found to show the decreasing trend. However, the average warp tensions of the yarns increase as the lifts of their heald shafts gradually increase. It is also observed from Fig. 7.19 that, the beat-up tensions of the warp yarns forming the bottom shed (BT-BS) and the top shed (BT-TS) though remain almost unaffected by the position of the heald shafts. Irrespective of the position of the heald shafts the warp yarns of a given shed line generally share the beat-up force almost

Effects of loom settings and other factors on warp tension

129

equally at the time of beat-up. Because of asymmetric shed however, (BT-BS) is always higher than (BT-TS). Bakulin [5] had also observed 25 – 30% lower warp tension of the front heald shaft than that of the yarns of the back heald shaft as, Badve & Bhattacharya [4] had observed greater warp tension of the back heald shafts, particularly with negative let-off motion. With the positive let-off motion on the contrary, the tensions of the warp yarns of the rear heald shafts have been found to be comparatively less [4]. This is possibly because, when the rear heald shafts are lowered, the back rest is depressed more under the resultant higher warp tensions and this in turn, registers lower values of tension. It has been observed [58] in a shuttleless weaving machine that, the geometrical change in length of the warp yarns from the warp beam to the cloth fell due to shed formation varies from 2.7 mm for the first heald shaft while forming the top shed to 15.6 mm for the 16th heald shafts while forming the bottom shed. This causes the change in warp tension as the result of extension, from 0.18% for the 1st to 1.04% for the 16th heald shafts. As discussed above, the difference in tensions of the warp yarns from the 1st to the say, nth heald shaft cannot generally be avoided, but if the difference in tension can be reduced to nil or as much as possible, all the yarns will be subjected to nearly the same loading which is desirable in the interest of good weaving performance. This is possible provided all the heald shafts have the same lift. Same lift of all the heald shafts will not allow the formation of clean warp shed and as a consequence, cause undesired interference particularly with the shuttle during its passage through the warp shed resulting in increased warp breakage or even shuttle trap. Nearly same loading on the warp yarns of a number of heald shafts can, however, be possible to some extent in shuttleless weaving machines where a much smaller weft insertion element than a shuttle is used. In these machines, the depth of shed is generally much lower than that required for shuttle and the heald shafts are placed very close to each other (that is, the pitch of the heald shafts is very low), as they are mostly driven by positive shedding mechanism. As a result, when a cloth is woven with less number of heald shafts, the difference in tension between the warp yarns of the back and front heald shafts often becomes hardly discernible [9].

7.5.4 Warp drafting pattern In accordance with the design to be produced on the cloth, different drafting patterns of warp ends through the heald shafts are adopted. In weaving plain construction with more than two heald shafts skip, draft is more commonly employed than straight draft. Reduced probability of warp breakage, particularly with high warp density, is the main reason along with ease of operation in tappet shedding mechanism. Studies [59] on straight and skip

130

Role of yarn tension in weaving

drafts with four heald shafts weaving plain cloth on a narrow loom with small pitch of 8 mm of the heald shafts have shown that contrary to general belief and practice, straight draft produces better shed. With skip draft as the adjacent heald shafts 1 and 2 move up and 3 and 4 move down, the yarn sheets are very dense and compact which cause high abrasion. It has also been observed that small heald shaft pitch with four shafts for plain weave is not ideal even for narrow loom. The small pitch can, however, be beneficial for weaving a fabric with longer floats of the yarns because, consequently less number of warp yarns at the crossing points in addition to smaller lifts owing to less heald shaft pitch will strain the yarns less.

7.5.5 Shed timing Setting of shed timing generally refers to the position of shed level with respect to beat-up. The shed can be set late or early. With late shedding, the changing warp yarns assume their level or closed position at beat-up or very close to it, while with early shedding, shed level position takes place well ahead of beat-up to the maximum extent of about 900 before beat-up. If we now consider plain weave for example, all the warp yarns in case of the late shedding are nearly at the same plane during beat-up, as shown in Fig. 7.20A, and the newly inserted pick is retained in place mainly by the frictional resistance with the warps yarns. As the heald shafts with this shed, timing are at the level position at beat-up and the yarns are therefore, at the lowest tension in the pick cycle, greater basic tension of the warp yarns is generally required for achieving proper beat-up force. In case of the early shedding on the contrary, the warp yarns of two shed lines have already crossed over the newly inserted pick at the instance of beat-up, as shown in Fig. 7.20B, and the pick is, therefore, well bound by the warp yarns. It may be observed from the illustrations in Fig. 7.20 that, when the shed timing is late (Fig. 7.20A), advancement of the reed towards the fell of the cloth during beat-up causes the resultant R of the warp tensions T1 and T2 gradually increase in magnitude and move towards the warp yarns, but the beat-up tension of all the warp yarns remain practically the same as the force exerted by the reed during beating is equally shared by them. When the shed timing is early (Fig. 7.20B), advancement of the reed during beat-up carries the pick of weft forward between the crossed warp yarns for a considerable fraction of the reed’s traverse and in the process angle α becomes more equal to angle β. As a result, the resultant R gradually moves away from the warp yarns and the pick is well held in its place at the cloth fell. If the back rest is now set at normal position so that the shed is symmetric, the beat-up tension of all the warp yarns will again be nearly the same, that is BTb = BTt, irrespective

Effects of loom settings and other factors on warp tension

131

of their positions, as the yarns of two shed lines are diverted to the equal distances from their closed position.

Figure 7.20 Warp yarn positions at beat-up with late and early shedding. These settings have, however, some shortcomings owing to which these are seldom maintained in practice. If the shed level position coincides with beatup (Fig .7.20A), the pick just beaten tends to spring back from the cloth fell as soon as the reed recedes after beat-up (since R moves away from the cloth fell) and the desired weft densities are often difficult to be obtained. Moreover, the cover of the cloth produced is also poor because of reediness. In the other case (Fig. 7.20B), the desired weft densities may be obtained in many cases as the picks cannot spring back after beat-up, but the problem of reediness still persists since the warp tension at beat-up is the same for both the yarns of the top and bottom shed lines. So the effective and more customary setting of the shed lines is the early shed timing along with asymmetric shed obtained by raising the back rest. This on one hand enables to achieve high weft densities and on the other, good cover of the cloths without reed mark. See Section 10.6.3 (Chapter 10) for detailed discussions on these. Tension variations of single warp yarn with late and early shed timings and raised position of the back-rest have been shown in Fig. 7.21 and Fig. 7.14A, respectively for plain weave construction and end-and-end leasing pattern with jute yarns [34]. In late shed timing the heald shafts are level at the time of beat-up and hence, the warp tension of either heald shaft is fairly low even at beat-up in each pick cycle, Fig. 7.21. The beat-up tension varies from 52 – 136 g. When the shed timing is early with shed level position set 900 before beat-up, the warp yarns are forming the next shed at the time of beat-up and are therefore, already under some tension at that instance. Moreover, as the

132

Role of yarn tension in weaving

shed is asymmetric, the beat-up thrust by the reed is shared more by the yarns moving down than those moving up. As a result, the beat-up tensions of the warp yarns forming the bottom shed are appreciably higher than those of the yarns forming the top shed for either heald shaft, as shown in Fig. 7.14A. The beat-up tension varies from 62 g when the yarns are moving up to 326 g when the yarns are moving down.

Figure 7.21 Warp tension variations with late shed timings.

7.6 Formation of clean warp shed It has been pointed out earlier in Section 1.3.2 of Chapter 1 that one of the prerequisites for maintaining proper tension of the warp yarns during weaving is the formation of clean warp shed for successful insertion of the pick of weft. Formation of clean warp shed depends on many factors such as the basic warp tension, the length of the front shed (from the fell of the cloth to the heald shafts), front shed angle, position of the back rest, distance between the backrest and warp stop motion and the distance between the heald shafts and the warp stop motion. Besides these, hairiness of the warp yarns tends to cause clinging and thereby, disturbance in forming a clear shed. Hairiness, which although depends on the type yarn and method of spinning, is caused due to abrasion of the yarns against the different loom parts like, drop wires, heald eyes, reed wires, etc., and also between the individual yarns. Abrasion between the warp yarns is influenced by warp density; higher the warp density, greater is the abrasion and hence, the yarn hairiness. Naturally, neither is it possible to completely eliminate frictional stresses during the process of weaving nor

Effects of loom settings and other factors on warp tension

133

is it possible to produce an absolutely smooth yarn from staple fibers. It is, therefore, aimed to lessen the abrasion of the yarn and reduce the resultant problem by effecting proper adjustments and settings of the different loom mechanisms and also carrying out the necessary treatments to the warp yarns. Shed geometry, which greatly determines the tensions of the top and bottom shed lines, has significant effect on shed cleanliness. With symmetrical shed, tensions of both the shed lines are nearly identical. The more asymmetric the shed setting, the greater the difference between the bottom and top shed tensions. Relatively high warp tension improves warp end crossing, resulting in a cleaner shed. If the shed is not formed cleanly and the yarns of the top and bottom shed lines tend to cling during picking, weft insertion will be affected. In shuttle loom, clinging of warp may not pose much problem as the heavy shuttle can generally overcome the hindrance and at the most break the clinging yarns, but in shuttleless looms, and particularly with fluid-jet picking, weft insertion will be affected very adversely. In jet picking, where the weft yarn is inserted by means of an air- or water-jet (and, to a good extent, in projectile picking also), an absolutely clean shed is essential for successful weft insertion. If the pick by any chance, catches on a warp end owing to clinging fiber or a broken filament, loom stoppage will result for short pick. Even with fully positive weft insertion system such as rapier, unclear shed formation resulting from yarn entanglement can also affect weft transfer by rapier [23]. Deeper warp sheds, earlier shed timing and increased loading on the back rest in positive let-off motion are commonly arranged to reduce the tendency of clinging of the warp yarns and thereby, assist in forming clear warp shed when weaving fibrous or closely sett warp yarns [49]. While the deeper shed and increased loading on the back rest directly increase the warp tension during the weaving cycle, early shedding causes considerable rubbing action between the warps and the weft during the process of beat-up. When the shed timing is early, each weft pick is carried forward between the crossed warp threads for a considerable fraction of the reed’s forward traverse during beatup. Thus, the warp yarns in this case are rubbed much more than when the timing of the shed change is normal or late and as a consequence, the clinging or stitching of the warp yarns of the two shed lines is broken. Again, increased warp tension causes greater abrasion of the warp [55], as shown in Fig. 7.22. It can also be observed from Fig. 7.22 that, besides the weaving tension, the magnitude of abrasion also depends on the type and count of yarn and method of spinning. For the same degree of sizing and warp density, all cotton and ring spun yarns are subjected to greater abrasion than the blended polyester/ cotton and open-end yarns, respectively because of higher hairiness, while the coarse yarns suffer greater abrasion than the fine yarns because of its increased surface area.

134

Role of yarn tension in weaving

Figure 7.22 Warp yarn abrasion with different tensions and yarn types. As loom speed increases the time span between separation of the warp ends for shed formation and pick entry becomes shorter and with very high speed latest weaving machines there is practically hardly any time available for the shed opening phase [59]. At very high speed, it may even so happen (particularly with the usable width of the reed fully utilized) that the shed fails to open sufficiently in time before weft insertion takes place. In view of this therefore, it is very important that, as soon as the shed starts forming the warp ends of the parting, heald shafts move away cleanly from each other without the slightest tendency of clinging. Maintenance of a clean warp shed is fairly easy when weaving a light or moderately dense fabric from well sized spun or smooth filament warp yarns, but the task becomes tough when weaving closely set dense fabrics, such as poplin for example, and/or with fibrous or hairy warp yarns where the tendency of clinging is high. The remedy in such cases is generally to maintain a higher than usual weaving tension of the warp yarns. Since yarn hairiness is one of the main reasons for shed entanglement, singles unsized warp yarns are generally hairier than other yarns, and these therefore show greater tendency of clinging when the shed opens, particularly for a plain weave where all the yarns move simultaneously in each pick [9]. For the same reason rotor yarn being much less hairy than ring yarn helps forming distinctly cleaner shed than the other even at lower warp tension. If the tension of the rotor warp yarn is increased the shed cleanliness is even better. Shed cleanliness also depends on the warp drawing-in pattern through the heald shafts as discussed above, denting pattern through the reed to reduce

Effects of loom settings and other factors on warp tension

135

frictional stresses [55] and weft density but not on count of weft yarn [59]. High weft density breaks the clinging tendency of the warp yarns at beat-up. Figure 7.23 shows the warp tension variations for a clinging and nonclinging 25 tex singles worsted wool warp yarns while weaving plain weave with 24.8 ends/cm and 23.2 picks/cm [9].

Figure 7.23 Warp tension variations with clinging and nonclinging warp yarns. Clinging naturally leads to higher tension during shedding but to a slight increase in tension at beat-up. The clinging can increase the average warp tension by up to 20%. With 2/2 twill weave on the other hand, as the inter yarn friction is much less compared to plain weave even with the same warp and weft settings, clinging has negligible effects on warp tension. Snarling and clinging of the warp yarns with consequent interference at weaving are mostly due to large differences in yarn tension caused by warp preparation or drawing-in processes [60]. At high level of warp tension, the differences in tension between the separate yarns may be evened out somewhat, as completely slack yarns are taken care of, and this may result in improved weaving performance. It is all the same more important to ensure even tension of all the warp yarns as early as in warp preparation. Clinging tendency of the yarns can naturally be avoided by adopting asymmetric shed by raising the position of the back rest. With asymmetric shed lines, there is a relative movement of the individual warp ends in their longitudinal directions and the effect is more pronounced as the heald shafts differ in their lifts. The higher the position of the back rest, the greater is the shed asymmetry (of

136

Role of yarn tension in weaving

course within certain limits) and thereby, the relative movements between the ends. In view of the fact that tensions of warp yarns are generally lower at the selvedges than at the body of the cloth (see Section 7.9), warp entanglement is more likely to occur at these regions. It has been observed [58] that higher the warp density, higher is the chances of warp entanglement; higher warp tension breaks the entanglement more, as indicated in Fig. 7.24.

Figure 7.24 Warp entanglement with different warp densities and warp tensions The warp entanglement can be prevented by maintaining the rear shed not longer than 30 – 35 cm (front shed also should not be too long), staggered shedding, asymmetric shed to such extent that the top shed does not become too slack. Extent of shed asymmetry with the ratio of 1.5 – 2.0 between the bottom shed and top shed tensions can be considered optimum [58]. The problem of yarn entanglement can be avoided by taking proper measures at spinning and sizing or even applying wax to the yarns, so that the projected fibers are bound on the yarn surfaces. These reduce the hairiness and enhance the binding strength also of the fibers. Bradbury and Worthington [10] studied the effects of warp tension and leasing pattern of warp yarns on the amount of fiber loss during shedding and for the purpose, they considered three levels of warp tension of 45, 24 and 10.5 g/end and three types of leasing pattern of 1 × 1, 2 × 2 and point, in which one lease rod is placed about 1.6 mm above the other. Figure 7.25 shows the results of total amount of loss of fiber as the function of shedding duration with different levels of warp tension and 1 × 1 leasing of warp yarns.

Effects of loom settings and other factors on warp tension

137

Figure 7.25 Fibre loss with different warp tensions. As Fig. 7.25 indicates, the amount of fiber loss always increases with shedding duration but the rate of fiber loss decreases at each level of warp tension as well as with decreasing warp tension, as the duration increases up to the end of the first 5 minutes of shedding, indicated by broken lines; thereafter, the rate of loss remains roughly constant. It can also been observed from the figure that contrary to general assumption, the fiber loss always increases with the magnitude of warp tension and hence, the chances of fiber loss are low if the warp tension is low. Similar observations on fiber loss were made by Lord [29] who found increase in extension of the warp yarns with tension up to a certain point beyond which the changes in both, the fiber loss and extension, are little. The warp yarns under higher tensions are thus more adversely affected by the abrasive actions of the healds and reed. It is further observed from Fig. 7.25 that, the rate of fiber loss decreases with decreasing warp tension. In regard to the effect of warp leasing, Bradbury and Worthington [10] found that when the warp tension is low, the type of leasing pattern has hardly any effect on fiber loss but when the tension is high, the 2 × 2 leasing has a much lower fiber loss than the other two. In case of point leasing, the length of rear shed from the lease to the heald shafts has no significant effect on fiber loss except at high warp tension. At high tension, the short rear shed produces greater loss of fiber, probably because as the shed is short the clinging yarns are more prone to break apart causing greater loss of fiber. Lord [29] had also found the effects of abrasion on the warp yarns at different levels of warp tension. At low warp tension, only the “skin” of the yarn, which

138

Role of yarn tension in weaving

mainly contains the size formed by the size paste on the yarn, is removed but at higher warp tensions, the droppings contain greater amount of fiber material removed from the yarns. As the beat-up action causes considerable abrasion to the yarns, this increases the total dropping from 0.05 – 0.073 g at high warp tension and in this, the fiber content increases from 0.04 – 0.055 g. In regard to the problem of yarn clinging, Weinsdorfer and Wimalaweera [55] had stated that measures taken during the weaving process to separate clinging threads will always cause an increase in fiber abrasion. It is not always advisable to increase the warp tension or reduce the distance between the drop wire supports and the heald frames to tackle the problem. As pointed out above, the best way to solve the problems of clinging of yarns is to take appropriate measures at the spinning and weaving preparatory stages.

7.7 Effects of warp denting pattern Warp yarns are dented one or more number of ends per dent of reed. Main consideration for denting pattern is the warp density or in other words ends/cm of the cloth. In case of weaving, a cloth with high warp density, two or more number of ends is passed through a dent and the count of reed used is such that it has correspondingly greater air space between the reed wires. The ends then have greater room to move against each other within a dent. Weinsdorfer and Salama [59] studied the effects of denting pattern of 1, 2, and 3 ends/dent on the shed formation with plain weave construction. They observed that, compared to 2 ends/dent, the denting pattern of 3 ends/dent creates more problem in forming the shed despite comparatively greater reed space available. In the latter case, the warp end at the centre of each reed dent has two adjacent ends which counter weave that is they move in the opposite direction to it in shed opening and therefore, obstruct it causing increased tension of the middle end. Increase in tension of the middle end is also because it has less room available in the cloth as it has two ends immediately adjacent and so has to interlace under more strain. As a consequence, the middle warp end is generally more susceptible to break for higher tension than the adjacent ends in the reed dent. In comparison, the other two ends lying next to the reed wires have more room in the cloth due to the exposed spaces occupied by the reed wires and have therefore, less tension as they interlace in the cloth more comfortably. For the same reasons, if 4 ends are drawn in a dent the two middle ends are supposed to be more tensioned, than the other two extreme ends in a dent. With denting pattern of 2 ends/dent the warp ends are better separated from each other and both the warp ends in one dent have practically identical warp yarn tension, provided the heald shafts are set correctly. Similar observations were made by Lord [29]. He observed that, as the number of ends is increased in a dent, the tension and also the amount of abrasion of the ends increase but, according to

Effects of loom settings and other factors on warp tension

139

him, the increase in abrasion is not severe until four ends are drawn through a dent, naturally because of overcrowding of the yarns in the dent. He also observed peculiar effect of denting pattern on yarn extensibility. The change in extensibility is the highest, 96%, at 4 ends/dent, but the lowest, 19%, at 3 ends/dent, while the single denting has shown 77% change in extensibility. Amongst the various denting patterns, single denting that is, one end/dent is seldom used as this needs very dense reed. In case of single denting, there is no possibility of inter-yarn friction within a dent but, reduced amount of space in the dent, in conjunction with more intensive abrasive action of the reed wires on the ends show adverse effects on the yarns. Moreover, a knot or thick place on the yarn cannot easily pass through the narrow reed space and the yarn is more likely to break because of abrupt rise in tension. When jute fabrics are woven in shuttleless looms, single denting is adopted. Warp densities of these fabrics are generally not very high which thus precludes the need for fine reeds and 2 ends/dents produces reed mark which in these modern weaving machines is very difficult to be avoided because of some limitations in the extents of asymmetric setting and early timing of the shed.

7.8 Effects of beat-up During the course of beat-up, the reed pushes the last inserted pick forward between the two layers of warp yarns to place it at the predetermined position at the fell of the cloth. While pushing the pick forward the reed moves it against the frictional resistance imposed by the warp yarns, the magnitude of which depends upon the coefficient of friction between the warp and weft. The pick of weft also experiences elastic resistance from the warp yarns due to imposition of crimp on them. As a pick moves towards the fell it forces the warp yarns to bend round it and this resists the movement of the pick. The resistance offered by the warp yarns against the penetration of the pick towards the cloth fell is the “weaving resistance” and this is the sum of frictional resistance and elastic resistance. When a new pick is forced at the cloth fell, the frictional resistance tends to keep it there while the elastic resistance tends to eject it from the cloth. From the time the reed meets the pick and pushes it forward both the resistances gradually increase and reach the maximum when the reed reaches the front dead centre. To overcome the weaving resistance, the reed has to apply force on the warps. The force applied by the reed during beat-up is the “beat-up force”. At any instant during beat-up, the weaving resistance and beat-up force are equal and opposite. The effect of reed on the fell of the cloth during the course of beat-up was recognized by Chamberlain and Snowden [12] and they observed that, depending on the type of fabric being woven, the reed experienced certain

140

Role of yarn tension in weaving

degrees of difficulty in beating the picks into a fabric, which was indicated by the peak tension of the warp yarn at beat-up. This they defined as “weaving difficulty”. Detailed study on the role of beat-up during formation of the cloth has first been made by Greenwood and Cowhig [17]. For the purpose, they considered the model shown in Fig. 7.26, of pick movement at the cloth fell during beat-up. According to the model, all distances beyond the front most position of the reed (that is, outside the sweep of the reed) are positive and those from this position of the reed towards the warp (that is, within the sweep of the reed) are negative, as indicated in Fig. 7.26. The distance of the reed from fell of the cloth, r and the instantaneous displacement of the cloth fell from its basic position Z are, however, always considered positive. The basic position of the cloth fell is the position it occupies immediately before it advances under the action of beat-up.

Figure 7.26 Cloth fell position during beat-up. During weaving, shed formation and beat-up cause cyclic change in warp tension and these in turn, cause cyclic variation in the position of the cloth fell. Rise in warp tension pulls the fell backward towards the reed and places it within the sweep of the reed and beating up action by the reed pushes the fell forward towards the weaver. Extent of backward movement of the fell (which under the stable weaving condition is of course, not very significant compared to the sweep of the reed) depends on the elastic characteristics of the warp and cloth, weft density, warp tension, etc., and that of forward

Effects of loom settings and other factors on warp tension

141

movement of the fell by the reed is always up to the front dead centre of the reed. From Fig. 7.26, at any instant during beat-up, the instantaneous distance of the cloth fell from the front position of the reed Y is given by (following the signs) – Y = – L – (+Z) or Y=L+Z

(7.17)

and the distance of the reed from fell of the cloth, r = – X – ( – Y) or r=Y–X

(7.18)

where L is the distance of the basic position of the cloth fell from the front position of the reed, and X is the instantaneous distance of the reed from its front position. Then from equations 7. 17 and 7.18 X = Y – r or X=L+Z–r

(7.19)

During the course of weaving, as the warp tension fluctuates with shed opening, beat-up and shed closing, the resultant fluctuations of warp tension are transmitted to the cloth as well. During the entire pick cycle, the tensions of the warp and cloth vary in the similar manner except at beat-up. At beatup, the warp stretches under the action of the reed and the warp tension rises sharply. In case of the cloth on the contrary, just the reverse happens. The cloth contracts at beat-up and the cloth tension falls sharply. This difference in warp and cloth tensions at beat-up is necessary to overcome the resistances to the beat-up and to the displacement of the newly inserted pick relative to the warp yarns, as the pick being beaten can be displaced against the frictional resistance from the warps only if the difference in tensions is larger than the resistance to beat-up. Actual nature of warp tension variations of both the warp and the cloth in a pick cycle have been illustrated in Fig. 4.2 (Chapter 4), while Fig. 7.27 shows the general trend of tension variations of the warp and cloth for the purpose of finding out relation between beat-up force and warp and cloth parameters. In Fig. 7.27, the warp tension and cloth tension have been indicated by the firm and broken lines, respectively.

142

Role of yarn tension in weaving

Figure 7.27 General form of tension variations of warp and cloth. The warp yarns are woven under a certain basic tension, say T0, determined by the setting of the let-off motion of the loom and the extent of shed opening. At beat-up, the warp tension rises from T0 to the peak tension T1 and the cloth tension falls from T0 to a minimum tension T2 (but not zero), Fig. 7.27, under the stable weaving condition. Under this condition, the cloth remains taut even at beat-up and T2, therefore, does not become zero. The peak warp tension T1, and the minimum cloth tension T2, can be obtained if the cloth fell is displaced by the reed during beat-up. At the time of beat-up, the force exerted by the reed and the tension of cloth tend to pull the fell back towards the weaver and this is balanced by the tension of warp which tends to pull the fell towards the back of the loom. This becomes possible if the cloth fell lies within the sweep of the reed just prior to beat-up. Thus, the forces acting on the cloth fell during beat-up will be in equilibrium when weaving resistance or beat-up force R is balanced by the excess of beat-up tensions of the warp over that of the cloth, which Greenwood and Cowhig [17] have expressed as “excess tension theory”. From this, therefore, R = T1 – T2

(7.20)

Just prior to beat-up, when the fell of the cloth is not displaced by the reed, the cloth fell position is L (Fig. 7.26) and the weaving resistance is equal to zero. At this stage, therefore, the cloth tension is equal to the basic warp tension T0, that is, T1 = T2 = T0. As the reed approaches the fell with a newly inserted pick to beat-up, the fell is moved forward by the distance Z. This causes extension of the warp yarns and contraction of the cloth by the same amount, provided no warp yarn is let-off at this stage. The extension of the warp will cause a rise in warp tension by dT1 and fall in cloth tension by dT2. If E1 and E2 are the elastic moduli of the warp yarn and cloth, respectively, and

Effects of loom settings and other factors on warp tension

143

l1 and l2 are the free lengths of the yarn and cloth, respectively, then according to Hooke’s law, dT1 = E1 Z / l1 and dT2 = – E2 Z / l2 Then, the instantaneous warp tension during beat-up is T1 = T0 + dT1 or T1 = T0 + Z E1 / l1

(7.21)

and the instantaneous cloth tension during beat-up is T2 = T0 – dT2 or T2 = T0 – Z E2 / l2

(7.22)

Substituting the equations 7.21 and 7.22 in equation 7.20, we get R = T1 – T2 = (T0 + Z E1 / l1) – (T0 – Z E2 / l2) or R = Z (E1 / l1 + E2 / l2)

(7.23)

From equation 7.23, we find that, at any instant during beat-up the weaving resistance (which is equal and opposite of the beat-up force) is proportional to the displacement of the cloth fell from its basic position in a pick cycle. When the reed reaches the front dead centre at the completion of beat-up, X = 0, and if the newly beaten pick does not recede from its position at the fell after beatup, r is then equal to the pick spacing, S of the cloth. Then, substituting for R, X, Z and r in equation 7.19 we get the maximum beat-up force Rmax as Rmax = Z (E1 / l1 + E2 / l2) = X – L + r (E1 / l1 + E2 / l2) = 0 – L + r (E1 / l1 + E2 / l2) (since X = 0) or Rmax = S – L (E1 / l1 + E2 / l2) (as r = S)

(7.24)

Equation 7.24 thus, indicates the relation between the beat-up force and the distance of the basic position of the cloth fell from the beat-up position of the reed at the end of beat-up on the basis of excess tension theory. It is to be noted here that if the fell position of the cloth is well beyond the front most position of the reed, like in case of very open net-like structure, the fell of the cloth will not be pushed forward by the reed during beat-up and there will be no difference between the warp and cloth tensions at beat-up. In such cases, the reed will experience no weaving resistance and will therefore, apply no beat-up force.

144

Role of yarn tension in weaving

It is worth noting from equation 7.23 that, it does not contain the term T0, the basic warp tension. This, therefore, indicates that under the stable weaving condition that is, when the cloth tension is always above zero, beat-up force is not dependent on the basic warp tension, provided the cloth fell remains at the same position. If, however, the cloth becomes so slack that it cannot offer any resistance to the reed during beat-up, “bumping” condition arises and the above condition does not hold good [17]. Under bumping condition, the equation 7.22 no longer holds, as T2 = 0 when the reed reaches the front centre and the cloth fell distance also reached the maximum value Zmax from its basic position. The equation 7.20 can then be written as Rmax = T0 + Zmax E1 / l1

(7.25)

Substituting Z in equation 7.19 by Zmax and considering that at beat-up X = 0 and r = S, as before Rmax = (S – L) E1 / l1 + T0

(7.26)

Equation 7.26 indicates the same relation as represented by equation 7.24 but under bumping conditions of the cloth. Comparisons of these two equations of beat-up forces under stable and bumping conditions reveal that, in the latter case the term, E2/l2 disappears and the warp tension,T0 appears. Thus, under bumping conditions, the elastic modulus of the cloth and its free length become unimportant, while the basic warp tension becomes important, with the result that, if the bumping conditions are owing to the decreases in warp tension, the stable weaving condition can be restored by increasing the weaving tension of the warp yarns. For a given type of fabric and weaving condition, there is a minimum warp tension that is sufficient to avoid bumping and weaving can be carried out satisfactorily under the stable condition. Unless the condition demands such as discussed above in Section 7.6, there is hardly any point in maintaining a higher warp tension than this, as that is likely to aggravate the chances of warp break. Greenwood and Vaughan [18] demonstrated the effects of weaving tension of the warp yarn on the cloth fell position, beat-up force and warp tension at beat-up, as indicated in Fig. 7.28. When the basic warp tension (measured with shed closed) is very low, there is no difference between the beat-up tension and the beat-up force and the bumping condition exists. At this region, the cloth fell moves significantly but as the warp tension increases bumping condition does not exist. Outside the bumping condition that is under the stable condition, as the basic warp tension increases there is although a tendency of increase in beat-up force at the initial stage, the increase is very insignificant and the warp tension at

Effects of loom settings and other factors on warp tension

145

beat-up increases significantly. This thus, substantiates that under the stable weaving condition the beat-up force is not affected by the basic warp tension.

Figure 7.28 Effect of warp tension on beat-up force. In some other studies [48, 39], however, it has been observed with negative let-off motion weaving plain cloth that, the beat-up force is dependent on the basic warp tension. One of these [48] indicates that, when the heald shafts are level, the maximum beat-up force increases linearly with warp tension as shown in Fig. 7.29. The beat-up force has been measured by two methods (i) load washer and (ii) strain gauge.

Figure 7.29 Beat-up force against different warp tensions.

146

Role of yarn tension in weaving

The other one [39] has also shown that if the warp tension is reduced, the beat-up force decreases, and the decrease is considerably greater for the fabric with high weft density than that with low weft density, as in Fig. 7.30.

