MECHANISMS OF YARN TWIST BLOCKAGE

Yarn twist blocking occurs when a yarn slides over a yarn guide or oth er machine part. This paper reviews previous investigations on twist b lockage, the influence of yam and guide conditions, and especially twi st blocking mechanisms. The twist blocking mechanisms so far suggested are all based on a continuum yarn model-a yarn treated as a flexible and laterally deformable cylindrical body. The validity and drawbacks of these mechanisms are discussed. A further mechanism based on the co ntinuum yam model-twist blocking due to biased normal pressure distrib ution across the yam-is proposed, and yam twist blocking phenomena are then re-examined by treating the yarn as an array of mobile fibers. T he yarn-to-guide frictional force pushes the fibers in front of the gu ide to increase their helix angles, and pulls the fibers behind the gu ide to decrease their helix angles. The new mechanism is put to the te st by comparing predicted twist blockage with previous observations, a nd is found to offer satisfactory interpretations.