The level of twist is usually expressed in number of turns per metre (tpm). Number of turns per inch or twist per inch (tpi) is also used in the industry. More twist gives greater radial component to any applied tension, so increases resistance of fibres to slip and the strength of yarn increases as a consequence. This is depicted by the ‘coherence curve’ in figure 13.6. On the other hand, if a bundle of parallel filaments is twisted, the twist will put the individual filaments under torsional stress. This stress weakens the filaments and the strength of the filament would decrease as the level of twist increases. This is depicted by the ‘obliquity curve’ in figure 13.6. For staple fibre yarns, these two curves combine to give the actual ‘twist-strength curve’ for a staple fibre yarn as shown by the heavy line in figure

indicates that for staple fibre yarn, increasing the twist level will increase yarn strength to a maximum level, beyond which further increase in twist will reduce yarn strength. It should be noted that for continuous filament yarn, the obliquity curve applies. In other words, twisting a continuous filament yarn only reduces the yarn strength, regardless of the twist level used. If a continuous multi-filament yarn is twisted, the reason for the twist is to keep the individual filaments together, not for strength.

Twist angle

This is the angle of fibres to yarn axis, and this angle varies throughout yarn, from zero at centre to maximum at yarn surface. The fibres on yarn surface are the most important, as they bind the others into the yarn (refer to self-locking effect discussed earlier).

While it is not common practice to measure the yarn twist angle, the surface twist angle made by the surface fibres in relation to yarn axis is a very important parameter. It determines the essential yarn characteristics such as yarn softness, yarn bulk etc, which in turn govern many essential fabric properties. The following example illustrates the point. In figure 13.7, yarn 1 and yarn 2 have the same twist level – one turn each. But the surface fibre on the thicker yarn is obviously stretched more to accommodate this twist.

This would mean the thicker yarn is more closely packed. As a consequence, yarn 2 will not be as soft as yarn 1. In other words, even though the twist level is the same in these two yarns, the yarn characteristics are quite different. Therefore, we can not simply use twist level to represent yarn character. However, the surface twist angles of yarn 1 (θ1) and yarn 2 (θ2) are different. They can better reflect the yarn characteristics, regardless of the difference in yarn thickness.

Twist factor (twist multiplier)

This is a very important factor that relates to the angle of twist helix the surface fibres have in a yarn. As we will see later, this factor is very important for a spinner because of the following reasons:

• Like surface twist angle, it governs the yarn characteristics

• It is used to work out the twist to use in spinning, in order to maintain the same surface twist angle and similar yarn characteristics when the yarn count is changed.

The twist worked out from twist factor is also needed for setting up the spinning machine. Twist factor is related to yarn count and the twist level in a yarn. This relationship is expressed in different ways for different yarn count systems.