This motion enables to stop the loom immediately after a weft break or weft running out. In case the loom is allowed to run even after the weft breaks there will be no woven cloth except long threads of warp.
There are two types of weft stop motions on a conventional weaving machine:
a) Side weft fork motion.
b) Centre weft fork motion.
Side weft fork motion:
The basic principle of the side weft fork lies in the fork and grate. A metal grate is placed between the end of the reed and the shuttle box mouth on the starting handle side as shown in the figure. A weft fork made of light metal which has three prongs bent at right angles is situated in front of the grate. The complete weft fork motion is illustrated at figure.
A weft fork A with a single tail hooked at the end is held by a weft fork holder B at C. the other end of the holder is held by knock-over lever D, which is in contact with the starting handle when the loom is running.
The tail end of the fork is slightly heavier than the forked end. A hammer lever E fulcrummed at X is connected to a greyhound tail lever F, the bottom end of which is resting on a weft fork cam G which is fixed on the bottom shaft. During the rotation of the bottom shaft the cam raises the greyhound tail lever on every two picks and causes the hammer lever to rock towards the loom front.
A channel is cut in the wooden raceboard H opposite the weft fork so that when the sley comes forward to beat-up position the weft fork prongs will remain below the raceboard level until it is touched by a weft thread lying across the channel from the selvedge to the shuttle.
In this case the shuttle should be on the starting handle side. If the weft thread is not broken or missing, it will push the weft fork prongs, thus lifting the hooked tail clear of the hammer lever E. At the same time the rotation of the cam G makes the hammer lever move towards the front rest. In case the weft is absent either through breaking or from running out, the weft fork remains horizontal and the prongs pass freely through the bars of the grate. Then the hook tail of the fork is caught in the notch of the hammer lever E as shown in figure and when this lever moves towards the front rest it carries the fork along with its holder resulting in the weft fork lever D pressing against the starting handle S and knocking off the loom.
One fault in the mechanism described early is that the weft fork lever and the holder move in an arc of a circle because of the fixed fulcrum of the weft fork lever. This sometimes causes the prongs of the fork to hit against the side wall of the channel in the raceboard and cause damage. In the British made Northrop looms this arc of movement does not exist since the weft fork acts directly upon the starting handle with a straight backward push.
In the mechanism illustrated in figure the weft fork A is mounted on a sliding bracket B which slides forward and backward in a fixed bracket C. As usual the hook tail of the fork is caught in the notch of the hammer lever on weft failure and backward movement of this lever will push the knock-over lever D, thus the releasing the starting handle E. The spring S returns the sliding bracket B to its original position.
Important points to note while setting the weft fork mechanism:
1. The weft fork must be a possible source of weft cutting if it protrudes too far through the weft fork grate.
2. The grate must be smooth.
3. The weft fork prongs, during the forward movement of the reed, should not touch the grate wires or any part of the grate or raceboard groove.
4. The weft fork prongs protrude neither too less nor too far through the grate.
5. The clearance between the hook tail of the fork and the notch of the weft fork hammer is very important. If the clearance is too wide the weft thread may not keep the hook tail raised till the tail is clear off the weft fork hammer notch. This will result in unnecessary knock off of the loom even though the weft has not broken. On the other hand if the clearance is too close the hammer notch might prevent the hook tail from lifting when the weft thread applies pressure on the prongs.
6. The fork must be properly balanced so that the tail end is slightly heavier than the forked end.
7. An accumulation of fluff at the base of the grate will unnecessarily press the prongs of the fork thus raising the tail end when no weft is present. This will make the loom run without the presence of weft.
8. The side-play in the rocking rail and sley might cause the grate foul the fork. Sometimes, loose cranks might also cause this trouble.
9. Weft thread catching on the prongs because of inadequate tension will cause the loom to run on.
10. Bent prongs, binding of the fork through rust on the fulcrum pin, fork fulcrum worn out etc. might affect the good working of the mechanism.
11. Faulty timing of the hammer lever may cause the loom running even after the failure of weft.
12. Weak or late picking from the off side of the loom may cause the shuttle to strike the prongs and damage it.
13. Insufficient tension in the weft fail to lift the fork sufficiently causes the loom stoppage.
14. If the hammer lever begins to move too soon before the weft has had time to lift the fork tail clear, the loom will keep stopping.
Since this mechanism is situated only at the starting handle side of the loom, the stopping is affected only when the shuttle reaches the starting handle side. This will result in missing a maximum of two picks when the weft breaks or exhausts as soon as the shuttle leaves the starting handle side.
In case such a device is to be provided on both sides of the sley the cost factor and the complicated knocking off arrangement has to be thought of.
Centre weft fork motion:
This motion has been designed to feel the weft thread every pick and stop the loom in case the weft thread breaks or runs out, no matter which way the shuttle is running at the time. The shuttle can be housed in any one of the boxes.