Figure 7.30 Effect of warp tension on beat-up force for different weft densities. If we now consider the cases of increase in beat-up force with increase in warp tension [48, 39] keeping in mind the “excess tension theory” [17], we find that the increase in warp tension will then pull the cloth fell more towards the reed and bumping will take place and to obtain a closer pick spacing a lowering of warp tension will be necessary. But, as observed above, it is the reduction in warp tension that causes bumping, and to obtain a closer pick spacing the warp tension has to be increased. When, however, the effect of warp tension on weaving resistance is small, the pick spacing is wider and the cloth fell remains in the same position with increase in warp tension. This is, thus, opposite to the effect observed in bumping conditions when an increase in warp tension produces closer pick spacing. Jui-Lung [22] observed that during beat-up, the reduction in cloth tension is less at the breast beam than at the fell of the cloth, as can be expected, and the extent of the difference depends on the structure of the cloth being woven and the settings of the loom. This aspect should, therefore, be taken

Effects of loom settings and other factors on warp tension

147

into account for more correct and precise calculation of warp tension at beatup. It is also important that to retain the useful properties of the warp yarn, the cloth tension at beat-up should be a minimum but not fall to zero. According to Efremov [14], the position of the cloth fell during shedding depends on a number of loom parameters like, shed dimensions, the number of heald shafts employed and their distances from the cloth fell, type of weave, elastic moduli of the warp and cloth, the basic warp tension, friction of the warp yarns and cloth on different guide elements of the loom and their geometric dimensions and the radius of the warp beam. These imply that contrary to what is generally assumed, the different picks in a weave repeat are beaten under different conditions in respect of the warp tension and the position of the cloth fell and the reasons for these are the positions of the heald shafts with respect to the fell of the cloth as well as the ratio between the number of ascending and descending warp yarns at the time of beat-up. Beat-up condition also depends on the diameter of the yarn build on the warp beam. Warp tension increases with an increase in the warp and weft densities of the cloth and the effect is more pronounced with the latter [26]. Because of the same reason, warp tension at beat-up increases sharply if the weft becomes coarser but the weft cover factor remains unaltered or if the warp becomes coarser but the warp cover factor remains unaltered. Irrespective of the warp and weft densities of the cloth, warp tension during beat-up reduces with an increase in the compressive deformation of the weft yarn and also in its coefficient of friction on the warp yarn. Lunenschloss and Schlichter [28] also found that the warp tensions at beat-up of both the upper and lower shed lines (BT-TS) and (BT-BS), respectively increase while the average warp tension (AWT) decreases with the increase in pick density, as indicated in Fig. 7.31.

Figure 7.31 Effect of weft density on warp tension.

148

Role of yarn tension in weaving

Coarser weft or higher weft density causes the fell of the cloth to creep more towards the back of the loom and the forward movement of the reed during the course of beat-up therefore, increases the warp tension more. Fall in average warp tension with increase in pick density (Fig. 7.31) is because, as the beat-up tension increases, the oscillating back rest is depressed more to let-off greater amount of warp yarn. In respect of the effect of warp linear density, they [28] have not observed any effect of warp count on warp tension. Other effects of beat-up action on formation of the cloth have been discussed in Chapter 10.

7.9 Effects of temple When a pick of weft, inserted in the warp shed, is beaten to the fell of the cloth the consequent interlacement between the warps and weft exerts forces on each other causing them to crimp. This results in shrinkage of both the warp and weft yarns. The extents of warp and weft way shrinkage of the cloth depend on various factors like weave, yarn tensions, yarn counts, yarn densities and so on. The effects of yarn tensions on cloth width and length have been discussed in Section 10.4.1 of Chapter 10. Because of the weft way contraction, the warp ends at the two selvedges of the cloth are severely rubbed against the reed wires during the reciprocating movement of the reed and become so susceptible to break that weaving becomes impossible. This untoward incident is annulled by the use of temples, shown in Fig. 7.32. In most cases, the fabrics are griped at the selvedges by ring temples (Fig. 7.32A) near the fell (see Fig. 5.3, Chapter 5) and stretched to, as near as possible, the width of the warp in the reed. In the more developed system, the cloth is trapped along the entire width by the full width temple (Fig. 7.32B).

Figure 7.32 Cloth temples. It is desired that the tension of all the warp ends of a cloth should vary similarly and be fairly uniform at any given place of the cloth and at any given time in a pick cycle during weaving. The uniformity in warp tension over the

Effects of loom settings and other factors on warp tension

149

whole width of the warp reduces the chances of warp breakage, improves the performance of the loom and the quality characteristics of the cloth woven. In actuality, however, tension of the warp yarns across the entire width of the cloth does not remain uniform. The warp yarns at the selvedges of the cloth held by the temples are comparatively under low tension than those at the body [56, 4, 9, 40]. Distributions of warp tension obtained with the conventional ring temple (Fig. 7.32A) and full width temple (Fig. 7.32B) have been indicated in Fig. 7.33 [40].

Figure 7.33 Warp tensions at the temple and body of the cloth. Warp tension variation with ring temples, indicated by the firm line in Fig. 7.33, shows that the warp yarns lying at the middle of the cloth have the maximum tension and those away from the centre of the cloth to the temples at both sides have gradually decreasing tension. At the temple zone where the selvedges of the cloth are held by the temples, the warp tension fluctuates a great deal with the minimum tension obtained here. The cloth selvedges gripped by the temple rollers and pressed under the temple covers are not allowed free forward and backward creep of the fell of the cloth and this is one of the reasons for low warp tension at the selvedges [4]. As the newly inserted pick is woven into the fabric it is held firm in the middle of the cloth by both sides and stretched taut while being beaten into the cloth fell. At the selvedges, the pick can relax a little from the selvedge inward, the extent of which depends on the type of weft insertion, type of temple etc. The weft is then, always more taut at the middle of the fabric than at the selvedges where the lowest tension of the warp yarn is observed [56]. The length of all the warp ends coming from the warp beam are, however, practically of the same length and additional firm binding of the warp at the middle compels these yarns to elongate more at this region. This results in higher tension of the warp yarns at the middle of the warp sheet.

150

Role of yarn tension in weaving

As the conventional temples grip the cloth at two selvedges only, the nonuniform distribution of warp tension is resulted. If, therefore, the full-width temple is used which grips the cloth along its entire width this shortcoming should not arise and the warp tensions at the body as well as at the selvedges of the cloth should nearly be the same. Distribution of warp tension across the width of the cloth measured with full-width temple has been indicated by the broken line in Fig. 7.33. As can be expected, significantly different warp tension behaviour is observed with the full-width temple. There is, now, a more even distribution of warp tension across the width of the cloth. Although the highest warp tension is observed at the centre of the fabric as usual, it is much reduced and the sudden considerable reduction in tension towards the selvedges observed with the ring temples is also not that prominent. There are albeit, fall in warp tension at the selvedges, but the curve of the tension distribution is spread further out at the two edges with the full-width temple. As a result, compared to the ring temples, the full-width temple enables to obtain more uniform distribution of warp tension over the whole width of the cloth, which is very important for wide and high speed looms. Lower warp tensions at the selvedge with ring temples may hinder the formation of clean shed during picking which in many of the shuttleless loom can cause faulty weft insertion. With the full-width temple, such a problem is much less acute as the tensions of the selvedge yarns are comparatively high.

Figure 7.34 Selvedge yarn tensions with and without temples. Despite the advantages obtained with the use of temples, the conventional jute looms are not provided with them. As a result, the warp yarns at the selvedges experience high stretching and abrasion particularly at beat-up [33].

Effects of loom settings and other factors on warp tension

151

Figure 7.34 shows the tension variations of selvedge yarns of front and back heald shafts of a jute loom operating with oscillating back rest and with and without temples. Suitable temples were designed for the purpose. In absence of temples, the tensions of the selvedge yarns are appreciably high at beat-up. When the temples have been used the beat-up tensions of the selvedge yarns have reduced by 55 – 85%. This clearly implies that the use of temples in the looms reduce considerably the chances of warp breakage at the selvedges during weaving, as mentioned above. It is also worth noting from the tension traces that, owing to the absence of dwell of the cam operating the back rest, there are considerable variations in warp tension during shed dwells, as discussed in Section 7.3.2.

7.10 Effects of looming-in lengths of warp and cloth In the process of weaving, the elastic system of the warp sheet and cloth in the weaving zone is subjected to cyclic deformation as a result of shed formation, beating-up and also cloth take-up and warp let-off. Shedding and beat-up naturally cause larger deformation than take-up and let-off. Tension of the warp yarns is higher at the front shed, from the heald shaft to fell of the cloth, than that at the back shed, from the heald shafts to the back-rest, because of sizes of the sheds (see Fig. 3.2, Chapter 3) and the warp tension at the back shed is again higher than that between the back-rest and the warp beam because of coil friction (see Fig. 1.8, Chapter 1). The free length of the warp yarns from the warp beam to the fell of the cloth has influential effects on warp tension during weaving. Lower free length and hence, higher elastic constant of warp yarns causes much greater tension at the front shed than that at the back resulting in excessive warp breakage [6]. This deleterious effect can be reduced to some extent by so timing the shedding motion that at the moment of beat-up the shed angle is small. This is very helpful particularly with asymmetric shed, where the beat-up force is not shared equally by the yarns of two shed lines. While the tension of the warp yarns owing to shedding is determined largely by the size of the warp shed, that owing to beat-up is determined primarily on the beat-up force which, in turn, depends on the type of cloth (that is, mainly the cover factor) being woven. Moreover, the warp tension is affected by the lengths of the warp sheet and the cloth as well, as mentioned above. Kulikova, [24] investigated the effects of warp and cloth tensions on the cyclic displacement of the fell of the cloth as a result of shedding and beatup with different lengths of warp sheet and the cloth. Fell displacement of a plain cloth was recorded by optical means and Fig. 7.35 shows the nature and magnitude of displacement with two different warp tensions.

152

Role of yarn tension in weaving

Figure 7.35 Cloth fell displacement with different warp tensions. At the instant of beat-up, the cloth fell moves towards the breast beam and the fell position at this instant is represented by A. Soon after the beat-up, the reed recedes and the shed opens with the result that the fell moves towards the reed. The shed then reaches the dwell period with the fell position momentarily at rest, indicated by B. Then the shed closes, the warp yarns are relaxed at the level position and the fell moves towards the breast beam, represented by C. When the shed again starts opening, the fell moves towards the reed till, it comes in contact with the reed at D. From this point, the fell is moved towards the breast beam by the reed during beat-up and reaches the maximum forward position at A1 again. If we now consider the displacement of the cloth fell along the ordinate in Fig. 7.35 we find that, the displacements of the fell from C to B1 at the next pick is because of shedding and that from D to A1 is because of beating that is, the width of beat-up strip. The upward trend of fell displacement in successive picks is due to the gradual movement of the fell towards the breast beam. From Fig. 7.35, it is evident that the displacement of the cloth fell as a result of shedding and beat-up increases with the decrease in warp tension set for weaving. The investigation has also shown that, the displacement of the cloth fell for shedding decreases with the increase

Effects of loom settings and other factors on warp tension

153

in the length of the warp sheet or the decrease in the length of the cloth in the weaving zone on the loom. Since the variation in warp tension is conveyed to the cloth in its free zone between the fell and the take-up roller during the course of weaving, the measurement of cloth tension is also important, particularly, for analyzing the beat-up force, as discussed above. In another study, Kulikova [25] measured the cloth tension from the breast beam by resistance strain gauge, as mentioned earlier in Section 3.5.1 (Chapter 3), whilst Lunenschloss and Schlichter [27] used a measuring plate pressed against the fabric from below and the fabric tension is recorded by a piezo-electric pressure sensor employing the strain gauge technique. Warp and cloth tension variations in two successive pick cycles measured from the back-rest and the breast beam respectively have been shown in Fig. 4.2 (Chapter 4) and some preliminary discussions on them have been made in Section 4.2.1 (Chapter 4). If we now consider the said tension forms of both the warp and the cloth in more details we find from Fig. 4.2 (Chapter 4) that, at the instant of shed change, the yarns are most relaxed and the tensions of both the warp and cloth, A/A1, are low. As the shed then starts opening, the tensions rise slightly till B/B1, when the reed comes in contact with fell of the cloth for beating-up. From here, the warp tension rises sharply to the peak tension, C and the cloth tension falls sharply to the minimum tension, C1 as the reed reaches the front position. Then, as the reed starts receding, the warp tension falls from C to D, while the cloth tension rises from C1 to D1 wherefrom both the warp and cloth tensions rise to E/E1 and remain high and fairly even till F/ F1 when the shed starts closing. Tensions E to F of the warp and E1 to F1 of the cloth indicate the dwell period of the warp yarns. Following the dwell period the shed begins to close and the warp and cloth tensions again reach the low values at A/A1. Warp tensions at different instances have been studied with different lengths of warp and fabric [25] and the results of variation of warp length with a given basic warp tension are shown in Fig. 7.36. Similar observations have been made with variation of cloth length. Warp tensions at beat-up, shed open and shed change generally decrease with the increase of the length of the warp sheet (with the length of cloth constant) or with the length of the cloth (with the length of warp sheet constant) and the rate of decrease is very sharp at the initial stages (Fig. 7.36). Such observations have been made irrespective of the basic warp tension at weaving. Coefficients of stiffness of the warp sheet or of the cloth can be decreased by increasing the length of the warp sheet or that of the cloth. Effect with the increase of the cloth length is of greater practical significance as the cloth has the larger resistance to mechanical actions on the loom.

154

Role of yarn tension in weaving

Figure 7.36 Warp tension variations with different warp lengths.

Figure 7.37 Cloth over the fixed and rotating breast beams. Dudochkin & Gordeev [13], however, made different observations in some cases. According to them, when the free length of the cloth from the fell to the take-up roller has been increased from 377 mm to 498 mm and the conventional fixed breast beam has been replaced by a rotating breast beam, as shown in Fig. 7.37, warp tension decreases when the shed is open but increases at beat-up. Increase in beat-up tension causes more uniform distribution of picks along the fabric and decrease in shedding tension reduces warp breakage by 7–16%. Other effects of free lengths of the warp and fabric, like on the starting mark of the fabric, have been discussed in Chapter 10.

Effects of loom settings and other factors on warp tension

155

7.11 Effects of other factors Besides the various settings of the weaving machine discussed above, there are also other factors which have distinct effects on warp tension during weaving.

7.11.1 Weave With the increase in basic warp tension, both the maximum and the mean warp tension rise proportionally. Owing to asymmetric setting of the shed lines, however, the resultant increase in tension in the upper shed is less than that in the lower shed and efforts should, therefore, be made to keep the basic warp tension as low as possible in order to ensure a clean shed formation. It has been observed [46] that changing over from one weave to the other causes significant changes in warp tension.

Figure 7.38 Warp tension with change of weave. Figure 7.38 illustrates the warp tension variations measured in the area of the changeover from 4/1 satin weave to 1/4 satin weave and vice versa. It is observed that when the weave is changed from 4/1 satin to 1/4 satin, the average warp tensions at shedding and beat-up increase (Fig. 7.38A) but, when the weave is changed from 1/4 satin to 4/1 satin they decrease (Fig. 7.38B). In comparison to 1/4 satin weave, 4/1 satin weave has a much greater number of warp ends at the top shed where they are under less tension. This is possibly the reason for the decrease in warp tension observed in the latter weave. From this, we thus find that in a weave, where more warp ends are

156

Role of yarn tension in weaving

on the face of the cloth during weaving, the average warp tension is less than those with less number of ends on the face of the cloth provided of course, the back rest is set at higher position.

7.11.2 Loom speed In regard to the effect of loom speed, the loom speed has not been found to have any conclusive effects on warp tensions either in projectile or in rapier weaving machine [28] or in narrow loom [57], as shown in Fig. 7.39. Figure 7.39 indicates the mean warp tensions of two heald shafts, 1st and 4th, observed in a narrow loom with change of speed from 296 to 791 picks/minute. Although the warp tensions of both the heald shafts remain more or less constant, the yarn tension of the 4th heald shaft farther from the fell of the cloth is found to be higher, as should be expected, because of greater lift.

Figure 7.39 Effect of loom speed on warp tension. Some of the studies [61, 2] have of course shown that, increase in loom speed results in increase in warp tension, which can however, be controlled by maintaining proper machine settings. With the machine settings remaining unaltered the increase of speed of a rapier weaving machine from 300 picks/ minute to 450 picks/minute has been found to cause distinct increase in warp tension [61] because of improper motion of the back-rest. When the let-off motion is properly set and the back-rest is heavily dampened, the extent of increase in warp tension has largely diminished and so, most of the modern weaving machines are now equipped with dampening system of the backrests.

Effects of loom settings and other factors on warp tension

157

7.11.3 Yarn join During the course of weaving, breakage of warp yarn is inevitable and the broken end is, in most cases, joined by a knot. When yarn with a knot passes through the dent of the reed, the knot causes undue rise in warp tension at beat-up. Depending on the dimensions of the knot, the beat-up tension of even the properly tied knot may be so high that the knot fails to pass through the reed dent. To study this aspect, tensions of unsized jute yarns of 214, 286, and 451 tex with seven different varieties of knot have been measured after each knot has traversed into the sweep of the reed of a pneumatic rapier loom running at 300 picks/min [37]. The knots considered are double-harness bend (DHB) of SS and SZ variants, weaver’s (WVR), fisherman’s (FM) of SS and SZ variants, barrel (BRL) and dog knot (DGN). For 214 and 286 tex yarns, single denting was arranged in the reed with 45 dents/dm, while the coarsest yarn was dented 2 ends/dent in the reed with 22.5 dents/dm to keep the ends/ dm the same. The findings of yarn tensions of the knotted and normal yarns are shown in Fig. 7.40.

Figure 7.40 Tensions of the normal and knotted yarns. Irrespective of the count of yarn and denting pattern, warp tension at beatup rises appreciably and in accordance with the dimensions of the knots considered. With respect to the warp tension at shedding on the contrary, although the tension shows the tendency to increase with the increase in dimensions of the knots, because of the obstructions faced by the knots while passing through the mail eyes of the healds, the extent of the increase is not so

158

Role of yarn tension in weaving

significant. It has also been observed particularly with the dog knot which has the maximum dimensions, that after the knot has reached the fell of the cloth and the new pick laid behind it is pushed forward by the reed during beatup, the knot creates obstructions to the movement of the pick, as indicated in Fig. 7.41. This also contributes much to the rise in beat-up tension of the knotted yarn. This situation continues for a few more succeeding picks, but with gradually diminishing magnitude, until the knot passes well away from the cloth fell.

Figure 7.41 Obstruction to pick movement at beat-up by knot. It view of the above observations made with knots, it is, therefore, expected that if the yarn is joined by splicing instead, tension of the warp yarn during beat-up will not be much affected by the yarn joint. This has been demonstrated by a study on the effects of knot and splice on warp tension during weaving of jute yarns [38]. Jute yarn of 290 tex was joined by dog knot and spliced by wrapping method and their tensions were examined during shedding and beat-up in the pneumatic rapier loom fitted with a reed of 45 dents/dm and operating at 300 picks/min, as stated above. Warp denting was single end per dent. As expected, spliced yarn has demonstrated much less variations in weaving tension than the knotted yarn, as shown in Fig. 7.42 which also shows the tension of the normal yarn for comparison. Shedding and beat-up tensions of the knotted yarn vary appreciably as compared to the same of the spliced as well as the normal yarns. Tension variations of the spliced and normal yarns are similar in nature with slightly higher values for the former. It is very interesting to note from Fig. 7.42 that, while the beat-up tension of the knotted yarn is much higher than those of the spliced and normal yarns as should be expected, the shedding tension of the knotted yarn is appreciably lower. During the major part of the shed opening period, the reed moves

Effects of loom settings and other factors on warp tension

159

backward, and when a knot comes within its sweep and cannot pass through the dent smoothly, it is at first pushed back by the reed, causing slackness in the yarn at that instance until the tension in the yarn in front of the reed increases sufficiently to enable the knot to pass through. Moreover, the repeated quick stretching and relaxation of the yarn with knot, as the result of to and fro movement of the reed, also causes some extension in the yarn and consequent fall in the normal level of tension except at beat-up. At the instant of beat-up, as the reed moves forward with the knot in front, it carries the knot with it causing a sharp rise in yarn tension until the knot squeezes past the dent. This continues till the knot reaches the fell of the cloth and thereby, moves out of the sweep of the reed.

Figure 7.42 Yarn tensions with knot and splice. Benefits of yarn joint by splicing in terms of yarn tension have also been demonstrated with all-wool warp yarns [31]. Rise in tension of the yarns of three different counts joined by fisherman’s knot, weaver’s knot and pneumatic splicing while passing through a guide are shown in Fig. 7.43. Spliced joint causes significantly less rise in tension than the knots for all yarn counts, which thus signifies that splicing has the least influence on the tension of the yarn during its processing. Moreover, irrespective of the type of joint, yarn tension rises with the linear density as well as input tension of the yarn. It has also been observed that, yarn joint has significant effect on yarn against yarn tension and the spliced yarns have the least effect.

160

Role of yarn tension in weaving

Figure 7.43 Effects of knots and splice on yarn tensions.

7.11.4 Weft tension Tension of warp yarns is also influenced by weft tension at the instant of formation of the cloth at the cloth fell. Azarschab [2] observed that, as the tension of the weft yarn is increased it causes an increase in warp tension also. We know that when a pick is beaten to the fell of the cloth, it causes the cloth to contract at the fell due to imposition of crimp in it. A highly tensioned pick tries to contract the cloth more and in the process increases the tension and stress of the warp yarns. Working performance of a loom is greatly determined by the behaviour of its warp yarns. Weaving operation imparts strain on the warp yarns of varying magnitudes, which is reflected in its pattern of tension variation. There are three types of tension on a warp yarn during weaving: constant mean tension, cyclic tension variations and random tension variations. The constant mean tension and its magnitude are generally determined by the take-up/let-off rate which in turn depends on the pick spacing of the cloth being woven. Cyclic tension variations are caused by shedding and beat-up. High tension at shedding or beat-up depends on fabric type and loom settings. Random tension

Effects of loom settings and other factors on warp tension

161

variations are generally caused by some obstructions during shed formation or passage of the warp yarns through their controlling elements. Greater strain on the warp yarns owing to unduly higher tension during the course of weaving naturally causes higher damage to the warp yarns. It is, therefore, of paramount importance that all the loom parts governing the movement of the warp yarns are so meticulously set and tuned that the warp yarns are woven under optimum tension varying within tolerable limits. Shed depth being one of the major reasons for the magnitude of rise in warp tension, utmost care should be taken to maintain it to the extent just adequate for the purpose of weft insertion and ensure that the movement of the warp yarns through the loom is as smooth as possible without the yarns being unduly strained.

References 1.

Adanur S., (2001). Hand book of weaving, Technomic Publishing Co. Inc, U.S.A., pp. 233.

2.

Azarschab M., (1985). Investigations into the behaviour of warp threads in weaving machine, Melliand Textilberichte, Volume 66, pp. 786–790.

3.

Badve N.P. and Bhattacharya U., (1960). Study of warp tension variations on Roper and Sakamoto automatic warp let-off motions, Part II, Textile Trends, Volume 3, pp. 55–63.

4.

Badve N.P. and Bhattacharya U., (1961). Study of warp tension variations on Roper and Sakamoto automatic warp let-off motions, Part IV, Textile Trends, Volume 3, pp. 27–32.

5.

Bakulin B.A., (1972). The warp tension of the AT – 120 – 5 loom, Technology of the Textile Industry U.S.S.R., No 2, pp. 61–63.

6.

Basu A.K., (1987). Effect of different let-off mechanisms on fabric formation and dimension of fabric in the loom, Textile Research Journal, Volume 57, pp. 379–386.

7.

Bhattacharyya P.K., (1995). Better back rest for jute loom, The Indian Textile Journal, Volume 106, pp. 112–116.

8.

Bhattacharya S.K. and Das K., (2006). Suitability of cycloidal cam in shedding mechanism, Indian Journal of Fibre and Textile Research, Volume 31, pp. 465–466.

9.

Blanchonette I (1996). Tension measurements in weaving of singles worsted wool yarns, Textile Research Journal, Volume 66, pp. 323– 328.

162

Role of yarn tension in weaving

10. Bradbury E. and Worthington A.P., (1954). The crossing of threads in shedding, The Journal of the Textile Institute, Volume 45, pp. T421– T437. 11. Bramma F., (1955). Some effects of shed timing and setting on weaving, The Journal of the Textile Institut,Volume 46, pp. 405–412. 12. Chamberlain N.H. and Snowden D.C., (1948). Loom study by means of the cathode ray oscillograph, The Journal of the Textile Institute, Volume 39, pp. T23–T43. 13. Dudochkin V.A. and Gordeev V.A., (1972). Experimental analysis of the fabric length of the loom as a factor in the conditions of weaving, Technology of the Textile Industry, U.S.R.R., No 2, pp. 64–67. 14. Efremov E.D., (1970). The movement of the cloth fell due to shedding, Technology of the Textile Industry, U.S.R.R., No 3, pp. 59–63. 15. Foster R., (1961). Positive let-off motions, Wool Industries Research Association, Leeds, U. K., pp 7–11. 16. Gloy Y-S and Gries T., (2012). Novel backrest systems and their influence on the weaving process, Melliand International, Volume 18, pp. 222–225. 17. Greenwood K. and Cowhig W.T., (1956). The position of the cloth fell in power looms, Part I – Stable weaving conditions, The Journal of the Textile Institute, Volume 47, pp. T241–T254. 18. Greenwood K. and Vaughn G.N., (1957). The beat-up force and pickspacing, The Journal of the Textile Institute, Volume 48, pp. T39–T53. 19. Gu H., (1984). Reduction of warp tension fluctuation and beat-up strip width in weaving, Textile Research Journal, Volume 54, pp. 143–148. 20. Ibragimov R.G., (1971). The variation of the warp tension during shedding on the STB loom, Technology of Textile Industry U.S.R.R., No 5, pp. 52–56. 21. Jeddi A.A.A., Nosraty H., Ordoukhany D., and Rashidian M., (1999). A comparative study on the performance of electronically- and mechanically- controlled warp let-off systems, Indian Journal of Fibre and Textile Research, Volume 24, pp. 258–263. 22. Jui-Lung C., (1960). Warp and cloth tension during the beat-up period, Technology of the Textile Industry, U.S.R.R., No 5, pp. 80–85. 23. Kalli F., (1990). Correlation analysis between fabric data and stoppage rate in rapier weaving machinery with reference to cotton fabrics, Melliand Textilberichte, Volume 71, pp. 35–37.

Effects of loom settings and other factors on warp tension

163

24. Kulikova N.A., (1966). Experimental analysis of the relative warp sheet and fabric lengths on some parameters of the weaving process, Technology of the Textile Industry U.S.S.R., No 3, pp. 71–75. 25. Kulikova NA (1966). The influence of the relative warp sheet and fabric lengths on the loom on the warp tension and end breakage rate, Technology of the Textile Industry U.S.S.R., No 4, pp. 69–73 26. Kuznetsov A.M., (1961). Warp and weft tension during beat-up in the formation of plain weave fabrics, Technology of the Textile Industry, U.S.R.R., No 5, pp. 83–93. 27. Lunenschloss J and Schlichter S (1987). The development of new measuring elements for electronically controlled warp let-off units, International Textile Bulletin, Fabric forming, No 3, pp. 56–71. 28. Lunenschloss J and Schlichter S (1987). Stress on weft and warp yarns in terms of the weft insertion speed and other weaving parameters, Melliand Textilberichte, Volume 68, pp. 93–98. 29. Lord P.R., (1966). Warp damage during the weaving process – part II, Textile Recorder, Volume 84, pp. 59–60. 30. Marks R. and Robinson A.T.C., (1076) Principles of Weaving, The Textile Institute, Manchester, pp. 178–181. 31. Mcmahon J.F., (1987). Tensions generated by different yarn joints in all-wool yarns, The Journal of the Textile Institute, Volume 78, pp. 316–322. 32. Mehmet D., Hakan C., and Cengiz K.M., (2009). Adaptive control of let-off system in weaving, The Journal of the Textile Institute, Volume 100, pp. 186–194. 33. Neogi S.K. and Mukherjee B., (1978). How to control tension variation of warp yarn, The Indian Textile Journal, Volume 88, pp. 149–155. 34. Neogi S.K. and Bhattacharya P.K., (1980). Effect of shed timing and leasing pattern on the warp tension, Textile Trends, Volume 23, pp. 61–65. 35. Neogi S.K., Bandyopadhyay A.K., and Banerjee N.C., (2000). Modified tappet shedding mechanism for improved performance of jute loom: Part I – Design, operation and heald movement analysis of the mechanism, Indian Journal of Fibre and Textile Research, Volume 25, pp. 108–114. 36. Neogi S.K., Bandyopadhyay A.K., and Banerjee N.C., (2000). Modified tappet shedding mechanism for improved performance of jute loom: Part II – Performance analysis of the mechanism, Indian Journal of Fibre and Textile Research,Volume 25, pp. 200–205.

164

Role of yarn tension in weaving

37. Neogi S. K., (1995). Physical properties and weavability of different types of knot with jute yarns, Indian Journal of Fibre and Textile Research, Volume 20, pp. 11–16. 38. Neogi S. K. and Bhattacharyya P. K., (1993). Splicing of jute yarn by wrapping method, Textile Research Journal, Volume 63, pp. 351–357. 39. Nyubikova K.G., (1969). The effect of the warp tension on the beat-up force in the processing of Calico ART. 4202, Technology of the Textile Industry, U.S.R.R., No 6, pp. 74–76. 40. Obolenski B. and Wulfhorst B., (1993). Influence of various temple systems on the running characteristics of weaving machines and on fabric quality, Melliand Textilberichte, Volume 74, pp. 25–29. 41. Osthus T., Wulfhorst B., Bosing T., Lanvermann G., and Potthoff F.J., (1995). Automated setting of backrest and drop-wires in mill trial, Melliand Textilberichte, Volume 76, pp. 826–829. 42. Owen A.E., (1928). The tension in a single warp thread during plain weaving, The Journal of the Textile Institute, Volume 19, pp. T365– T388. 43. Poole E.J., (1947). Wool yarn weaving, with particular reference to the controlling warp tension, The Journal of the Textile Institute, Volume 38, pp. P286–P297. 44. Porat I., Greenwood K., Eren R., and Roy A.K., (1994). Development of hybrid type warp let-off systems for weaving and warp knitting, Indian Journal of Fibre and Textile Research, Volume 19, pp. 114– 124. 45. Ramaswamy B.R. and Paliwal M.C., (1965). Warp tension in weaving: its effect on performance and quality, Proceedings, 6th Technological conference of ATIRA, BTRA & SITRA, pp. 87 - 95. 46. Schlegel W., Glawe A., and Greiz V., (1995). Thread tension analysis in weaving under mill conditions, Melliand Textilberichte, Volume 76, pp. 1077–1081. 47. Sheikhzadeh M., Hosseini S.A., and Darvishzadeh M., (2007). Theoretical evaluation of warp tension variations during weaving process, Indian Journal of Fibre and Textile Research, Volume 32, pp. 377–380. 48. Shih Y., Mohamed M.H., Bullerwell A.C., and Dao D., (1995). Analysis of beat-up force during weaving, Textile Research Journal, Volume 65, pp. 747–754.

