Objective of centre weft fork motion:
The weft fork motion stops the loom when the pick gets absent in the warp shed. If this motion is mounted at one side of the loom, then this motion performs its work at every alternative pick. There is always a possibility of missing two picks in the fabric and these missing picks create either dense marks or crack during restarting of the loom.
The centre weft fork motion is mounted at either middle of the race board or nearly the middle of the race board. This mechanism stops the loom immediately if the weft breakage occurs mechanism doesn't wait for the next round of loom. Thus this mechanism helps to improve the quality of the fabric.
Structure of center weft fork motion:
A slot is cut at either middle or near the middle of the race board. The prongs of the weft fork fall in this cut-out slot on the race board. The weft fork with prongs gets fulcrummed at the weft fork support bracket. This weft fork support bracket is attached to the front of the sley. A return spring is mounted at the one end of the knock-off rod as shown in the figure. A weft fork cam is mounted on the knock-off rod near the weft fork support bracket. This weft fork cam touches the bowl mounted on the weft fork. The driving collar is mounted on the knock-off rod. The driving collar is connected with the driving rod. There are two guide collars fixed on the sley. The one end of the knock-off rod passes through these two fixed guide collars. A knock-off lever is mounted near the outer guide collar under the end of the knock-off rod.
working principle of center weft fork motion.
The movement of sley is used to reciprocate the weft fork cam by having the knock-off rod fixed to a point on the loom frame which is often attached to the inside of the loom frame near its base. When the sley makes a reciprocating motion, the distance from the fulcrum varies. this distance variation in the fulcrum causes the knock-off rod to be driven to the left direction as shown in the figure. The returning of the knock-off rod is achieved by return spring.
As the weft fork cam moves in the left direction, the bowl of the fork rides up the weft fork cam surface and the prongs of the fork lift up. Now the shuttle travels from one shuttle box to another shuttle box.When the fork is supported by weft yarn, the bowl is carried over the knock-off notch, and the knock-off rod makes complete movement toward the right side. In this situation, the left side of the knock-off rod doesn't protrude and it does not strike the knock-off lever. Thus loom gets continues to run.
If weft yarn gets absent in the shed the prongs of the weft fork fall in the cut out of the sley and the bowl gets trapped in the knock-off notch and limits the distance that the knock-off rod can move to the right. A small length of knock-off rod is now protruding from the left side of the guide collar. As sley moves toward the beating position, the protruding portion of the knock-off rod strikes with a projection from the starting handle and knocks off the loom. the loom is stopped before the beating position.
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