Basic Functions of the Weaving Loom Functions
The basic functions of a loom are shedding, filling insertion, beat-up, and warp control.
The weaving process starts with the sized yarn coming off the loom beam in a single sheet or layer of yarn. The yarn then proceeds over a tension or whip roll that is designed to maintain a constant level of tension on the warp yarns throughout the entire weaving cycle. Each yarn then travels through its own drop wire that serves as a stop motion detector. When yarn tension drops too low or when the yarn breaks the metal drop wire will complete an electric circuit and the loom will stop.
Next, each warp yarn then passes through its own heddle that is suspended in the harness. The heddle has an eye through which the yarn passes and allows for exact control for each yarn. The harnesses control the raising and lowering of the warp yarns.
The yarns then proceed through the dents of the reed. The reed is a comb-like device that maintains the position of the warp yarns as well as performs the beating up of the filing yarn into the body of the cloth. The point at which the yarn is beaten up into the cloth is the fell of the cloth. This is the transition point where the yarn becomes fabric.
The cloth now winds over the take-up roll, sometimes called the sand roll or press roll which, when combined with the let-off of the loom beam, controls the number of picks per inch in the fabric. Finally, the cloth is rolled onto a cloth roll.
Shedding is the process of creating a path across and through the warp yarns by raising some warp threads through the harness and leaving other threads down by keeping their harnesses in the down position.
There are three methods of creating a shed: cam shedding, dobby shedding, and jacquard shedding.
Cam shedding is the most basic of shedding motions, it uses profiled cams to move the harnesses up and down. Cam looms usually have 6–8 harnesses. Cam looms are best suited for simple weaves like plain weaves, basic twills, and satin weaves. Pattern changes require changing the harness cams which is a simple process.
Dobby shedding uses an electronic or mechanical device to select or move individual harnesses. Up to 28 harnesses can be used to weave complex twills, satins, small geometric figures, and pattern stripes.
Jacquard shedding is the most complex shedding system. It is named after its inventor Joseph-Marie Jacquard who developed the system in 1801 to be used on hand looms. The jacquard has advanced significantly since then but it still works on the same principle today. Jacquard shedding exhibits control on each individual warp yarn independent of all other warp yarns. Up to 12,000 warp ends can be controlled, these systems can employ mechanical or electronic devices to actuate the warp ends. Design capabilities are virtually endless. Pattern areas can equal the width of the fabric and are almost unlimited in the length. Fabrics made on jacquard looms are normally more expensive because of their complexity and lower weaving speed. There are no harness frames on this style of loom. Each end is activated independently, being controlled by its own harness cord from the jacquard head. The harness cord is attached to a hettle which is pulled down by a spring in most cases but in some of the older and slower models, weights are used to pull the hettles down to achieve the pattern.
Several systems are used to insert the filling yarn into the fabric, including shuttle, rapier, projectile, air-jet, and water-jet.
The shuttle loom was the first type of power loom developed. It uses a shuttle which is a boat-shaped device that holds a small quill of yarn. The shuttle is the only powered insertion system that carries its own yarn supply across the warp. The maximum speed of these looms varies with the width of the fabric but most produce between 150 and 200 picks per minute.
Rapier filling insertion is one of the most versatile insertion types. This is due to the rapier head gripping the yarn and carrying it across the width of the shed, therefore any yarn from extremely fine to very coarse can be woven. Also, very different yarns can be woven from pick to pick without readjusting the rapier. There are four types of rapier systems—single rigid, double rigid, double flexible, and double telescoping rapiers.
The single rigid rapier uses a solid rod material that carries the yarn completely across the warp. This loom footprint requires twice the floorspace of the rapier due to the fact that when the rapier is not in the shed it is housed in a sheath external to the shed.
Double rigid rapiers use two rapiers that are just over one-half the width of the warp. Each transfers the yarn halfway across the warp shed. This still takes up the same amount of floorspace as a single rigid rapier because each rapier is stored on either side of the loom. However, the speed can be increased because it takes less time to insert a pick since each rapier only has half the distance to travel in the shed.
Double flexible rapiers use a flat metal tape or high-tech composite material to transfer the yarn across the warp shed. The advantage of this system is that the floor space is reduced since the rapier storage space is reduced. Because the rapier is flexible, it is coiled into a circular sheath when not inserting the filling. The rapiers are much lighter resulting in higher processing speeds. Flexible rapier looms are capable of reaching 850 picks per minute on single-width fabrics.
Double telescoping rapiers use sliding or telescoping devices to insert the filling yarn. They are similar to double rigid rapiers but since they telescope much of their length is stored inside the rapiers themselves as the rapiers collapse, thereby reducing the with of the loom. This type of rapier loom can only achieve a maximum of 350 picks per minute due to the weight of the rapier and the time required to extend and contract it.