Effects of loom settings and other factors on warp tension

165

49. Snowden D.C., (1949). Some factors influencing the number of warp breakages in woolen and worsted weaving, The Journal of the Textile Institute, Volume 40, pp. P317–P330. 50. Strazd G.E., (1971). The dropper motion as a factor in the deformation of the warp yarns, Technology of the Textile Industry U.S.S.R., No 5, pp. 61–64. 51. Svyatenko M.V., (1971). The warp tension on the Sulzer loom, Technology of Textile Industry U.S.R.R., No 3, pp. 84–88. 52. Talavasek O. and Svaty V., (1981). Shuttleless weaving machines, Elsevier Scientific Publishing Co., Amsterdam, p. 81. 53. Turton G. and Greenwood K., (1954). Weaving faults: avoiding defects in filament rayon, The Textile Weekly, Volume 53, pp. 975–978. 54. Turton G., (1954). Weaving faults: avoiding defects in filament rayon, The Textile Weekly, Volume 53, pp. 904–908. 55. Weinsdorfer H and Wimalaweera W (1987). Investigation on abrasion and roughening of warp yarns during weaving, textil praxis international, June, pp. 611–617. 56. Weinsdorfer H., Wolfrum J., and Stark U., (1991). The distribution of the warp end tension over the warp width and how it is influenced by the weaving machine setting, Melliand Textilberichte,Volume 72, pp. 903–907. 57. Weinsdorfer H. and Salama M., (1992). Study of shedding and warp end crossing, Part 1: Effect of machine parameters Melliand Textilberichte, Volume 73, pp. 717–722. 58. Weinsdorfer H., (1994). Effects of shed formation on the loading of warp yarn, Indian Journal of Fibre and Textile Research, Volume 19, pp. 139–146. 59. Weinsdorfer H and Salama M (1993). Study of shedding and warp end crossing, Part 2: Effect of warp and cloth parameters, Melliand Textilberichte, Volume 74, pp. 740–744. 60. Wimalaweera W and Weinsdorfer H (1988). Warp thread clinging and snarling in spun yarns during weaving, Melliand Textilberichte, Volume 69, pp. 412–417. 61. Wulfhorst B, Bohm D, Jager W and De Weldige E (1993). Computer simulation of warp yarn loading in the weaving process, Melliand Textilberichte, Volume 74, pp. 613–617.

8 Effects of loom settings and yarn characteristics on weft tension

Abstract: In shuttle picking, abrupt rise in weft unwinding tension from almost empty pirn with straight base can be avoided. Fairly uniform tension can be obtained all through by using the pirn body with conical base and/ or lining the inside walls of the shuttle with nylon furs or lamb’s wool. The final weft tension at the completion of a pick depends on shed timing. In shuttleless picking, if the weft is inserted directly from the stationary package, the package dimensions have adverse effects on weft tension, which are evaded by the use of weft accumulator. Weft tensions are of course, affected by the type and settings of the accumulator. Weft tension in any shuttleless picking system increases with machine speed while, the yarn characteristics have significant effects on weft tension in air-jet picking system. Keywords: weft tension; shuttle picking; pirn; shuttle; shed timing; shuttle checking; shuttleless picking; weft package; weft accumulator; loom speed; yarn characteristics.

8.1

Introduction

In comparison to the warp yarns, the path taken by the weft yarn in each pick, from its supply package to the fell of the cloth, is much shorter and, therefore, relatively less effects of different loom settings on the weft tension. In shuttle picking system, as the weft package that is, the pirn is carried by the shuttle, the design of the wooden base of the pirn and the shuttle are supposed to have some effects on weft tension during exhaustion of the pirn, while the shed timing and shuttle checking may show their influences on weft tension during insertion of a pick. In any shuttleless picking system, the weft is dragged from its stationary supply package and the weft insertion tension is therefore likely to be affected by the different loom settings governing the weft insertion. Again, since the weft in air-jet picking is inserted by drag force of air, the yarn characteristics are likely to influence the weft insertion behaviour more in air-jet picking than in others. Weft feeder or accumulator is now an integral part of any shuttleless picking system. Its type and settings can also show their effects on weft tension, but the supply package of the weft yarn is hardly likely to have any effect on weft tension if the weft is fed through the accumulator.

168

8.2

Role of yarn tension in weaving

Shuttle picking

We have observed in Chapter 6 that the unwinding tension of weft associated with shuttle picking system has two distinct features; one, during exhaustion of the pirn and the other, during insertion of a pick. While the former is affected by the design of the wooden pirn base and produces long term tension variation, the latter is influenced by the timing of the shedding motion with respect to picking and the efficiency of the checking mechanism of the shuttle and produces short term variation.

8.2.1 Effects of pirn and shuttle modifications Modifications in pirn and/or shuttle either in the design of the pirn base or in the design or internal fittings of the shuttle are more likely to reflect their effects on the weft unwinding tension during the exhaustion of the pirn rather than on the weft unwinding tension during insertion of a pick.

8.2.1.1 Pirn modifications It has been discussed in Section 6.2.1 and illustrated in Fig. 6.1 (Chapter 6) that if the weft is unwound from the pirn with straight base then the weft tension rises sharply when the pirn is nearly empty. The reason for this sudden high rise in tension is the shape at the base of the pirn body. If therefore, the body of the pirn is so modified that it has a conical shape at the base in conformity with the profile of the nose of the pirn, the tension of the weft will remain fairly the same through out the unwinding of the weft from the pirn [23, 12].

Figure 8.1 Weft unwinding tension from pirn with conical base. Figure 8.1 shows the unwinding tension of weft from the pirn with conical base. The pirn with conical base has also been shown in the figure. Here, when unwinding takes place from the base of the nearly exhausted pirn, the conical shape of the base of the pirn, now, allows the weft to be unwound smoothly

Effects of loom settings and yarn characteristics on weft tension

169

with less amount of licking of the weft on the bare pirn body even when the pirn is empty. The average tension of the weft yarn is now fairly uniform throughout weaving out of the pirn.

8.2.1.2 Shuttle modifications Increase of the final unwinding tension with straight-tube pirn (see Fig. 6.1, Chapter 6) can also be reduced to some extent by modifying the internal shape of the shuttle so that it fits close around the nose of the full pirn and using lamb’s wool or nylon furs inside the shuttle [22]. With proper lining of the shuttle as stated, the unwinding tension of a 29.5 tex cotton weft yarn has been found to increases by 5 times only, from 4 to 20 g, while the same with the ordinary shuttle without lining increases by 21 times, from 3 to 63 g at the same unwinding speed of 10 m/sec. Filament rayon has the propensity to be stretched very easily and so the control of filament weft during weaving is very important. Each pick should be inserted under substantially uniform tension. For the purpose, the shuttle is fitted with fur inside and woolen mop at the shuttle eye and the weft should be passed over the mop rather than through it, to achieve more uniform withdrawal tension of weft [24]. The fitting of the pirn in the shuttle is, however, very crucial in regard to weft withdrawal tension. If the pirn does not sit concentrically in the shuttle, adverse effects on weft unwinding tension cannot be evaded even with the properly shaped pirn or proper fittings inside the shuttle. The pirn must be fitted concentrically in its holder or on the tong (in case of non-automatic loom) of the shuttle and remain in perfect alignment with the shuttle eye. Relation between the withdrawal and final tensions of weft depends on the type of shuttle eye. With self-threading eye of the shuttle for automatic loom, the final tension is low and changes only slightly with withdrawal tension but if the shuttle is fitted with spring eye, the final tension increases substantially. In case of the automatic that is, pirn-changing loom, there is a further effect on weft tension during the change of a pirn [22]. When the shuttle is picked from the battery side following the change of a pirn, the weft yarn is partly threaded through the shuttle eye and the withdrawal tension of this pick is much less than when the weft is fully threaded through the eye in the succeeding picks. Although the use of heavier furs in the shuttle increases the drag on the weft during its withdrawal from the shuttle, the final tension of weft is not much affected by it [23], the withdrawal tension of weft falls drastically if the shuttle is not lined with fur. This results in loop formation, ringing off, and weft break during weaving [13]. The withdrawal of the weft from the nose or shoulder of a pirn has no significant difference in weft tension [13]. Foster [12] had shown that the various dimensions of the shuttle and pirn have their effects on the unwinding tension of the weft. The relevant dimensions of the shuttle and pirn have been shown in Fig. 8.2. Here, the

170

Role of yarn tension in weaving

normal shuttle eye which is fitted on the shuttle front wall at its one end has been replaced by a yarn guide located on the axis of the shuttle so that the distance between the pirn and the yarn guide can be varied to study the effect of the distance of the shuttle eye on weft tension.

Figure 8.2 Shuttle modifications As indicated in Fig. 8.2, if W is the internal distance between the shuttle walls, l is the length of pirn from the tip to the yarn base, df is the diameter of the full pirn, de is the diameter of the pirn body at the base that is, empty pirn, d is the mean diameter of the wound pirn {d = (df – de) / 2 + de}, D is the distance of yarn guide from the tip of the pirn, (with all the above dimensions in inches) S is unwinding speed of weft from the pirn in yd/min, C is the yarn count in indirect (Yorkshire Skein Woolen) system, and K is a constant obtained from different unwinding speeds of the pirn. Then T1, the weft tension at the body of the pirn is T1 =

0.9 S W g d (C + 6.4)

(8.1)

and T2, the weft tension at the base of the pirn is T2 =

K (1 + D) 4 W g d e (C + 6.4)

(8.2)

It is to be noted that instead of presently officially accepted metric units the traditional imperial units have been used here because otherwise the equations derived will not be valid.

Effects of loom settings and yarn characteristics on weft tension

171

Considering the yarn guide, instead of the shuttle eye, placed along the shuttle axis and ignoring the frictional effects on the yarn in the shuttle, the following inferences can be drawn from equations 8.1 and 8.2: 1.

Unwinding tensions of weft from both the body and base of a pirn are directly proportional to the distance between the side walls of the shuttle, W and inversely proportional to the count of the weft, C in indirect system of yarn counting;

2.

The weft tension at the body of the pirn, T1 is directly proportional to the unwinding speed of the yarn, S and inversely proportional to the effective or mean diameter, d of the pirn ; and

3.

The weft tension at the base of the pirn, T2 is directly proportional to the distance between the base of the pirn and the yarn guide in the shuttle (l + D). From these above discussions in respect of the unwinding tension of the weft from the pirn in the shuttle it may be stated that, besides using the pirn with conical base and lining the shuttle with lamb’s wool or nylon furs, the weft tension can also be reduced by making some modifications in the dimensions of the normal pirn with straight base and the plain shuttle. These are: 1.

by using the pirn base diameter as large as possible so that the required amount of weft yarn is wound on the pirn but the diameter of the full pirn should be a little less than the distance between the side walls of the shuttle so that the yarn surface of the wound pirn does not touch the shuttle walls,

2.

by using a pirn long enough to utilize to the full the space in the shuttle, and

3.

by utilizing the full length of the shuttle but maintaining a distance of at least 25 mm between the tip of the pirn and the shuttle eye.

Apart from these, neither the winding tensions nor the number of winds per traverse during winding of a pirn have any significant effects on the unwinding tension of weft from the pirn during picking [12].

8.2.2 Effects of shed timing As Fig. 6.3 of Chapter 6 exhibits, soon after the shuttle comes to the opposite box following its traverse through the warp shed, the weft tension starts falling till it is arrested by the closing of the shed. This tension of the pick occurring at the closing of the shed is the final tension. The final weft tension, thus, depends to a good extent on the shed timing, which has marked effects on the final tension but not on the initial or withdrawal tension of the weft [13]. If

172

Role of yarn tension in weaving

the shed closes early soon after the shuttle enters the box, in other words, if the shed closing and shuttle arrival time in the box coincide, final weft tension will be almost equal to the withdrawal tension. This shed timing is generally set for weaving heavy cotton fabrics. On the contrary, if the shed closes well after the shuttle enters the box and is at rest, the weft tension will fall from the withdrawal tension and the final tension at trapping of the shed will be much lower. This shed timing is suitable for weaving continuous filament yarns. From this we may, therefore, assume that, the same final tension of weft can be obtained by the combination of early closing of warp shed and low withdrawal tension of weft or a later closing of shed along with high withdrawal tension of weft [22]. We thus find that besides shed timing, the final weft tension also depends on the withdrawal tension of the weft. Higher the withdrawal tension higher will be the final tension of the pick.

8.2.3 Effects of shuttle checking Checking of shuttle in the shuttle box has significant effect on the final tension of the pick. The rate at which the final pick tension falls is influenced by the efficiency of the checking mechanism of the shuttle also [22, 13]. If the shuttle rebounds from the picker after checking because of incorrect settings of the shuttle box or malfunctioning of the checking mechanism, the final weft tension will fall to zero, no matter when the shed is closing. The most harmful effect of rebounding of shuttle is the possibility of shuttle trap in the next or the following picks. It is therefore very important that to obtain a correct final tension of weft after the completion of a pick, the shuttle should be properly checked in the shuttle box without the slightest tendency of rebounding from its picker and also the timings of shedding and picking are set correctly as desired or demanded by the type of cloth being woven.

8.3

Shuttleless picking

Thanks to the process of weaving, the yarns (both warp and weft) are subjected to stresses of rather complex nature. The nature, intensity, and duration of the stress or load are albeit much higher and more acute on the warps than on the wefts, with the advent of the shuttleless picking systems, the weft yarns are subjected to much higher loading than what their counterparts experience in shuttle picking. Any improvement in performance of a shuttleless loom through an increase in speed increases the acceleration of the weft. These results in significant increase in drag forces and thus, the stresses on the weft yarn. In all the weft insertion systems (in shuutleless looms), there are peak forces located at the yarn acceleration phase, like in projectile and rapier and/ or at the yarn braking phase, like in projectile and air-jet, as discussed in

Effects of loom settings and yarn characteristics on weft tension

173

Section 6.3 of Chapter 6. With a view to overcoming the problems of high stress imposed on the weft yarns in the present high speed shuttleless weaving machines, the yarns are fed through suitable weft storage systems and not directly from their stationary supply packages, although the earlier projectile and rapier looms used to operate without the weft storage systems.

8.4

Effects of weft package

When the first projectile and the rapier weaving machines with loop transfer and then tip transfer were launched commercially in around early 1950s and early 1970s, the wefts used to be fed to the picking mechanisms directly from the weft packages installed at the suitable positions outside the weaving areas. This type of weft feeding was possible as both the weft insertion elements grips the weft during insertion and at the end of picking the projectile is checked like in case of shuttle, and the rapiers exercise positive control on the weft throughout the weft insertion period and move to the predetermined distance in each pick. For these reasons, it was thought unnecessary to make use of any weft storage system in those days [10]. The situation is however, not the same in the cases of fluid-jet picking systems, the air-jet, and water-jet. In fluid-jet picking, the weft insertion is subject to force control as the weft is carried forward by drag force of air in air-jet and of water in water-jet looms and air or water cannot pull the weft directly from the weft package such as the projectile or rapier. No air-or water-jet loom can therefore work without weft storage system and the first air-jet weaving machines, “Maxbo” and “Kovo” and the water-jet weaving machine “Kovo”, the first air-jet weaving machines were employed commercially in early 50s and were equipped with weft storage systems. In these machines, a pre-measured length of weft needed for the width of the cloth to be woven is stored prior to insertion. As the weft is withdrawn from the package the yarn forms balloons, whose characteristics depend on many factors like the weft count, balloon length, withdrawal speed, etc., as discussed in Section 2.3.1 of Chapter 2. For successful withdrawal of the yarn from the package, the built of the package and yarn withdrawal speed are very important. In the shuttleless picking systems, the weft has to be accelerated from zero to a high velocity with fairly high acceleration and that too intermittently within a fraction of a second in a pick cycle during weft insertion. This is expected to create severe problems particularly at high working speeds of the weaving machines leading to increased weft breakage. It has been found in projectile weaving machine that besides weak places on the weft yarn, movement and slippage of yarn coils of the weft package are the main causes for weft break [19]. Coil movement and coil slippage depend on the shape and size of the package as well as the type

174

Role of yarn tension in weaving

of yarn. Smooth or slippery yarns are more liable to cause coil movement and slippage than the coarse and rough yarns. Ivitz [15] had studied the effects of different characteristics of yarn package such as, shape, size, method of winding, etc., on weft tension during picking in projectile weaving machine. He had also observed that frictional properties of the yarn play an important role in yarn unwinding tensions from the package. Besides that, the bobbins wound by spindle drive (that is, precision wound bobbins) perform better than those wound by surface drive (see Fig 2.7, Chapter 2) and the conical packages are better than the cylindrical packages in regard to unwinding of the yarn. Crossing angle of the yarn in the package is also important. Yarn crossing angle of 22 – 280 has been found to help better withdrawal of the yarn from its package. Another important aspect is the storage duration of the yarn package. Stability of the cross wound package deteriorates with increasing storage time and increasing amount of synthetic fibre contained in the blended yarn. In case of normal blended yarn, weft withdrawal is affected if the package is stored for more than 60 – 70 hours. So far as conicity of the package is concerned, cross wound package of 00 – 8040’ has been found suitable for projectile [15] and that of 20 made o in precision winding has been found to work better than that of 0 in air-jet weaving machines [7] (see Fig. 8.3 B and Fig. 8.11). Direct feeding from the weft package may work satisfactorily so long as the package is full and the machine speed is moderate. As the package is gradually depleted the unwinding tension of the weft increases, as Fig. 8.3 illustrates for cotton open-end spun yarn of different counts, weft packages and for different weft insertion systems. Figure 8.3 A indicates the results of different counts of yarn in projectile [15] and Fig. 8.3 B shows the results of a 30 tex yarn prepared in different types of package in air-jet weaving machines [7]. The increase in weft tension with depletion of yarn package is because of reduction in the package radius, as indicated by the equations 2.2 and 2.3 of Chapter 2.

Effects of loom settings and yarn characteristics on weft tension

175

Figure 8.3 Relation between weft tension and package characteristics. Feed tension of the weft increases as the diameter of the package decreases. Under the circumstances it becomes difficult to empty the entire package without increasing the weft breakage and sacrificing the quality of the cloth produced. Bertucci and Scholze [7] had observed with air-jet picking that the maximum tension and transfer time of weft are not always steady but scatter and the scatter is lower at the small package diameters than at the large ones. This is because of yarn balloon which is large when the yarn runs off the package of large diameter. From the above discussions we, thus, find that to avoid the problems of high weft tension variations associated with direct feeding of weft from the package, which naturally become more acute if the speed of the machine is increased, it is now almost mandatory to feed the weft yarn to the loom through a suitable weft storage system. In spite of this the built of the package still shows some influences on the weft tensions even after the weft is fed through the storage system, as discussed in Section 8.5.2.

8.5 Weft storage system (weft accumulator or weft feeder) Weft is inserted during a fraction of the pick cycle. Ever since the successful introduction of the different weft insertion systems without shuttle, the prime aim has been to increase the speed of the loom to achieve higher weaving productivity. As the speed of the loom increases so does the weft velocity. The weft velocity depends on its driving force which again, is determined by the type of weft insertion system and the structural design of the machine. High insertion speeds of weft associated with the different types of shuttleless weaving machines impose high stresses on the weft yarns and at certain phases undesirably high tension peaks. The consequent effects of these are increased weft breakage and/or disruption of the surface layer of the weft package if the yarn is drawn directly from it. Moreover, if the weft is made

176

Role of yarn tension in weaving

to be withdrawn directly from the package, withdrawal tension of the weft will be influenced significantly by the package parameters, as seen in Fig. 8.3, and the resultant variations in weft tension will affect the insertion of the pick. All these will have adverse effects on the weaving performances of the looms and the qualities of the fabrics produced and much of the purpose of the shuttleless picking system will be foiled. To solve these problems a suitable weft storage system known as weft accumulator or weft feeder is interposed between the weft package and the weft picking mechanism of the shuttleless weaving machine. A weft accumulator is shown in Fig. 8.4.

Figure 8.4 Weft accumulator. Leaving the package, usually the cross wound cone, the weft enters the accumulator. The weft accumulator draws off yarn from the weft package intermittently at a speed not much above the average rate of weft utilization in the loom and stores enough in advance for the insertion of each pick. The weft is stored in a form that allows it to be supplied without difficulty at the maximum speed of insertion but at comparatively low tension. The main advantages of the weft accumulator are thus, to 1.

supply the weft yarn to the weaving machine smoothly and at a constant and proper tension as the diameter of the supply package diminishes,

2.

reduce the weft tension and variation in weft tension during the course of its insertion,

3.

compensate for different withdrawal conditions when using different weft packages, and

Effects of loom settings and yarn characteristics on weft tension

4.

177

prevent sloughing-off of the yarns from the package.

By maintaining proper coordination with weft insertion, the weft accumulator ensures high weaving performance by reducing the chances of weft break and lowering the yarn waste. The weft storage system assists particularly the jet picking where the weft is inserted by drag force of air or water and the weft velocity is so high that the jet can not have the ability to pull the weft intermittently from the supply package overcoming the severe retarding force to its own velocity. There are basically two types of weft storage system, (i) drum type, where the reserved length of weft is stored winding on a drum (Fig. 8.4) and (ii) loop type, where the measured length of weft is stored under air current in a closed tube. The loop storage is suitable for air-jet picking, but it requires a large space, especially for wide looms and faces some problems in controlling the wefts of high twist or high stiffness. For these reasons weft accumulator of drum type is more universally accepted in the shuttleless looms of all types of weft insertion systems. Irrespective of the type, the weft accumulator system should fulfill the following purposes [21]: 1.

As the weft is unwound from its supply package a minimum suitable tension, free of impacts, must be applied to it,

2.

The system must allow an easy withdrawal of the weft from the prepared weft supply package, and

3.

The passage of the weft through the feeder must be smooth and continuous and it must allow easy rethreading of the weft when it breaks between the package and the accumulator.

It should, however, be remembered that a weft accumulator with proper adjustments and settings can minimize the problems of weft tensions associated with weaving machine or yarn or package but can not resolve them on its own. Leaving the weft accumulator as the yarn enters the picking area of the weaving machine, it is still subjected to increased tension due to yarn guides, tensioners or braking units and also yarn acceleration. etc [14]. Moreover, all types of weft accumulator cannot control the weft tension variations in identical manner.

8.5.1 Forces acting on the yarn at weft accumulator As discussed in Section 6.3 of Chapter 6, the tension characteristic of the weft during insertion depends on (i) peak acceleration force, (ii) insertion tension,

178

Role of yarn tension in weaving

and (iii) peak deceleration force, all related to insertion speed of the weft. The peak acceleration and peak deceleration forces increase with the square of the speed of yarn withdrawal and the insertion tension is influenced by friction which retards the yarn at different deflection points depending on the method of weft insertion. The friction increases with speed. Again, the weft tension during insertion is also influenced by acceleration processes which the yarn is subject to after being withdrawn from the weft accumulator, and higher the acceleration higher is the yarn tension. During the withdrawal of the weft from the drum of the accumulator the weft is subjected to centrifugal force which has a critical influence on friction. If the centrifugal force is F then,

F = (m × va2 ) / r

(8.3)

F = (c × l × va2 ) / r

(8.4)

or

where m is the mass of the yarn, va is the withdrawal speed of the yarn, r is the radius of the drum, c is count of yarn, and l is the length of yarn. From the equation 8.4 we find that the force acting on the yarn is inversely proportional to the radius of the drum and directly proportional to the mass of the yarn and its withdrawal speed. So, the resistance to withdrawal that is, the friction of the weft during withdrawal from the drum will be lower with the increased diameter of the drum. The reduced friction in turn, reduces the withdrawal tension of the weft from the feeder. The mass that is, the count and length of the weft yarn and its withdrawal speed that is, the speed at which the machine operates however, increase the force exerted on the yarn. It is to be noted here that the effect of yarn balloon has not been considered in the equations 8.3 and 8.4. The feeder is fitted with a ring (brush or lamellar tensioner, etc.) (see Fig. 8.4) at its exit and that limits or reduces the formation of normal balloon of the unwinding yarn. In air-jet picking where the insertion of a pick is solely dependent on the drag force created by air-jet, the motion of the yarn during insertion is fundamentally governed by Newton’s Second Law which states that for a body, the rate of change of momentum is equal to the total force acting upon

Effects of loom settings and yarn characteristics on weft tension

179

it. Adanur and Mohamed [3] had analyzed the motion of the yarn in a closed guide tube and employing single nozzle for insertion with both the drum and loop storage systems. In case of drum storage system, the total force ∑F acting on the yarn is

∑ F = d (MV ) / dt = F − F 1

2

(8.5)

where M is total mass of yarn, V is velocity of yarn, t is time, F1 is air-friction force on the yarn, and F2 is yarn tension at the yarn guide. Air-friction force, F1 depends on many factors such as, yarn diameter, air density, air/yarn friction coefficient, air velocity at the air nozzle, yarn velocity, yarn length between the yarn guide and the nozzle, length of air tube of the nozzle, distance between the nozzle exit and the cutter, the distance between the cutter and the guide tube, length of the closed guide tube, and the distance the yarn tip travels from the entrance of the guide tube. As the yarn is unwound from the feeder drum and passes through the yarn guide, its tension, F2 is affected by the capstan effect of the guide and so, F2 = 0.5 mV2 eµα

(8.6)

where, m is linear density that is, count of the yarn, µ is coefficient of friction between the yarn and guide and α is angle of wrap around the guide. In case of loop storage system, ∑F = F1 – FL

(8.7)

where, F1 is the same as in equation 8.5 but with the length of the yarn between the loop and the nozzle instead of that between the yarn guide and the nozzle, and FL is the total force acting on the yarn loop. Deriving the equation of motion it has been found that the yarn velocity can be increased by increasing the air/yarn frictional force (i.e., by increasing the air velocity) and by decreasing the yarn tension.

180

Role of yarn tension in weaving

8.5.2 Effects of weft accumulator Usefulness of weft accumulator in controlling the weft tension during insertion has been demonstrated by Kohlhass [16] with the help of simulation and by others [15, 9, 17, 18, 29] in actual operations of the shuttleless weaving machines with different types of weft insertion systems. Kohlhass [16] worked with different accumulators of different manufacturers had observed that the tension of the yarn between the weft package and the weft accumulator or feeder was highly influenced by the unwinding speed and insertion angle of the yarn. But, between the accumulator and the simulator (that is, the weft insertion unit) there were no effects of package diameter and the yarn insertion angle on yarn tension. This implies that the accumulator evens out most of the yarn tension fluctuations Ivitz [15] and Chahal and Mohamed [9] studied the weft tensions on projectile weaving machines running at 205 and 258 picks/min, respectively, with and without weft accumulator. In the former case, the measurements of weft tension have been made between the weft package and the first yarn guide of the machine and between the compensator arm and the projectile, when the accumulator has not been used. When the accumulator has been used, the measurements have been made between the package and the accumulator, between the accumulator and the first yarn guide and between the compensator arm and the projectile. It has been found that the average weft insertion tension reduces significantly by the use of accumulator for any type of weft package or weft yarn.

Figure 8.5 Weft tensions with and without weft accumulator in projectile loom.

Effects of loom settings and yarn characteristics on weft tension

181

In the study carried out by Chahal and Mohamed [9], weft tension was controlled by adjusting the weft brake shoe and the tension was measured first without any weft feeding system and then with drum type weft accumulator of two makes. When no accumulator was used the weft package was placed at a distance of 400 mm from the yarn guide element of the machine and when accumulators ware used they were placed at a distance of 450 mm from the weft package and 300 mm from the machine. Both the accumulators were fitted with ring brush brakes made of animal hair. Weft tension variations without weft accumulator (A) and with weft accumulators (B and C) are shown in Fig. 8.5. Irrespective of the type of control exercised on the weft yarn, the form of weft tension variation is in general, similar to that shown in Fig. 6.5 of Chapter 6. The major fluctuations in weft tension are because of projectile acceleration, pick insertion, projectile checking, and tension compensation. Weft tension variations are nearly identical in nature in all three cases (A, B and C), without weft accumulator and with two types of accumulator, except at the start of pick insertion, which is evident from Fig. 8.5. In this region, the tension variation differs significantly when the tension compensator supplies the stored weft for insertion. Differences in the path of weft yarn with accumulators of two makes and without accumulator have also shown their effects on weft insertion tension soon after picking. As can be expected, the average weft tension during the insertion of a pick is the highest when the weft is fed directly from the package that is, without accumulator. The weft accumulators also neutralize the pick to pick variations and the irregularities in weft tension occurring during unwinding from the supply package. As a consequence, the quality or the size of the weft package is least likely to have any adverse effect on the cloth produced. It is well known that rapier picking system exercises positive control on the weft yarn during the entire period of weft insertion and hence, the rapier weaving machines are sometimes observed to be operating without weft feeding system, as pointed out above. Lunenschloss and Schlichter [17] had shown the benefits achieved in using weft feeder in tip transfer system in rapier picking. Their findings are shown in Fig. 8.6. The tension trace indicated by the broken lines is without weft feeder and that by the firm lines is with feeder. As observed from Fig. 8.6, the weft feeder reduces significantly the peak tensions as well as the maximum tension levels of the pick observed during the course of its insertion by the giver and taker rapiers.

182

Role of yarn tension in weaving

Figure 8.6 Weft tensions with and without weft feeder in rapier loom. Similarly, the weft feeder helps to abate the increase of tension peaks of the weft during insertion, resulting from the increase in speed of the rapier loom, as demonstrated in Fig. 8.7. The braking ring of the weft feeder prevents formation of a yarn balloon and reduces yarn movement in conjunction with the delivery point and the withdrawal direction, which remains constant. McMahon’s [18] observations with rapier weaving machine working on tip transfer system have also revealed that the weft accumulator reduces the maximum transient tensions and tension variation during weft insertion. Peak tensions occurring during insertion by the donor rapier and withdrawal by the receiver rapier as well as the mean weft insertion tension are reduced by the weft feeder. The feeder also reduces even the minimum tensions observed during transfer of the pick and at the end of weft insertion with medium or high setting of the tensioner. He has observed that the transient tension peaks and the minimum tension increase with the increase in mean weft tension.

Figure 8.7 Effect of weft feeder in reducing weft tension with increasing loom speed.

Effects of loom settings and yarn characteristics on weft tension

183

The problem of high variations in weft tension during insertion, which naturally imposes restriction on increase in loom speed, can be reduced significantly by employing Coaxial Tensioner (CAT) integrated on the weft accumulator. In this system, the yarn passes through two tensioning discs mounted at the exit of the accumulator. An adjustable tensioning spring regulates the base tension exerted by the discs which allow the setting and maintaining of tension levels and compensate yarn tension fluctuations during the process of weft insertion. Study of CAT on a rapier loom [29] has revealed appreciable reduction, to the extent of about 30%, in peak tension and general reduction in weft tension during insertion, except at weft presentation and weft transfer which are however fairly low in any case, The results of weft tensions with CAT are shown in Fig. 8.8.

Figure 8.8 Reduction in weft tension with coaxial tensioner. Significant reductions in weft insertion tension with CAT make it possible to achieve higher weft insertion rate by increasing the loom speed. It also permits gentler control on tender weft yarns with low strength and difficult yarns such as crimped, bulked or elastomeric type yarns, which require more constant control of tension for successful insertion. As mentioned earlier, no air-jet weaving machine can operate without weft feeding system. In air-jet picking, the weft is dragged by air only for insertion and hence, unless the weft of the required pick length is stored prior to picking, weft insertion is not possible. To increase the weft velocity the friction between air and yarn should be increased and the tension that retards the weft movement should be decreased. These studies referred above, thus, demonstrate the efficacy of the weft storage system in achieving the desired performances of the different shuttleless weaving machines by reducing the effective weft loading and thereby, the weft tension in comparison to the direct withdrawal from the

184

Role of yarn tension in weaving

weft package during the course of weft insertion. This is especially crucial with ever increasing operating speeds of the weaving machines. However, as pointed out above, the dimensions and hardness of the weft packages do affect the weft tensions even after the weft feeders [7, 25]. Withdrawal of the weft yarn from the package causes ballooning and the balloon becomes large when the yarn is unwound from the large diameter of the package. The large yarn balloon causes high and wide scatter in feed tension of the weft to the accumulator. Weft tension also increases with the hardness of the package [7]. Again, as the weft package decreases in diameter the withdrawal force of the weft yarn from the cross wound package increases which, in turn, increases the tension at which the weft is fed to the accumulator [25]. Owing to this high feed tension, the weft is wound more tightly on the accumulator drum. This as a consequence, may elongate the weft yarn and affect its elasticity. Proper control of feed tension of the weft is, therefore, very important to ensure better tension characteristics of the weft yarn during insertion. Yarn winding tension of the package or the feed tension of the weft yarn from the supply package will have no adverse effects on weft tension beyond the accumulator if the yarn coils are properly built on the accumulator drum [14].