It is for this reason the mechanism is situated in the centre of the raceboard. The loom is brought to a stop before the beat-up action takes place. It is not necessary that the shuttle should always be in the starting handle side box for effecting the loom stop. Therefore this device will not allow two missing picks before the loom stops. This device is useful for looms weaving pick and pick colored wefts. If there are two different colored picks weaving alternately and if one of the colored thread is broken, it is necessary to stop the loom immediately before another colored thread of the second pick is inserted in the shed. It also helps to weave faultless cloth free from pick finding marks or broken picks. With effective braking system, the loom can be stopped dead on the broken pick. In addition a device is incorporated to turn back the loom, opening the previous shed with a broken pick laid inside, so that the weaver can rethread without making any bad mark on the cloth. Centre weft fork motion is, therefore, suitable for weaving fabrics made of filament yarns, e.g. polyester, nylon and yarns made out of other delicate fibers. Though several types of centre fork motions are designed the basic principle remains the same. A channel is cut in the raceboard, at or near the centre depending upon the length of the weft from the shuttle eye to the fork and also on certain attachments like pirn changing and box changing. The weft fork with prongs is fulcrummed on a bracket fixed to the front of the sley. When the sley moves towards the back centre, the fork tilts upwards through the warp far enough to allow the shuttle to pass underneath and the weft is laid under the fork. During the forward movement of the sley the fork drops downwards upon the weft and is held from moving further down in the channel by the grid effect of the warp threads belonging to the bottom shed, supporting the weft thread against the light pressure of the fork. In this condition the weft fork holds the knock-off arm away from the knock-off lever. The fork is pulled out of the shed just before the reed reaches the fell of the cloth for the beat up of the weft. If, however there is no weft underneath the fork as the sley moves forward, the fork drops into the channel in the sley and the knock off arm D is moved into contact with the knock-off lever G thus stopping the loom. One important device which is necessary in all the centre weft fork motions is to enable the loom to restart after the weft replenishment without the presence of a weft thread across the shed. This means that the knock-off arm should be made ineffective for the first pick without the help of the weft thread.
A shield has been provided in all such motions to enable the sley to move forward, on the first pick, without stopping the loom. On successive picks the shield moves out to enable the weft thread to act as a preventive device to knock-off the loom.
The centre weft fork motion shown in figure has two important parts. The first part, the weft fork, is attached to the sley and moves with it. The second part consisting of, cam, knock-over lever, brake lever, the rod that connects the mechanism to the shipper lever, all attached under the breast beam, which is stationary.
The weft fork A is pivoted in a stud and is connected to a lever B pivoted in a bracket on the lower end of the stand by a connector rod C. An adjustable knock-off arm D which is connected to the lever B slides over the face of the cam E projecting from the breast beam assembly. The knock-off arm D is held against the cam face by a special spring S on the opposite end of the lever.
During the backward movement of the sley the fork is raised, and during the forward movement it drops down. The projecting stand F mounted under the breast beam has a knock-off lever G on one side and a first pick shield M on the other. The knock-off lever projects above the lug stop of the stand F. If the weft thread is not holding the fork from falling down in the sley channel, that is the absence of the weft, the knock-off arm D will follow the cam E during the forward movement of the sley and engage the knock-off lever G. When the knock-off lever is pushed back by the knock-off arm, a round bracket on the lower part of the lever will press a brake tube lever, turn the brake and stop the loom.
Immediately the loom is knocked-off, a flat spring J clamped to the shipper shaft pushed back the first pick shield through an intermediate lever N. Since the shield M is held by pins that follow the curved shape of the cam slots, a push at the back will enable it to rise above the top of the stand F and also above the top of the knock-off lever G. When the loom is started after the repair of the broken pick, the flat spring is moved away from the lever N but the shield F stays in position owing to the dwell in the cam slots. On the first pick the advancing knock-off arm, strikes the end of the cut out O in the shield pushing it forward into the normal position.
Improved centre weft fork motion:
One disadvantage of the motion described is that the whole mechanism is difficult to approach for any adjustments. The looms, namely, the German Zang, the Swiss Ruti and the American C & K for silk have designed to have all the working parts in an easily accessible position at the side of the loom. Therefore setting and adjustment of the parts is made easier.
Problems associated with centre weft fork motion:
1. Weft curls in the middle of the cloths.
a) The prongs of the forks press the weft through the bottom shed.
b) Early or strong picking.
c) Irregular loom speed.
a) Correct tensioning of weft in the shuttle.
b) Shortening the prongs a little in case of rayon weft.
c) A longer setting for the prongs in case of nylon weft.
2. Loom stopping constantly although weft has not broken.
a) Slack warp.
b) Slack weft.
c) Fibrous or hairy warp.
Remedies: Correct tensioning of warp and weft.