Mechanisms of yarn twist blockage caused by frictional contact

Abstract

This thesis is concerned with an investigation on the mechanisms of yarn twist blockage caused by frictional contact. As there has been limited published work in the literature, both theoretical and experimental studies were conducted in order to describe various processes of twist blockage, and determine the effects of machine and yam parameters quantitatively. Two mechanical models have been developed to predict yam twist distribution in the frictional contact zone of a cylindrical guide and a revolving surface, respectively. The first mechanical model was developed for a cylindrical yarn guide where dynamic equilibriums of forces and moments are considered. This model is capable to describe, quantitatively, distributions of four functions in the frictional contact zone, namely, the deviate angle, yarn tension, frictional force and yarn twist. It can be used to characterize twist blockage between the yarn and guide in terms of yarn, machine, and operational parameters. The effects of these factors are discussed for the cases that the yarn twist is independent of time and the yarn speed is much lower than that of strain wave propagation. The second mechanical model was developed for a revolving surface, where the yarn twist blockage, yarn tension and deviate angle can be determined as functions of wrap angle, contact surface factor, torsional factor, and rotary speed factors from the navel frictional contact zone. A passive rotary navel in rotor spinning was examined to reduce yam twist blockage between yarn and frictional contact surface of navel. A measurement system has been developed to determine distributions of yarn diameter and twist. The binary image conversion technique was applied to evaluate the yarn average diameter, and the two-dimensional Fourier transform technique was used to assess major fiber orientation angle on the yarn surface. An angular function derived from the power spectrum was used to determine the major energy in a power spectrum image. Five ring yams of 100% cotton spun by a lab-spinner have been used to test the measurement system. Twist values obtained by the conventional twist-untwist method agree well with those measured by the present method. The relationship between the yarn torque and twist was studies for the case where there is a frictional contact between yarn and guide. A new device has been set up which the yarn torque and twist can be measured simultaneously. The influences by the wrap angle of yarn over the yam guide, yarn tension and input twist were investigated. The effect on yarn twist blockage caused by yarn flattening was identified. An experimental investigation of yam twist blockage in rotor spinning has been carried out. The twist level inside the rotor for different types of navels was investigated by using the high-speed photography technique. The yarn tension variation affected by different frictional surface of navels and rotor speeds was also investigated. The surface structure of result yarns spun with various different kinds of navels was examined by the two-dimensional wavelet analysis.