Yarn hairiness assumes a significance which should not be underestimated. This refers to the length and frequency of fiber ends that are not integrated in the yarn and therefore protrude from the yarn bundle.
High yarn hairiness (primarily of hairs longer than 3 mm) can have a negative impact, both due to a diffuse fabric appearance lacking in clear structure and also in downstream processing due to a tendency to cling and fiber deposits on the machines. If these deposits reach the final fabric they are usually rated as disturbing defects. However, high yarn hairiness (hairs shorter than 3 mm) is positive where it contributes to soft fabric hand. Soft, flexible fabric hand is preferred in knitted fabrics for underwear, T-shirts and leisurewear. High – albeit precisely defined – yarn hairiness is required for processing denim yarns using the rope dyeing method. However, in this case the high hairiness does not contribute to the hand, but acts as a spacer between the yarns when the dyed rope is opened.
Rotor-spun yarns display significantly lower hairiness than comparable 
ring-spun yarns. The reason given for this by Bunk/Trommer (see references) is that the fiber ends facing away from the yarn take-off direction point toward the interior of the yarn and the number of free fiber ends is therefore about half that in ring-spun yarns. Furthermore, the wrapper fibers wound crosswise around the yarn help to „bind-in“ loose fiber ends. Abrasion resistance and pilling tendency are positively influenced by these wrapping fibers. The clinging tendency, fiber abrasion and fiber fly of rotor yarns in downstream processing are less critical than for comparable ring-spun yarns. Put simply, the higher hairiness of ring-spun yarns is caused by the uncontrolled passage of edge fibers in cylinder drawframes and in the spinning triangle at the drawframe delivery end.
However, low hairiness is a drawback where a large number of protruding fiber ends contributes to a soft hand, for example in knitted fabrics for underwear and leisurewear. Here ring-spun yarn has an advantage, since its high hairiness is especially beneficial for a soft hand in the final fabric. However, the disadvantage of rotor-spun yarn‘s lower hairiness can – and this is in turn an advantage over other spinning systems – be varied in a wide range by means of spinning elements (rotor groove and draw-off nozzle). First of all, rotor-spun yarns can now be produced with significantly lower twist multiplyers than previously, without any adverse effect on spinning stability. Yarn hairiness and yarn bulk can also be adapted to the end product by the skillful choice of technology components, for example:
- By the rotor groove: the larger the groove angle, the groove radius and the rotor diameter, the bulkier and hairier the yarn. If rotor speed and thus spinning tension are also reduced, further gains in hairiness and bulk are achieved. If rotor speed is reduced, the twist multiplyer can also often be reduced – since spinning tension declines – which offsets the loss of production due to the reduction in speed. Conversely, tighter groove angles and smaller groove radii and rotor diameters mean that leaner and more compact yarns with lower hairiness can be produced. And the higher the rotor speed and thus the spinning tension, the greater the effect.
- By the shape and design of the nozzle surface: the more structured the nozzle surface and the longer the contact surface (nozzle radius) on which the yarn unwinds, the higher the yarn hairiness and the yarn bulk. Additional inserts in the throat of the nozzle and additional TWISTstop elements in the draw-off nozzle further increase the hairiness of the yarn. The smoother the draw-off nozzle, the smaller the nozzle radius and thus the contact surface, and the fewer twist accumulation elements affect the passage of the thread, the lower the hairiness and bulk of the yarn.