Side Weft Fork
Motion:
Side weft fork motion check the
presence of weft in every two picks. It is situated at the side of the reed. It
is used for producing medium and heavy fabric. Basically, it is used in modern
automatic loom.
Principle of 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.
Side weft fork motion
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Where,
A= weft fork
B= weft fork holder
C= Fulcrum
D= knock over lever
E= Hammer lever
F= greyhound tail lever
G= Weft fork cam
S= Starting handle
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.
A= weft fork
B= weft fork holder
C= Fulcrum
D= knock over lever
E= Hammer lever
F= greyhound tail lever
G= Weft fork cam
S= Starting handle
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.
Mechanism of Side Weft Fork Motion
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.
Disadvantage of Side Weft Fork Motion:
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.
