18—THE DEVELOPMENT OF TWIST IN A FALSE-TWIST TEXTURING MACHINE WITH A FRICTION TWISTER

Abstract

In a false-twist texturing machine, the yarn can be rotated by means of either a pin or a friction twister. Irrespective of the type of twister used, the yarn twist can only be stable when it is numerically equal to the ‘speed ratio’ (the ratio of rotational to linear speed). When the machine is started up, the yarn has no twist, and a certain length of yarn has to be processed before the twist attains its stable value. The nature of this initial development of twist depends on the type of twister used.

With a pin twister, the speed ratio is a machine setting and is independent of the yarn twist. When the machine is started up, the speed ratio attains its stable value almost instantaneously, and the yarn twist approaches this value exponentially.

With a friction twister, the speed ratio is not a machine setting and is inversely related to the yarn twist except under certain theoretical limiting conditions, which hardly ever occur in practice. Since the yarn twist equals zero when the machine is started up, the initial value of the speed ratio is higher than its stable value. The subsequent increase in twist is accompanied by a decrease in the speed ratio, and stable conditions are reached when the two quantities have converged to a common value. The difference between the initial and final speed ratios depends on the magnitude of the rotational slip present under stable running conditions. The transition from the initial to the final speed ratio is discontinuous when the longitudinal slip is small or absent. Under these conditions, the development of twist can be calculated by a simple analytical method. When the longitudinal slip is large, the transition from the initial to the final speed ratio is continuous, and the relevant differential equations cannot be solved analytically. For these conditions, numerical solutions are found by means of a computer.