rpm variable motor 20 based on the different circumferential velocities of the roll pairs the slivers are drafted and the extent of draft is a function of the velocity relationships between the roll pairs the rpm changes of the motor 20 thus result in a change of the extent of draft such a change is controlled by the control device 22 with the aid of a setting signal which is correlated with the mass fluctuations of the slivers in the input zone of the draw unit 0024 fig4 shows a regulating device associated with the opening unit of an open end spinning machine and includes a feed roll 27 a feed table 44 and an opening roll 28 the fiber material entering the device is designated at 29 as a first step the sliver mass fluctuations are detected by a measuring member 30 subsequently based on the detected values the optimal points of regulation are determined by a regulator 31 with the aid of a computing algorithm or a transfer function so that as a result with each sliver measurement a point of regulation is associated the point of regulation is thus variable during operation of the regulated draw unit in fig5 the device according to the invention is associated with the draw unit of a ring spinning machine the sensor 38 detecting mass fluctuations of the sliver 37 applies its signals to a regulator 39 which in turn controls a motor 40 driving the two roll pairs 33 a 33 b and 34 a 34 b the motor 41 drives the output roll pair 35 a 35 b with a constant speed on the basis of the detected values the optimal points of regulation are.