The collecting groove of the rotor combines the fibers delivered to it into a ring of fibers which changes into the twisted thread at the integration point (refer to Fig. 17), while the integration point moves forward relative to the rotor collecting groove at yarn take-off speed. The integration point starts immediately after the point at which the yarn is lifted out of the rotor groove. The fiber ring formed in the rotor consists of individual layers of fiber. A thin layer of individual fibers – their number corresponding to so-called back-doubling – is deposited in the rotor groove with each revolution of the rotor:
The number of fiber layers from which the spun yarn is formed results from the rotor diameter, the twist multiplyer and the yarn count. Since back-doubling increases and declines in a straight line relative to the rotor diameter, using smaller rotor diameters implies a reduction, using larger rotor diameters an increase in the number of fiber layers from which the yarn is formed (refer to section Fiber collection in the rotor groove (back-doubling)). Doubling linear bundles of fibers, i.e. forming a sliver or yarn from several layers, implies in principle an improvement in the regularity of the fiber bundle, with back-doubling exerting a positive influence on variations that amount to no more than the length of the rotor circumference.
When the number of fibers required for a given yarn count have been deposited in the rotor groove, the end of the yarn already spun, which extends into the rotor groove and rotates with the rotor, transmits the twist to the fiber ring. The integration zone operating with constant overfeed is described as the „twist zone“, the zone in which the thread leaves the rotor groove as the „lift-off point“ (Fig. 17).
Rotor spinning is an open end process which generates a genuine yarn twist. In this case the component imparting the twist is the rotor, which twists the thread around its axis. The resulting yarn twist is the decisive factor for yarn tenacity. However, in order to maintain the spinning process, i.e. integrate the fibers in the rotor groove, a spinning twist is required, which as a rule must be higher than the yarn twist required for yarn tenacity. This means that an additional twist must be imparted to the radial section of yarn (imparting false twist). This false twist is imparted by the unrolling motion of the yarn on the draw-off nozzle, which is therefore much more than a thread guide. Depending on spinning conditions, the false twist can be up to 60% of the set yarn twist.
The false twist effect generated between the draw-off nozzle and the yarn unrolling from it has Z twist between the draw-off nozzle and the rotor groove and S twist between the draw-off nozzle and the nip of the take-off shaft and the pressure roller. At this nip the false twist effect has again reached its zero point and the yarn body has only the preset genuine Z twist. The false-twisting effect of the draw-off nozzle can be increased by inserting a twist accumulating element in the draw-off tube immediately following the draw-off nozzle (refer to section Genuine and false twist).
All rotor spinning machines are designed to spin yarns with Z twist. Z twist is the customary direction of twist used in practice. Manufacturing yarns with S twist would imply redesigning the rotor drive, sliver feed into the spinning box and fiber feed to the rotor.
In light of the large quantities of fibers a rotor has to cope with, the centrifugal forces already referred to and the abrasive components sometimes present in the material or the fibers themselves, rotors and also the clothing of the opening rollers are subject to natural wear and tear. Solid steel rotors, usually protected against wear by boron, diamond or boron/diamond-coated surfaces, currently offer exceptionally long service lives of up to 30 000 hours for rotors and opening rollers, depending on fiber throughput volumes.