A Study of the Mechanism of Loop Formation on Weft-Knitting Machinery

Abstract

A detailed study was made of the effects of yarn friction on the magnitude of the tensions developed in the knitting zone of the standard weft-knitting machine and the in fluence of these tensions on loop formation and fabric quality. Variations in loop length of ±10% due to changes in yarn friction may be experienced at given cam settings and input tensions and these variations can be explained by the phenomenon of "robbing-back." A geometrical model of loop formation is suggested as a means of mathematically predicting the values of the developed tensions, and the position and magnitude of the maximum knitting tensions T_k within the knitting zone, knowing the coefficient of yarn friction μ. This analysis suggests that the knitted loop length depends on the position of T_k rather than on the magnitude of T_k, but that fabric holes and "press-offs" are caused by T_k exceeding the breaking load of the yarn. For high values of μ, T_k exceeds the breaking load more readily than for low values. It is predicted that redesign of the conventional cam system to decrease the number of yarn/metal contact surfaces may reduce the values of T_k by 70% for high-friction yarns and by 50% for low-friction yarns, for the same cam setting and input tension.