8.5.3 Effects of type and setting of weft accumulator It can be noticed from Fig. 8.5 that the weft tensions on a projectile weaving machine with the same drum type weft accumulator or feeder but of two different makes are not exactly the same. One (B) produces higher tension peak than the other at the initial stage of weft acceleration although, both the accumulators minimize the overall weft insertion tension, as discussed in the preceding section. This is because of higher pull on the yarn during this period [9]. Difference in weft tension values with different types of weft accumulator has been observed by Kohlhass [16] also, as indicated in Fig. 8.9, which shows the peak weft tension of 167dtex viscose multifilament yarn between the package and the accumulators. It is, however, to be noted that the weft feeding system affects the weft tension only during insertion of the pick and not after that. The various settings of weft feeder have some significant effects on weft tension. The weft feeder of drum type (which, as stated above, is more popular and used in all types of shuttleless weaving machines) is provided with a ring like braking device for removing any snarl present in the yarn and controlling the tension of the pick by limiting the yarn balloon during insertion. The braking device may be brush (see Fig. 8.4), metal lamella, flex brake, etc. Effects of various aspects of different types of weft accumulator on

Effects of loom settings and yarn characteristics on weft tension

185

weft insertion tension in the rapier weaving machine working on tip transfer system have been revealed by many studies [14, 25 - 27].

Figure 8.9 Weft tensions with different weft accumulators. Hubner and Frenzel [14] found that the brush ring of the weft accumulator, made from soft materials, is more suitable for controlling the weft tension, especially for the fancy yarns. Brush rings they have considered for their study are also of animal hair. If the brush rings made of harder materials are used they cause excessive peak tensions during weft insertion. Tension peaks during weft insertion can also be reduced by employing self-compensating yarn tension or braking unit like “Flex Brake’ or CAT system, discussed above. Similar observations have been made by Weinsdorfer, Lange and Scholze [25 - 27] in respect of hardness of the brush ring of two types of weft accumulator. Irrespective of the type of accumulator, softer brush brakes cause lower peak acceleration than the harder brushes as the severity of friction is less with softer brush although, the softer brush tensioners are more likely to wear out quickly. Tension increases distinctly with the increase in hardness of the brush. Softest possible brush tensioner and harder setting of the disc tensioner than the brush tensioner of the weft accumulator ensure smoother weft tension. The brush rings or other elements fitted on the weft accumulator to limit the yarn balloon should also be set precisely to control the weft insertion tension. If the brush tensioner of the accumulator is not set perfectly concentric it causes high tension peaks [14] and also periodicity in weft withdrawal tension from the accumulator, which increases with machine speed. Rise in weft tension at higher speed (Fig. 8.9) is because of higher wind-on tension of the weft from the cross-wound package and increased friction at brushes and guideeyes of the accumulator. Besides the correct settings of the brush ring and other yarn controlling elements of the accumulator, the emerging yarn from the accumulator should also follow a relatively straight path from the brush ring to the yarn guide of the weaving machine with the fewest possible yarn

186

Role of yarn tension in weaving

deflection points beyond the accumulator. This will ensure better control of weft tension during insertion. In one of the studies [26] it has been observed that one of the weft accumulators has registered a lower weft tension than the other, because of the manner in which the yarn is distributed on the accumulator drum, the hardness of the brush tensioners and their central alignment. The winding drum of one accumulator is not round [25] and when the yarn is wound on the drum it becomes hexagonal in shape. In this case, the yarn is not always withdrawn from the same diameter. When the yarn is withdrawn from the shoulders of the hexagon the tension is higher than when it is withdrawn from the sides. This type of accumulator drum, therefore, cannot produce smooth tension of the weft during its acceleration. Entry tension that is, the tension of the weft yarn entering the accumulator or the distance between the weft package and the accumulator have no significant effects on weft tension [25] but, location of the accumulator with respect to the weft entry point in the machine has significant effects on weft tension [25 - 27]. Amount of the yarn wound on the accumulator drum has also some effects on the weft tension. An increase in the reserve of the weft yarn on the accumulator drum below the photocell causes a slight increase in weft tension. Leaving the accumulator the weft yarn enters the picking area of the weaving machine. The length of weft extending from the accumulator to the machine forms balloon as the weft is withdrawn from the accumulator for insertion. The tension experienced by the weft when leaving the accumulator is the product of the friction force exerted by the brush brake and the force due to ballooning of the free length of the weft yarn between the accumulator and the weaving machine. Figure 8.10 shows the effects of distance of the weft accumulator from the weaving machine on weft tension.

Figure 8.10 Effects of weft accumulator distance from the loom on weft tension.

Effects of loom settings and yarn characteristics on weft tension

187

Both the insertion and acceleration tensions of the weft increase with increasing distance of the weft accumulator from the entry guide of the weaving machine. When the accumulator is placed farther away from the machine, mass of the yarn to be accelerated and also the ballooning of the accelerating yarn increase because of correspondingly longer length of the yarn. Increase in yarn mass increases the acceleration tension and the increase in ballooning increases the insertion tension of the weft. The longer the length of the yarn to be accelerated, the longer the duration of peak acceleration forces. For reducing the weft tension, therefore, the weft accumulator should be placed as near to the weaving machine as possible so that the unrestricted length of the weft is reduced and the yarn is deflected as little as possible while entering the weaving machine. The problem is more critical for heavy weft yarn, which is more likely to form balloon during withdrawal. For such yarns, the accumulator is generally fitted with balloon breaker in place of the normal eyelet at its front to eliminate the possibility of balloon formation. However, with the maximum weft pattering capacity of 16 with the modern rapier loom, how close the accumulators can be placed to the machine is a matter of consideration. Contrary to the findings of Weinsdorfer, Lange, and Scholze [25], Bertucci and Scholze [7] have found that the location of the weft accumulator or feeder with respect to the weft supply package has significant effects on weft tension during picking in air-jet weaving machine. Both the feed and braking tensions of the weft generally increase with the distance between the yarn package and weft accumulator, as illustrated in Fig. 8.11, for two types of precision wound package of 00 and 20conicity. The results of the former type are indicated by the broken lines while those of the latter are indicated by the firm lines.

Figure 8.11 Effects of weft accumulator distance from the weft package on weft tension. Increase in weft tensions has been observed with both the 20 and 00 precision wound packages although, the tensions with the former are lower,

188

Role of yarn tension in weaving

as stated in Section 8.4. Increased distance between the weft package and the accumulator increases the length and thereby, the mass of the yarn which as a result, increase the size of the yarn balloon and experience greater air friction. Beyond a certain distance, however, indicated by the broken vertical lines in Fig. 8.11, the single balloon is transformed into the double balloon and the yarn tensions decrease. This is because of the reasons discussed earlier in Section 2.3.1 of Chapter 2. Besides the drum feeder, loop storage system is also used as weft feeding system in air-jet weaving machine. Adanur and Mohamed [1, 2] studied the effects of these two types of weft feeding system on weft tension in air-jet weaving machine. They have observed different natures of tension variations of the same cotton weft yarn with loop and drum storage systems [1]. The findings of their study are shown in Fig. 8.12. Weft tensions with loop and drum storage systems are indicated by broken and firm lines, respectively in Fig. 8.12.

Figure 8.12 Weft tension variations with loop and drum storage systems. After the yarn is released from the clamp, the tension starts rising as the length of the yarn, under the control of air inside the closed tube considered in the study, increases. With loop storage system there are three tension peaks during insertion of the weft by the jetting nozzle, Fig. 8.12. The first tension peak is reached when the length of weft in the reserve loop in the loop storage system is consumed. Just before this tension peak the tension becomes low because the length of the yarn in the reserve loop decreases and so its resistance to motion also decreases. The second tension peak occurs when the weft is held by the clamp. The third tension peak is because of straightening of the weft in the insertion channel. With drum storage system on the other hand, there are two tension peaks during insertion of the weft by the jetting nozzle. The first tension peak is when the clamp closes on the yarn and the second tension peak is for the straightening of the weft. In regard to the first tension

Effects of loom settings and yarn characteristics on weft tension

189

peak, the clamp closes at the same time for both the storage systems but in drum storage system, even after the clamp is closed the yarn continues to fly until it is stopped by clamp pin of the drum, while in loop storage system, the yarn is held as soon as the clamp is closed. As a result, in case of drum storage system, there is a relative delay for the occurrence of the tension peak due to clamp closing. In regard to the second tension peak, here also the tension peak due to yarn straightening takes place earlier for loop storage system because the weft velocity is higher in loop storage system. Comparisons of the two tension traces in Fig. 8.12 also reveal that the average weft tension is higher with drum storage but the distribution of tension is more even with this. The loop storage produces higher tension peaks than the drum storage. In drum storage system, the yarn tension depends on the frictional force between the drum surface and the yarn, the mass linear density that is, count of the yarn and the unwinding speed of the yarn in other words, the speed of the loom. High weft insertion tension should result in a long insertion time which depends mainly on the yarn characteristics viz., count, structure, twist, surface characteristics, and surface area of the yarn subjected to air friction and also relative velocities of air and yarn [2]. As the drum storage system exercises better control on the yarn and produces more uniform weft tension than the loop storage system, it is more capable of reducing the velocity fluctuations for different yarns. The initial acceleration of the pick is higher in case of drum storage, than with loop storage but, the latter gives a higher yarn velocity. In spite of the advantage of higher velocity offered by loop storage, the drum storage is used on most weaving machines, because of some problems associated with the former, as stated in Section 8.5. The drum storage also exercises better control on the weft yarn than the loop storage. From these above discussions we, thus, find that for achieving improved weaving efficiencies of the shuttleless weaving machines there should be optimum coordination between the weft accumulator and the weft insertion.

8.6

Effects of other loom mechanisms

Barring the weft storage system that is, weft accumulator or feeder, other weft controlling mechanisms also influence the characteristics of weft tension during the course of weft insertion in shuttleless weaving machines. In projectile looms, the setting of weft brake is very critical in controlling the weft tension [15, 9]. Whether the weft accumulator is used or not, the overall weft tension is lower at lower brake pressure, but there is a sharp rise in tension during checking of the projectile. When the brake pressure is increased the peak tensions at checking disappear. Peak tensions of weft during insertion can also be reduced significantly by proper adjustments of the weft stop motion and the torsion bar of the projectile machine [17].

190

Role of yarn tension in weaving

In rapier weaving machines with universally adopted tip transfer system, the transfer of weft is accomplished in negative or positive manner during insertion. In positive transfer system with positively operated clamps of both the giver and taker rapiers, the trajectories of the transfer point on the giver and the gripping point on the taker rapiers are tangential and so, the velocity change and therefore, drop in weft tension at the transfer can be avoided [11]. Moreover, in this system, the tip of weft yarn does not slide inside the clamps of either the giver or the taker rapier during the process of transfer and therefore, the weft is not subjected to dragging at this moment. This is very beneficial for weak and delicate yarns. In negative transfer system on the contrary, overlapping of the trajectories is necessary at the instant of transfer of the weft. During transfer, the weft clamped by the giver rapier (basically in the same manner as that in case of the positive transfer system) is snatched away from it by the spring loaded clamp of the taker rapier. As the giver rapier reaches the transfer point inside the warp shed, the withdrawal of weft from the supply package is temporarily withheld and the weft velocity falls to zero until the taker rapier starts retreating following the transfer. During the process of transfer the tip of the weft slides inside the clamp of the giver rapier and the transfer occurs during withdrawal of the taker rapier. After transfer, the point of weft formerly at the transfer point is located at, and moves with, the clamping point of the taker rapier. Soon after the transfer there are sudden changes in velocity and tension of the weft. Figure 8.13 A illustrates in simple form, the clamping force exerted by the giver rapier on the weft. Suppose, the clamping lever is pivoted at A and the spring force S on the lever exerts a force N on the weft clamped by the rapier. If the distance at which the force S is applied on the clamping lever from the pivot is ds and the distance at which the clamping force N is exerted on the weft is dn then, N dn = S ds

(8.8)

or N = S d s / dn

(8.9)

Thus, for a given spring pressure S acting at a given distance ds from the clamp pivot, further the weft is inside the rapier clamp smaller the distance dn and consequently, grater the clamping force N. The weft is then held more securely in the clamp of the giver rapier, which is desired. During transfer of the weft from the clamp of the giver rapier to that of the taker rapier, the weft is pulled free from the clamp of the giver rapier and is therefore, under tension. If the weft is subjected to tension T then, T = 2µ N = 2µ S ds / dn where,

(8.10)

Effects of loom settings and yarn characteristics on weft tension

191

µ is the coefficient of friction between the weft and the clamp.

Figure 8.13 Weft clamped by rapiers in negative transfer system. As the tip of the weft approaches the exit of the clamp of the giver rapier, the distance dn increases, the tension T decreases and consequently the force needed to pull the weft free from the rapier clamp reduces. This allows the weft to be transferred without suffering much damage. In case of the taker rapier, its clamp grips the weft and pulls it away from the clamp of the giver rapier. Let us now consider what happens to the weft in the clamp of taker rapier. Figure 8.13 B shows the clamp, again in simple form, of the taker rapier with weft clamped in it. As Fig. 8.13 B indicates, after the weft is clamped in the taker rapier after the transfer, the distance dn1 of the weft from the pivot gradually increases as the weft moves further inside the clamp. According to the equations 8.9 and 8.10, the clamping force and therefore the tension of the weft then gradually decrease, which are not desirable as the weft will then tend to slip off the rapier clamp. To evade this, the design of the clamp of the taker rapier is such that the clamping surfaces converge into the clamp (see Fig. 8.13.B). As a result, as the weft slides inside the clamp, it increases the separation of the spring loaded clamping surfaces and consequently increases the clamping force N1, to counteract the adverse effect of dn1. The increase in dn1 is reduced if the general direction of the clamping surfaces is at an angle to the direction of insertion [11].

192

Role of yarn tension in weaving

In air-jet weaving machines the weft velocity during insertion depends a great deal on its tension; high tension causes longer insertion time and hence lower weft velocity. Several factors such as friction between yarn and still air before the air nozzle, mechanical friction between the yarn and guides and airfriction force on the yarn inside the insertion channel determine the tension of the weft yarn during insertion. It has been discussed earlier in Section 6.3.3 (Chapter 6) and illustrated in Fig. 6.7 (Chapter 6) that, unlike in projectile and rapier picking systems, in air-jet picking, the maximum weft tension during weft insertion occurs at the braking phase that is, at the completion of insertion of the pick. After the weft yarn has been fully inserted in the warp shed, the stopper pin on the weft accumulator closes to stop further supply of yarn. This causes the yarn to stop abruptly from its high insertion velocity and generates a high tension in the yarn at the end of insertion. The magnitude of this peak tension is most likely to be determined by the speed at which the weft is moving when braking begins. The maximum peak tension will occur if the braking process is initiated to bring the yarn to standstill when the yarn is moving at its full insertion speed. As an effect, the portion of the yarn near the stopper at the insertion end experiences the maximum loading which is propagated along the length of the pick and the resulting stressing of the weft yarn increases with the speed of the loom. It is well known that the yarn breaks at its weakest place and therefore, each weak place is more critical, the nearer it is to the insertion end. If the weft tension in air-jet picking is too high due to stopping of the yarn, the weft may break or jump back resulting in stoppage of the machine and developing fault in the fabric. The tendency of the pick to jump back inside the warp shed can be reduced by stretching it to the full width of the cloth by suction nozzle located at the off side of the weaving machine. The problem of abrupt rise in weft tension during braking can be avoided if the speed drop of the pick during the braking process can be diminished with the help of controlled weft brake. There are however, two factors that make the braking of weft at the precise moment at each pick very difficult [6]. One is the deviations in weft insertion speed. The weft can hardly be expected to be inserted at a given exact speed each time that is, there may be some deviations in arrival time of the pick (see Section 6.3.3, Chapter 6). The fixed setting of weft brake will cause short pick if the pick traverses slow. This can be avoided if the information of the actual status of weft insertion in each pick instead of the preset status is available so that the braking system can act accordingly. The other one is the differences in yarn characteristics. Differences in the type of material and spinning technique of the weft yarns produce the yarns with considerably different surface characteristics and these can show significant effects on braking action of the weft. Having taken care of these, the modified electronically controlled braking system has been found to be very effective.

Effects of loom settings and yarn characteristics on weft tension

193

Study on 36 tex wool weft yarn has shown that it reduce the peak tension at braking by 30 – 60% [6]. Effect of electronically controlled weft braking system in reducing the weft tension at braking is illustrated in Fig. 8.14 [20].

Figure 8.14 Weft Tension with electronic braking Systems. Figure 8.14 compares the weft insertion tensions without the controlled braking system, indicated by broken lines and with controlled braking system, indicated by firm lines. In both the cases, the weft tensions rise relatively gently and nearly in the similar manner during the acceleration phase of the yarn when the weft is transported by frictional resistance. Following this there is a sudden rise in tension due to braking of the weft on completion of insertion of the pick and the peak tension is reached when the weft is braked to a standstill, as discussed earlier and shown in Fig. 6.7 (Chapter 6) and by the broken lines in Fig 8.14. This peak tension of the weft at braking can be reduced considerably by employing the electronically controlled weft braking system, shown by firm lines in Fig. 8.14. This is particularly beneficial for fine and delicate yarns.

8.7

Effects of loom speed

Increase in operating speeds of the weaving machines increases the insertion tension of the weft yarn in all types of weft insertion system. Increased weft tension is likely to increase the weft breakage, which in turn, affects the weaving efficiency at higher weaving speeds as well as the qualities of the cloths produced. It has been observed with a 216 cm wide projectile weaving machine that increase in projectile speed from 18.6 m/s to 22.3 m/s increases the weft tension substantially [5], as shown in Fig. 8.15.

194

Role of yarn tension in weaving

. Figure 8.15 Weft tensions with different projectile velocities. At lower projectile speed the maximum weft tension is only about 60% of that obtained with higher projectile speed. With increase in weft insertion tension with the increase in machine speed there may be high variations in tension at the braking of the projectile because of machine adjustments. The increase in weft tension with higher projectile velocity may not be of much problem to strong weft yarns but the weak yarns are certainly very vulnerable at the higher tension levels.

Figure 8.16 Weft tensions with increasing speed of rapier loom.

Effects of loom settings and yarn characteristics on weft tension

195

Lunenschloss and Schlichter [17] had observed that the weft tension in rapier weaving machines increases by 70% with the increase in machine speed from 250 to 400 picks/min, as indicated in Fig. 8.16. The increase in peak tension is, however, not regular at the higher speeds probably due to machine and rapier vibrations caused by irregular running of the machine at these speeds. Increase in the operating speeds of the weaving machines increases the insertion tension of the weft yarn because of corresponding increase in velocity of the giver rapier at the instant it picks up the weft for insertion. Similar observations of increase in weft tension as a result of increase in operating speed of the rapier weaving machine have been made with different hardness of the brush tensioner fitted on a given type of weft feeder [27]. The results are shown in Fig. 8.17.

Figure 8.17 Weft tensions with increasing rapier loom speed with different brush tensioners. Although the weft insertion tension increases almost linearly with insertion speed with any type of tensioner (Fig. 8.17), the hardness of the bristles of the tensioners show their effects on the tension levels. As can be anticipated, the weft tension is consistently higher with harder bristles. The increased weft tension is likely to affect the weaving efficiency at higher weaving speeds, because of resultant increase in weft break. The efforts should therefore be made to reduce the weft tension. It has been suggested [26] that the reduction in weft tension can be achieved by (i) optimum adjustment of the machine and the weft accumulator and (ii) modifications of constructional designs of the machine and the accumulator. In respect to the former, it has been proposed that positioning of the accumulators, like placing one above the other, and as we can see from equation 8.4, the dimensions

196

Role of yarn tension in weaving

of the accumulator which is determined by the diameter of the accumulator drum should be considered. In regard to the constructional design, it has been proposed that, the time of presentation of the weft to the rapier clamp should be earlier to reduce the inertia force and at the time of presentation, the weft should be under taut condition. If the weft remains taut at presentation it will be subjected to acceleration with the rapier speed at the instant of pick-up and not later, when the speed of the rapier is high. Moreover, smaller angles of deflection of the weft through the guides to reduce friction, opening of the weaving machine tensioner at the instant of pick-up of the weft by the giver rapier to reduce the acceleration tension peak, proximity of the weft entryguide of the weaving machine to the weft pick-up point of the giver rapier to reduce the mass and thereby, the acceleration force of the weft, etc., should also be maintained. Reduction of weft tension during insertion, particularly at the acceleration phase, will permit higher machine speeds or conversely, the use of weaker weft yarn without sacrificing the desired weaving efficiency.

Figure 8.18 Weft tensions with increasing speed of air-jet loom. In air-jet weaving machine also, insertion tension of weft increases with the increase in speed of the machine because of higher yarn acceleration. Figure 8.18 shows the tensions of two rotor-spun cotton weft yarns of 20 tex and 60 tex as function of machine speed [28]. From Fig. 8.18 we see that the weft tension increases with the increase in insertion speed and also the linear density of the yarn. Load is higher on the yarn with higher mass which therefore, is subjected to higher tractive force during insertion by air-jet and thus, experiences higher tension at insertion. Blanchonette [8] had studied the weft tension of a 25 tex worsted wool yarn during picking in three shuttleless weaving machines, projectile, rapier

Effects of loom settings and yarn characteristics on weft tension

197

(flexible), and air-jet, running at different speeds and the findings are shown in Table 8.1. Table 8.1

Weft tension at different looms running at different speeds

Weft insertion type

Loom speed, picks/min

Weft insertion rate, m/min

Maximum tension change, N/s

Rapier (flexible)

370

625

60 – 120

Projectile

400

648

350 – 700

Air-jet

600

1038

700 – 1000

It is observed that even with different types of weft insertion system the rate of change of maximum tension of a given weft yarn increases with the loom speed (and also, the weft insertion rate). Comparison of weft tensions observed in Table 8.1 with the three weaving machines indicates that the lowest range of tension change of 60 – 120 N/s is obtained with the rapier loom running at the lowest speed and the highest range of tension change of 700 – 1000 N/s is obtained with the air-jet loom running at the highest speed. The lowest value as well as the range of variation of weft tension with the rapier picking system evinces the gentler action on the weft yarn by the rapiers during insertion. However, as discussed above in Sections 8.5.2 and 8.5.3, the selfcompensating yarn tension system or braking units such as, Flex Brake, Coaxial Tensioner (CAT), etc., can obviate, to some extent, the possibilities of increase in weft tension caused by the increase in working speed of the weaving machines.

8.8

Effects of weft yarn characteristics

Yarn characteristics in terms of linear density, compactness, surface structure, type of constituent fiber, etc., of the weft yarn play important roles in the behaviour of weft tension during insertion more in shuttleless picking systems than in shuttle picking system. In the case of the latter, the shuttle simply lays the weft ejecting through an eye much larger in size than the diameter of the yarn during picking and the yarn characteristics therefore, have hardly any discernable effects on weft tension during insertion. In any shuttleless picking system in contrast, the weft is dragged during picking and its insertion tension is, therefore, affected by the axial velocity and the characteristics of the yarn. Again, amongst the different shuttleless picking systems, as air is the weft insertion element in air-jet picking, the effects of the yarn characteristics are naturally supposed to be more pronounced in this than in the others.

198

Role of yarn tension in weaving

Axial velocity of the weft yarn caused by dragging of the yarn by the weft insertion element, whether tangible like projectile and rapier or intangible like air, imposes strain and thus, increases tension of the yarn. The insertion tension of a weft yarn is, therefore, supposed to be affected by its linear density and the type of weft insertion element employed, besides the insertion speed. It has been found in cases of projectile [15, 17] and rapier [17] that, the tension of a given type of yarn increases with the yarn tex. Figures 8.3 A and 8.19 A show the tensions of different counts of weft yarn with the projectile and rapier weaving machines, respectively.

Figure 8.19 Weft tensions with different weft counts.

Effects of loom settings and yarn characteristics on weft tension

199

In both the cases, the weft tension increases with yarn tex. The rise in tension is because, the greater mass of the coarser yarn experiences greater frictional resistance when being dragged by the projectile or rapier. In case of air-jet picking, however, the situation is different, as has been discussed later. If we make the direct comparison between the yarn count and yarn tension like in Fig 8.19A, the actual picture of loading on the yarn may remain concealed to some extents as, the capability of a yarn to bear the amount of load imposed on it is dependent primarily on its linear density. It has, therefore, been suggested [28] that if we consider the specific or count related loading on the yarn, which is the ratio of loading and the count of the yarn (cN/tex), it is independent of the yarn characteristics and insertion system and comparable with the yarn tension. Figure 8.19 B indicates the count related loading on the weft yarn as the function of yarn count observed on projectile weaving machine [17]. It is now observed from Fig 8.19 B that the fine weft yarns with lower tenacity are stressed more. The practical significance of this is that, assuming the same insertion rates and the same yarn quality, finer a yarn is more heavily it is loaded relative to its natural resistance properties and is therefore more likely to fail under a given tension. As the loading on the weft yarn during its insertion is primarily determined by the insertion speed, stresses during insertion differ between spun and filament yarns and also amongst the different types of spun yarn.

Figure 8.20 Weft tensions with different weft types.

200

Role of yarn tension in weaving

Ivitz [15] has shown significant differences in weft insertion tensions amongst a raw open end spun yarn, a coloured open end spun yarn and a multifilament yarn, each of 37 tex, in a projectile loom operated with and without weft accumulator. The findings are shown in Fig. 8.20. Diagrams marked A are the results without accumulators and those marked B are with accumulators. While the multifilament yarn has the lowest tension, the coloured open end spun yarn has registered the highest tension either with or without the accumulator, although the tension in each case is higher when the accumulator has not been used, as is expected. Working with polyester/wool spun and textured filament yarns on projectile loom Chahal and Mohamed [9] have observed that, whether the weft accumulator is used or not, the overall weft tension is always higher for the textured yarn during insertion. The effect is also more pronounced at higher setting of the weft brake which, as stated above, increases the weft tension. When both the yarns are fed to the picking area directly from their supply packages, that is without accumulator, weft tension is higher for spun yarn but, when they are fed through the weft accumulators the tension is higher for the textured yarn. The stretchability of the textured yarn is much higher than that of the spun yarn and hence, when it is fed directly from the package the excess stretch helps in lowering the tension. When the accumulators are used, the yarn is first unwound from the packages and then wound on the accumulator drums before being fed to the loom for picking. In the process, the textured yarn loses some of its stretch, which causes rise in tension during insertion. In case of the spun weft yarn, higher insertion tension when fed directly from the package is mainly because of the frictional effects. When the accumulators are used the tension reduces because of the beneficial effects rendered by the accumulators, as discussed earlier. Irrespective of the mode of feeding that is, whether through the weft accumulators or direct, weft tension at checking of the projectile is always higher for both types of yarn, with the peak tension more pronounced for the textured yarn than for the spun yarn. High weft tension at checking is because of the braking action along with the movement of the compensator, as discussed in Section 6.3.1 (Chapter 6). In regard to air-jet picking, dynamometric properties of the weft yarn are extremely important with respect to the resistance offered by the yarn to the loads imposed on it during insertion. In this context yarn strength and elongation are very significant. Assuming comparable yarn qualities, properties of viscose and modal staple fiber open end spun yarns permit higher insertion rates than the cotton rotor spun yarns in air-jet picking system [20]. Wulfhorst and de Weldige [30] have observed that the weft yarn with higher tension can be inserted more quickly by air-jet picking. According to them, while the method adopted for spinning the yarn influences the weft insertion behaviour, hairiness, and twist of the yarn have only little and the diameter of the yarn has no influence on the yarn tension. These findings [30]

Effects of loom settings and yarn characteristics on weft tension

201

are somewhat surprising as the surface characteristics of the weft yarn affect the drag force of air, which in turn, influences the weft insertion tension, as observed by Adanur and Qi [4]. In the course of analyzing the various properties of a 3/1 left-hand denim twill fabrics (see Chapter 10) with 65/35 polyester/cotton warp yarn of 37 tex and different types of weft yarn in an air-jet weaving machine, Adanur and Qi [4] have studied the effects of different parameters of the open end and ring spun cotton weft yarns on the weft tensions also. The air pressure maintained for weft insertion is 5.5 bar (80 psi). It has been observed in this study that, for any type of weft yarn or for any given yarn parameter the maximum tension and the fluctuations of tension of a single pick during insertion are always higher than when the tensions of a large number of picks are considered (see Section 6.3.3, Chapter 6). The average tensions of 35 picks are, therefore, shown in Figs. 8.21 to 8.25.

Figure 8.21 Effect of weft count on weft tension. Weft insertion tension depends on weft count and in general, the average weft tension increases as the weft becomes finer (from 41.4 to 25.7 tex), Fig. 8.21, with however, a slight dip in tension at the initial stage. This decrease in weft tension is probably because the yarns in this region have comparatively higher twists and are therefore smoother on the surfaces.

Figure 8.22 Effect of twist multiplier on weft tension.

202

Role of yarn tension in weaving

Application of twist in forming a yarn binds its constituent fibers together to make the yarn compact and strong. As the twist is increased, the fibers are bound more tightly and as a result, the yarn becomes more compact, its diameter reduces and its surface becomes smoother. In air-jet picking, a pick is inserted by the friction between the yarn surface and the air. As the yarn becomes smoother and its surface area reduces with the increase of twist, the weft tension reduces during insertion, as indicated in Fig. 8.22, because of consequent decrease in propulsive force.

Figure 8.23 Effect of hairiness on weft tension. Increase in hairiness of the yarn increases the frictional resistance and this causes increase in weft insertion tension. Figure 8.23, thus, shows increase in maximum weft tension with hairiness.

Figure 8.24 Effect of yarn elongation on weft tension. In respect of elongation of the weft yarn, when the yarn is subjected to a drag force it first stretches out before being tensioned. Therefore, the weft yarns with higher elongation property experience lower insertion tension, Fig. 8.24.

Effects of loom settings and yarn characteristics on weft tension

203

We know that, when the jet of air flows past the weft surface beyond the nozzle, friction between the air, and the weft produces the drag force on the weft and this carries the weft forward. This drag force that carries the weft depends on the yarn mass, coefficient of friction between the air and the yarn and the relative velocity between the air and yarn. If the drag force is F then, F = 0.5 ρ µ π d l (vj – vy ) 2

(8.11)

where, ρ is the air density, µ is the coefficient of friction of the yarn surface, d is the diameter of the yarn, l is the inserted length of the yarn, vj is the jet velocity, and vy is the yarn velocity. From the equation 8.11 we find that, to maintain the same drag force for a given yarn and air density, the higher the relative velocity between air and yarn (vj – vy ), the smaller the coefficient of friction between the two and this in turn, causes a decrease in weft tension, as shown in Fig. 8.25.

Figure 8.25 Relations amongst µ, (vj–vy) and weft tension.

204

Role of yarn tension in weaving

Many technical and technological problems associated with shuttle picking system, particularly pertaining to the speed of the loom, have given birth to the various types of shuttleless picking systems. The speeds of these shuttleless weaving machines have reached the levels not possible to be attained by even the fastest shuttle loom. Owing to the principles of weft insertion, the modern high speed shuttleless weaving machines impose high strain on the weft yarns and higher the speed higher is the strain. Concurrently, various nonconventional spinning techniques have also been evolved which produce the yarns with vastly different structural and physical characteristics than those of the conventional spun yarns. All these non-conventional spun yarns are often not equally suitable in all types of shuttleless weft insertion systems because of the magnitudes of strain they experience during insertion when being used as wefts. Judicious measures should, therefore, be taken while developing the weaving machines so that neither the operating speed nor the novelty of the yarns poses a problem in increasing the speed and thereby, the productivity of the modern weaving machines.

