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Why is the thread broken in bag sewing machine?
Nov 09, 2021

Thread broken is the most common fault encountered in bag sewing machines. The suture quality is unqualified, the machine is not adjusted properly, the thread passing parts have sharp edges, the operation method has problems, the needle cooling is not sufficient, etc. all of these will lead to thread breakage. When sewing products with uneven thickness with lock stitch woven bag sewing machine, in addition to considering and analyzing the above reasons one by one, Pay attention to wire breakage caused by twice hooking.

It is well known in the sewing machine industry that the transmission ratio is 1:2 between the upper shaft and the rotary shuttle, due to the need of distributing the action cycle in the design, that is, the upper shaft rotates for one cycle and the rotary shuttle shaft rotates for two cycles. Therefore, in the process of woven bag sewing, each time the needle pricks the material, the rotary shuttle rotates for two weeks, completes the hook expanding thread ring action one week, and idles one week. If the rotary shuttle hooks the surface thread every week, that is, the machine pricks the material once and the rotary shuttle hooks the thread twice, it will conflict with the retraction and release of the surface thread and stretch the suture.

woven bag sewing machine

This situation may occur when sewing uneven thickness sewing material: the reason can be discussed from the formation process of lock stitch:


Lock stitch formation process

The lock stitch is formed by the interweaving of the upper thread and the lower thread in the sewing material. When the woven bag sewing machine works, the needle guide belt upper thread passes through the sewing material and rises upward after reaching the lower limit position: due to the friction between the sewing material and the sewing thread, the upper thread cannot be lifted synchronously with the needle, but remains under the sewing material and expands to both sides of the needle under the action of elasticity to form a ring. Then, the shuttle tip reaches the needle during the movement, passes through the top thread ring, and continues to expand the hooked thread ring in the rotary movement. After winding to its radius, it "jumps" over the expanded thread ring-like skipping rope. The next action is to take up the thread by the thread take-up rod and feed the dog. In order to make these actions go smoothly in enough time, the rotary shuttle still rotates at the original speed for one cycle, however, the thread is no longer hooked and only idling for one circle. Due to the take-up effect of the thread take-up rod, the surface thread ring originally expanded and "jumped" by the rotary shuttle is rapidly reduced until the bottom thread from the rotary shuttle is pulled into the sewing material to form a primary interleaving. When the needle reaches the upper limit position and leads down again, the woven bag sewing machine repeats the above action process, again and again, to form a continuous and regular lock stitch on the sewing material


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