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 Tk within the knitting zone, knowing the coefficient of yarn friction μ. This analysis suggests that the knitted loop length depends on the position of Tk rather than on the magnitude of Tk, but that fabric holes and "press-offs" are caused by Tk exceeding the breaking load of the yarn. For high values of μ, Tk 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 Tk by 70% for high-friction yarns and by 50% for low-friction yarns, for the same cam setting and input tension.