What is Loom | Define Loom | Shuttle Loom | Shuttle less loom | Modern Loom | Classification of Modern Loom | Projectile Loom | Rapier Loom | Water Jet Loom | Air Jet Loom | Circular Loom

An apparatus for making fabric by weaving yarn or thread. A loom is a device used to weave cloth. The basic purpose of any loom is to hold the warp threads under tension to facilitate the interweaving of the weft threads. The precise shape of the loom and its mechanics may vary, but the basic function is the same.

Shuttle Loom: The shuttle loom is the oldest type of weaving loom which uses a shuttle which contains a bobbin of filling yarn that appears through a hole situated in the side. The shuttle is batted across the loom and during this process, it leaves a trail of the filling at the rate of about 110 to 225 picks per minute (ppm). Although very effective and versatile, the shuttle looms are slow and noisy. Also the shuttle sometimes leads to abrasion on the warp yarns and at other times causes thread breaks. As a result the machine has to be stopped for tying the broken yarns.

Classification of Modern Loom:

Shuttle less loom: Many kinds of shuttle less looms are used for weaving such as Projectile Looms; Rapier Looms; Water Jet Looms; and Air Jet Looms.

Projectile Loom: It is sometimes called missile loom as the picking action is done by a series of small bullet like projectiles which hold the weft yarn and carry it through the shed and then return empty. All the filling yarns are inserted from the same side of the loom. A special tucking device holds the ends of the wefts in place at the edge of the cloth to form the selvage. This loom needs smooth, uniform yarn which is properly sized in order to reduce friction. Projectile loom can produce up to 300 ppm and is less noisier then the shuttle loom.

Rapier Loom: Rapier loom comes in many types. Early models of it use one long rapier device that travels along the width of the loom to carry the weft from one side to the other. Another type of rapier loom has two rapiers, one on each side of the loom. They may be rigid, flexible or telescopic. One rapier feeds the weft halfway through the sheds of warp yarns to the arm on the other side, which reaches in and carries it across the rest of the way. Rapier looms are very efficient and their speed ranges from 200 to 260 ppm. These looms can manufacture a variety of fabrics ranging from muslin fabric to drapery fabrics and even upholstery fabrics.

Water Jet Loom: In it, a pre measured length of weft yarn is carried across the loom by a jet of water. These looms are very fast with speeds up to 600 ppm and very low noise. Also they don’t place much tension on the filling yarn. As the pick is tension less, very high quality of warp yarns are needed for efficient operation. Also, only yarns that are not readily absorbent can be used to make fabrics on water jet looms such as filament yarn of acetate, nylon, polyester, and glass. However, it can produce very high quality fabrics having great appearance and feel.

Air Jet Looms: In the air jet weaving looms, a jet of air is used to propel the weft yarn through the shed at speeds of up to 600 ppm. Uniform weft yarns are needed to make fabrics on this loom. Also heavier yarns are suitable for air jet looms as the lighter fabrics are very difficult to control through shed. However, too heavy yarns also can’t be carried across the loom by air jet. In spite of these limitations, air jet loom can produce a wide variety of fabrics.

Circular Looms: These looms are particularly used for making tubular fabrics rather than flat fabrics. A shuttle device in it circulates the weft in a shed formed around the machine. A circular loom is primarily used for bagging material.

Modern Weft Stop Motion Device of Loom | Control Mechanism of Loom

Projectile Weaving Machine | Mechanism and Working Principle of Projectile Weaving Machine

Main parts of Projectile Weaving Machine

1. Torsion bar A: As shown in fig it has splinted ends as seen in the fig one end is secured firmly at the clamping flange with provision for adjusting twisting angle. The twisting length of the torsion is 721 mm. It is diameter is 15, 17 or 19 mm depending upon the model. Larger the diameter higher the initial projectile speed. The angular twisting of torsion bar at commencement of picking is 28-30O.

2. Picking shaft B: The free end of the torsion bar is linked with the picking shaft through spines.

3. Picking lever C: The picking lever is clamped on the picking shaft.

4. Picking shoe D: The picking lever carries the picking shoe at its top end.

5. Picking shaft lever E: It is a rigid part of the picking shaft.

6. Toggle plates F: The toggle plates center at o carry a roller G and connected to the picking shaft lever E through a link H .They are covered at the bottom.