References 1.

Adanur S. and Mohamed M.H., (1991). Analysis of yarn tension in air-jet filling insertion, Textile Research Journal, Volume 61, pp. 259–266.

2.

Adanur S. and Mohamed M.H., (1992). Analysis of yarn motion in single-nozzle air-jet filling insertion Part II: Experimental validation of the theoretical models and statistical analysis, The Journal of the Textile Institute, Volume 83, pp. 56–68.

3.

Adanur S. and Mohamed M.H., (1992). Analysis of yarn motion in single-nozzle air-jet filling insertion Part I: Theoretical models for yarn motion, The Journal of the Textile Institute, Volume 83, pp. 45–55.

4.

Adanur S. and Qi J., (2008). Property analysis of denim fabrics made on air-jet weaving machine, Part II: Effects of tension on fabric properties, Textile Research Journal, Volume 78, pp. 10–20.

5.

Anon (1960). Weaving with the gripper shuttle loom – part II, Textile Recorder, Volume 77, pp. 46–48.

6.

Anon, (1991). Reduction of the maximum weft yarn tension during weft insertion on air-jet weaving machines, Melliand Textilberichte, Volume 72, pp. 732–735.

7.

Bertucci G., and Scholze G., (1990). Research on air jet weaving machines, Melliand Textilberichte, Volume 71, pp. 964–967.

Effects of loom settings and yarn characteristics on weft tension

205

8.

Blanchonette I., (1996). Tension measurements in weaving of singles worsted wool yarns, Textile Research Journal, Volume 66, pp. 323– 328.

9.

Chahal V. and Mohamed M.H., (1986). Measuring filling yarn tension and its influence on fabrics woven on a projectile weaving machine, Textile Research Journal, Volume 56, pp. 324–333.

10. Duxbury V., (1962). Air jet weft insertion, Modern development in weaving machinery, Edited by V. Duxbury and G.R. Wray, Columbine Press, Manchester & London, pp. 115–116. 11. Dawson R.M., Georgiadis N., Jelveh-Moghaddam A., and Songelaeli K.W., (1996). Filling velocity changes at tip transfer on rapier looms: a simple analysis, Textile Research Journal, Volume 66, pp. 739–746. 12. Foster R., (1959). Tension variations occurring during the unwinding of cops and pirns, The Journal of the Textile Institute, Volume 50, pp. 7–35. 13. Greenwood K. and Vaughan G.N., (1958). Weft tension during weaving, The Journal of the Textile Institute, Volume 49, pp. T247–T264. 14. Hubner M. and Frenzel I., (1995). Yarn tension investigations on weft accumulators, International Textile Bulletin, Volume 41, pp. 57–66. 15. Ivitz R., (1979). Features of the weft draw-off from cross-wound bobbins for projectile weaving machines, Melliand Textilberichte, Volume 60, pp. 843–848. 16. Kohlhass O., (1979). Investigation on weft storage systems, Melliand Textilberichte, Volume 60, pp. 920–923. 17. Lunenschloss J. and Schlichter S., (1987). Stress on weft and warp yarns in terms of the weft insertion speed and other weaving parameters, Melliand Textilberichte, Volume 68, pp. 93–98. 18. Mcmahon J.F., (1986). Weft tension during double-rapier weft insertion in weaving, The Journal of the Textile Institute, Volume 77, pp. 356–358. 19. Ormerod A., (1962). The theory and practice of operating Sulzer shuttleless weaving machines, Modern developments in weaving machinery, Edited by V. Duxbury and G.R. Wray, Columbine Press, Manchester & London, pp. 92–93. 20. Pfister H., Weissenberger W. and Frick E., (1989). Aspects of weft insertion of viscose and modal staple fibre yarns on air-jet weaving machines, Chemiefasern/Textilindustrie, Volume 39/91, pp. 1194– 1198.

206

Role of yarn tension in weaving

21. Talavasek O. and Svaty V., (1981), Shuttleless weaving machines, Elsevier Scientific Publishing Co., Amsterdam, Oxford, New York, p. 245 22. Thomas I.H., (1957). Behaviour of weft during unwinding from a shuttle, The Textile manufacturer, April, pp. 163–166. 23. Townsend M.W.H., (1955). Weft tension in weaving, The Journal of the Textile Institute, Volume 46, pp. 699–712 24. Turton G., (1954). Weaving faults: avoiding defects in filament rayon, The Textile Weekly, Volume 53, pp. 904–908. 25. Weinsdorfer H., Lange A., and Scholze U., (1990). Weft thread tensions on a rapier weaving machine, Melliand Textilberichte, Volume 71, pp. 432–438. 26. Weinsdorfer H., Lange A. and Scholze U., (1990). Weft thread tensions on a rapier weaving machine, Melliand Textilberichte, Volume 71, pp. 507–512. 27. Weinsdorfer H, Lange A and Scholze U., (1990). The rise in weft yarn stress with further increase in production rate, Melliand Textilberichte, Volume 71, pp. 760–763. 28. Weissenberger W. and Frick E., (1989). Yarn stress and high performance weaving, Textile Month, December, pp. 60–63. 29. Whitefield T., (1992). Eliminating weft tension problems, Textile Month, January, pp. 38, 39. 30. Wulfhorst B. and De Weldige E., (1995). Novel test method for predicting weft insertion behaviour in air-jet, Melliand Textilberichte, Volume 76, pp. 830–831.

9 Effects of yarn tensions on loom performance

Abstract: Weaving operation entails strain on both the warp and weft yarns, albeit the magnitude is higher on the warp, and the yarn strain increases with the increase in speeds of the weaving machines. Rapid cyclic loading on the warp yarns due to shedding and sudden thrust on the fell of the cloth due to beating impart high load on the warp yarns and make them vulnerable to breakage. Proper shed timing and shed type, effective temple system, proper warp let-off tension, etc. can reduce the chances of warp breakage. Similarly, correct fitting of the pirns in the shuttles in shuttle looms, proper picking and checking of the picks and use of weft accumulator in shuttleless looms can reduce the strain and hence, the chances of weft breakage. Keywords: warp break; warp tension; shedding; let-off; temple; beat-up; loom speed; relative humidity; weft break; shuttle picking; shuttleless picking

9.1

Introduction

In the preceding chapters the various aspects of yarn tensions have been discussed along with how the different settings of the looms, with shuttle and without, affect the weaving tensions of the warp and weft yarns and how they can be controlled or regulated to the benefit of loom operation. During operation of any type of weaving machine, tension is imposed on the warp and weft yarns and if a yarn is strained beyond its capability to withstand it, it breaks resulting loss in weaving efficiency. Yarn tensions at weaving, therefore, have distinct effects on the operating performances of the looms and the effects are product, yarn and loom dependent. Indeed, it is true that between the warp and weft, the warp yarns are more subjected to strain than the weft yarns and hence the chances of warp break and, thereby, the loss in efficiency due to warp break are much higher in both the shuttle and shuttleless looms. In regard to the weft on the other hand, any shuttleless picking system strains the yarn much more than the shuttle picking and this as a consequence causes weft breakage.

208

9.2

Role of yarn tension in weaving

Loom performance

The process of formation of a cloth by interlacement between the warp and weft yarns in weaving imposes strain on both sets of yarn and this generally reduces the yarn strength. The reduction in strength and thereby, the breakages of the yarns are supposed to be more at higher tensions. Performance of a loom therefore depends on the behaviour of its warp and weft yarns during the process of weaving. In regard to loss in strength of the warp yarns due to weaving, Blanchonette [5] has observed that although prediction of warp breakage is very complicated by cyclic tensioning and yarn abrasion, weaving without weft insertion has shown 10% decrease in average breaking strength of the yarns and the weaker sections of the yarn may be only half as strong as a result of weaving process. He has also observed during the trial on a rapier weaving machine running at 370 picks/min that, about 95% of weft breaks occur at very thin and inextensible places in the yarn while only 15% of warp breaks occur owing to these. Amongst the various causes for loss in weaving productivity short stoppages which include yarn break and warp changes account for more than 90% of down time in high performance weaving machines and between the two, the losses in production caused by short stoppages are usually the larger [18]. In any type of weaving machine, chances of warp break are much higher than those of weft break because of much higher strain imposed on the warp yarns and the ratio of warp stoppage to weft stoppage is 60:25% [15]. In regard to the warp yarns, the free length of the yarn sheet particularly between the back-rest and the fell of the cloth is subjected to rapid and repeated stretching and relaxation due to shed formation and beating, as discussed earlier, and this weakens the yarns. At the fell, as the warp yarns are interlaced with the wefts to form the fabric, they are transformed from the yarn state to the fabric state and the deleterious effects of shedding and beating on them are minimized considerably. Stretching and relaxation as well as abrasion on the warp yarns against their different controlling parts of the loom and also amongst themselves have fatiguing effects on them. Yarn breaks at its weakest place and any weak place or point of low extensibility in the yarn will naturally be further weakened and diminished in extensibility by these actions. The adverse effects on the yarns commence at the start of the back shed near the back-rest and continue till the yarns reach the cloth fell with the effects being the maximum at the healds. Each small section of the yarn is subjected to sudden tensions and extensions a great many times before it passes into the cloth. Therefore, quicker the yarns pass through the zone between the warp beam and the cloth fell lesser the duration of torture on them. One of the possibilities of this is the lower weft density that is, greater pick spacing of the cloth. In case of weft yarns, the situation is not that alarming as for the warp yarns. If the weft is inserted by shuttle, the weft is simply unwound from its pirn and

Effects of yarn tensions on loom performance

209

there is not much strain on it to cause it to break due to tension, under the normal situation. If the weft is inserted by other means like, projectile, rapier or air-jet, fairly high load is imposed on it as it is dragged by the insertion element and unless this is properly controlled, weft breakage may occur.

9.2.1 Warp break Greater or lower tension of the warp yarns than what is necessary augments the likelihood of warp breakage by straining the yarns more or by not allowing clear formation of the warp shed and thus affect the weaving performance, particularly in shuttleless weaving machines. Mean tension is usually not the cause for warp breakage. Repeated extensions and relaxations and friction on the yarns during weaving have deleterious fatiguing effects on the yarns. In addition, the highest peak tension and random variations in tension caused by improper knot, entanglement of warp yarns, etc. are also often the cause for warp break.

9.2.1.1 Warp tension The property of a yarn that plays the most vital role in weaving is the extensibility. Application of tension causes the yarn to stretch and affects its extensibility. It has been observed that, increase in warp stretch by 2.73% increases the loss in its breaking elongation by 23%, warp break by 39.2% and decreases the weaving efficiency by about 7.9% [1]. Naturally, a yarn will break when it is subjected to a load greater than its breaking strength and the warp tension therefore has direct effects on warp breakage. The breakage is found to be consistently and significantly lower at the normal level of warp tension set for weaving a given type of fabric than that at the higher or lower level [13]. There is, therefore, an optimum tension at which there is a minimum end breakage. Higher warp tension will unnecessarily strain the warp yarns more and lower tension will cause excessive buckling of the cloth at beat-up, as stated earlier, and both will augment the possibilities of warp breakage. High tension on the warp yarn may be owing to the improper settings of the loom, the hindrance on the passage of the yarn caused by knot, entanglement, etc. or high friction between the yarn and the machine. Although the main cause for warp break is generally accepted to be the amount of load applied on it, as stated above, the weakness of the yarn is also an important reason for breakage. The weakness is the consequence of damage suffered by the yarns during weaving. Lord [10] has shown the distinct change in load-elongation behaviour of the warp yarns caused by weaving, as indicated in Fig. 9.1.

210

Role of yarn tension in weaving

Figure 9.1 Effect of weaving on load-elongation behaviour of warp yarn. Fig. 9.1 indicates the hysteresis diagrams of the warp yarns before and after weaving. The yarn becomes much less stiff with the hysteresis loop much flatter after weaving than before weaving. This is because, the component fibres of the yarn are now more free to slip, which in turn indicates that the reinforcing and binding effects of the size are greatly reduced due to weaving.

9.2.1.2 Shedding We know that, the warp yarns are subjected to severe loading during shedding operation because of rapid stretching and relaxation and abrasion of the yarns. As a result of these, the strength of the warp yarn is reduced during weaving. In weaving fibrous yarns, the effect of yarn loading is more due to rubbing of the yarns against each other during shed change rather than from the tensions [17]. The extent of shed formation, that is, the depth of shed, depends largely on the dimensions of the weft insertion element employed and higher the depth of shed higher the strain on the yarns and thereby, the chances of warp breakage. The depth of shed in a shuttleless loom is generally much smaller than that of a shuttle loom and this helps reduce the likelihood of warp breakage. However, for proper formation of the small shed in a shuttleless loom the warp tension is needed to be set higher and the shuttleless looms operate at comparatively higher speeds than the shuttle looms, which again augment the chances of warp breakage. To combat these, the warp beams should be prepared properly with all the yarns wound under the same uniform tension. Besides the depth of shed, movements of the warp yarns during shedding also impose fair amount of strain on the yarns and the consequent repeated

Effects of yarn tensions on loom performance

211

stretching and relaxation of the yarns in quick succession make them susceptible to break. Therefore, utmost care should be taken to not strain the yarns unduly during shedding. It has been observed that by incorporating fully positive tappet shedding mechanism in place of the conventional negative tappet shedding system [11] or employing cicloidal shedding cams in place of the conventional simple harmonic cams [4], much improved natures of warp tension variation during shedding can be obtained, as discussed earlier in Section 7.5.2 of Chapter 7. These improved natures of warp tension variation with modified shedding mechanism and the shedding cams reduce the warp breakages by about 38 and 8%, respectively, as they can minimize the undue strain of the warp yarns during shedding.

9.2.1.3 Shed timing and shed configuration It has been stated in Section 7.3.1 of Chapter 7 that, if the back-rest is set at higher level (indicated by firm lines in Fig. 7.4 of Chapter 7), the warp yarns forming the bottom shed are stretched and, therefore, strained more than those forming the top shed (Fig. 7.5 of Chapter 7). If beat-up now occurs with the warp yarns positioned in this manner, as is the case with early timing of shedding motion, the sudden strong thrust during beat-up will be shared more by the yarns tensioned more that is, those forming the bottom shed with the result that, these yarns will be more susceptible to breakage. On the contrary, if the shed timing is so arranged that at the instant of beat-up all the yarns are at the same level, as is the case with late timing of shedding motion, the beatup force will be shared equally by all the yarns even though the back-rest is at higher level and the chances of warp breakage will be much less. Again, if the back-rest is set at normal position (indicated by broken lines in Fig. 7.4), the yarns of both the top and bottom shed lines will experience nearly the similar tensions at any given time in a pick cycle and all the yarns will therefore share the beat-up force almost equally, no matter what the timing of shedding motion is. This will minimize the chances of warp breakage. So, if we compare the two situations shown in Fig. 7.20 of Chapter 7 we observe that, while with late shed timing the warp tension at beat-up is the same for all the warp yarns irrespective of the position of the back-rest, with early shed timing the same is the case irrespective of the extent of opening of the warp shed at the time of beat-up provided of course, the shed is symmetric. The warp yarns are, however, rubbed much more in early shedding than when the timing of the shed change is late. Main advantage with these shed settings is the reduced chances of warp breakage, as the sudden intense thrust at beat-up imposed by the reed on the warp yarns is equally shared by all the yarns. From these, we thus find that the raised position of the back-rest along with early shed timing aggravates the chances of warp breakage because of unequal share of the beatup force by the warp yarns. If, however, the back-rest is allowed to oscillate

212

Role of yarn tension in weaving

in accordance with the movements of the warp yarns during shedding, like with spring-loaded back-rest, the warp break can be expected to reduce. A study with jute cloth of plain weave construction has shown that employment of spring-loaded back-rest with early shedding has minimized the warp break by 34–47% [3] because of significant reduction in warp tension variation (see Section 7.3.2 of Chapter 7). Increased likelihood of warp breakage with early shedding coupled with raised position of back-rest is true for the weaves in which the float length of the warp yarn is the same on both the face and back of the cloths like, in plain, matt, etc. If the weave is such that the yarns have longer float lengths on the face of the cloth, much less number of yarns will then be under high tension at beat-up and the chances of warp breakage will be even more. Again, with the fabric with longer float lengths, there will be less number of warp yarns at the crossing points and hence, they will be subjected to less strain at shed crossing. Therefore, whether the warp breakage will be more or not in case of a fabric with longer floats will perhaps depend on its warp density and the type of weave. In regard to the effects of shed timings, Lord [9] has observed that the early shed timing imparts low warp tension but the extent of abrasion on the yarn is very high. The shed timings between 305° and 310° (with beat-up at 0°) cause the minimum abrasion and change in yarn extensibility. Any timing later than 350° or earlier than 270° causes sharp rise in yarn extensibility. This is possibly because of the cloth fell distance, which determines the extent of movement of the picks against the abrasion of the warp ends during beat-up.

9.2.1.4 Let-off motion With the progress of weaving the warp tension tends to increase with weaving out of the warp beam. Unless the rise in tension is properly controlled this will give rise to warp breakage and the breakage will be more as the beam becomes nearly empty with its diameter much reduced, because of exponential rise in warp tension, Fig. 7.1 of Chapter 7. In negative let-off motion, the tension control is effected by gradual shifting of the dead weights towards the fulcra of the weight levers so as to allow the beam to rotate more freely. In case of positive let-off motion, the back-rest mechanism initiates the control, but the rises in warp tension during shedding and beat-up cannot cause the back-rest to respond immediately to compensate for the amount of warp pulled forward due to the inertia of the back-rest and the warp yarns are therefore unduly stretched [16]. This enhances the chances of warp breakage.

9.2.1.5 Temple Another important factor which determines the warp breakage is yarn crimp. Interlacement between the warp and weft yarns causes crimp in both sets

Effects of yarn tensions on loom performance

213

of the yarn, owing to which the cloth contracts both warp and weft ways. Crimp in the weft yarns tends to bring the warp ends closer together and the ends at the selvedges suffer high abrasion against the reed wires during the reciprocating movement of the reed. Unless the fell of the cloth is stretched to the full dented width of the warp sheet at the reed by the temples, there are considerably high tension fluctuations of the selvedge ends (see Fig. 7.34 of Chapter 7) and the chances of warp breakage at the selvedges aggravate. Effects of different types of temple on warp tensions and superiority of the full-width temple over the ring temple in controlling warp tension have been discussed in Section 7.9 of Chapter 7. Greater uniformity in warp tension along the entire width of the warp with full-width temple imposes uniform load on all warp ends during weaving which as a consequence, is expected to reduce the chances of warp breakage. Correlation between the warp tension distribution across the cloth widths with ring and full-width temples and the consequent warp breakages with them has been shown in Fig. 9.2 [12].

Figure 9.2 Warp breakages with ring and full width temples.

214

Role of yarn tension in weaving

In Fig 9.2, the optimum working range of warp tension for satisfactory weaving performance with regard to warp breakage for a given type of yarn has been indicated by the area ABCD for both types of temple. In the areas where the warp tensions depart from the optimum working range, which is beyond ABCD, significant changes in warp breakage rate occur. In case of ring temples, Fig 9.2A, the warp tensions at the centre as well as at the selvedges of the cloth are well beyond the optimum range and hence the warp breakages are fairly high at these regions. In case of full-width temple, Fig 9.2B, the warp tension distribution over the greater part of the cloth including the middle portion is within this optimum range and hence, the overall warp breakages are comparatively much less with the benefit of 1–1.5% increase in weaving efficiency observed with rapier looms [12]. Thus, the full-width temple helps to reduce the warp breakage by ensuring more uniform warp tension and thereby, the dynamic load on the warp yarns across the width of the warp sheet. At the selvedges, however, the tension with full-width temple falls below the range and this, therefore, leads to an increase in the breakage rate. Since the overall warp tension distribution is more uniform and the warp breakages are less with the full-width temple, a slight increase of the overall level of warp tension can be considered for this type of temple for reducing the warp breakages at the selvedges. Any increase in the yarn breakage at the centre of the warp as a result of increase in warp tension can be offset if significant reduction in breakages at the selvedge areas can be achieved. For these reasons full-width temple appears to be more beneficial in high speed shuttleless weaving machines.

9.2.1.6 Beat-up In the course of processing of a yarn, its breaking modulus rises with the amount of work it receives, as the fatigue plays an important part in the process. One of the two loom functions that cause significant rise in warp tension in a pick cycle is the beat-up. Beat-up action alone has deleterious effect on warp breakage. If the warp yarns are processed in a loom without actually beating the wefts to the fell of the cloth, the warp yarns have been found to suffer 36% reduction in breaking load as against 42% reduction in breaking load of the woven yarns when the picks are beaten [10]. Considerably high beat-up tension is likely to show its effect on warp breakage. Although the beat-up force is not the main parameter which influences the warp breakage rate, the warp break increases with the beat-up force. Beat-up factor multiplied by the loading/unloading cycle influences the warp breakage rate to a considerable extent [7]. Therefore, if the cloth fell distance (Y in Fig. 7.26 of Chapter 7) can be reduced, the beat-up force will reduce, and this in turn, will reduce the warp breakage. Beat-up tension also depends on the elastic constant of the warp yarns, which again depends on the free length of the yarn from its point of let-off

Effects of yarn tensions on loom performance

215

from the warp beam to the fell of the cloth [2]. An increase in the elastic constant of the warp yarns increases the beat-up tension. It also causes the front part of the shed to be tensioned more than the back, the result of which is supposed to increase the warp breakage. This adverse effect can be reduced to some extent, by so timing the loom that the shed angle at the moment of beatup is small and thereby, the intensity of the beat-up thrust on the warp is low.

9.2.1.7 Warp length Lengths of the warp sheet and the cloth have also been found to affect the warp breakage [8]. Warp breakage decreases with the decrease in the length of warp sheet and increase in the length of the cloth on the loom. This is because, shorter length of warp becomes less fatigue as it is then subjected to correspondingly less number of repeated stretching and relaxation due to shedding and beating and less mechanical actions on the loom. This may, however, be a matter of argument as, with a given lift and frequency of movement of the warp yarn, longer free length of warp is possibly more capable of withstanding the rigorous treatment meted out to the warp, as discussed earlier.

9.2.1.8 Loom speed Increase in loom speed increases the reciprocating movement of the warp yarns. This imposes greater load and causes distinct increase in strength loss of the yarns. It has been found that the reduction in strength is more significant in the compact polyester/cotton yarn that in the pure cotton yarn [17].

9.2.1.9 Relative humidity Relative humidity of the weaving shed has an important role in weaving behaviour of the warp yarns. Depending on the type of yarn there is an optimum relative humidity for achieving the desired weaving performance of the loom. Relative humidity of 78–80% is considered normal for weaving in most cases. Lower relative humidity than this causes increase in warp tension and thereby, the warp breakage [13].

9.2.2 Weft break 9.2.2.1 Shuttle picking It has been discussed in Section 6.2.2 and shown in Fig. 6.4 of Chapter 6 that because of non-symmetric location of the shuttle eye, the weft unwinding tension is not equal in each pick. If the eye is at the right end of the shuttle, the weft tension is higher when the shuttle flies to the right hand box. Weft break increases with weft tension. A study [14] on a loom running at about

216

Role of yarn tension in weaving

206 picks/min has therefore, shown that with this type of shuttle and 9.1 tex weft yarn, larger number of weft break to the extent of 58% of the total break has occurred when the shuttle traverses to the right hand box. It has also been observed that the weft break varies along the length of the pirn because of corresponding variations in weft tension. Most of the weft break occurs at the start of unwinding and when unwinding takes place from the centre layers of the pirn. Weft break in shuttle loom may also occur due to undue strain imposed on it mainly because of improper fitting of the pirn in the shuttle or incorrect built of the pirn, when the yarn of the outer layers abrades against the shuttle walls. The weft may also break if there is a big knot, dirt, high irregularity in the yarn or sloughing off of the yarn, which cause abrupt high withdrawal tension of the weft. The reasons for these lie in improper preparation of the pirn. Another reason for significant increase in weft withdrawal tension as the pirn becomes empty is the pirn with straight base, but this in most cases, is not so high as to cause weft break. Even that much rise in tension can also be taken care off by using the pirn with conical base, as discussed earlier.

9.2.2.2 Shuttleless picking As pointed out above, compared to warp, the weft experiences much less strain even in shuttleless weaving machine as it is exposed to strain during a single passage across the warp shed. However, if the width of the cloth being woven is large, length of each pick is correspondingly greater and chances of weft break may be more, particularly depending on the type of weft insertion. Although the weft in any shuttleless picking system is generally fed through the weft accumulator, projectile and rapier weaving machines can operate with direct feeding of wefts from the weft packages, as discussed earlier. In such cases cones are normally used and their qualities of winding are very crucial for ensuring efficient performances of the machines. The angle and wind are critical and winding patterns of the packages can cause increase in weft breakage rate because of high tension rise and fluctuations. The weft breakage with a given wind of package also varies according to the yarn in use; a smooth and slippery yarn is susceptible to coil slippage and thus, causes frequent yarn breakage. It is well recognized that peak load on the weft increases with an increase in insertion speed because of higher yarn acceleration. Blanchonette [5] has observed that depending on weft insertion system and quality of the weft yarn, up to 0.05% of weft insertions may fail on high speed looms, as weft tension increases with the loom speed. When the weft is inserted in projectile picking system, it is subjected to friction against the weft guide, yarn compensator, etc. which in turn, enhances the chances of weft breakage if, in particular, the

Effects of yarn tensions on loom performance

217

weft is inserted directly from the package. Use of weft accumulator reduces the weft breakage. Because of the effect of coefficient of friction, Ivitz [6] has observed while working with 37 tex open end spun yarn on a projectile weaving machine that, the number of weft break is reduced considerably when the yarn is coloured. Weft insertion by rapier in tip transfer system exercises more gentle action on the weft yarn than the other weft yarn insertion systems and hence, the weft breakage is likely to be less in this than in the others. However, there are the chances of transfer failure which cause loom stoppage. The transfer failure is more likely to occur in the negative tip transfer system than in the positive one. In case of the loop transfer and loop insertion systems on the contrary, the weft insertion velocity is double the rapier velocity with the result that the weft tension is very high during weft insertion. The wefts in these systems are thus highly prone to breakage. In the loop transfer system, high weft velocity and therefore the tension is experienced in the first half only of each pick insertion and in case of the, loop insertion system (which inserts a double pick in each insertion by single rapier), the weft is subjected to very high strain during the entire insertion phase and not the withdrawal phase of the rapier. However, the loop transfer system is now obsolete and the use of loop insertion system is restricted to the weaving of sacks only in the jute industry and that too in small proportion of the total production of the jute sacking cloth. These do not, therefore, warrant any further discussion. It has been discussed in Section 6.3.3 of Chapter 6 that in case of air-jet picking system, the weft tension is the maximum at the completion of weft insertion when the stopper pin on the weft accumulator closes to stop further supply of yarn. If the tension of the weft is too high at this moment, the weft may break and/or jump back resulting in stoppage of the machine and creating fault in the fabric. In order to avoid this, the brake should slow down the yarn before the stopper pin actually closes. The relay nozzles and the suction nozzle at the off side of the machine help to keep the pick stretched at the end of the insertion and thus, avoid the problem of short picks. The most effective and pragmatic means of increasing the weaving productivity is to increase the operating speeds of the weaving machines. Increase in machine speed implies an increase in axial as well as transverse speeds of the warp and an increase in axial speed of the weft yarns, particularly in shuttleless picking systems for the latter. Increase in axial speed of the warp is because of quicker traverse of the yarns from the warp beam to the cloth fell, while the increase in transverse speed of the warp is because of faster shed formation. In case of the weft, the increase in speed of the shuttle loom may not however impose much higher strain, but that of a shuttleless loom does, as the weft is subjected to higher drag. These, as a consequence, impart greater loads on both the warp and weft yarns and increase the propensity of

218

Role of yarn tension in weaving

yarn breakage. It would be a matter of great regret if the benefits expected by the increase in operating speeds of the weaving machines are marred by the increase in yarn breakage. Various efforts are, therefore, made and the methods are adopted, as discussed earlier, to counter that problem.

References 1.

Ajgaonkar D.B., Talukdar M.K. and Wadekar V.R., (1982). Sizing materials methods machines, Textile Trade Press. Ahmedabad, India, pp. 389–390.

2.

Basu A.K., (1987). Effect of different let-off mechanisms on fabric formation and dimension of fabric in the loom, Textile Research Journal, Volume 57, pp. 379–386.

3.

Bhattacharyya P.K. (1995). Better back-rest for jute loom, The Indian Textile Journal, Volume 106, pp. 112–116.

4.

Bhattacharya S.K. and Das K., (2006). Suitability of cycloidal cam in shedding mechanism, Indian Journal of Fibre and Textile Research, Volume 31, pp. 465–466.

5.

Blanchonette I., (1996). Tension measurements in weaving of singles worsted wool yarns, Textile Research Journal, Volume 66, pp. 323– 328.

6.

Ivitz R., (1979). Features of the weft draw-off from cross-wound bobbins for projectile weaving machines, Melliand Textilberichte, Volume 60, pp. 843–848.

7.

Kalli F., (1990). Correlation analysis between fabric data and stoppage rate in rapier weaving machinery with reference to cotton fabrics, Melliand Textilberichte, Volume 71, pp. 35–37.

8.

Kulikova N.A., (1966). The influence of the relative warp sheet and fabric lengths on the loom on the warp tension and end breakage rate, Technology of the Textile Industry U.S.S.R., No 4, pp. 69–73.

9.

Lord P.R., (1966). Warp damage during the weaving process – Part II, Textile Recorder, Volume 84, pp. 59–60.

10. Lord P.R., (1966). Warp damage during the weaving process, Textile Recorder, Volume 83, pp. 56-67. 11. Neogi S.K., Bandyopadhyay A.K. and Banerjee N.C., (2000). Modified tappet shedding mechanism for improved performance of jute loom: Part II – Performance analysis of the mechanism, Indian Journal of Fibre and Textile Research, Volume 25, pp. 200–205.

Effects of yarn tensions on loom performance

219

12. Obolenski B. and Wulfhorst B., (1993). Influence of various temple systems on the running characteristics of weaving machines and on fabric quality, Melliand Textilberichte, Volume 74, pp. 25–29. 13. Ramaswamy B.R. and Paliwal M.C., (1965). Warp tension in weaving: Its effect on performance and quality, Proceedings 6th Technological Conference of ATIRA, BTRA & SITRA, pp. 87–95. 14. Redozubova E.P., (1961). Weft tension during weaving, Technology of the Textile Industry U.S.S.R., No 2, pp. 84–91. 15. Schlegel W and Glawe A., (1995). Thread tension analysis in weaving under mill conditions, Melliand Textilberichte, Volume 76, pp. 1077– 1081. 16. Snowden D.C., (1949). Some factors influencing the number of warp breakages in woolen and worsted weaving, The Journal of the Textile Institute, Volume 40, pp. 317-330. 17. Weinsdorfer H., (1994). Effects of shed formation on the loading of warp yarn, Indian Journal of Fibre and Textile Research, Volume 19, pp. 139–146. 18. Weissenberger W. and Frick E., (1989). Yarn stress and high performance weaving, Textile Month, December, pp. 60–63.

