Basics of Weaving

BASICS OF WEAVING

1.1 Introduction

     The process of producing a fabric by interlacing warp and weft threads is known as weaving. The machine used for weaving is known as weaving machine or loom. Weaving is an art that has been practiced for thousands of years. The earliest application of weaving dates back to the Egyptian civilization. Over the years, both the processes as well as the machine have undergone phenomenal changes. As of today, there is a wide range of looms being used, right from the simplest handloom to the most sophisticated loom.

     In this rang, the most widely prevalent loom, especially with reference to India, is the ubiquitous “plain power loom”. In this and in the chapters that follow, the various mechanisms associated with the plain power loom are discussed in elaborate detail.

1.2 Basic Mechanisms in a Plain Power Loom

     In order to interlace wrap and weft threads to produce a fabric, the following mechanisms are necessary on any type of loom:

1. Primary mechanisms
2. Secondary mechanisms
3. Auxiliary mechanisms

1.2.1 Primary Mechanisms

     These are fundamental or essential mechanisms. Without these mechanisms, it is practically impossible to produce a fabric. It is for this reason that these mechanisms are called ‘primary’ mechanisms. The primary mechanisms are three in number.

1. Shedding mechanism
2. Picking mechanism
3. Beat-up mechanism

1. Shedding mechanism

     The shedding mechanism separates the warp threads into two layers or divisions to form a tunnel known as ‘shed’

2. Picking mechanism

     The picking mechanism passes weft thread from one selvedge of the fabric to the other through the shed by means of a shuttle, a projectile, a rapier, a needle, an air-jet or a water-jet. The inserted weft thread is known as “pick”.

3. Beat-up mechanism

     The beat-up mechanism beats or pushes the newly inserted length of weft thread (pick) into the already woven fabric at a point known as “fell of the cloth”. These three mechanisms namely shedding, picking and then beat-up are done in sequence.

1.2.2 Secondary Mechanisms

     These mechanisms are next in importance to the primary mechanisms. If weaving is to be continuous, these mechanisms are essential. So they are called the ‘secondary’ mechanisms. They are:

1. Take-up motion
2. Let-off motion.

1. Take-up motion

     The take-up motion withdraws the cloth from the weaving area at a constant rate so as to give the required pick-spacing (in picks/inch or picks/cm) and then winds it on to a cloth roller.

2. Let-off motion.

     The let-off motion delivers the warp to the weaving area at the required rate and at constant tension by unwinding it from the weaver’s beam. The secondary motions are carried out simultaneously.

1.2.3 Auxiliary Mechanisms

     To get high productivity and good quality of fabric, additional mechanisms, called auxiliary mechanisms, are added to a plain power loom. The auxiliary mechanisms are useful but not absolutely essential. This is why they are called the ‘auxiliary’ mechanisms. These are listed below.

1. Warp protector mechanism
2. Weft stop motion
3. Temples
4. Brake
5. Warp stop motion (Predominantly found in automatic looms)

1. Warp protector mechanism

     The warp protector mechanism will stop the loom if the shuttle gets trapped between the top and bottom layers of the shed. It thus prevents excessive damage to the warp threads, reed wires and shuttle.

2. Weft stop motion

     The object of the weft stop motion is to stop the loom when a weft thread breaks or gets exhausted. This motion helps to avoid cracks in a fabric.

3. Temples

     The function of the temples is to grip the cloth and hold it at the same width as the warp in the reed, before it is taken up.

4. Brake

     The brake stops the loom immediately whenever required. The weaver uses it to stop the loom to repair broken ends and picks.

5. Warp stop motion

     The object of the warp stop motion is to stop the loom immediately when a warp thread breaks during the weaving process.

1.3 Passage of Warp and Cloth through a Plain Power Loom

     Figure 1.1 shows the passage of a warp sheet and cloth through a plain power loom.

     A warp sheet A from a weaver’s beam B passes around a back rest C and is led around lease rods D to healed shafts E & F which are responsible for separating the warp sheet into two layers to form a shed. The purpose of the back rest and the lease rods is to separate the warp yarns uniformly and precisely, and reduce entanglement and tension in the yarns during the opening of the warp shed.

	A - Warp sheet
	B - Weaver's beam
	C - Back rest
	D - Lease rods
	E - Heald shaft
	F - Heald shaft
	G - Reed
	H - Cloth
	I - Weft yarn
	J - Temples
	K - Front rest
	L - Take-up roller
	M - Guide roller
	N - Cloth roller

Figure 1.1 Passage of warp and cloth through a plain power loom In the CD-ROM, watch Animation No. WFP - l 1.1

The warp yarns then pass through a reed G, which holds the yarns at uniform spacing and is also responsible for beating-up the weft yarn I into the fell of the cloth. After the weft is beaten up, the warp yarns interchange positions in the shed and thereby cause interlacing to be achieved. At this point, cloth is formed and is held firmly by temples J to assist in the formation of a uniform cloth. The cloth H then passes over a front rest K, around an emery roller or take-up roller L and a guide roller M and is finally wound on to a cloth roller N.

