Basic Principles of Weaving Loom

Weaving Mechanism of Loom:
Weaving is a process in which the lengthwise yarns, called warp, and width-wise yarns, called weft, are interlaced. The interlacement of warp and wept yarns results in a material called fabric having tensile strength, flexibility and other essential properties. Each thread or yarn in a warp is called an end, and each thread in the weft is called a pick. The American term for weft is filling. Warp yarns are usually stronger and more compact than the weft or filling yarns. In the loom, the warp threads are led through eyes in metal rods known as heddles, one for each thread. Alternate heddles are joined together in a frame. There are at least two frames, together making up the harness. The purpose of the harness is to move the warp threads up and down.

The following six basic principles are essential for continuous weaving:

1. Shedding
2. Weft insertion
3. Beat-up
4. Warp let-off
5. Fabric take-up
6. Stop motions

Shedding, picking/weft insertion and beat-up are called main or primary motions, and warp let-off and fabric take-up are called secondary motions. There are three other motions, namely warp protector, warp stop and weft stop motions, necessary for good weaving. These motions are called auxiliary motions. Figure 1 shows the schematics of weaving.

A. Shedding:
Shelling is one of the most important principles of weaving. Shedding is the process of separating the sheet of warp into two layers one at the top and another at bottom making a V-shape opening for the filling (weft) yarn to pass through as shown in Figure 2. This open­ing is called a shed. Figure 2 shows the geometry of warp shed. Different mechanisms are used for shedding. They may be grouped as tappet mechanism, Dobby mechanism and Jacquard mechanisms.

The shed height H is given by

H = B tan A,

where B is the distance between the heald eye and the fell of the cloth and A is the angle between warp shed and fabric plane. It is advantageous to have a small H in order to reduce the stress on the warp. The magnitude of H is determined by the weft insertion device. Each warp yarn drawn through the eye of a thin metallic bar called a heald is guided by that heald. Healds that guide the warp yarns with the same pat­tern are attached to the same heald frame. When a heald frame moves up or down, all the yarns attached to that heald frame moves up or down together along with that heald frame. Changing the order of lifting the heald frame will result in a different weaving pattern.

B. Weft Insertion / Picking:
Weft insertion is the second important motion after shedding in the weaving process. Weft insertion is the process of inserting the weft yarn in between the top and bottom layers of warp shed. This can be achieved by means of a fly shuttle in shuttle weaving and by means of a projectile, rapier and a jet of air or water in shuttleless weaving. Figure 3 shows the different weft insertion methods in weaving machines.

C. Beat-Up:
Beat-up is the process of pushing the newly inserted weft yarn into the fell of the cloth by means of reed. Reed is a rectangular closed comb of fat metal strips placed closely with small spacing. This spacing corresponds to the spacing between the warp yarns. The spaces between two metal strips are called dents. The fell of the cloth is the meeting point of the warp yarn and fabric. Figure 4 shows the beat-up process in a loom.

Beating up of weft yarn requires a considerable amount of energy. At the final stage of beat-up, the bending of warp and weft yarn takes place due to crimp inter­change. To overcome the frictional reactions, the pushing of the weft yarn to the fell of the cloth is done in a harsh manner that gives the name beat-up. During beat-up as the weft yarn is being pushed to the fell of the cloth warp tension increases and the fabric tension decreases.

D. Warp Let-Off:
A warp let-off mechanism releases the warp yarn from the sized warp beam at uni­form tension as the weaving proceeds. The let-off mechanism controls tension in the warp yarn by controlling the rate of flow of warp yarn. The warp beam diminishes in diameter as the weaving continues, necessitating a gradual increase in the angular movement of the beam in order to maintain a constant flow of warp.

An increase in warp tension decreases warp crimp and increases weft crimp. Crimp is defined as the ratio between extra length of yarn present to length of fabric where extra length of yarn is the length of yarn present in the sample minus the sample length. The crimp ratio of warp and weft crimp determines the fabric quality. Crimp affects the weight, flexibility and thickness of fabric. Hence, maintaining a uniform warp tension throughout weaving is very important to maintain fabric qual­ity and shrinkage level.

