Factors Contributing to Hook Failure of Latch Needles in Weft Knitting

Abstract

One of the problems that occurs in high-speed knitting machinery is the event of hook breakage of the knitting needle. Many causes have been suggested for this. This article deals with a theoretical analysis into the nature of the cause of failure and describes a needle design feature that has proven to be effective in reducing the problem. It also reports a sequence of testing that supported the theoretical analyses. The basic premise of the report is that acceleration forces imparted to the knitting needles via the cams on the machines are major factors contributing to hook failure. Upon selection of a stress level required to achieve fatigue failure at the needle hook, an acceleration force is calculated, and the value of the acceleration required to produce this force is readily obtained. This value for the acceleration is then used as the input for a dynamic loading analysis, which demonstrates how changes in the design of the main body of the needle can reduce the acceleration at the hook end of the needle, thereby reducing the forces that can cause fatigue failure. By placing scallops or cutouts in the body of the needle near the driving butt, the stiffness-to-weight ratio of the needle is reduced, yielding lower accelerations at the hook. The conclusion is that the forces accompanying these accelerations are reduced, and that hook breakage can be reduced. The report also demonstrates how cams can affect needle life. Clearly, it is the cam design that determines the level of acceleration initially imparted to the needle butt, which in turn can be transmitted to the hook end of the needle.