10 Effects of yarn tensions on fabric properties

Abstract: Formation of a cloth by weaving is not completed until beatup takes place. Beat-up action causes stuffing of the weft picks against the properly tensioned warp ends and in the process, imparts crimps in both sets of the yarn. Changes in weaving tensions of the yarns cause changes in the magnitudes of the yarns crimps which in turn, affect most of the properties of the cloths and also the prominency of some weaves. Besides the successful operation of the weaving machine and achieving the desired properties of the cloth, yarn tensions also play important roles in rectifying a number of cloth defects produced at weaving. Thus, while on one hand, the tensions of the warp and weft yarns during weaving ensure proper loom operation and desired properties of the woven cloth, on the other, judicious application and control of yarn tensions help overcome many of the cloth faults generally generated at weaving. Keywords: yarn tension; yarn interlacement; beat-up force; cloth properties; structural and dimensional properties; physical properties; comfort properties; weave prominence; cloth defects; starting mark; cloth relaxation; repping; reed mark; stitching; selvedge faults

10.1

Introduction

It is well known that the fundamental of weaving is to produce a cloth by interlacement of warp and weft yarns and the formation of the cloth is completed only after beat-up operation. Interlacement of two sets of yarn naturally imposes crimps on them, the magnitudes of which depend to a great extent, on the tensions at which they have been woven, besides other factors. As crimp is the main parameter in deciding many of the properties of a woven cloth, the various structural, dimensional, physical as well as comfort properties of a cloth are determined largely by the tensions of the warp and weft yarns maintained during the process of weaving. All these properties are however, not relevant in all types of cloth, but their features are significantly changed by the changes in weaving tensions of the yarns. Yarn tension also plays a significant role in creating weave prominence in some types of cloth.

222

Role of yarn tension in weaving

Moreover, a number of faults are often generated in the cloth during weaving and some of them are related to the warp and/or weft tensions. Such tensionrelated faults can therefore be rectified by prudent control of the warp and/or weft tensions during the course of weaving.

10.2

Mechanics of cloth formation in relation to yarn tensions

The woven cloth is formed as a pick of weft inserted in the warp shed, is beaten to the cloth fell and bound by the warp ends crossing over it. The structural stability of a cloth is however, not attained till the woven cloth is taken off the loom and allowed to relax for quite some time so that the warps and wefts assume their desired wavy configurations, that is, crimps. The waviness or crimps of the two sets of yarn depend largely on the tensions of the two interlacing yarns. It is, therefore, more appropriate first to discuss the functions of the tensions of the yarns in forming the net like structure of the cloth before considering the effects of yarn tensions on the various cloth properties.

Figure 10.1 Crossing of a warp and weft yarns. Let us first consider a single cross-over of warp and weft as shown in Fig. 10.1 and assume for simplicity, that both the yarns are of the same diameter and remain circular in shape even under pressure at the cross-over point. As each yarn crosses over the other and thus crimps, the tensions at each end of the yarns act at various angle to the plane, AB of the cloth. These tensions can be resolved into two directions in the plane of the fabric, viz., T1 and T4 for warp and T2 and T3 for weft and one perpendicular to it. The resolved components of the four tensions must balance to give equilibrium. Now, consider first the components lying in the plane of the fabric and for simplicity, assume that the tension along a given yarn is always constant (which factually may not be so

Effects of yarn tensions on fabric properties

223

because of friction, but this does not invalidate the discussions made here). On the basis of these assumptions T1 = T4 for warp and since T1 cos θ1 = T4 cos θ4 to give equilibrium in warp direction, then θ1 = θ4. Similarly for the weft yarn T2 = T3 and T2 cos θ2 = T3 cos θ3 whence θ2 = θ3. Since it has been assumed that the tensions and the angles made by the yarns with the plane of the fabric are the same, then T1 and T4 can be replaced by Te and θ1 and θ4 by θe for warp, while T2 and T3 can be replaced by Tp and θ2 and θ3 by θp for weft. In the direction perpendicular to the plane of the fabric

T1 sin q1 + T4 sin q 4 = T2 sin q 2 + T3 sin q3 or 2Te sin qe = 2Tp sin q p

(10.1)

From this basic conception of fabric formation in relation to warp and weft tensions let us now analyze the actual characteristics of the yarn tensions as the two sets of the yarns are interlaced at the fell of the cloth and the woven cloth moves forward during the course of weaving. At the instant of beating when the newly inserted pick lying between the two warp sheets is forced against the fell of the cloth with the warp ends crossing behind the pick, crimp is imparted to the warp and weft. Immediately prior to beat-up, suppose the warp yarns are under tension Te and the weft yarn is under tension Tp. The warp tension Te is determined and controlled primarily by the warp let-off motion and also the extent of shed opening at the time of beat-up. The weft tension Tp is controlled by the tension device in the shuttle or by the weft tensioning systems employed in the shuttleless picking systems.

Figure 10.2 Cross section of unit cell of yarn interlacement. Fig. 10.2 illustrates the interlacement of a warp (or weft) yarn with two weft (or warp) yarns along the fabric cross section in the warp (or weft) direction. Owing to crimping, the length L of a yarn observed on the cloth surface varies from its actual length L1 with L1 > L. Although this is true for both the warp and weft yarns, there is a fundamental difference. Each weft pick has to crimp around all the warp ends simultaneously, whereas each warp end has to crimp around one pick only at each beat-up.

224

Role of yarn tension in weaving

Before beat-up the weft remains straight and its length between the two adjacent warp ends Le is 1/Er, where Er is the warp density in reed. After the weft is crimped to the extent of Cp, this length Le is extended to Le (1 + Cp). This imparts strain on the yarn and the resulting tension Tpf of the weft in the fabric is given by

Tpf = Tp + E p

Le (1 + C p ) – Le (10.2)

Le

= Tp + E p C p where Ep is the elastic modulus of weft. To counter this increase in weft tension Tpf the cloth will tend to contract width way, but the tendency will be prevented by the action of the temples which hold the cloth to its dented width Wr at the reed. Similarly, the length of a warp between two adjacent weft picks Lp is 1/P where P is weft density. This length is extended to Lp (1 + Ce) as the warp is crimped to the extent of Ce and the resulting tension Tef of the warp in the fabric appears to be

Tef = Te + Ee

Lp (1 + Ce ) – L p Lp

(10.3)

where Ee is the elastic modulus of warp. However in case of the warp, the extension of the warp is spread over the whole length l of the warp and hence,

Tef = Te + Ee = Te + Ee

Lp (1 + Ce ) – L p l

(10.4)

Lp Ce l

The second term of the right-hand side of the equation 10.4 is usually too small in comparison to the first term and hence, it can be ignored and equation 10.4 can be rewritten as

Tef = Te

(10.5)

The similar simplification cannot, however, be done in case of the weft as the length of the weft affected is much less. If the practical values are

Effects of yarn tensions on fabric properties

225

inserted as appropriate in equation 10.2, it is found that the second term in this equation is higher than the first term and can not therefore be ignored. During weaving the tension of the weft is generally much less than that of the warp and hence, the increase in tension due to imposition of crimp is generally much more pronounced in the weft than in the warp. This indicates that, in a low picked open fabric where the yarns are crimped less the crimp of the warp tends to be lower than that of the weft, while in case of a dense fabric where the yarns are likely to be more crimped, the warp crimp tends to be higher than the weft crimp. However, as soon as the woven cloth comes out of the influence of the temples at the fell there is no longer any force to counteract the weft tension Tpf and the cloth is free to contract width way. This increases weft crimp, reduces the width of the cloth, reduces the warp crimp and as a result, increases the pick spacing. Again, when the cloth is taken off the loom it is no longer under the influence of the warp tension and the cloth is thereby fully relieved either of the warp or of the weft tension maintained on the loom. This grey state of the cloth therefore causes some reversal of the crimp amplitudes of the yarns with the result that, the warp crimp increases, the weft crimp decreases, the pick spacing reduces and the cloth width again increases to some extent. Similarly when the cloth is subjected to further processing like dying and finishing where it is stretched width way, there are further changes in its structural parameters, but all these changes are interrelated as discussed above, so that no single parameter can be changed without affecting the others.

10.3

Beat-up force

As stated above, the formation of a cloth is accomplished by the third primary motion of a loom that is, beat-up. The beat-up action which pushes the newly inserted pick to fell of the cloth is directly related with warp yarns and creates the beat-up force. Heavy fabrics need a violent or double beat-up with long front dwell while, the light fabrics with weak warps need gentle beat-up. While the beat-up force is affected by the various settings of the loom, it influences the efficiency of the weaving machine and many of the fabric characteristics. Effects of warp tension on the beat-up action have been discussed in details in Section 7.8 of Chapter 7 and now this section deals with other aspects of beat-up force. It has been observed earlier that, beat-up force depends on the difference between the rise in warp tension and the fall in cloth tension during beatup, and these two tensions are equal and opposite to each other under the stable weaving condition. It is difficult to measure the beat-up force directly in actual weaving conditions, but it is possible to measure the cloth fell distance (Y in Fig. 7.26 of Chapter 7) wherefrom the beat-up

226

Role of yarn tension in weaving

factor and consequently the beat-up force can be calculated, as the cloth fell distance is proportional to the weaving resistance (or beat-up force) of a given fabric. Again, the cloth fell distance is not constants during a pick cycle and hence, Kalli [16] has suggested to measure the cloth fell distance at closed position of the shed when the warp and cloth tensions are equal. According to him the beat-up factor, Bf, which is closely related to beat-up force, is Bf = Pe Pp fm/D

(10.6)

where Pe is end spacing in mm; Pp is pick spacing in mm; f is average yarn float of a given fabric construction; m is weave value depending upon the fabric construction; D is sum of warp and weft yarn diameters in mm. The relation between the cloth fell distance, Y and Bf is Y = 15.96 − 41.2 Bf

(10.7)

with correlation coefficient of 0.91. The Bf value (beat-up factor) can be used to forecast the beat-up force which will be exerted on the warp ends and the fabric. For plain weaves, f and m are equal to one and then equation 10.6 can be written as Bf = Pe Pp/D

(10.8)

That is, in plain weave construction, the beat-up factor is the ratio of the product of end and pick spacing and the sum of end and pick diameters. As the beat-up force is equal to the weaving resistance, it is dependent on the shed timing and back-rest position, in other words, the back shed geometry. It has been observed [23] that, with the given warp tension and weft density if the shed timing is advanced from the shed level position at beat-up and the back-rest is raised from below the position of the front rest, the beat-up force first decreases and then increases as shown in Fig. 10.3. With advancement of shed timing, the shed angle of the crossed warp yarns behind the pick at beat-up increases and hence, the contact angle of the pick between the warp ends also increases. This causes increase in the deflection of the pick in the vertical direction, pressure and friction between the warp yarns and the pick, which beyond a certain limit increases the beat-up force. In regard to the effect of back-rest setting, at the normal position of the back-rest that is, at 0 setting, the pick experiences equal pressure from the yarns of both

Effects of yarn tensions on fabric properties

227

the shed lines as discussed earlier, but as the back-rest is raised the tension of the bottom shed increases. This exerts more downward pressure on the pick and creates greater friction between the pick and the ends during beat-up. As shown in Fig. 10.3, for weaving a given type of plain fabric the minimum beat-up force has been obtained with the shed level position about 50° before beat-up and the back-rest position about 5 mm above the front-rest.

Figure 10.3 Beat-up force with different shed timings and back-rest positions. With the conventional sley driving mechanism, if the cloth fell is, for any reason, shifted forward that is away from the reed, the level of beat-up force for the first few picks becomes very low before the beat-up force reaches the normal value needed for the type of fabric being woven. Conversely, if the cloth fell is shifted backward that is towards the reed, greater beat-up force than the normal is required for the few picks before reaching the desired level. This is because, with the conventional sley driving system beat-up occurs at the same crank angle, 360°/0°, Fig. 10.4A. This naturally disturbs the pick spacing and produces fault in the fabric in the form of starting mark (discussed in Section 10.6.1). To get rid of this problem a microprocessor controlled hydraulically driven sley has been developed [29] in which the

228

Role of yarn tension in weaving

front position of the sley is adjusted on-line according to the level of beat-up force developed in the pick cycle.

Figure 10.4 Beat-up forces with conventional and modified sley motions. Fig. 10.4B shows the beat-up force with the modified sley driving system with the displaced positions of the cloth fell. If the cloth fell is shifted to the maximum extent of 1.5 mm backward or forward, the beat-up occurs

Effects of yarn tensions on fabric properties

229

at slightly different crank angles depending on the disturbed position of the cloth fell with the result that, it maintains the beat-up force constant and keeps the pick spacing same. The constant beat-up force thus reduces the pick spacing variation created by bar of increased or decreased weft density, normally observed when the loom with the conventional beat-up system is restarted. With conventional crank driven beat-up mechanism, where the sley does not have a dwell, the beat-up force starts to rise after the sley has completed about 85% of its forward motion before reaching the peak. It has been observed [28] that, during beat-up with the conventional sley driving mechanism, there is double peak, as shown in Fig. 10.5A. The first one occurs slightly before the reed reaches the front most position, while the second one occurs exactly at that poison. This is because, the rapid beat-up action causes the reed to bounce back after hitting the cloth fell, which thus momentarily reduces the pressure on the fell and thereby, the beat-up force. Sudden sharp beat-up peak imparts high strain on the warp yarns particularly in heavy fabrics. A study [28] has therefore been made with the modified beat-up mechanism [29] discussed above, but with dwell at beat-up position. The system has been so developed that the beat-up timing, speed and front position of the sley can be altered through microprocessor.

230

Role of yarn tension in weaving

Figure 10.5 Beating-up with conventional (A) and modified (B) sley driving systems. The beat-up force with the latter system indicated in Fig. 10.5B shows that, after the initial double pick the beat-up force reduces and gradually increases again but not to the initial maximum level. This may possibly be owing to a shock wave travelling through fabric in front of the reed. The maximum beat-up force with the sley with front dwell is also less than that without dwell (that is, the conventional system) and the increase in period of dwell reduces the beat-up force. The reasons for these are, the front dwell of the sley prevents the pick’s initial slipping back, and therefore, less beat-up force is required in the next cycle. The beat-up speed has no significant effect on the maximum beat-up force. It is suggested that if the pick is inserted as close to the cloth fell as possible and the front swing of the sley is as short as possible, the beat-up force can be reduced. Beat-up force and also the width of beat-up pulse, which is the function of cloth fell displacement, increase significantly with the increase in coarseness of the weft yarn and decrease in pick spacing [6, 26] and the relationship between the beat-up force and the weft linear density is linear

Effects of yarn tensions on fabric properties

231

[26]. Fig. 10.6 illustrates the effects of weft count and pick spacing on the beat-up force.

Figure 10.6 Effects of weft count and pick spacing on beat-up force. As the weft yarn becomes coarser and/or the pick spacing decreases, the cloth fell moves more towards the back of the loom and the beat-up pulse increases. Moreover, as the weft becomes coarser, the warp crimp increases and hence the warp tension increases. It can also be noted from Fig. 10.6 that, the crank angle at which the beat-up occurs differs with the changes in these two. Beat-up in case of the coarser weft occurs slightly earlier than with the finer weft. In this context, the author likes to mention that taking a cue from Greenwood and Vaughan [12] he once made a preliminary study (unpublished) to find out the relationship between the pick spacing and the beat-up tension of the warp yarns. He carried out the study on a shuttleless weaving machine weaving jute cloths. He considered plain square cloths with 47.2 yarns/dm from the warp of 293 tex and weft of 207, 310 and 413 tex. He measured the pick spacing at the cloth fell and beat-up tension, first with each weft count separately from the start of weaving of the cloth (that is, with the fell of the cloth well away from the front most position of the reed) and then, changing the weft counts in the descending and ascending orders during the course of normal weaving without stopping the loom. In the first part of the study, it was observed that

232

Role of yarn tension in weaving

as expected, initially the pick spacing was fairly large and then gradually decreased and became steady on reaching the desired value. At the same time, the peak tension at beat-up was initially very low and then gradually increased to reach a steady value. The desired pick spacing and the steady beat-up tension were obtained simultaneously after a certain number of picks were inserted in the cloth. This was true for any weft count and there was a significant relationship between the changes in pick spacing and the peak tension at beat-up. In the second part of the study it was observed that, as the weft count was changed abruptly from the high to low or vice versa, the pick spacing and also the peak tension at beat-up varied significantly as soon as the weft count was changed and then both reached the desired steady values concurrently after a number of picks had been inserted. Here also there were significant correlations between the pick spacing and the beat-up tension for the different weft counts under the disturbed weaving conditions. While studying the warp way breaking strength of the sample cloths made with different weft counts but with the same picks/dm, very interesting observations were made. For each cloth sample woven with two different weft counts, the majority of the break occurred at the place where the count of weft was changed. This is probably because of the consequent changes in the warp crimps at these transition zones which thus produce potential weak places in the cloths. Beat-up force is not affected by the change in loom speed. In regard to shed timing, the beat-up force has not been found to be significantly affected by change in shed timing in one of the studies [6] referred to above. If the shed is timed early, the weft yarn tends to crimp and flatten more with the result that the angle of wrap of the warp yarns around the weft reduces causing decrease in the beat-up force. On the other hand, earlier crossing of the heald shafts at the time of beat-up causes higher tension of the warp yarns at beatup, which increases the beat-up force. Therefore, whether the beat-up force increases or decreases with early shed timing possibly depends on the count and type of the weft yarn. In the other study [26] however, the beat-up force has been observed to increase with earlier timing of shedding motion possibly because of corresponding increase in warp tension at that instant. The beat-up force also increases as the back-rest is lowered, because the extent of shed asymmetry and therefore, the difference in yarn tensions between the two shed lines decreases. In respect of the effect of shed timing on beat-up force, Hepworth [15] states that the beat-up force is affected by the degree of unbalance (or asymmetry) of the shed at the time of beat-up. If the back-rest is set at raised position, the shed is unbalanced and the ratio of warp tensions in the two shed lines at beat-up then depends on both the shed timing and the manner of yarn interlacement that is, the weave. In case of plain weave for example, if the shed timing is such that the shed is just crossing at beat-up it is in fact

Effects of yarn tensions on fabric properties

233

a balanced shed, in spite of the raised position of the back-rest, because all the yarns follow the same path at the time of beat-up, as mentioned earlier in Section 9.2.1 of Chapter 9, and the beat-up force remains fairly unaffected. He has also stated that, the weaving resistance or beat-up force also depends on the number of picks that slip back a little after being beaten into the cloth. An early crossing of the shed at beat-up naturally helps to prevent the picks slipping back immediately after beat up, but for a plain weave, in which all the yarns change positions in each pick cycle, the most vulnerable time may be just before the next beat up, when re-crossing shed may release the last beaten up pick before the succeeding one is pushed near enough to hold it in place. For other weaves the situations are different as the beaten pick is held in place by some warp yarns not changing positions. The best shed timing from this point of consideration is, therefore, likely to be a function of both the type of weave and the pick spacing.

10.4

Cloth properties

The very fact that a woven cloth is formed by the interlacement of the warp and weft yarns implies that, the characteristics of the woven fabric network depend a great deal on the weaving tensions of these two sets of yarn. Besides the weave and yarn parameters like count, twist, etc., yarn tensions determine the magnitudes of crimp of both the warp and weft yarns and these show their effects on the structural, physical (or mechanical) as well as comfort properties of the cloth.

10.4.1 Structural and dimensional properties Structural parameters like warp and weft densities and dimensional properties like width, length, thickness and weight of a finished cloth are of immense importance for the end use the cloth is made for. Weaving tensions of the warp and weft yarns play key roles in deciding these characteristics of the cloth. Again, these structural properties of a given type of cloth differ at the loom and grey states, because of changes in the tensions of its constituent yarns within the cloth, held taut on the loom and relaxed off the loom.

10.4.1.1 Cloth width and length The finished width of a cloth is an important dimensional property for its end use. The specified width of a cloth is obtained primarily by adjusting the width of the warp sheet at the reed and the final width of the woven cloth at the grey state that is, when it is free to relax after removal from the loom, is always less than that at the reed. This is because of imposition of crimps on the constituent yarns resulting from interlacement, as stated above. Irrespective of the type

234

Role of yarn tension in weaving

of weave or the constructional parameters, as a cloth is formed, wound on the cloth roller and finally taken off the loom and allowed to relax, the crimp amplitudes of its interlaced warp and weft yarns undergo changes and the effects are reflected on the changes in length and width of the cloth. The main reason for these dimensional variations of a woven fabric being the changes in the warp and weft crimps, the weaving tensions of the warp and weft yarns have significant effects on the cloth width.

Figure 10.7 Effect of warp tension on cloth width. Effects of warp tension on the width of a plain worsted fabric woven with 28.4 ends/cm and 19.7 picks/cm from 12.3 × 2 tex warp and weft yarns [34] have been shown in Fig. 10.7 at the various stages on the loom and also at the grey state. For any level of warp tension as the cloth is woven and traverses forward towards the cloth roller and is then taken off the loom, its width gradually changes from the fell to the grey state. The width of the cloth at the fell is slightly less than the width of the warp at the reed and then the cloth width reduces significantly at a distance of about 30.5 cm from the cloth fell or at the cloth roller. The width of the cloth then increases when the cloth is at the grey state, but the increase is not so much as to attain the width at the fell. Thus, there is a certain amount of contraction in the cloth width in each case, Fig. 10.7. At each stage the cloth generally contracts more as the weaving tension of the warp yarns increases. At higher warp tension the warp yarns crimp less at the fell and hence stretch the weft yarns to take a share of yarn crimp at that stage of weaving. With the given constructional parameters and the weave of the cloth, higher warp tension imposes higher crimp on the weft yarns causing greater contraction in cloth width. However, the extent of width contraction from the fell to the grey state decreases as the magnitude

Effects of yarn tensions on fabric properties

235

of weaving tension of the warp yarns increases. Morton and Williamson [18] have also observed greater contraction in width of plain cotton cloth with increasing warp tension, Table 10.2. As soon as the cloth is formed by interlacement of the warp and weft yarns, each set of yarns tends to impose crimp on the other and the cloth contracts in width. At the fell of the cloth the extent of width contraction is restricted by the temple which stretches the cloth fell to the dented width of the warp sheet at the reed and hence the difference in width between the fell and the reed is very small. As the cloth moves forward from the fell, the picks in the cloth become free to crimp and contract the cloth in its width. After the cloth is doffed from the loom, the warp yarns are no longer under tension and hence, the weft can shed some of its crimp to the warp causing increase in the width. It has been stated in Section 7.9 of Chapter 7 that the conventional jute looms are not equipped with temple. The nature of width variation of the cloth as well as the effects of warp and weft tensions on the cloth width are therefore significantly different from those fabrics woven on the looms fitted with temples.

Figure 10.8 Effects of yarn tensions on width of jute cloth. Fig. 10.8 depicts the widths of jute cloths of plain construction at different places on a jute loom and at grey state, woven with different warp and weft tensions [19]. The cloths were woven with 51 ends/dm from 292 tex warp and 47 picks/dm from 310 tex weft yarns on a over-pick shuttle loom with negative let-off motion. Different warp tensions were set by varying the gripping force of the warp beam and the different weft tensions were set by varying the pressure of the shuttle spring with the warp to weft tension ratio varying from about 1.7 to 12.5. It is observed from Fig. 10.8 that the magnitude of width contraction of the cloth depends on the weaving tensions of the warp and weft yarns. As

236

Role of yarn tension in weaving

observed in the previous cases, width of the cloth always first decreases from the width of warp at the reed and then increases at the grey state. In this case, irrespective of the magnitudes of yarn tensions, the width gradually decreases till the cloth is on the breast beam (or front-rest) and then increases when the cloth is taken off the loom. What is rather of greater importance to note here is the consequence of the absence of temple. It is distinctly evident in Fig. 10.8 that, for all the warp and weft tensions considered, the maximum contraction in width of the fabric takes place between the dented width of warp at reed and that at the fell of the cloth. The extent of this contraction is so high that the width of the cloth, as soon as it is formed, nearly equals its final width that is, when the cloth is fully relaxed after being taken off the loom. As mentioned earlier, this high degree of unrestricted contraction in cloth width at the fell (because of absence of the temples) makes the warp yarns highly susceptible to break which, as a result, also imposes limitation on the loom speed. Warp and weft tensions have shown significant effects on cloth width at each stage. With the same fabric the highest width has been obtained with the lowest warp and weft tensions (Wl/Fl) and the lowest width has been obtained with the highest warp and weft tensions (Wh/Fh) at all stages from the cloth fell to the grey state. When both the warp and weft are under high tension they are not much free to crimp and hence, each tends to pull the other closer to crimp and thereby, contracts the fabric more in width. Conversely, when the warp and weft yarns are very slack they can crimp easily and hence the minimum contraction in cloth width is observed. Fig. 10.8 also clearly exhibits that the width contraction of the fabric varies in accordance with the tension ratio of the warp and weft yarns at the each stage. With the same weft (or warp) tension the cloth width decreases with the increase in warp (or weft) tension. The same is most likely to happen in regard to the cloth length also, but this aspect has not been studied in the study referred to [19]. Ramaswamy and Paliwal [24] however, have not observed any conclusive effects of warp tension on the widths of a plain cotton cloth of warp cover factor of 19 and weft cover factor of 11, at any stage from the loom to the finished state. No significant contractions in widths have been noted from the width of warp sheet at reed to those at the grey and finished stages as well as between the widths of the grey and finished cloths with change in warp tension. In regard to the effect of warp tension on the cloth length, they have observed that for the same tape length, longer lengths of cloth are obtained at the grey and bleached stages with increase in warp tension. However, while the contraction in length from tape to grey stage decreases with higher warp tension, the corresponding gain in length from grey to finished stage also decreases with the result that there is no significant effect of warp tension on the cloth length at the finished stage. On the whole, therefore, there is no significant trend of cloth shrinkage with change in warp tension. According to Snowden [27] also decreases in cloth width from that at fell to that at take-

Effects of yarn tensions on fabric properties

237

up roller increases with the increase in warp tension, but after the cloth is removed from the loom and allowed to relax the relaxation is such that the effect of the warp tension on cloth width is not that marked and after de-sizing the differences in widths are evident only slightly. Relaxation causes increase in width of the cloths from the loom to grey state and that at higher warp tension is accompanied by a decrease in fabric length. Nichiporchik [22] has also observed increase in widthwise contraction of the cloth with increase in warp and weft tensions and lengthwise shrinkage of the cloth with the ratio of yarn tensions. It has been discussed in Sections 7.3.1 and 7.5.5 of Chapter 7 that the backrest setting and shed timing affect the warp tension during weaving and here we have seen that the width of the cloth is affected by yarn tensions. From these we can therefore expect that, cloth width is affected by the settings of the back-rest and shedding motion during weaving. Wagh and Snowden [34] have observed that while the width contraction of the cloth reduces as the position of the back-rest is lowered from its normal position, shed timing has not been found to have any conclusive effect on the cloth width. Neogi and Bhattacharyya [20] have of course found significant effects of both the back-rest position and shed timing on the width of the plain jute cloth. With a given leasing pattern of the warp yarns (which however, has not been found to show any conclusive effect on the width contraction) contraction of cloth width at different stages from the reed to the grey state increases as the backrest is raised or the shed level position is advanced towards beat-up. It may, therefore, be taken into consideration that if the back-rest is set higher to obtain better cover of the cloth (see Section 10.6.3) or the shedding is timed late to reduce the strain on the yarns at beat-up (see Section 9.2.1 of Chapter 9), the finished width of the cloth may decrease to some extent even though the weaving tensions of the warp and weft yarns remain unaltered. Working with projectile picking system Chahal and Mohamed [7] have examined the effects of weft tension, with and without weft accumulators, on the various properties of polyester filament and spun polyester/wool blended cloths in grey and scoured states. They have observed that while the weft tension has not shown any significant effect on the width of the dense filament cloth, increase in weft tension decreases the width of the spun cloth woven with relatively low warp density. As a result, increase in weft tension causes higher warp way shrinkage of the filament cloths and higher weft way shrinkage of the scoured spun cloth as the higher weft tensions impose higher crimps on the warps. The extent of cloth shrinkage however depends on the method of weft feeding system adopted, as the magnitude of weft tension has been found to vary significantly when the weft is fed through an accumulator or direct (see Section 8.5.2 of Chapter 8). Working with air-jet picking system Adanur and Qi [1] have not observed any significant effect of weft tension on the weft way shrinkage of the denim fabrics of different

238

Role of yarn tension in weaving

constructions after one laundering, but after five launderings greater weft way dimensional stability have been achieved at lower maximum weft tensions than at higher tensions.

10.4.1.2 Yarn density Yarn density in warp or weft direction in the cloth is the number of ends or picks lying in a certain unit length, usually inch or centimeter, in the cloth. From this it implies that, when a certain number of yarns, warp or weft, is employed to weave a cloth with a given calculated number of ends or picks per unit length, the actual number of ends or picks in the said unit length may change if the yarns come closer or move apart when the cloth is in the process of making on the loom and then relaxes after being removed from the loom. The warp and weft densities of the cloth are therefore likely to increase with the contraction in width and length, respectively of the cloth. As the cloth is taken off the loom and allowed to relax, the relaxation causes increase in width and decrease in length of the fabric and these are supposed to be associated with changes in warp and weft densities. It has been observed that the weft density increases when the cloth is woven under higher warp tension, and the increase continues till de-sizing of the cloth [27] or at finished stage and irrespective of the level of warp tension, the weft density decreases from the grey to the finished stage of the cloth [24]. In respect of the warp density on the contrary, it has not been found to be affected by the change in warp tension albeit, the warp density increases from the grey to the finished stage at each warp tension [24]. These are because of changes in yarn crimps at the different stages of the cloths consequent to the change in warp tension. The reason for higher weft density with increased warp tension is that, under high tension the warp yarns remain taut and during beat-up this helps the newly inserted picks to be placed closer to the preceding picks at the fell of the cloth. High tension of the warp yarns is thus more beneficial at the time of beat-up when the formation of the cloth takes place rather than at the time of shed formation. Again, high warp tension causes reduction in warp crimp and increase in weft crimp (see Section 10.4.2). As a consequence, the width of the cloth reduces and hence the warp density increases. With regard to the weft density however, the increase in warp tension has two-fold effects, as stated by Greenwood [11]. From Section 7.8 of Chapter 7 we find that the increased warp tension causes an extension of the woven fabric and this causes the cloth fell to move towards the reed that is, away from the weaver. Again, an increase in warp tension also causes an increase in weaving resistance. The movement of the cloth fell towards the back will cause an increase in weft density, while the increase in weaving resistance will cause a decrease in weft density. These two effects thus oppose each other in regard to the weft density (or

Effects of yarn tensions on fabric properties

239

in other words, pick spacing) of the cloth and whether the net effect is an increase or a decrease in weft density depends on the structure of the fabric under consideration. For instance, in case of a low picked fabric, the weaving resistance is not that dominant and an increase in warp tension will increase the weft density. In case of a dense fabric on the contrary, the weaving resistance plays the most important part in affecting the pick spacing and an increased warp tension will tend to reduce the weft density. It has been mentioned in Section 7.5.5 of Chapter 7 that, higher position of the back-rest in conjunction with early shed timing increases the weft density of the cloth. When the back-rest is raised from its normal position the warp ends of a heald shaft become alternately taut and slack during shedding and the resultant variations in warp tension help obtain higher weft density. Let us now see how it happens.

