The significance of the high torsional rigidity of the false-twist threadline, and also of the variation of its torque with changes in tension, is discussed. The magnitudes of the rigidity and the torque-tension gradient are explained by treating the yarn as an anisotropic solid rod, and it is shown that the stresses involved can, where necessary, be supported by interfilament friction. The origin of the steady threadline torque is examined (a) in the zone in which the yarn first twists on entering “the system, and (b) as the yarn cools between the heater and the spindle.
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