13—A STUDY OF TORSIONAL STABILITY IN PLIED YARNS

In this paper, a study is made of the self-plying mechanism, by which torsional stability is spontaneously achieved in plied yarns. It is proposed that the state of twist balance, or zero torque, is reached when the torsional energy released by the effective untwisting of the yarn during plying is equal to the yarn-bending energy used in the plying process. This hypothesis is supported by extensive experimental evidence reported in the paper. Calibration curves are presented that give the ply twist of a balanced two-, four-, or seven-ply yarn as a function of the single-yarn twist factor. These curves are characterized by the basic yarn structure (i.e., multifilament or staple-fibre), the degree of relaxation of the torsional stresses in the single yarn before plying, and the constituent fibre type (specifically, the ratio of fibre torsional to bending rigidity.)