Twist is the number of turns per unit length (cm, m or inch). In the manufacture of staple fibre yarns, twist is inserted into the fine strand of fibres to hold the fibres together and impart the desired properties to the twisted yarns. Without twist, the fine strand of fibres would be very weak and of little practical use. A change in the level of twist also changes many yarn properties, such as strength and softness. This section discusses the nature of yarn twist, the effect of twist on yarn properties, as well as twist measurement.

Nature of twist

Types of twist

There are two types of twist:

real twist and false twist.

(1) Real twist To insert a real twist into a length of yarn, one end of the yarn should be rotated relative to the other end, as indicated in figure 13.3(a). Spun yarns usually have real twist, which holds the fibres together in the yarn.

(2) False twist When inserting false twist into a length of yarn, both ends of the yarn are clamped, usually by rollers, and twist is inserted with a false twister between the clamping points, as indicated in figure 13.3(b). If the yarn is not traversing along its axis, the twist will be in opposite directions above and below the false twister.

If the false twister is removed, the opposite twists will cancel out one another, leaving no real twist in the length of yarn. If the yarn is traversing along its axis, then the section of the yarn moving away from the false twister would have no net twist, as indicated in figure 13.3(b).

False twisting is a very important phenomenon, which has considerable practical implications in yarn technology. False twisting is featured in many key processes that we will discuss later, including woollen ring spinning, open-end rotor and friction spinning, air jet spinning, and filament yarn texturing

Twist direction

A twist can be either in Z direction or S direction as indicated in figure 13.4, depending on the orientation of the surface fibre in relation to yarn axis.

It is worth noting that twist direction affects fabric properties. For example, Figure 13.5 shows two identical twill-weave fabrics with the warp yarn of different twist direction. Fabric A will be more lustrous than fabric B, because light reflected by fibres in the warp and weft is in the same direction. Fabric A will be softer while fabric B firmer, because in Fabric B, the surface fibres on the warp and weft in the region of contact are aligned in the same direction and they may ‘get stuck’ inside each other and reduce the mobility of the intersection. Whereas for fabric A, the surface fibres on the warp and weft in the region of contact are crossed over, and they can move about easily. The freedom of movement at the yarn intersections is the key for fabric softness.