TWIST GEOMETRY AND TWIST LIMITS IN YARNS AND CORDS

Abstract

In the first part of this paper, the geometry of twisted yarns is analysed and, by the introduction of the term “basic” twist (defined as twist per unit of untwisted length), some interesting and apparently new relations are obtained which show why there is a limit to twist and why snarling is inevitable above this limit, even when the yarn is held taut. This theoretical onset of snarling may have useful applications in the analysis of the twisted zone in false-twist bulking processes, where the highest twist is used and temporary snarling is intentionally induced. In the second part, the geometry of primary cording, i.e., the twisting together of two or more twisted yarns, is investigated with the approximation of neglecting their curvature whilst taking proper account of their true length and true torsion. This approximation allows the results of the first part of the paper to be applied and leads to a fairly simple solution without need to consider the path of individual filaments. The resultant equations show that there is a cord twist at which the “de-twist” (more accurately, reduction of torsion) of the plies is a maximum: further increase of cord twist beyond this level increases ply torsion towards the pre-twisted level. This explains the minimum found experimentally when outside filament-angle relative to the ply axis is plotted against cord twist. In the third part, the existence of a limit to cord twist is demonstrated, above which snarling must occur. This is when the plies in the cord are corded as close together as possible, very similar to a close-coiled spring, and though it seems likely that the existence of this limit must be known, at least experimentally to some, yet there appears to be no mention of it in the literature.