Figure 10.9 Pick movement at the cloth fell with high back-rest position. Fig. 10.9 depicts the nature of movement of the picks at the fell of the cloth with the back-rest set at higher position and the shed timing set early. With the said settings of back-rest and shed timing the warp ends become taut when forming the bottom shed and slack when forming the top shed. As the newly inserted pick ‘1’ is pushed forward by the reed during beating it is forced downward since the ends which pass over it are at the bottom shed and are therefore, under higher tension. Conversely, the preceding pick ‘2’ is forced upward, but to a lesser extent, as the taut ends are under it. It is quite reasonable to assume that a few more picks farther from the fell of the cloth also behave in the similar manner but with gradually diminishing amount of movement in the vertical direction. Thus, every time beat-up takes place the picks, in the vicinity of the cloth fell, move not only in the horizontal direction to move forward but in the vertical direction also. Now, it may be observed from Fig. 10.9 that, the last inserted pick ‘1’ can be pushed much nearer to its preceding one,’2’ with the result, p1 < p, where p is the pick spacing of the cloth. This could not have been possible had the warp yarns of both the shed lines been under the same tension, generally obtained with the normal position of the back-rest (see Fig. 7.4 of Chapter 7) and all the warp yarns remain nearly at the same level at the time of beat-up. From this we thus find that, there

240

Role of yarn tension in weaving

are mainly two reasons which enable to achieve closer pick spacing or in other words, higher weft density. One is the difference in tension of a given warp yarn forming the top and bottom sheds in two pick cycles and the other is the crossing of the warp yarns behind the newly inserted pick at the time of beat-up. Therefore, even with the normal position of the back-rest increase in pick density can still be achieved if the shed timing is changed from late to early, so that the picks cannot slip back after beatup, although the extent of increase may not be as much as with the raised position of the back-rest. This is true for any type of weave. It has also been found that the type of warp let-off motion affects the warp and weft densities of the cloth woven under different levels of warp tensions [3]. Increase in warp tension at shed level position increases the warp and weft densities and the rate of increase of weft density depends on the type of warp control system. No effect of weft unwinding tension has, however, been found on the yarn densities of the cloths woven with any type of warp let-off system [3] possibly because, as discussed in Section 8.2.2 of Chapter 8, the tension of the weft which matters more during the formation of the cloth is the final weft tension at the time of shed closing rather than the unwinding weft tension. From the above discussions we thus find that, during the course of weaving the cloth first contracts in width as it is woven and traverses forward and then expands when it is taken off the loom. Such behaviour of the cloth width is owing to crimp interchange of the warp and weft yarns and the final width of the cloth, in most cases, depends on the weaving tensions of the warp and weft yarns. Thus, by proper manipulation of yarn tensions in weaving, a cloth with predominantly warp crimp can be changed into one with predominantly weft crimp. Again, as increase in warp tension increases weft crimp, the resultant contraction in width of the cloth increases the warp way density of the cloth. The asymmetric nature of warp shed in which the warp yarns in two shed lines become alternately tight and slack in consecutive picks because of raised position of the back-rest along with early shedding help to obtain higher weft density. The weft way compactness of the cloth can also be increased by maintaining high warp tension. Raised position of the back-rest, early shedding and high warp tension are thus essential for weaving dense fabrics.

10.4.1.3 Cloth thickness Thickness of a cloth is primarily determined by the diameters, in other words, counts and crimps of the warp and weft yarns. As yarn crimp is dependent on the weaving tensions of the constituent yarns, yarn tension may show its effect on the fabric thickness. Generally speaking, the thickness of a cloth is the minimum when the crimps of the warp and weft yarns are most evenly distributed. Greater crimp in any set of the yarns increases the cloth thickness. Snowden [27] has observed that an increase in warp tension at weaving

Effects of yarn tensions on fabric properties

241

decreases the thickness of the grey fabric and the effect continues into the de-sized state only in fairly dense fabrics and then to a reduced extent, while Adanur and Qi [1] have not observed any significant effect of weft tension on the cloth thickness.

10.4.1.4 Cloth weight As the yarn tensions show their effects on the yarn densities of a fabric, the weight of the fabric is also supposed to be affected by changes in yarn tensions. Chahal and Mohamed [7] have found that the increase in peak tension of the weft that is, the maximum tension during insertion of the pick in projectile loom has caused increase in the weights of the spun fabrics and reduction in the weights of the filament fabrics although the average weft tensions have no effects on the weights of the filament fabrics. They have also observed that, the weights of the filament fabrics are not affected by the different weft feeding systems adopted, but the weights of the spun fabrics are. One of the weft accumulators has been found to cause an increase in the weight at high weft tensions and the direct weft feeding (that is, without weft accumulator) has increased the weight at low weft tensions. Ronz and Scholze [25] have studied the effects of warp tension on some of the properties of 5-end satin cotton fabrics of different constructions woven on air-jet weaving machine. Fig. 10.10 illustrates that the weight of the cloth in a unit square area (g/m2) increases with the increase in warp tension. This is because, the increased warp tension causes greater contraction of the cloth, as mentioned above and hence, the same mass of the yarns are contained in a smaller area. For any warp tension weight of the cloth increases with the coarseness of the weft yarn also, naturally because of correspondingly greater mass of the yarn.

Figure 10.10 Cloth weight with different warp tensions.

242

Role of yarn tension in weaving

10.4.2 Physical properties Yarn tensions have significant effects on the various physical properties, particularly the tensile properties and crimp, porosity, abrasion resistance, etc. which determine the serviceability and durability of the fabric.

10.4.2.1 Yarn crimp Crawshaw, Morton and Brown [8] are possibly the first to examine the effects of warp tension on some of the fabric properties, way back in 1931, while Morton and Williamson [18] have made more detailed study on the effects of warp tension on the various tensile properties of the fabrics in 1939. Crawshaw, Morton and Brown [8] have studied the effects of high and low warp tensions on the yarn crimp and breaking strength of plain cotton fabrics with different yarn densities and counts in warp and weft. The results of their study are given in Table 10.1. Table 10.1

Effects of warp tensions on cloth properties

Cloth type

Warp tension

Yarn density/cm

Yarn count, tex

30 × 27.5

2/11.8 × 18.5

31 × 28 28 × 28

18.5 × 14.8 16.4 × 14.8

Crimp, %

Breaking strength, kg/cm

Warp

Weft

Warp

Weft

High

10.9

8.5

20.4

9.2

Low

19.4

5.3

17.1

8.0

High

5.4

5.1

8.9

8.2

Low

7.3

5.3

8.7

7.2

High

2.9

11.9

5.7

7.2

Low

3.6

11.6

5.7

7.1

Different warp tensions have been obtained by adjusting the weights of the negative let-off motion of the loom. From Table 10.1 it is seen that, the increased warp tension changes both the warp and weft crimps considerably in the heaviest fabric with the highest cover factor, while the effects of warp tension are hardly perceptible in case of the lightest fabric with the lowest cover factor. When the cover factor of the cloth is the lowest, the weft crimp is higher than the warp crimp and as the cloth becomes denser the warp crimp becomes higher than the weft crimp. Irrespective of the type of fabric, the warp crimp decreases and the weft crimp generally increases as the warp tension increases. High tension of the warp yarns tends to keep the warp yarns

243

Effects of yarn tensions on fabric properties

straight and hence, imposes greater crimp on the weft yarns. This is naturally more pronounced in case of the fabric with the highest cover factor. Similar effects of warp tensions on the yarn crimps have been observed by other workers [18, 19, 24, 27, 22]. Morton and Williamson [18] have also varied the warp tension by adjusting the load of the negative let-off motion and the results of their study are indicated in Table 10.2. Plain cotton cloths of four different cover factors, from 13.2 to 16.2 and one with the highest cover factor of 16.2 but different twist factors of the warp and weft yarns were considered. Each cloth of a given cover factor was woven with the same count and twist of warp and weft. All the cloths were woven with the same yarn densities of 22/cm in both the warp and weft and under three different warp tensions with the total load on the warp beam ranging from 86.5 to 198.5 kg. Cover factor of the cloths was thus varied by varying the warp and weft counts. Weft tension was maintained the same but as small as possible so that all the stresses were borne by the warp only. Increased cover factor with the same yarn densities were obtained by employing coarser yarns in both the warp and weft. Table 10.2 Cloth cover 13.2

14.0

15.0

16.2

16.2 (yarns with higher twists)

Dimensional and physical properties of the cloth with different warp tensions

Warp Cloth Crimp, % tension width, cm Warp Weft Min 52.5 8.1 8.1 Med 52.7 6.6 8.8 Max 52.4 5.5 9.3 Min 52.5 10.9 7.3 Med 52.3 8.8 8.8 Max 52.0 6.6 9.1 Min 53.2 17.6 6.8 Med 52.3 11.3 8.6 Max 52.0 8.7 9.9 Min 53.4 21.8 6.1 Med 52.5 16.7 7.5 Max 52.3 14.8 8.6 Min 53.0 21.0 6.4 Med 52.2 15.1 8.9 Max 51.7 12.7 9.9

Breaking strength, kg/cm Warp Weft 15.2 14.5 15.2 15.0 15.1 14.7 18.7 18.4 18.5 18.0 19.0 19.1 21.1 22.8 21.7 22.3 22.9 22.6 23.8 25.2 24.7 25.8 26.1 26.0 19.1 22.1 22.5 22.1 21.8 23.1

Breaking extension, % Warp Weft 14.0 15.0 11.1 15.9 10.0 16.1 17.6 15.0 15.4 15.7 12.3 17.0 28.0 13.6 19.3 16.3 16.3 17.6 30.4 13.7 24.1 15.3 21.4 16.7 29.4 14.4 23.0 16.4 19.6 18.4

Bursting pressure kg/cm2 17.2 15.5 15.0 19.3 19.9 17.7 19.4 22.0 22.2 21.4 24.7 26.2 19.6 21.9 23.6

244

Role of yarn tension in weaving

For the cloth of a given cover factor that is compactness, the warp crimp decreases and the weft crimp increases with the increase in warp tension as the warp yarns become more taut. As a result, the total crimp of a cloth decreases with the increase in warp tension. Again, for the given warp tension the warp crimp increases and the weft crimp decreases with the increase in cover factor of the cloth. As the cloth woven under a certain warp tension becomes denser, owing to increase in the coarseness of the yarns, the warp yarns crimp more and consequently the weft yarns crimp less. Similarly, when the cloths of the same cover factor but different yarn twists are woven with the same warp tension, the more compact warp yarns with higher twist become more rigid. As a result, the warp yarns being under higher tensions than the wefts crimp less and impose more crimp on the weft yarns. Besides the usual nature of decrease in warp crimp accompanied by increase in weft crimp with increase in warp tension, Snowden [27] has further observed that on de-sizing these effects of yarn crimps considerably reduce and there is no effect on weft way crimp in loosely woven fabrics. Ramaswamy and Paliwal [24] too have observed loss of warp crimp of both the grey and finished cloths woven with increase in warp tension with the crimp always considerably lower at the finished state. In regard to weft crimp, they have not found any definite effect of warp tension variations on the weft crimp of the grey fabric. However, after the cloths have been finished the weft crimp has been found to increase for any warp tension and at the low warp tension the increase is considerable. Thus, following the balancing of crimp phenomenon the loss in warp crimp from grey to finished state has been accompanied by a corresponding gain in weft crimp. As the structure of the loom-state fabric depends on the ratio of the warp and weft tensions, Nichiporchik [22] has observed that the increase in the ratio causes decrease in warp crimp (or wave height) and increase in weft crimp. Compared to the magnitudes of yarn crimps near the fell of the cloth, the largest variation in yarn crimps in the fabric occurs with the highest weft tension and the lowest warp tension, while the smallest change occurs with the lowest weft tension and the highest warp tension. Neogi and Bhattacharya [19] have studied the effects of both the warp and weft tensions on the different properties of plain jute cloths of the constructions mentioned in the preceding section. The results of the study are produced in Table 10.3. Table 10.3

Effects of yarn tensions on properties of jute cloths

Yarn tension, g

Crimp, %

Tensile strength, kg

Breaking extension, %

Warp

Warp

Warp

Warp

Weft

Weft

Weft

Weft

Bursting strength g/cm2

245

Effects of yarn tensions on fabric properties 500

40

4.3

5.6

65.0

52.0

4.0

5.8

27.0

500

80

4.3

5.2

62.0

57.0

4.1

5.4

26.0

278

40

5.5

6.6

65.0

60.0

4.7

6.3

27.0

278

80

6.0

4.3

62.0

65.0

5.0

5.0

26.0

135

40

6.2

6.7

65.0

52.0

5.5

5.4

27.0

135

80

8.1

4.6

61.0

63.0

7.7

4.5

27.0

From Table 10.3 it is observed that the effects of warp and weft tensions on the warp and weft crimps, respectively, are similar to those obtained with the cotton fabrics that is, the crimp decreases with the increase in weaving tension of the concerned yarns. Although the change in warp tension has not been found to show any definite effect on weft crimp, the increase in weft tension with a given warp tension increases the warp crimp and the effects are more prominent at lower warp tensions. It is also interesting to note that the cloths of the same constructional parameters have higher crimp in weft than in warp till the tension ratio of warp to weft is 6.95 (that is, when warp tension is 278 g and weft tension is 40 g) and then the situation reverses in the most cases. This evinces that by suitably altering the warp and/or the weft tensions a given type of fabric with predominantly warp crimp can be changed into one with predominantly weft crimp and vice versa. If we now compare the results of width of the cloths woven under different warp and weft tensions shown in Fig. 10.8 and the results of yarn crimps of the same cloths indicated in Table 10.3, we find that the cloth with the least width contraction has high warp and weft crimps and that with the highest width contraction has the lowest warp crimp and moderate weft crimp. Thus, as should be expected, the changes in weaving tensions of the warp and weft cause changes in warp and weft crimps which in turn, show their effects on the width of the cloth. In case of projectile loom [7], increase in weft tension has been found to increase the warp crimp of the spun polyester/wool blended cloths and reduce the weft crimp of the polyester filament cloths. There is, however, no significant effect on the weft crimp of the spun cloths at the scoured state, most likely because of crimp interchange between the warp and weft yarns after scouring process. It has also been observed that the use of weft accumulators is more significant at the low tension range than at the high range in respect of yarn crimps.

10.4.2.2 Yarn breaking strength (cloth) One of the most important properties of a cloth, whether meant for apparel or technical purpose, is its ‘strength’, that is, the breaking strengths in its warp and weft directions. It is, therefore, always endeavoured to maneuver the constructional parameters of a cloth in such manners as to make it suitably

246

Role of yarn tension in weaving

strong and durable for its end uses. It is true indeed that the major factors that determine the strength of a fabric are the counts and strengths of its constituent yarns, yarn densities, weaves, etc. but, the weaving tensions of the yarns can also play a role in this regard as the yarn tensions affect the crimps of the yarns. It is observed from Table 10.1 that, with change in warp tension the warp way breaking strength of the cloths of different yarn densities and counts varies in the similar manner but in the reverse order as the warp crimp, while the weft way breaking strength varies in the similar manner as the weft crimp and the effects of yarn tension are more pronounced with denser cloths. Cloths woven under higher warp tensions have higher warp and weft way breaking strengths. With increased warp tension the warp yarns remain more straight and, therefore, are more capable to share the tensile load to register higher strength. The cloths with high cover factors have high warp and weft strength with the maximum strength obtained with high warp tension. From Table 10.2 we find that, the increase in warp tension generally increases the warp way breaking strength of the cloths but the effects on the weft way strength are not very conclusive and from Table 10.3 we find that for a given warp tension the warp strength always decreases and the weft strength always increases with the increase in weft tension, while the effects of warp tension on the warp or weft strength are not clear. Ramaswamy and Paliwal [24] have also found increase in warp way tensile strength of the cloth at both the grey and finished stages with increase in warp tension. The effect of warp tension on the weft way strength of the cloth however is somewhat confusing; while there is no significant difference in strength of the cloth at the grey stage, the strength is unexpectedly low at the low warp tension at the finished stage and surprisingly, the weft way tensile strength per pick of the finished cloth is significantly higher at the medium than at the high or low warp tension. Snowden [27] also has not observed any positive effect of warp tension on the tensile strengths of the fabrics. In regard to the shuttleless picking system, increase in weft insertion tension in air-jet picking has been found to decrease the breaking load of the cloth [1]. From the above findings it appears that, the strength of the fabric network is so complex, as it depends on so many constructional parameters and weaving conditions, that a precise prediction of the breaking strength is possibly not very easy. However, as the increase in warp and weft tensions has in general been found to increase the warp and weft ways breaking strengths of the cloths, it may be stated that higher yarn tensions help to produce stronger fabrics.

10.4.2.3 Yarn breaking elongation (cloth) Breaking elongation of the warp or weft yarns of a cloth is attributed mainly to the crimps and elongation property of the yarns. As the fabric is subjected

Effects of yarn tensions on fabric properties

247

to the tensile loading, the affected yarns are straightened and consequently elongated. The extent of straightening, therefore, largely determines the extent of elongation of the yarns before they break. We can observe from Table 10.2 that irrespective of the compactness of the cloth, the breaking elongation of the warp yarns decreases and that of the weft yarns increases with the increase in warp tension, although the effect is more pronounced for the warp yarns. Again, for a given level of warp tension higher is the cloth cover greater is the warp breaking elongation, while there is no conclusive effect on weft breaking elongation. It is also interesting to note that for the same cloth cover, when the yarns are more compact because of increase in twist, breaking elongation of the warp yarns decreases and that of the weft yarns increases at any warp tension. This corroborates with the similar trends of yarn crimps. Fairly similar effects of varying yarn tension on yarn breaking elongation have been obtained by Snowden [27] and also with the jute cloths woven with a given construction (Table 10.3). As should be expected, the breaking elongations of the yarns vary in accordance with the yarn crimps as determined by the warp tension arranged during weaving. In case of the jute cloths, Table 10.3 shows that, increase in warp tension decreases the warp breaking elongation and shows the tendency to increase the weft breaking elongation when the weft tension remains unaltered, while the increase in weft tension increases the warp breaking elongation and decreases the weft breaking elongation with the same warp tension. Here also the breaking elongation of weft is higher than that of warp till the tension ratio of warp to weft is 6.95 and then the situation reverses, as observed with yarn crimps. Ramaswamy and Paliwal [24] have not always observed very significant effects of warp tension variations on yarn breaking elongation. They have observed that at the grey state there is significant decrease in warp elongation with increase in warp tension but there is barely any effect on weft elongation. At the finished state on the other hand, there is no clear effect on warp elongation but the weft elongation increases slightly with warp tension. However, there are significant decrease in warp way elongation and increase in weft way elongation after finishing of the cloth woven at each level of warp tension. This is because of the stretch the cloths are subjected to at finishing process. Adanur and Qi [1] have also not observed significant effect of weft tension in air-jet loom on the elongation property of the cloth.

10.4.2.4 Bursting strength Unlike in tensile test, where the load is applied in one direction, in bursting test, the loads are applied concurrently in both the warp and weft directions of the cloth. The bursting strength of the cloth thus depends on the two sets of yarn breaks which down first. Therefore, if both the sets of yarn have

248

Role of yarn tension in weaving

nearly the same crimp they will break down simultaneously and the cloth is supposed to have the maximum bursting strength. It may be noted in Table 10.2 that, those cloths in which the change in warp tension causes the crimps of both the warp and weft yarns to become more alike, their bursting strengths are generally on the higher side. No such definite effects of warp or weft tension on the bursting strength of the cloths have however been observed by Snowden [27] or Neogi and Bhattacharya [19] (see Table 10.3). The reasons for this may be that the cloth considered in either study [19] or [27] has been woven with one yarn density and yarn type only in both the warp and weft and the differences in warp and weft crimps in Table 10.3 are not as much as in some cases observed in Table 10.2.

10.4.2.5 Tear strength Like the tensile strength, the tear strength of a fabric depends fundamentally on the strength of the yarn under question and the strength of the fabric. During the process of tearing if the yarns are allowed to group together they become more capable of sharing the stress to register higher tear strength of the cloth. Adanur and Qi [1] have observed that the fabrics woven with lower weft tensions register higher tear strength. With long weft float of 3/1 denim fabric as the weft tension decreases the picks can assemble together more easily under stress and thus, increase the tearing strength. Gu [14] has also found that change in warp tension, as the result of improved oscillatory movements of the back-rest produced by the modified cams, shows significant effects on tear strength of the plain cotton fabric considered in the study. It has been observed that the cams which have caused higher beat-up and shed-open tensions and lower fell movements at beat-up than the others have produced higher tearing strength of the cloths. However, Gu has not found any significant effects of change in warp tension caused by change in back-rest movement owing to modification in cam design, on other fabric parameters like weight, thickness, yarn densities and abrasion resistance.

10.4.2.6 Abrasion resistance Abrasion resistance of a cloth is determined by many factors like, type of the constituent yarns, type of fibre, type of weave and so on. If the yarns are less twisted with the surface fibres not secured tightly or the yarns are more crimped or the type of the weave is such that the yarn crowns are well exposed, the fabrics are more vulnerable to abrasion. Even, given the fabric parameters being the same, the fabrics woven with different yarn tensions can also behave differently in regard to abrasion resistance, as demonstrated by Adanur and Qi [1]. They have shown that with increase in weft tension the warp crimp increases and a greater area of the cloth is exposed to abrasion resulting in higher losses in weight and thus, decrease in abrasion resistance

Effects of yarn tensions on fabric properties

249

of both the grey and scoured denim fabrics. The effect caused by change in weft tension, as observed by Adanur and Qi [1], is likely to be the same for change in warp tension also, although Snowden [27] has not observed any significant effect of warp tension on the resistance to abrasion of the cloth. If we consider Table 10.2 we can observe that, as the warp tension increases the weft crimp increases and for the cloths of low cover factors, the weft crimps, in most cases, are even higher than the warp crimps. The weft yarns of these cloths are therefore more liable to be abraded. As a matter of fact, with the same increased magnitude of crimp, the weft yarns are expected to be more adversely affected by abrasion than the warp yarns as, for the same yarn count, the weft yarn is generally less twisted and hence, its fibres are less secured in the yarn.

10.4.3 Comfort properties As indicated above, there are some properties which are more pertinent to the cloths meant for apparel purposes. These are handle, stiffness, drape, air permeability, etc. For the same constructional parameters, weaving tensions of the warp and weft can bring about some changes in these properties. Experimenting with plain cotton fabrics Ramaswamy and Paliwal [24] have observed that as expected, the same type of cloths woven with different warp tensions differ in handle and feel and that woven with lower tension is less stiff. The decrease in warp tension increases the warp crimp and makes the cloth more suppler. According to Ronz and Scholze [25] also, the qualities of a given type of fabric are liable to be changed significantly by the change in weaving tension of the warp. Air permeability (an important property for technical textiles also) of a fabric has been found to decline with increase in warp tension. Increased warp tension increases the contraction of the cloth, as discussed above, and this in turn, makes the cloth more dense and hence less permeable to air. Snowden [27], however, has not observed any significant effects of warp tension on the bending length and air permeability of the cloth. As mentioned above, Adanur and Qi [1] have studied the effects of weft tension on the various properties of denim fabrics woven on air-jet weaving machine. Contrary to the findings of Ronz and Scholze [25] stated above, Adanur and Qi have observed greater air permeability of the cloth with increase in weft tension. Increase in weft tension makes the yarn more compact and produces larger openings to increase the air permeability but after the fabrics are scoured they become denser after laundering and hence the air permeability reduces at this latter state. While studying the effects of weft tension on the other comfort properties, Adanur and Qi have observed that increase in weft tension increases flexural rigidity, which determines the stiffness and hence drape coefficient, but decreases the wrinkle recovery angle

250

Role of yarn tension in weaving

of the fabrics mainly at the grey state. Sizing operation makes the constituent yarns more rigid and hence reduces the flexibility of the fabrics at the scoured state. Again, fabrics with higher stiffness are not easily wrinkled, as greater pressure is needed to crease a stiff fabric. But, once the wrinkles are formed in these types of fabric, they are not easily recovered, which is not so in the cases of less stiff that is, more flexible fabrics. From the above discussions on the effects of yarn tensions on the various cloth properties we thus find that, although the yarn tensions may not always have been found to show their precise effects on some of the properties possibly because of some unaccounted factors, this can nevertheless be inferred that, for achieving the desired properties of a given type of cloth woven on several weaving machines at the same time or at different times identical, not only the maintenance of proper machine settings but also the maintenance of correct weaving tensions of the warp and weft yarns are equally important.

10.5 Weave prominence Besides the different properties of the cloths discussed above, the yarn tensions affect the prominence of the weaves of the cloths also. In gabardine cloths the diagonal lines formed by the weave should be clear and well defined. This can generally be achieved by setting the back-rest well down [5] and thus, creating variation in warp tensions between two shed lines, with the tension of the yarns forming the top shed much higher than that of the yarns forming the bottom shed. This causes the stuffing of the picks a little more difficult as the weft contraction slightly reduces and the warp contraction increases thus, making the warp more predominant than the weft on the face of the cloth. The resultant variation in warp tension caused by lowering the back-rest thus enables the warp floats to be slightly longer on the face of the cloth than on the back and the warp yarns therefore become more prominent.

Figure 10.11 Warp-ribbed and repp cloths.

Effects of yarn tensions on fabric properties

251

Similarly, in the warp-ribbed cloth, the rib effect is produced by bringing about difference in weaving tensions amongst the warp yarns of even the same linear density. The yarns with lower tension crimp more and create prominent ridges on the cloth surface (Fig. 10.11A). Another variety of ribbed cloth, known as repp cloth, which is much firmer and stronger than the ribbed cloth, is also woven with significant difference in tensions between two sets of warp yarns during weaving to create very prominent furrows. In this case, however, one set of yarns is finer and stronger and is woven under higher tension than the other which is coarser and softer and woven under lower tension, to produce the desired effects (Fig. 10.11B). To weave these types of cloths two warp beams, whether the yarn count is the same or not, are employed to enable the yarns to be woven under considerably different tensions.

10.6

Roll of yarn tensions on the remedy of some cloth defects

There are numerous defects of the cloth generated during weaving on the looms. Most of these are generally observed at the body, but some occur at the selvedges also, of the cloths. Many of these faults are again related to the tensions of the warp and weft yarns and these can, therefore, be rectified by taking proper care of yarn tensions during the process of weaving.

10.6.1 Starting mark or setting-on place During the normal operation of a loom the desired pick spacing of a cloth, as set by the take-up mechanism, is obtained by the fell of the cloth occupying the correct position in each pick. If for any reason, the position of the cloth fell is disturbed there will be disturbance in the desired pick spacing and a weft way fault, in the form of “starting mark” or “setting-on place”, will be produced in the cloth. This may occur when the loom has been stopped to rectify a fault and the cloth fell has not been positioned correctly before restarting the loom. This is owing to the operational error of the weaver. Another reason is the stoppage of the loom generally for a longer period say, over the night. The cloth fell then tends to creep away from its correct position. The cloth will tend to stretch towards the back of the loom that is, away from the weaver and the warp will tend to stretch towards the front of the loom that is, towards the weaver. The actual movement of the cloth fell will therefore depend on the difference in stretch between the warp and fabric. When the loom is restarted the pick spacing of the cloth will now vary from the correct one generating starting mark. If during the stoppage of the loom, the warp tension falls from the running tension, the weft density at the starting mark will be less and if the warp tension rises above the running tension the weft density will be more than the normal.

252

Role of yarn tension in weaving

Figure 10.12 Starting mark of cloth. Wulfhorst and Obolenski [35] have observed that the starting mark of a fabric consists of three characteristic zones: (a) the part of the cloth woven shortly before the loom stoppage, (b) the actual stopping point which is due to relaxation of the yarn/fabric combination, and (c) the part of the cloth woven on restarting of the loom, as illustrated in Fig. 10.12. These three zones compose of minimum of two picks inserted prior to the stoppage and five picks inserted after the stoppage. As can be expected, the fall in warp tension increases with the duration of loom stoppage and the stoppage of over 1 min usually leads to starting mark. The fundamental requirement for preventing a starting mark is, therefore, to carry out any repair work of yarn break (which generally is the major cause for loom stoppage) within 1 min.

10.6.1.1 Cloth relaxation phenomenon The creep or displacement of the cloth fell during loom stops is because of the difference in relaxation between warp yarns and the cloth. This is determined by a number of parameters like the modulus of elasticity of the warp yarn and the woven fabric, their relative lengths on the loom, yarn characteristics, construction parameters of the cloth, settings of the loom and so on. The cloth relaxation is therefore a complex phenomenon. During the course of weaving both the warp yarns and the cloth remain under tension forming together a flexible system. Both have their own elastic properties which again depend on their free lengths on the loom. The elastic property of the cloth naturally differs from that of the yarn because of yarn interlacement. Let us now see what may happen to the yarn-cloth system if the loom remains stopped.

Effects of yarn tensions on fabric properties

253

Figure 10.13 Yarn-cloth system on a stationary loom. Fig. 10.13 shows the woven fabric with the warp yarns, as mounted on a loom, with the fell of the cloth at F. Suppose the cloth is held at C and the warp sheet is held at W. Since the loom is at rest, both the cloth and the warp will relax as they are not under working state, but let us assume that the fell remains fixed at F even when the relaxation takes place. Now, suppose the basic weaving tension of the cloth and the warp is To and as the cloth and the warp relax, their tensions will fall but, the extent of fall will be different because of the difference in their elastic properties. Suppose the tension of the cloth falls from To to Tc and that of the warp from To to Tw. If the fell of the cloth is now released, it will drift to the extent say, ΔL to take up its equilibrium position where the tensions of both the cloth and the warp will again be equal to a new basic tension. If the cloth fell drifts towards the weaver, then

ΔL =

Tc − Tw Ew / lw + Ec / lc

(10.9)

where Ew is the elastic modulus of warp; lw is the free length of the warp from F to W; Ec is the elastic modulus of cloth; lc is the free length of the cloth from F to C. From equation 10.9 we find that (i) the equation holds good so long as the relaxation of the cloth or of the warp is not taken up during the process of relaxation, (ii) if the cloth and the warp relax equally there will be no drift of the cloth fell and (iii) the drift increases with the free length of the cloth or the warp or the both. During the period of relaxation the resultant fall in tension of the cloth cannot be taken up (unless of course, the loom is equipped with the negative take-up motion, which is very rare) but that of the warp can be taken up if the warp beam can rotate backward or the suspended back-rest can swing outward. The warp beam can rotate backward with the negative let-off motion of the type shown in Fig. 3.3A of Chapter 3 and the back-rest can

254

Role of yarn tension in weaving

swing outward with the positive let-off motion shown in Fig. 3.3B of Chapter 3. In such cases the warp tension remains constant at To during the stoppage of the loom and the drift of the cloth fell can be written as

ΔL =

To − Tc Ec / lc

(10.10)

It is usually observed that the fabric relaxes more than the warp yarns and so the cloth fell drifts away from the weaver during a loom stoppage. The extent and rate of relaxation depend on the duration of loom stoppage and the weaving tension of the warp yarns. The magnitude of cloth fell displacement increases with the duration of loom stoppage and consequently aggravates the intensity of the starting mark. If the loom remains stopped with the reed away from the cloth fell, the fabric relaxes more than the warp yarns generating a dense starting mark. On the other hand, if the loom is stopped with the reed at the beat-up position against the cloth fell, the warp yarns relax more than the fabric generating a light starting mark. With the type of let-off motion where the warp beam cannot move backward during the period of loom stoppage, the cloth fell always moves away from the weaver and the warp tension falls considerably. As the loom is now started, the warp beam cannot rotate forward immediately to deliver the warp until a fair number of picks is inserted and the warp tension rises well above the normal preset value. The rise in warp tension causes a thick place which ends abruptly when the warp beam starts moving [31]. We therefore find that, the propensity of developing the starting mark in the cloth due to loom stoppage can be evaded by eliminating the drift of the cloth fell and the rise in warp tension as the loom is restarted after stoppage. Drift of the cloth fell can be avoided if the loom is stopped with the reed at front position so that some tension is taken off the fabric and it will stretch less. Rise in warp tension can be avoided if the warp beam can be made to rotate immediately at the reduced tension when the loom is started. The occurrence of starting mark can also be avoided if very low warp tension (which however, should not be so low as to give rise to clinging tendency of the warp yarns or cause other problems) can be maintained [35]. If the warp tension, the cloth tension and the position of the suspended back-rest at the start-up phase of the loom remain the same as those on the looms operating normally, there will be no significant fabric fault. If the free lengths of warp and cloth are large, correct pick spacing becomes less dependent on the correct cloth fell position and the weaver may not have to adjust the fell of the cloth very precisely at restarting of the loom after a stoppage [13]. In this regard the negative type let-off with dead-weight (Fig. 3.3A of Chapter 3) seems preferable because it allows an adjustment of the cloth fell position without changing the warp tension. On the other hand, if the lengths of warp and fabric are short, the cloth fell displacement caused by relaxation is reduced. This is advantageous where the weaver is not expected to take any special action to

Effects of yarn tensions on fabric properties

255

adjust the fell position. Nevertheless, the type of fabric and the type of let-off motion employed are also critical for cloth fell position. When a loom is stopped for some time and then restarted it takes about 4 weaving cycles before the amplitudes of warp and cloth tension at beat-up attain their normal values [35]. The take-up motion rectifies the defect, generated by the disturbance in cloth fell position, automatically by gradually bringing the fell to its correct position. At every pick the action of the take-up motion causes the cloth fell to move towards its correct position by a distance equal to the difference between the actual and the correct pick spacing. Therefore, when the cloth fell is very far from its correct position, so that the actual pick spacing differs greatly from the correct value, the cloth fell will initially move very quickly towards the correct position but as the cloth fell approaches the correct position the movement will be slower. The movement of the fell will thus vary exponentially as shown in Fig. 10.14, which can be used to predict pick spacing from the fell position with accidental displacement of the cloth fell [9].

