If someone twists your head, it is your neck that suffers most. That is because the neck is a ‘thin’ place and offers little resistance to being twisted. By analogy, if a yarn of varying thickness is twisted, it is usually the thin spot in the yarn that gets twisted the most. Invariably, yarns spun from staple fibres (e.g. wool, cotton) are not perfectly uniform, and there are thick and thin spots along the yarn length. This variation in yarn thickness will lead to variation in the twist level along the yarn length, because twist tends to accumulate in the thin place.

The fact that twist tends to accumulate in the thin spot along the yarn has several important implications:

• It exacerbates the variation in yarn linear density While variation in yarn linear density is the fundamental cause of twist variation, concentration of twist in the thin places will make those places even thinner, exacerbating the problem of yarn unevenness.

• It improves the evenness of a fibre assembly during “drafting against twist” In the drafting stage of woollen ring spinning, the woollen slubbing is drafted while twist is inserted into the slubbing (drafting against twist) to control fibres during drafting. Because twist tends to accumulate in the thin spots, the fibres in thin regions in the slubbing are more difficult to draft than those in the thick places, which have less twist. As a result, the thick places are drafted more than the thin places, thus improving the evenness of the drafted material. This is depicted in figure

It has implication for twist measurements Because the twist level varies along the yarn length, the twist measured at a short length of yarn may not reflect the true average twist of the yarn. Standard test procedures should be followed to measure the yarn twist accurately

Twist contraction

When a bundle of parallel fibres is twisted, the distance between the two ends of a fibre will decrease, particularly for fibres near the surface of the twisted bundle. As a result, the overall length of the twisted bundle is shorter than its length before twist insertion. The reduction in length due to twist insertion is known as twist contraction. The following formula is used to calculate the amount of twist contraction:

It should be noted that because of twist contraction and the associated change in length, the count of a yarn will change slightly when twist in the yarn is changed. Twist contraction increases yarn count (tex), because the weight of the yarn is distributed over a shorter length. The following formula can be used