# Rieter

### Twist and strength

#### Index

Fig. 59 – Relationship between the number of turns of twist and the strength of a yarn; F, strength; T/m, turns of twist per meter in the yarn; PES, polyester fibers; Co, cotton fibers

The strength of a thread twisted from staple fibers increases with increasing twist. In the lower portion of the curve (Fig. 59), this strength will be due solely to sliding friction, i.e. under tensile loading the fibers 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 criticaltwist 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:

$Yarn\ twist = \frac {spindle\ speed \left( rpm \right)}{delivery\ speed \left( m/min \right)}$

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.