Power Loom


  • Classification of Weaving Machines

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

a. power looms

These looms have only the basic mechanisms, viz. primary, secondary and some auxillary 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

b. 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.

c. Shuttle-less 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.

d. 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.

A warp sheet A from a weaver’s beam B passes around a back rest C and is led around lease rods D to heald 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. 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.

  • Details of various parts of the loom

1. Heald shaft

This part is related to the shedding mechanism. The heald shaft is made of wood or metal such as aluminium. It carries a number of heald wires through which the ends of the warp sheet pass. The heald shafts are also known as ‘heald frames’ or ‘heald staves’. The number of heald shafts depends on the warp repeat of the weave. It is decided by the drafting plan of a weave. The main function of the heald shaft is as follows:
(i) It helps in shed formation
(ii) It is useful in identifying broken warp threads
(iii) It maintains the order or sequence of the warp threads
(iv) It determines the order of lifting or lowering the required number of healds for a pick. In other words it helps in forming the design or pattern in a fabric.
(v) It determines the warp thread density in a fabric, i.e. the numbers of heald wires per inch determine the warp thread density per inch.

2. Sley

It is made of wood and consists of the sley race or race board, reed cap and metal swords carried at either ends. The sley mechanism swings to and fro. It is responsible for pushing the last pick of weft to the fell of the cloth by means of the beat up motion. The sley moves faster when moving towards the fell of the cloth and moves slower when moving backwards. This unequal movement is known as ‘eccentricity of the sley’. It is needed in order to perform the beat up and also to give sufficient time for passage of shuttle to pass through the warp shed. The beat up of the lastly laid pick of weft is accomplished through a metal reed attached to the sley.

3. Shuttle

It is basically a weft carrier and helps in interlacement of the weft with the warp threads to form cloth. The shuttle which is made of wood passes from one end of the loom to the other. It travels along the wooden sley race and passes between the top and bottom layers of the warp sheet. The shuttle enters a shuttle box fitted at either ends of the loom, after passing through the warp shed. A shuttle normally weighs about 0.45 kgs.

4. Shuttle box

It is the housing for the shuttle and is made of wood. It has a spindle and a picker. It may also accommodate the picker without spindle. The top and side of the box towards the sley race are open. The shuttle dwells inside the box for the intermediate period between two successive picks.

5. picker

The picker is a piece made either of leather or synthetic material. It may be placed on a spindle or grooves in the shuttle box. It is used to drive the shuttle from one box to another. It also sustains the force  of the shuttle while entering the box.

6. Reed

It is a metallic comb that is fixed to the sley with a reed cap. The reed is made of a number of wires and the gap between wires is known as dents. Each dent can accommodate one, two or more warp ends. The count of the reed is decided by the number of dents in two inches. The reed performs a number of
functions which are enumerated as follows:
(i) It pushes the lastly laid pick of weft to the cloth fell
(ii) It helps to maintain the position of the warp threads
(iii) It acts as a guide to the shuttle which passes from one end of the loom to the other.
(iv) It determines the fineness of the cloth in conjunction with the healds.
(v) It determines the openness or closeness of the fabric. There are various types of reed such as ordinary reed, gauze reed, expanding reed, V reed etc.

7. Warp beam

This is also known as the weaver’s beam. It is fixed at the back of the loom. The warp sheet is wound on to this beam. The length of warp in the beam may be more than a thousand metres.

8. Back beam

This is also known as the back rest. It is placed above the weaver’s beam. It may be of the fixed or floating type. In the first case the back rest merely acts as a guide to the warp sheet coming from the weaver’s beam. In the second case it acts both as a guide and as a sensor for sensing the warp tension.

9. Breast beam

It is also known as the front rest. It is placed above the cloth roller at the front of the loom and acts as a guide for the cloth being wound on to the cloth roller. The front rest together with the back rest helps to keep the warp yarn and cloth in horizontal position and also maintain proper tension to facilitate
weaving.

10. Cloth beam

It is also known as the cloth roller. The woven cloth is wound on to this roller. This roller is placed below the front rest.


  • 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 overlooker 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 centres are used to correspond to the 00, 900, 1800 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 of indicating Loom Timings

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.

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 and fro motion. The reed thus beats up the weft into the fell of the cloth.

  • 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.

  • 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 weft-way 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.

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