In the course of development, rotor speeds have been increased from approx. 30 000 rpm originally to 160 000 rpm today. However, this has only been possible by simultaneously reducing rotor diameter. It can be demonstrated that all rotor speeds and diameters introduced in recent decades are closely related, as can be shown by a graph of the same centrifugal force. Fig. 18 shows centrifugal force cN/tex as a function of rotor speed and the resulting spinning range for different rotor diameters. Assuming that the centrifugal force acting on the thread in the rotor can never exceed yarn tenacity, this represents a theoretically absolute spinning limit which cannot, however, be reached in practice and is also not intended to be reached. Spinning tension must always lie with a sufficient safety margin below the „normal“ variations in inherent strength existing in the yarn, otherwise economical running behavior cannot be achieved.
However, not only a maximum, but also a minimum possible speed is allocated to each rotor. If the rotor speed and thus spinning tension decline to such an extent that the centrifugal force in the rotor groove is no longer sufficient to generate the twist retention and false twist effect (between nozzle and rotor groove) on the draw-off nozzle which are necessary for spinning stability, twist integration in the rotor groove is seriously disturbed or interrupted and a thread break occurs. This situation is clearly apparent when calculating the so-called minimum twist multiplyer (αmin), from the course of which the optimum speed range for each rotor diameter can be derived (refer to Fig. 19).
Reducing rotor diameter for the sake of higher rotor speeds and thus higher output has worked surprisingly well as a rule. The repeatedly predicted (lower) limits for rotor diameter have consistently been breached by development, with the result that quality yarns are spun nowadays with 28 mm diameter rotors operating at speeds of up to 160 000 rpm (and suitable raw material). It should also be mentioned in this context that the frequently prophesied need to increase twist when reducing rotor diameter has not materialized.
However, the fundamental relationship between rotor diameter and fiber length, although not invalidated, is decisively modified by the considerable development advances in rotor technology (refer to section Technology).