Figure 10.14 Return of cloth fell to the correct position after disturbance. The effect of the disturbance in the cloth fell position on pick spacing will be more adverse in a cloth with low weft density than that with high weft density and hence, maintenance of correct position of the cloth fell is more important with light fabrics than with heavy fabrics [9]. Again, as discussed earlier in Section 7.10 of Chapter 7, the free lengths of the warp and fabric and also the type of let-off control play crucial roles in controlling the setting on places by manual adjustment of the cloth fell. Difference in relaxation between the warp yarns and fabrics is much greater in case of filament yarns than in case of the staple fibre yarns [32] mainly because of larger fall in tension of more stretchable filament warp yarns and the effect is aggravated by relative humidity [31].

256

Role of yarn tension in weaving

Vangheluwe and Kiekens [33] have calculated dynamic modulus relative to the count of warp yarns for each loading cycle during cyclic loading.

Ed =

Fu – Fl ( Eu – El )nTw

(10.11)

where Ed is dynamic modulus in cN/tex; Fu is the force at the upper limit during loading (cN); Fl is the force at the lower limit during loading (cN); Eu is the elongation at the upper limit during loading; El is the elongation at the lower limit during loading; n is number of warp yarns in the sample; Tw is the linear density, i.e. count of warp (tex). With a view to studying the cloth fell displacement, relaxation and cloth fell displacement have been measured for 300 s under different magnitudes of loading and three kinds of cloth fell displacement have been observed, as indicated in Fig. 10.15.

Figure 10.15 Cloth fell displacement characteristic. The displacement is (a) positive and increases with time when relaxation starts from a high load, (b) negative and decreases with time when relaxation starts from a low load and (c) mixed where the cloth fell displacement initially increases and then decreases with time. The cloth fell displacement is, therefore, the function of the force from which relaxation is started, fabric geometric parameters and the properties of the yarns in the fabric. The weave has significant influence on cloth fell displacement. Plain weave fabric shows the largest cloth fell displacement. As constructional properties of the fabric

Effects of yarn tensions on fabric properties

257

(weave, yarn counts and yarn densities) influence warp and weft crimps in the fabric, warp crimp has significant correlation with cloth fell displacement, particularly at high load, but not the weft crimp. Weft yarn type and weft density of course, have significant influence on cloth fell displacement. There is also significant correlation between residual tension and interlacement of warp and weft yarns [32]. Fabrics with greater warp interlacement have lower residual tension (and therefore more relaxation). Although the correlation between fabric elastic modulus and warp yarn interlacement is significant on cloth relaxation, the effect of weave is more important than that of yarn interlacement. Severity of starting mark depends on the weave [35], type of shedding motion used and as stated above, the length of loom stoppage time [2, 17]. It has been observed that plain cloth woven from viscose filament yarn is more susceptible to starting marks than those of twill and satin weaves, because of comparatively fewer number of interlacing points in the latter [35]. Studies with two plain cloths woven on jacquard looms and one plain cloth woven on dobby loom with the same weft density but different yarn counts, types and materials have shown that the loom stoppage time of 1, 5 and 10 min produce starting mark on all the cloths [2]. While the duration of stoppage has significant effect on the starting mark of the cloths woven on jacquard looms, the effect is not so significant on the one woven on dobby loom. Jacquard loom fabrics were woven with ring spun cotton and polyester filament warps and rotor spun cotton weft and that made on dobby loom was woven with ring spun cotton warp and rotor spun cotton weft yarns. Due to relaxation of warp/cloth during loom stoppage, the warp tension falls and consequently the sensing back-rest of the positive let-off motion rises. Lunenschloss and Schlichter [17] have recorded the nature of fall in warp tension vis-a-vis rise in back-rest position owing to warp relaxation during loom stoppage (Fig. 10.16), having measured the tension of the group of warp yarns. They have found that as the changes can be recorded much quicker and more accurately, it is possible to make correct placement of the cloth fell before restarting the loom to reduce the starting mark.

Figure 10.16 Back-rest movement and warp tension fall during loom stoppage.

258

Role of yarn tension in weaving

From the above discussions we thus find that, the reason for starting mark of the cloth at the restart of the loom after a stoppage is the displacement of the cloth fell from its correct position, as determined by the weft density of the cloth, because of relaxation of the cloth/warp combination resulting from the fall in warp tension. As the displacement of the cloth fell depends on the free lengths of the warp and cloth in the loom, it is generally desirable to have a long length of warp (within reason of course) and a short length of fabric. There may be three means to eliminate the starting mark or setting-on place [9]: (a) modification of the loom drive, so that the loom attains its full running speed quickly after stoppage, (b) modification of the warp control system in the let-off motion so that warp tension is kept constant during a stoppage, and (c) introduction of some kind of cloth fell indicator which enables the weaver to restore the cloth fell to its correct position before starting the loom. Modern weaving machines are equipped with appropriate devices to take control of this problem. The possibility of starting mark of the cloth on restarting the loom with the disturbed position of the fell of the cloth can also be avoided by incorporating modified system of sley drive [29], discussed above in Section 10.3.

10.6.2 Repping Stoppage of loom for some long time, particularly with shed open and asymmetric shed, can cause another fault of the cloth called, “repping”. As we have known, in asymmetric shed with the back-rest set at raised position, the yarns of the bottom shed are tensioned much more than those of the top. If the loom is now stopped for an appreciable time with the shed open, relaxation of the yarns will occur. The yarns under higher tension will naturally relax more than those under lower tension. As a result, the yarns of the bottom shed will experience greater fall in tension than those of the top shed during the period the loom remains stop. When the loom is now started the mean tensions of the alternate warp ends will no longer be equal and the wefts are displaced vertically, as indicated in Fig. 10.17, producing corrugated appearance, the repping fault of the cloth.

Figure 10.17 Mechanism of repping fault of the cloth.

Effects of yarn tensions on fabric properties

259

The repping fault is thus, the result of two different values of tension of the alternate warp yarns which in turn, produce two different values of warp and weft crimp. The warp yarns which are now under higher tension (originally lower tension at the top shed), shown by firm lines in Fig. 10.17, generate more weft crimp and have less crimp themselves and conversely, those warp yarns which are now under lower tension (originally higher tension at the bottom shed), shown by broken lines, generate less weft crimp and have more crimp themselves. As a result, more yarn is used for the warps under lower tension till all the yarns are again tensioned equally and the fault disappears. Before this, however, the repping fault is already produced in the cloth. In this context it may be noted here that, the vertical displacements of the picks occur both while producing the repping fault in the cloth and for achieving higher weft densities, as discussed above and shown in Fig. 10.9. In regard to the latter, the pick movement is because of asymmetric setting of the shed and the rapid reversal of tension level of alternate high and low tensions of the warp yarns in the successive pick cycles ultimately results in equal mean tension of all the warp yarns and thus, laying of the picks in the same level but closer to each other. In regard to the former on the contrary, the vertical displacements of the picks persist as the warp yarns under high and low tensions remain so for a long time and thus causes the fault in the cloth. Greenwood [11, 10] has shown how to measure the intensity of the repping fault. If a straight line is drawn across the warp sheet at a place where it leaves the warp beam and allowed to be woven into the cloth, it will, under the normal condition, appear as one straight line, may be slightly blurred, on the cloth. Slightly blurred because, all the warp yarns of the sheet can hardly be expected to be unwound from the beam under the exactly equal tension. In case of repping fault on the other hand, the single straight line is split into two parallel lines in the cloth because of unequal tensions resulting from differential relaxation of the alternate warp ends owing to long loom stoppage. The distance between the two parallel lines indicates the intensity of the fault. The portion of the cloth containing the repping fault has nonuniform warp crimp which may lead to about 50% loss in warp strength in addition to non-uniformity of fabric thickness [11]. It may be recalled here that, for the same reason of change in warp crimp the author has also observed fall in the warp way breaking strength of the cloths woven with two significantly different weft counts, as discussed above in Section 10.3. Taking into consideration the cause of the fault, two measures should be taken to avoid it [10]; whenever the loom is to be stopped for some times it should be stopped with the shed closed and the shed should be so adjusted that there is no difference in tension between the warps of two shed lines. The first measure is now adopted in all the modern looms where, in the event of warp or weft break, the looms are automatically stopped with the shed closed. The second measure of maintaining equal warp tension of two shed lines however,

260

Role of yarn tension in weaving

has the propensity of producing another fault, the reed mark of the cloth, as discussed below. So a compromise has to be met.

10.6.3 Reed mark or reediness One of the serious defects of the cloth, particularly in case of plain weave construction, is the “reed mark” or “reediness”. The groups of warp ends as drawn through the individual dents of the reed run in the similar manner in the cloth producing pronounced warp way streaks along the entire width of the cloth. As the ends do not lie at the equal spacing, the cloth is produced with lack of good cover. The reason for this is the inadequate tension difference between the ends of a reed dent at the instant of formation of the cloth. The desired tension difference is generally achieved by raising the back-rest with respect to the positions of shed level and the front-rest that is, by arranging asymmetric shed line. The warp yarns are then, tightened and slackened alternately as the heald shafts move up and down and thus spread in the cloth to cover the bare wefts opposite the reed wires. Neogi and Bhattacharyya [21] have made a detailed study to examine the effects of different loom settings and the role of warp tension in eliminating the reed mark of a plain jute cloth woven with 39 ends/dm of 275.6 tex warp and 35.6 picks/dm of 292.8 tex weft yarns. Denting pattern of the warp is 2 in a dent for the body and 4 in a dent for the selvedge of the cloth. As stated above, the reed mark is produced because of persistence of unequal distance between the two adjacent warp ends drawn through a reed dent and between those two separated by the reed wire in the cloth. The reed mark of the cloth has been expressed in quantitative form as ‘reed mark percentage’ (RMP) where

RMP =

( A – B) × 100 B

(10.12)

where A is the distance between two adjacent warp ends separated by a reed wire, and B is the distance between two successive warp ends within a dent. Different positions of the back-rest, timings of shedding motion and the leasing patterns of the warp yarns have been arranged to bring about the variations in warp tension needed for annulling the reed mark. Different backrest heights ranging from 3.5 to 13.5 cm (maximum of 11.5 cm with twoby-two leasing) above the front-rest, shed timings with shed level at 45° and 90° before beat-up and three types of warp leasing patterns, two of which are shown in Fig. 7.13 of Chapter 7 and the third, a modified two-by-two, is illustrated in Fig. 10.18.

Effects of yarn tensions on fabric properties

261

Figure 10.18 Modified two-by-two warp leasing. The basic difference between the two types of two-by-two leasing pattern shown in Figs. 7.13B of Chapter 7 and 10.18 is the manner the two warp ends of the two heald shafts pass through the reed dent and the lease rods. In the conventional type shown in Fig. 7.13B of Chapter 7 (type-1), if a given pair of ends (of the two healds) of a given dent passes over the front and under the back lease rods, the next pair of the adjacent dent passes in the reverse order that is, under the front and over the back lease rods and so on. In the modified type shown in Fig. 10.18, a given pair of ends of the two heald shafts passing through the two lease rods in the similar manner passes through the two successive dents of the reed and not through the same dent like in Fig. 7.13B of Chapter 7 (type-2). The modified twoby-two leasing is thereby, expected to produce even greater variations in warp tensions during weaving.

Figure 10.19 Effects of shed timing and back-rest position on reed mark.

262

Role of yarn tension in weaving

RMP for each loom setting has been calculated from the measurements made on the cloth at the grey state. Irrespective of the type of leasing pattern of the warp yarns, reed mark decreases with earlier timing of shedding motion, Fig. 10.19A, and increasing height of the back-rest, Fig. 10.19B. From Fig. 10.19 it is thus observed that, the reed mark is the least when the weft is beaten with the shed timing arranged as much early as possible and the back-rest is set at the highest possible position. With these settings of shed timing and back-rest, the warp yarns of the bottom shed line are under high and those of the top shed line are under low tensions at beat-up that is, at the instant of formation of the cloth.

Figure 10.20 Warp yarn movements at the cloth fell. Fig. 10.20 demonstrates the behavior of the warp yarns at the fell of the cloth while producing very high and almost no reed mark in the cloths, which

Effects of yarn tensions on fabric properties

263

persisted even at the grey state. When neither the shed timing nor the backrest setting is conducive for eliminating the reed mark the warp yarns in Fig. 10.20A are clearly seen to run in pairs in the cloth in the same manner as they pass through the reed dents and there is no evidence of side-wise movement of either of the yarns in a pair to cover the bare spaces created by the reed wires. But, when both the shed timing and the back-rest setting are appropriate for eliminating the reed mark every alternate end is seen to have moved away from its partner, indicated by arrow in Fig. 10.20B, and thus, reduces the reed mark. It can also be noted from the same figure that, the side-ways movements of the ends took place when the yarns were forming the top shed and therefore, slack. If we now suppose, that the back-rest is at the highest position but beat-up occurs when the shed is level, all the warp yarns will experience the same beatup tension and reed mark will not be reduced. Similarly, if the shed timing is as early as possible, say 90° before beat-up, but the back-rest is set at the low level so that the shed configuration is nearly symmetric all the warp yarns will again be under the same beat-up tension and there will be no remedial effect on reed mark. The prime cause for elimination of reed mark is thus, the difference in tensions amongst the warp yarns at the time of beat-up. The same benefit can also be obtained by lowering the back-rest with respect to the position of the front-rest or by raising the front-rest, but these are seldom done in practice. It is further noted from Fig. 10.19 that, for any shed timing and back-rest height, reed mark reduces when the warp leasing pattern is changed from one-by-one (Fig. 7.13A of Chapter 7) to the conventional two-by-two (type1) (Fig. 7.13B of Chapter 7) and then to modified two-by-two (type-2) (Fig. 10.18). With one-by-one leasing, warp tensions of the yarns, A and B of the two heald shafts (Fig. 7.13A of Chapter 7) are different at beat-up with early shedding and raised back-rest but, the tensions of the yarns A or B of a given heald shaft, are fairly the same and hence the magnitude of difference in beatup tension is limited. Let us now see what happens with two-by-two leasing patterns shown in Figs. 7.13B of Chapter 7 and 10.18 with the front heald shaft forming the bottom shed and the back heald shaft forming the top shed. For the convenience of discussion the path of the yarns through two lease rods, as can be seen from the end of the shed, have been shown in Fig. 10.21.

Figure 10.21 Warp path with 2-by-2 leasing.

264

Role of yarn tension in weaving

In the conventional two-by-two leasing (type-1) indicated in Fig. 7.13B of Chapter 7, half of the total warp ends, abc in Fig. 10.21, of the front heald shaft forming the bottom shed are diverted from the front lease rod located nearer to the cloth fell and therefore the yarns abc have the maximum beatup tensions, say ‘Maxh’, (abc = Maxh). Also see Fig. 7.14B of Chapter 7 for this part of discussion. In the same dent, half of the total warp ends, aef (Fig. 10.21) of the other heald shaft are diverted from the back lease rod located further away from the cloth fell to form the top shed and hence, the yarns aef have the minimum beat-up tensions, say ‘Minl’(aef = Minl). In the other dents at the same instant the rest of the ends, abd of the front heald shaft are diverted from the back lease rod to form the bottom shed and the yarns abd will have the beat-up tensions, which are lower than ‘Maxh’. Let us assume the beat-up tensions of the ends abd are ‘Maxl’ (abd = Maxl) and Maxl < Maxh. Similarly, the rest of the ends, aeg of the other heald shaft forming the top shed are diverted from the front lease rod and the yarns aeg will have the beat-up tensions, which are higher than ‘Minl’. Let us assume the beat-up tensions of ends aeg are ‘Minh’ (aeg = Minh) and Minh > Minl. From this we find that in half of the total number of reed dents which holds the ends, abc and aef, the difference in beat-up tension of the yarns is Maxh − Minl = High and in the rest of the dents which holds the ends, abd and aeg, the difference in beat-up tension of the yarns is Maxl − Minh = Low. Thus the differences in beat-up tensions between the warp yarns of the two shed line are much higher in half of the total reed dents and much lower in the rest. Owing to this unbalanced nature of beat-up tension difference, the beneficial effect of high beat-up tension difference between the ends abc and aef in reducing the reed mark is greatly subdued by the low beat-up tension difference between the ends abd and aeg. If, therefore, the ends abc are dented with the ends aeg and the ends aef are dented with ends abd, as shown in modified two-by-two warp leasing pattern (type-2) in Fig. 10.18, the beat-up tension difference between the ends abc and aeg will be Maxh − Minh = Medium and that between the ends abd and aef will be Maxl − Minl = Medium. More balanced but significant difference in beat-up tensions between all the warp yarns of two shed lines are now more effective in reducing the reed mark of the cloth. The same is true with the front heald shaft forming the top shed and the back heald shaft forming the bottom shed, but the levels of the warp tension will differ depending on the positions of the heald shafts and the resultant diversions of the warp ends from the lease rods. Photographs in Fig. 10.22 clearly indicates the gradual reduction in reed mark or in other words, the improvement in cover of the cloth with a given shed timing (shed level position at 90° before beat-up) and back-rest height (highest position of the back-rest) but change in warp leasing pattern from one-by-one (A) to conventional two-by-two (B) and then to modified two-bytwo (C).

265

Effects of yarn tensions on fabric properties

\

Figure 10.22 Effect of warp leasing pattern on cloth cover.

266

Role of yarn tension in weaving

Thus, the well known method of reducing or eliminating the reed mark of the cloth is to arrange asymmetric shed and early shed timing. These settings have another merit of increasing the weft density of the cloth (see Section 10.4.1). But the main draw back of these are the chances of increase in warp breakage, as discussed in Section 9.2.1 of Chapter 9, because of unequal share of beat-up force by the warp yarns. From this point of consideration therefore, it may be assumed that if the modified two-by-two leasing pattern is adopted, reasonably good cover of the cloth can be achieved with relatively later timing of shedding motion and/or lower position of the back-rest because of its ability to create greater variation in tension between the yarns of each reed dent. This may of course not help much to achieve high weft densities in some dense fabrics, but this will in most cases help to achieve increased weaving efficiency because of reduction in warp breakage. Depending on the necessity, a compromise can therefore be made.

Figure 10.23 Relationship between RMP and beat-up tension difference for different shed timings and back-rest positions. Since the difference in tension between the yarns of two shed lines at beatup is the cause for reduction of reed mark, the relationships between the RMP at grey state of the fabric and the beat-up tension difference for different shed timings and back-rest heights are shown in Fig. 10.23. For both the cases of shed timing and back-rest position, RMP is related inversely with beat-up tension difference and the relations are statistically significant. The reed mark

Effects of yarn tensions on fabric properties

267

of the cloth reduces linearly with the increase in the difference in beat-up tension between the yarns of two shed lines but the effect of the back-rest setting is more pronounced. In another study [4] with the same loom and fabric parameters, the authors have found that irrespective of the loom settings and leasing pattern of the warp yarns, reed mark reduces with the increase in basic warp tension and the distance of the lease rods from the heald shafts. With the increase in warp tension the difference at beat-up tension also increases and resultantly the reed mark decreases. The reasons for the decrease in reed mark with the increase in the distance of the lease rods from the heald shaft with no definite effect on the beat-up tension difference are possibly that, as the lease rods are moved away from the heald shafts the consequent greater length of jute yarns, which have high flexural rigidity, become more free to move sideways under whatever the difference in beat-up tensions obtained and the proximity of the lease rods from the back-rest produces nearly the similar effects as by the raised backrest. These together help improve the cover of the cloth. The weft tension however, has no effect on reed mark, as can be expected.

10.6.4 Stitching In the preceding section relating to reed mark it has been stated that the asymmetric shed line with the top shed slack and the bottom shed taut is beneficial for remedy of the problem. A very slack top shed may however cause the pick pass over the slack yarns of the top shed during weft insertion creating the defect called “stitching”. This problem is more acute when weaving warp way dense cloths with hairy yarns which tend to cling together. From this it appears that the usual means to avoid this is to keep the yarns of the top shed evenly taut particularly during picking. The backrest should therefore be so set as to avoid the problem of stitching but at the same time extract the advantages of the asymmetric shed. Increasing the shed depth, the warp tension and early timing of the shedding motion are also helpful as during beat-up the picks are then carried forward by the reed to the cloth fell against the considerable friction between the crossed warp yarns and in the process detach the adjacent yarns held together by the surface fibres. Care should however be taken while increasing the warp tension as, excessive warp tension may cause uneven letting-off of the warps [5] in addition to increased warp breakages. Staggering of heald shafts to prevent overcrowding of the warp ends during crossing is also a useful means to prevent stitching. For the purpose there are heald shafts in which the mail eyes are set at slightly different levels [5] and thus avoid crossing of all the warp ends at one level.

268

Role of yarn tension in weaving

10.6.5 Selvedge distortion Various defects at the selvedges of the cloths like loopy, curly, kinky selvedge etc are mainly owing to the tensions of the warp or weft yarns. Structure of the selvedge, more often than not, differs significantly from that of the body of the cloth and that creates complexity, as the yarn fabric relationship in light of yarn tension plays different role in these two parts of the cloth. It is even more so in cases of the cloths made on shuttleless weaving machines, as the selvedges are formed with the help of separate selvedge forming mechanisms often using the materials different from those at the body of the cloth. There are also differences in the relaxation behavior between the warp yarns and the fabric because of their different elastic modulus and these may cause selvedge distortion. Again, as we have observed in Section 7.9 of Chapter 7, the warp tensions at the selvedges are much less than that at the body of the cloth because of the temples. This usually causes entanglement of the warp yarns at the selvedges. Too long rear shed and high shed asymmetry augment this problem even further. These have detrimental effects on the cloth selvedges, which again, are rather more acute for the cloths woven in shuttleless looms because of small depth of shed maintained there. From this point of view, full width temple is more appropriate as it minimizes the difference in warp tensions between the selvedge and the body of the cloth. It has been observed earlier in Section 8.2.2 of Chapter 8 that in case of shuttle picking, late timing of shedding motion produces fairly low final tension of the weft at shuttle checking. This low weft tension generally causes loopy and curly selvedges of the cloths. These can be rectified if the weft withdrawal tension is increased or shed timing is made early. Final weft tension will be high with high withdrawal tension. Early shed timing with the given withdrawal tension of the weft will also increase the final weft tension. These will rectify these faults and improve the selvedges formed. While increasing the weft withdrawal tension along with early shedding care should be taken. If the withdrawal tension is set high that, with early shedding will further increase the weft tension at the selvedges and result in selvedge breakage or distortion of the selvedges. Another reason for fault of the selvedge, like formation of kinky selvedge, is the sudden change in weft tension at the change of pirn. We have observed that, there is considerable rise in weft unwinding tension at the end of the pirn with straight base and as soon as the pirn is changed the weft is unwound from the full pirn and the withdrawal tension becomes low. This sudden change in tension is very likely to cause distortion of the selvedges at the change of the pirn. Pirn with conical base can eliminate this problem as it does not cause much change in weft tension when the pirn is nearly exhausted. From these we find that the main reason for distortion of the selvedges of the cloths woven on shuttle looms is the final weft tension. Taking this into consideration, fitting of spring

Effects of yarn tensions on fabric properties

269

eye in the shuttle will help minimize the change in withdrawal tension and consequently distortion of the cloth selvedge [30]. It is quite obvious that the properties of a cloth are basically determined by the physical parameters of the constituent yarns of the cloth and the weave. Weaving, in true perspective, does not mean merely to weave a cloth on a loom. The woven cloth must be of the desired quality characteristics commensurate with the purpose it is made for. Here comes the importance of yarn tensions. While the primary necessities for tensions of the warp and weft yarn are to weave a cloth, changes in tensions during the process of weaving can significantly influence the various properties as well as the appearance of the fabrics made with the same constructional parameters. Yarn tensions directly affect the crimps of the yarns and thereby, many of the dimensional, physical and comfort properties of the cloths. Cloths produced on even the best of the weaving machines cannot always be guaranteed to be completely fault free and many of the faults are found to be yarn tension related. Judicious control of yarn tensions and that too depending on the type of cloth being woven thus often become the prime factor for ensuring the desired qualities of the cloths being woven and the performances of the weaving machines under operation.

References 1.

Adanur S. and QI J., (2008). Property analysis of denim fabrics made on air-jet weaving machine, Part II: Effects of tension on fabric properties, Textile Research Journal, Volume 78, pp. 10–20.

2.

Ayala A.L. and Govindaraj M., (2001). Detecting and quantifying set marks on woven fabrics, Textile Research Journal, Volume 71, pp. 587–595.

3.

Basu A.K., (1987). Effect of different let-off mechanisms on fabric formation and dimension of fabric in the loom, Textile Research Journal, Volume 57, pp. 379–386.

4.

Bhattacharyya P.K. and Neogi S.K., (1990). Reed mark of fabric with different weaving parameters, Journal of the Textile Association, Volume 50, pp. 302–304.

5.

Bramma F., (1955). Some effects of shed timing and setting on weaving, The Journal of the Textile Institute, Volume 46, pp. P405– P412.

6.

Bullerwell A.C. and Mohamed M.H., (1991). Measuring beat-up force on a water jet loom, Textile Research Journal, Volume 61, pp. 214– 222.

270

Role of yarn tension in weaving

7.

Chahal V. and Mohamed M.H., (1986). Measuring filling yarn tension and its influence on fabrics woven on a projectile weaving machine, Textile Research Journal, Volume 56, pp. 324–333.

8.

Crawshaw H., Morton W.E. and Brown K.C., (1931). Experiments in fabric wear testing, The Journal of the Textile Institute, Volume 22, pp. T64–T76.

9.

Greenwood K., (1956). The theory of pickspacing in woven fabrics, Skinner’s Silk and Rayon Records, Volume 30, pp. 1154–1160.

10. Greenwood K., (1957). Relaxation phenomena in weaving, The Journal of the Textile Institute, Volume 48, pp. 206–213. 11. Greenwood K., (1975). Weaving: control of fabric structure, Merrow Publishing Co. Ltd., England, pp. 42–56. 12. Greenwood K. and Vaughan G.N., (1959). The effect of weft yarn irregularity on pickspacing and weft cover factor, The Journal of the Textile Institute, Volume 50, pp. T337–T352. 13. Greenwood K. and Cowhig W.T., (1956). The position of the cloth fell in power looms, Part II – Disturbed weaving conditions, The Journal of the Textile Institute, Volume 47, pp. T255–T265. 14. Gu H., (1984). Reduction of warp tension fluctuation and beat-up strip width in weaving, Textile Research Journal, Volume 54, pp. 143–148. 15. Hepworth K., (1992). Comments on “Measuring beat-up force on a water jet loom”, Textile Research Journal, Volume 62, pp. 121–122. 16. Kalli F., (1990). Correlation analysis between fabric data and stoppage rate in rapier weaving machinery with reference to cotton fabrics, Melliand Textilberichte, Volume 71, pp. 35–37. 17. Lunenschloss J. and Schlichter S., (1987). The development of new measuring Elements for electronically controlled warp let-off units, International Textile Bulletin, Fabric Forming, No 3, pp. 56–71. 18. Morton W.E. and Williamson R., (1939). The influence of varying warp tension on some physical properties of plain cotton cloth, The Journal of the Textile Institute, Volume 30, pp. T137–T156. 19. Neogi S.K. and Bhattacharya P.K., (1979). Effect of varying yarn tension on jute fabric, The Indian Textile Journal, Volume 90, pp. 127–132. 20. Neogi S.K. and Bhattacharyya P.K., (1983). Setting the width, Textile Asia, Volume 14, pp. 42 21. Neogi S.K. and Bhattacharyya P.K., (1988). Reed mark of fabric and its relation with warp tension at beat-up with different loom settings, Indian Journal of Textile Research, Volume 13, pp. 151–157.

Effects of yarn tensions on fabric properties

271

22. Nichiporchik L.D., (1967). The variation of the structure of a plainweave fabric with the ratio of the warp and weft tensions, Technology of the Textile Industry U.S.S.R., No 2, pp. 58–61. 23. Nyubikova K.G., (1969). The effect of the warp tension on the beat-up force in the processing of Calico ART. 4202, Technology of the Textile Industry U.S.S.R., No 6, pp. 74–76. 24. Ramaswamy B.R. and Paliwal M.C., (1965). Warp tension in weaving: Its effect on performance and quality, Proceedings of the 6th Technological conference of ATIRA, BTRA & SITRA, pp. 87–95. 25. Ronz C. and Scholze G., (1993). Influence of warp tension on fabric quality in air-jet weaving, Melliand Textilberichte, Volume 74, pp. 1112–1113. 26. Shih Y., Mohamed M.H., Bullerwell A.C. and Dao D., (1995). Analysis of beat-up force during weaving, Textile Research Journal, Volume 65, pp. 747–754. 27. Snowden D.C., (1950). Some aspects of warp tension, The Journal of the Textile Institute, Volume 41, pp. P237–P249. 28. Sternheim A. and Grosberg P., (1991). The effect of sley motion on the beat-up force, The Journal of the Textile Institute, Volume 82, pp. 325–331. 29. Sternheim A. and Grosberg P., (1991). Weaving with constant beat-up force, The Journal of the Textile Institute, Volume 82, pp. 317-323. 30. Townsend M.W.H., (1955). Weft tension in weaving, The Journal of the Textile Institute, Volume 46, pp. P699–P712. 31. Turton G. and Greenwood K., (1954). Weaving faults: avoiding defects in filament rayon, The Textile Weekly, Volume 53, pp. 975–978. 32. Vangheluwe L., (1995). Simulation of woven fabric relaxation under different weaving conditions, Melliand Textilberichte, Volume 76, pp. 612–619. 33. Vangheluwe L. and Kiekens P., (1995). Test method for cloth fell displacement caused by relaxation, Textile Research Journal, Volume 65, pp. 540–544. 34. Wagh A.S. and Snowden D.C., (1976). Width contraction during weaving, Textile Highlights, Volume 3, pp. 27–32. 35. Wulfhorst B. and Obolenski B., (1990). Assessment of measures for preventing starting marks in woven fabrics, Melli and Textilberichte, Volume 71, pp. 24–34.