Let-off mechanisms are classified into positive and negative let-off mechanisms. In the negative let-off mechanism, the tension in the warp yarn is used to release the warp yarn from the warp beam. When the tension in the warp yarn exceeds the frictional forces in the let-off mechanism, the beam rotates, and warp yarn is released. Negative let-off mechanism are used in nonautomatic looms. Heavier fab­rics with high picks per cm are difficult to weave with a negative let-off mechanism. Figure 5 illustrates the principle of a negative let-off mechanism. The frictional force can be increased or decreased by moving the dead-weight away or toward the fulcrum point.

In a positive let-off mechanism, the release of warp yarn is controlled by con­trolling the rotation of the warp beam positively by a separate mechanism. The mechanism may be either mechanical or electronic. In a positive mechanical let-off motion, the tension variation in the warp is sensed by a lever, and this is fed in to the let-off mechanism through a ratchet wheel, which, in turn, will rotate the beam accordingly. A few examples of positive mechanical let-off motions are the Bartlett let-off motion, the Ruti let-off motion and the Sulzer Ruti let-off motion. In electronic let-off, a servomotor is used to rotate the beam to the required extent.

E. Fabric Take-Up:
Fabric take-up is an important function in a loom because it controls the quality of the fabric in terms of picks per unit length. Uniform fabric take-up will result in uniform pick density. There are two types of take-up mechanisms: (1) mechanical and (2) electronic.

1. Mechanical take-up
The rotation of the fabric roller is determined by the change and standard wheels in the fabric take-up gear. The required amount of pick density is achieved by changing the change and standard wheels in the fabric take-up gear in a conventional shuttle loom. In automatic shuttle looms the standard and change wheels are dispensed with instead levers are used to change pick density. The schematic diagram of a mechani­cal take-up mechanism used in an automatic shuttle loom is given in Figure 6 .

A double-through take-up cam 2 is mounted on the picking shaft 1, and as the cam rotates, it oscillates the follower bracket 4. This motion is transmitted to the actuating lever 6 loosely mounted on the ratchet wheel stud. The left end of the connecting rod 5 rests on the slot of the actuating lever. The up and down adjustment of the connecting rod in the slot determines the pick density. An upward adjustment results in more angular movement of the ratchet pawl and decreases the pick density. A downward adjustment results in increased pick density. No change wheels are required to alter the pick density.

2. Electronic take-up
The rotation of the fabric roller is done by a servomotor. The weft density can be altered by changing the frequency of the servomotor. The schematic diagram of an electronic take-up mechanism used in an air-jet weaving machine is given in Figure 7. The fabric is drawn over a spreading roll then under the fabric roll before it is wound on a cloth roll. A press roller is used to prevent the fabric from slipping back.

F. Stop Motions:
Warp and weft yarns are liable to break during weaving operation. During weav­ing, the warp and weft yarn is subjected to tension, and whenever the tension in the yarn exceeds the strength of the yarn, the yarn breaks. Yarn faults, such as weak places, snarls, slubs and so on, present in the yarn also leads to yarn breaks. Hence, it becomes necessary to stop the loom as soon as either the warp or weft yarn breaks. This stopping mechanism may be mechanical or electrical.

The mechanisms used to stop the loom whenever a weft breaks or weft exhausts are called weft stop motions. Similarly, mechanisms used to stop the loom whenever a warp yarn breaks are called warp stop motions.

Conclusion:
Weaving is a highly coordinated process that involves the systematic interlacement of warp and weft yarns to produce fabric. Basic weaving principles include primary motions (shedding, weft insertion, beat-up), secondary motions (warp let-off, fabric take-up), and auxiliary motions (warp and weft stop mechanisms) which ensure consistent and high-quality fabric production. Each mechanism has specific functions that contribute to fabric properties like strength, flexibility, and texture.

References:

1. Fabric Manufacturing Technology: Weaving and Knitting by K. Thangamani and S. Sundaresan
2. Handbook on fabric manufacturing (grey fabrics: preparation, weaving to marketing) by B. Purushothama
3. Principles of Fabric Formation By Prabir Kumar Banerjee

You may also like: Comparison between Weaving and Knitting

Share This Article!