As described in the previous section, in the rotor spinning process fibers are continuously fed into the rotor groove and the yarn is also continuously withdrawn from the rotor groove. The fibers laid parallel and untwisted in the fiber collecting groove of the rotor are given the necessary twist via the finished yarn being withdrawn from the rotor. A finished end of yarn must therefore be fed into the rotor – in the opposite direction to yarn take-off – at the start of the spinning process. The yarn end is also twisted by the rotating rotor. The yarn end is pressed into the rotor groove by the rotor‘s centrifugal force and is thus connected to the fiber ring fed into the rotor groove. The yarn twist penetrates into the fiber ring in the collecting groove, where the fibers are to be bound together to form a yarn. Each revolution of the yarn inserts one turn of twist.
The zone in which the yarn end inserts twist into the fiber ring is described as the twist or binding-in zone (Fig. 94). The length of this binding-in zone is of some significance for the spinning conditions and the yarn characteristics. If this length is too short, the ends down rate will be high; if it is too long, twistingin will be very tight, and there will be many wrapping fibers.
Accordingly, in rotor spinning, it is not possible under given conditions to reduce the coefficient of yarn twist below a certain value (αmin) because otherwise the length of the binding-in zone will be reduced to zero (refer to 
Rotor speed and rotor diameter). The yam-twist momentum will then be negligible, and transmission of twist to the fibers in the ring will not be assured. The parameter αmin is therefore independent of yarn strength.
Dragging of the yarn from the rotor arises at the yarn lift-off point. The yarn is continually withdrawn at this point, which therefore shifts continuously forward within the rotor in the direction in which the rotor itself rotates, i.e. the yarn liftoff point has a higher peripheral speed than the rotor. The exact twist formula for the yarn would thus have to be represented as follows:
The lead relative to the rotor speed is, however, so small that it can be ignored on a percentage basis and it is possible to use the usual form of twist formula in relation to the rotor spinning machine as well:
The process involved in imparting twist is far from simple. To assist in understanding the procedure, the reader can imagine a manually operated winch mechanism (see Fig. 95), in which:
- (a) represents the take-off rollers,
- the yarn on the stretch (b) represents the axis of the winch, and
- the yarn on the stretch (c) represents the hand-operated crank with the handle (d).
If yarn section (c) is now rotated like a crank at handle (d), the axis (here section (b)) rotates as in the case of the winch. However, since – in contrast to the winch – the rollers cannot rotate around the yarn axis in this model, the result is twisting only of yarn section (b). The turns imparted by this process are all in section (b); section (c) remains temporarily untwisted.
Section (c) nevertheless contains turns of twist running from section (b) by twist transmission; some of the turns generated in section (b) travel into section (c) (evening out of torsion forces).
As in the case of ring spinning, twist is transmitted against the direction of movement of the yarn. In rotor spinning, bending of the yarn at the nozzle acts as a brake for twist transmission. This means that the system itself transmits fewer turns into section (c) than were generated in section (b). Under such conditions, spinning at high speeds and normal twist coefficients would not be possible, because the twist momentum available from the yarn would be inadequate to twist the fibers together in the rotor groove (the twist momentum transmissible from the yarn is a function of the twist coefficient).
In practice, however, yarn section (c) must exhibit more twist turns than section (b). This is, in fact, the case and arises from the false-twist effect and from tension variations in the yam.
Fig. 94 – Inserting twist in the rotor groove
Fig. 95 –Inserting twist in the yarn