1.4. Motion of Heald Shafts, Shuttle and Sley

     In a plain power loom the heald shafts, shuttle and sley are operated by mechanisms that are set in motion by a motor through a crankshaft and a bottom shaft. The heald shafts move up and down by the shedding mechanism. The motion is obtained from the bottom shaft or counter shaft that carries the tappets. So the warp sheet is divided into two layers and it forms a shed.

     The shuttle is pushed into the warp shed by a picker that gets activated by a picking mechanism. Normally the shuttle is kept in a shuttle box. When the shuttle is pushed, it reaches the opposite box. The arrival of the shuttle in the opposite box is confirmed by shuttle checking devices. The picking mechanism is set in motion by the bottom shaft.

The crankshaft operates the sley through the crank and crank arms. The sley gets a to and -fro motion. As the sley reciprocates, the reed, which is fixed to the sley, also gets a to-andfro motion. The reed thus beats up the weft into the fell of the cloth.

1.5 Warp and Cloth Control

     The shuttle is pushed into the warp shed by a picker that gets activated by a picking After beating up the weft into the fell of the cloth, a take-up motion draws the cloth forward and winds it on to a cloth roller. At the same time the warp is delivered from the weaver’s beam by a let-off motion.

     These two motions are operated simultaneously and at a constant rate. i.e. the rate of cloth take-up is so set as to be equal to the rate of warp let-off. The take-up motion is operated through a sley stud and gear mechanism. The let-off motion operates by the pulling action of the cloth.

     The two temple pieces located at the selvedges of the cloth control width.

1.6 Stop Motions

     To ensure good productivity and quality of cloth, the following stop motions are used: The warp protector mechanism protects the warp from breakages during shuttle trap and stops the loom immediately.

     The weft stop motion stops the loom if a weft thread breaks or the weft yarn gets exhausted, and thereby prevents the formation of weftway cracks in the fabric. The brake stops the loom instantaneously at any desired moment. The warp stop motion stops the loom when a warp thread breaks during weaving.

1.7 Methods of Driving a Plain Power Loom

     Power loom are driven by the following types of drives :

1. Individual drive
2. Group drive

1.7.1 Individual Drive

In this method, each power loom is driven by an individual motor. The power required to drive a plain power loom is 0.75 HP.

Figure 1.2 shows a simple driving arrangement commonly found in mills. A single motor is used to drive the loom. Motor A, via motor pulley B and loom pulley or fast-andloose pulley C and D, drives the top shaft or crank shaft E. A crank shaft gear wheel F and a bottom shaft gear wheel G drive the bottom shaft H.

By means of a starting handle, a belt fork can be used to change the position of the belt on the fast-and-loose pulley arrangement. When the belt is on the loose pulley D the pulley will rotate but the crank shaft will not rotate. Therefore the machine can be stopped. By moving the belt to the fast pulley C the loom can be started or stopped at any time.

In the latest looms, a motor with an electro-magnetic clutch drive is used. This is more reliable and stops the loom instantaneously by a push-button control system.

	A - Motor
	B - Motor pulley - 2" diameter
	C - Fast pulley - 16" diameter
	D - Loose pulley
	E - Crank shaft
	F - Crank shaft gear wheel (48 teeth)
	G - Bottom shaft gear wheel (96 teeth)
	H - Bottom shaft

Figure 1.2 Individual drives in a loom In the CD-ROM, watch Animation No. WFP - l 1.2

From the figure, it is clear that:

1)    Speed of the crank shaft = motor speed x

      = 960 x                        = 120 revolutions per minute (rpm)

Speed of the = Speed of the x

Bottom shaft    crank shaft

= 120 x            = 60 rpm

Note

1. The ratio of the number of teeth on the gear wheels i.e. the ratio of the number of teeth on the crank shaft gear wheel to that on the bottom shaft gear wheel is 1:2. The actual number of teeth in the two gear wheels could be 36:72, 45:90, etc.
2. Since the ratio of the number of teeth on the gear wheels is 1:2, the ratio of the speeds of the crank shaft and the bottom shaft will be 2:1. If the crank shaft has a speed of 50 rpm, the bottom shaft will have a speed of 25 rpm.
3. When the crank shaft makes one revolution, one pick is inserted. If it has a speed of 75 rpm, 75 picks will be inserted in a minute. Therefore the crank shaft speed in rpm also indicates the picks per minute (ppm), i.e. a crank shaft speed of 75 rpm indicates a pick insertion rate of 75 ppm.
4. Crank shaft speed indicates the loom speed.

1.7.2 Group Drive

In the de-centralized weaving sectors, a group of looms is driven by means of a common motor and an overhead shaft and belt-drive arrangement.