7. Picking cam I: It is mounted on a shaft J and rotated by bevel wheels K once every pick. It rotates in the direction of the arrow shown in fig. It carries a roller R after the nose part.

8. Oil break L: The shock of the picking is taken by the oil break.

A=Torsion bar , B=Picking shaft ,C=Picking lever , D=Picking shoe ,E=Picking shaft lever ,F=Toggle plate , G=Antifriction bowl , H=Link , I=Picking cam , J=Shaft ,K=Bevel wheels, L=Oil brake , P=Projectile ,R=Projectile guide.

       Fig: Picking mechanism on projectile weaving machine.

Working Principle:

Weft is withdrawn from the package through a tension device, weft tensionar, shuttle feeder, scissor, and weft end gripper. The picking arm has released the projectile which is shown in the guide teeth at the mid-shed position. At the receiving side the weft end gripper is positioned to grip the weft after reception. The shuttle break is shown in its operating position with the shuttle returner ready to push the projectile to the release and tucking position.Illustrates the torsion bar picking system of the machine. Strain energy is developed in the bar and released in such a way as to transfer the maximum possible strain energy to the projectile before it separates from the picker shoe. 

 

The torsion bar (A) has its splined ends rigidly constrained in an adjustable housing with provision for adjusting the maximum angle of twist and projectile initial velocity. The other end of the torsion rod is splined into the picking lever (C) which carries the picking shoe (D) at its extremity. The projectile (P) is illustrated in the shuttle lifter with the projectile spring opener. The bevel wheel (K) rotates the picking cam shaft (J) which carries the picking cam (I). The picking shaft lever (E) is rigidly connected to the torsion bar and through a short linkage to the toggle plate (F) center at anti friction bowl (G).The action of the cam is for the small roller to bear against the toggle rotate it anti clockwise about anti friction bowl (G), thus withdrawing the picking shoe to its rearmost position. In this position the center of the toggle arrangement are in line and the torsion bar is twisted to its predetermined angle. 

 

The nose of the picking cam then bears against the roller carried between the toggle plates and moves the central pivot of the toggle system off line center, thus permitting the strain energy in the rod to be transmitted instantaneously to the projectile. The projectile separates from the shoe after 6.4 cm travel in 0.007 s as a velocity of about24.4 m/s after being subjected to a maximum acceleration of about 6700 m/s2 at a point 1.5 cm inboard of the rest position. The residual energy in the picking system, some 62% of the whole is absorbed in the hydrolic buffer the body and plunger of which are shown at (L).

Primary Motions of Weaving Loom Mechanism | Shedding Mechanism | Picking Mechanism | Beating Mechanism

Primary Motions:
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.
a. Shedding mechanism
b. Picking mechanism
c. Beat-up mechanism

(a) Shedding Mechanism
The shedding mechanism separates the warp threads into two layers or divisions to form a tunnel known as ‘shed’. The shed provides room for passage of the shuttle. A shed may be formed by means of tappets, dobby and jacquard.

Fig: Shedding motions.

 

Types of Tappet Shedding Mechanism:
Generally there are two types of shedding
1. Negative tappet shedding
2. Positive tappet shedding

Negative Tappet Shedding:
In a tappet shedding mechanism, if the tappet controls only one movement, either an upward or downward movement of the healed shafts, then the shedding is known as “negative tappet shedding”. The healed shafts are returned by some external devices like springs, dead weights, rollers, etc.

Positive Tappet Shedding:
In a tappet shedding mechanism, if the tappet controls both upward and downward movements of the healed shafts, then the shedding is known as positive tappet shedding.

(b) 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”.

Picture: Picking Mechanism

 

(C) 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. 

Picture: Beating mechanism with sley

Motions of Loom | Weaving Loom Motion | Primary Motions of Loom | Secondary Motions of Loom | Tertiary Motions of Loom

In order to interlace wrap and weft threads to produce a fabric, the following motions are necessary on any type of loom:
1. Primary motions
2. Secondary motions
3. Tertiary motions

1. Primary Motions:
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.

a. Shedding mechanism
b. Picking mechanism
c. Beat-up mechanism

2. Secondary Motions:

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

3. Tertiary Motions:

To get high productivity and good quality of fabric, additional mechanisms, called auxiliary mechanisms, are added to a loom. The auxiliary mechanisms are useful but not absolutely essential. This is why they are called the ‘auxiliary’ mechanisms. These are listed below.
a. Weft stop motion
b. Warp stop motion
c. Warp protector mechanism
d. Weft replenishment / Warp mixing motion
e. Cutter
f. Temples
g. Brake
h. Selvedge

Principle of Negative Tappet Shedding Mechanism

Principle

A tappet is given a rotary motion so that it depresses a follower and a lever, known respectively as the anti-friction bowl and the treadle arrangement, by means of which the heald shaft is operated.

