In contrast to ring spinning, twisting during rotor spinning takes place from the inside outwards. The rotating, brush-like open yarn end first catches fibers in the core and then with further rotation gradually takes up fibers toward the periphery. In the interior, where the fibers cannot avoid the twist, the fibers become more compact. On the other hand, toward the exterior, the compactness falls off to an increasing degree, since here the fibers are able partially to avoid being twisted in. In addition to the fiber-specific properties (fiber strength, elongation, length, friction, etc.), yarn tenacity depends primarily on how often the yarn has been rotated around its axis. In this process the angle of inclination of the fibers being twisted in plays a crucial role for the degree of tenacity (see Fig. 90).
This means that twice as many twists have to be imparted to a fine count yarn in order to achieve the same angle of inclination and thus the same level of tenacity as in a yarn twice as thick. The absolute number of yarn twists therefore gives an indication of the degree of tenacity only if this is related to yarn count. However, twist multiplyer α/m or α/e can be used to describe the degree of twist in a yarn, regardless of yarn count. The higher the twist multiplyer, the higher the degree of twist and the higher the yarn tenacity, and vice versa. For detailed explanations of yarn twist, refer to the volume entitled: The Technology of Short-staple Spinning, section Twist formulas.
However, the required twist multiplyer α/m or α/e for maximum tenacity of a given yarn varies widely with the raw material being processed. Depending on the type of fiber and its key physical characteristics an average twist multiplyer is enough to reach a certain tenacity in one case, in other cases a higher twist mutiplyer must be selected. This means that higher twist multiplyers must be selected for cotton yarns (fiber strength 20 - 30 cN/tex) than for blended yarns (fiber strength mix of 30 - 40 cN/tex) and for these in turn higher twist multiplyers than for viscose, polyester or polyacrylic yarns with fiber strengths of 40 - 60 cN/tex. NB: overtwisting yarns reduces yarn tenacity!
Furthermore, the twist multiplyers for warp and weft yarns (in all raw materials) are generally higher than those for knitting yarns, since yarn bulk, yarn hairiness and a soft hand take priority for knitting yarns rather than the highest possible yarn tenacity, such as is required for yarns processed in weaving preparation and weaving.
Higher twist multiplyers are used,
- to increase yarn tenacity and yarn elongation;
- to produce lean yarns with low hairiness;
- to improve spinning stability;
- to obtain a clean-cut fabric appearance; and
- to improve the shifting resistance of the yarns.
Lower twist multiplyers are selected, presupposing adequate yarn tenacity,
- to achieve a soft hand in the final fabric;
- to produce bulky and more hairy yarns;
- to reduce a yarn‘s tendency to snarl; and
- increase output with the same rotor speed.
The twist multiplyers for weaving and knitting yarns naturally differ, as do the twist multiplyers for yarns made from different raw materials (see twist multiplyers customary in mill practice for weaving yarns in Fig. 91 and for knitting yarns in Fig. 92).
Calculation of yarn twist:
* conversion factor dtex/Macronaire
The number of turns inserted in a yarn depends on how long a given length of yarn spends in the rotor. If a yarn is withdrawn from the rotor more rapidly at a given rotor speed, fewer turns can be inserted (by the rotating rotor) in the yarn per unit of time than at a lower take-off speed when the yarn therefore spends more time in the rotor groove:
The specified number of turns for a given yarn is thus inserted, at a given rotor speed, by adjusting the delivery speed. The feed rollers are driven by infinitely adjustable inverters.