The projectile filling system uses a small bullet-shaped object to carry the filling yarns across the warp shed. The bullet or projectile is similar to the much larger shuttle in shape however the projectile does not have a self-contained yarn supply. The projectile must be presented with a yarn that it grips just before it is propelled across the warp shed. The projectile is presented to the picking arm and is fed a yarn, it is then fired across the loom through guide teeth to where it is caught in a breaking device. It is then returned to the picking station by way of a conveyor chain. Several projectiles are in use at any time so that rapid pick insertion can occur.
Air-jet filling insertion uses a stream of high-pressure air to insert the filling yarn into the warp shed. This is the fastest conventional method of weaving. Air-jet looms consistently reach production speeds of up to 1,000 to 1,200 picks per minute on single-width fabrics. Air-jet looms require very expensive profiled reeds to channel the air and yarn across the shed. Air-jet weaving inserts the filling from an off loom auxiliary supply system that accumulates the exact amount of yarn needed to travel across the shed. An initial burst of air starts the yarn on its way and several auxiliary nozzles then help propel the yarn completely across the warp. These auxiliary nozzles are evenly spaced and fit into the tunneled or profiled section of the reed.
Water-jet filling insertion uses the same concept as the air-jet. Water is used instead of air to project the filling yarn across the warp shed. Due to the higher density of the water-jet as compared to the air-jet, booster nozzles and profiled reeds are not needed. Speeds of water-jet insertion rival those of air-jet weaving. One shortcoming of water-jet weaving is that only hydrophobic fibers can be used.
Multi-phase insertion is the newest technology for inserting picks into the warp shed. This system uses a series of four air-jets on a rotary drum to reach a pick insertion rate of up to 2,800 picks per minute on basic plain weaves and twills. Due to its complexity and its ability to do only simple weaves, very few multi-phase systems are in use.
The beat-up motion is the loom motion that pushes the last filling pick into the fell of the cloth. This happens after the pick has been inserted and while the harnesses are changing position allowing for the trapping of the pick. The reed is moved forward with force to push the filling tightly to the cloth structure.
The reed is responsible for several different functions in weaving. The first function of the reed is to keep the ends parallel and in the same position for each picking cycle. The reed also provides some control for the filling insertion. This is especially true for air-jet weaving where the shape of the reed assists the insertion.
Finally, the reed is used to beat the filling yarn into the fabric. The reed number of a reed is the dents per inch. Therefore, if a reed has a number of 21.5, this means that there are 21.5 slots per inch in the reed. For example, if the yarns are reeded 2 ends per dent (21.5 times 2), then there would be 43 ends per inch in the reed. This is calculated by multiplying the two ends per dent by the reed number of 21.5 which would equal 43. Three ends per dent would mean that there are 64.5 ends per inch in the reed.
Warp control is the relationship on the loom between warp let-off (release of the warp yarn from the loom beam) and take-up of the formed fabric. The rate at which the loom beam turns and releases the warp yarn to the loom determines the production rate.
The let-off on the loom, which is the drive mechanism, controls the rate that the loom beam turns and releases or feeds the warp yarn to the loom. This is how the warp tension is controlled.
The take-up, on the other hand, controls the amount of warp used yarn pulled forward after the pick insertion. This determines the number of picks per inch in the cloth.
The let-off must be synchronized with the take-up to have proper tension control during the weaving process. Therefore, the pick spacing and delivery rate on the loom are controlled by the relationship between the let-off and the take-up.
TERMS TO KNOW (click to flip)
The process of forming the warp shed and the mechanical system that creates the shed. There are three types of…view in glossary
The type of harness drive that uses cams to actuate the up and down movement on a loom. This is…view in glossary
The process of using a dobby device to produce the warp shed on a loom.view in glossary
The form of shedding that is the result of using a jacquard head to control individual heddles to form the…view in glossary
The shuttle is a boat like device that contains a quill (or spool) of filling yarn. In hand weaving, the…view in glossary
Type of filling insertion that uses a mechanical arm to transfer the filling yarn across the warp shed.view in glossary
A type of loom that uses projectiles to insert filling yarns. This loom is somewhat like the shuttle loom, except…view in glossary
A shuttle-less filling insertion system developed in the 1950s that uses a jet of air to propel the filling yarn…view in glossary
Type of loom that uses a jet of water to propel the filling yarn across the warp shed. Not used…view in glossary
The process of simultaneously inserting several picks at one time into multiple sheds. Only applicable to Sulzer Textil M8300 weaving…view in glossary
The process common to all looms where the loosely inserted filling yarn is pushed up into the fabric forming the…view in glossary
The function on the loom that controls the take-up of the cloth and the let-off of the warp.view in glossary