Twist is produced in the yarn with the aid of spindles, rotors, rollers, and so on. Since two twist directions, left and right, are always possible, the fiber windings can also have two directions. The direction of the twist is indicated as Z- or S-twist depending on the transverse orientation of the fibers, i.e. the orientation relative to the diagonals of the letters Z and S (Fig.1). Z-twist is normally used in short staple spinning, though in some cases yarns with S-twist are also produced.

Twist and Strength

The strength of a thread twisted from staple fibers increases with increasing twist. In the lower portion of the curve (Fig.2), this strength will be solely due to sliding friction, i.e. under tensile loading the fibers tend to slide apart.

Cohesive friction arises only in the middle-to-upper regions of the curve. This is caused by the high tension, and thus high pressure, and finally becomes so considerable that fewer and fewer fibers slide past each other and more and more are broken.

This continues up to a certain maximum, i.e. to the optimal exploitation of the strength of the individual C) – is dependent upon the raw material.

Normally, yarns are twisted to levels below the critical twist region ( A – knitting, B – warp); only special yarns such as voile ( C) and crêpe ( D) are twisted above this region.

Selection of a twist level below maximum strength is appropriate because higher strengths are mostly unnecessary, cause the handle of the end product to become too hard, and reduce productivity. The last effect arises from the equation:

Since the spindle speed is always pushed to the maximum possible limit (and thus may be considered as constant), higher yarn twist can only be obtained through reduction in the delivery speed and hence in the production rate.

Deformation of the yarn in length and diameter

·         Fibers can be wound in spirals around other fibers only by increasing their length through exploitation of fiber elongation.

·         When a fiber is extended, its elasticity tries to draw it back. This constant tendency to return to the unextended condition results in a high tension directed towards the core and thus to increase pressure continually towards the yarn interior.

·         These tensions cause the strong compression, and hence great density of the yarn body. The compression leads to a reduction in the diameter of the yarn.

·         Diameter is thus inversely proportional to twist. However, the tendency to relax also leads to shortening of the yarn (twisting-in, spinning-in).

The same effect is produced by the inclined disposition of the fibers relative to the yarn axis. Hence, the length of the spun yarn never corresponds to the delivered length measured at the front roller.

The degree of shortening is also dependent upon the raw material and especially upon the number of turns. Fig.3  shows how the degree of shortening depends upon the yarn linear density and the twist .