Construction
Figure shows a negative tappet shedding mechanism. A pair of tappets A and B are fixed to the bottom shaft C at 180 degrees to each other. Two treadle levers D and E are connected to the loom back-rail by a bracket F.

The bracket acts as a fulcrum for the levers. The two treadles have teeth to carry the lamb rods G and H respectively. Two heald shafts J and K are connected to the lamb rods. A top reversing roller shaft Q carries two rollers of different diameters. The roller of small diameter N is connected to a leather strap L to which the front heald shaft J is connected. The roller P of large diameter is connected to a leather strap M to which the back heald shaft K is connected. The tappets A and B touch the anti-friction bowls or followers R and S respectively, which are fixed to the treadle levers.

Figure : Negative tappet shedding mechanism

The heald shafts have heald eyes T and U through which the war p threads pass X is the war p sheet and Y is the cloth. The odd ends are passed through one heald shaft while the even ends are passed through the other heald shaft.

Working Process:
When the bottom shaft is rotated in the clockwise direction as shown in the figure, the tappets are also rotated. The tappet will depress the anti-friction bowl and the treadle. Being fulcrumed at one end, the front portion of the treadle moves down. This action is transferred to the lamb rod, the heald shaft and the leather strap. So one heald shaft is lowered and the threads connected to this heald shaft are lowered and form the bottom layer of the shed.

The leather straps attached to the reversing rollers are connected in opposite directions, i.e. when leather strap is pulled down, it is unwound from its roller. The shaft therefore rotates in the clockwise direction and the other leather strap is wound on to its roller. The heald shaft is raised and therefore the lamb rod and treadle lever are also raised. The threads connected to the heald shaft are also raised and form the top layer of the shed.

For the next shed, the other tappet works with the other set of bowl, treadle, lamb rod, heald shaft, strap and roller and the other heald shaft is lowered. The first heald shaft is raised by the top reversing rollers, and the positions of the healds shafts are thus interchanged. Thus, for one rotation of the bottom shaft, two sheds are formed.

In this type of tappet shedding therefore, one tappet depresses the concerned treadle and the corresponding heald shaft is lowered. But the other heald shaft is raised by means of the top reversing rollers. So this type of shedding mechanism is known as “negative tappet shedding mechanism”

Timings and Settings:

1. Turn the crank to the top centre position.
2. Fix the anti-friction bowls to the treadle levers; they should move freely in the slots.
3. Fix the treadle levers with a bracket to the back rail of the loom.
4. Set the grid and grid bracket to the front rail of the loom in the slots of the grid.
5. Make sure that the tappet with the lower throw is fixed to the bottom shaft at the starting handle side.
6. Fix the top reversing rollers to the top reversing roller shaft to be equidistant from the ends and at the same time ensure that the connecting screws of the rollers are symmetrical about the central axis of the shaft when the heald shafts are at the same level. The roller of smaller diameter is always connected to front heald shaft.
7. The heald shafts are connected to the top reversing rollers by means of cords and leather straps. The leather straps are connected to the rollers, such that when one of them winds on its roller the other strap unwinds from its roller and vice versa.
8. Lamb rods are connected to the heald shafts by cords.
9. Adjust the tappets on the bottom shaft and make sure of the following points :

• The tappet with a bigger throw should be connected to the back heald shaft.
• The bowls should have perfect contact with the tappet surfaces.
• The treadles should be at the same level and parallel to each other at the top centre position.

Heald shafts : The hook of the lamb rod of the front heald shaft should be connected to the first notch of the treadle lever while that of the back heald shaft should be connected to the third notch. If the depth of shed is altered, the connections of the hooks to the treadle levers can be changed.

Points to be Observed:

1. Turn the crank shaft through two revolutions and make sure that the bowls are always in contact with the tappets.
2. The heald shafts should not touch the side frames or the sley.
3. Turn the crank shaft to the bottom centre and check the size of shed. The bottom line of warp sheet or the heald eyes of the lowered heald shaft should have a clearance of 1 mm from the race board and the top.