This method of driving power looms is found in the de-centralized weaving sectors, It can be seen in Figure 1.3 that in this system, a common motor A drives an overhead shaft D via pulleys B and C, which is in fact the main shaft of the system. The main shaft runs from one end of the loom shed to the other. A number of pulleys E, are fixed on this shaft, one for each loom. Each loom has a fast-and-loose pulley G which is connected to the corresponding main shaft pulley by means of a belt F. The belts can be shifted on the corresponding fast-and-loose pulley, either to run the loom or to stop it.

1.7.3 Advantages and Disadvantages of Different Loom Drives

Individual drive

The advantages of individual drive are listed below:

1. In case the motor of any particular loom fails, that loom alone will stop running, while the other entire loom keep running.
2. Power losses in individual loom drive are much less than the losses in a group drive system. There is therefore a considerable saving in power.
3. The life of the transmission belt is comparatively greater in individual drive.
4. In the individual drive system, there will be a clear view of all the looms in the shed. Due to the absence of a overhead shafts and moving belts, the lighting in the shed will be brighter and more uniform.
5. The possibility of accidents is considerably minimized in the individual drive system as each loom and its drive is compactly arranged, without any inter-loom connection.
6. The shed plan and layout of looms is neat and easy.

A - Common motor	E - Main shaft pulleys
B - Motor pulley	F - Belts
C - Overhead shaft driving pulley	G - Fast-and-loose pulleys
D - overhead shaft or main shaft

Figure 1.3 group drive in a loom shed

The disadvantages of individual drive are :

1. Initial cost is high.                     2. High maintenance cost.

Advantages of group drive:

1. Initial cost is low.                        2. High maintenance cost.

Disadvantages of group drive;

1. Higher power consumption.
2. One motor drives a number of looms. So, if it fails, all the looms it drives are affected. This results in poor loom-shed efficiency.
3. There are greater chances of accidents due to the overhead and other interloom connections.
4. The large number of pulleys and belts in the loom shed will reduce the effective amount of light in the loom shed.
5. The layout for a group-drive system is complicated and presents a clumsy overall appearance.

1.8 Classification of Weaving Machines

Looms are classified mainly into handlooms and power looms. The power looms are classified further into the following categories.

1. Non- power looms

These looms have only the basic mechanisms, viz. primary, secondary and some auxiliary mechanisms. The following are examples of non-automatic power looms.

1. Tappet looms
2. Dobby looms
3. Jacquard looms
4. Drop box looms
5. Terry looms
1. Automatic looms or conventional automatic looms

To get high productivity and good quality of fabric, additional mechanisms are added to ordinary non-automatic power looms. These looms are becoming popular because of their advantages of versatility and relative cheapness.

Examples:

1. Pirn changing automatic loom
2. Shuttle changing automatic loom.
2. Shuttleless looms or unconventional looms

In the non-automatic and automatic looms, shuttles are used for inserting the weft yarns. In these shuttle-looms, preparation of weft yarn and the weft insertion mechanism itself limit the loom production and fabric quality; they are also prone to mechanical problems in propelling the shuttle. Hence loom manufacturers have developed looms with various innovative and alternative means of weft insertion.

These modern looms are known as “shuttleless looms” and some examples of the looms are:

1. Air-jet loom
2. Water-jet loom
3. Projectile loom
4. Rapier loom
5. Needle loom
6. Various other methods include rectilinear multiphase looms.

4. Circular looms

These looms achieve higher weft insertion rates because more than one shuttle is delivered at a time. In these looms, the shuttles move simultaneously in a circular path and tubular fabrics are produced.

1.9 A Method for Indicating Loom Timing

In a loom, all the mechanisms must be set at correct timings in relation to each other. We therefore need a simple and unambiguous method for identifying and stating these timings.

The loom over looker or jobber often adjusts the loom settings. This is generally done by keeping the reed or sley at a particular distance, as measured by a steel rule or a gauge, from a fixed mark on the loom frame. This is convenient for practical purposes but not for studying the principles of weaving.

To study and set the mechanisms, it is better to state their timings in terms of the angular positions of the crank shaft which activates both the sley and the reed.

This can be done conveniently by means of a circle, the radius of which is equal to the length of crank and in which the centre represents the centre of the crank shaft. The circle is known as crank circle or timing circle. Figure 1.4 shows a timing circle. The circle is graduated in the direction of rotation of the crank and is divided into four quarters; the terms top, front, bottom and back canters are used to correspond to the 0o, 90o, 180o and 2700 positions of the circle. Also, in these timings the crank positions correspond to the top, front, bottom and back respectively.

Figure 1.4 Method for indicating loom timing
In the CD-ROM, watch Animation No. WFP.I 1.3

By stating the crank position in terms of degrees, the mechanisms like shedding, picking, etc. can be set and studied without any difficulty. The timings are graduated on a wheel fixed to the crank shaft in degrees and a fixed pointer enables settings to be made in relation to the angular position of the crank shaft.