rigidly connected to one another releasably at different spacings with respect to one another in accordance with fig9 in contrast to fig1 each lower roller i ii and iii is driven by its own drive motor 20 52 and 19 respectively as shown for example in de os 38 01 880 the motor 20 drives the toothed belt wheel 55 of the lower roller i by way of the toothed belt 56 the motor 52 drives the toothed belt wheel 41 of the lower roller ii by way of the toothed belt 57 and the motor 19 drives the toothed belt wheel 40 of the lower roller iii by way of the toothed belt 47 attached to the slider 36 a in addition to the smooth guide pulley wheel 46 is a further smooth guide pulley wheel 51 the endless toothed belt 47 loops around in succession the pulley wheels 44 46 40 51 and 43 the toothed belt wheels 44 40 and 43 are in engagement with the teeth of the toothed belt 47 whereas the smooth guide pulley wheels 46 and 51 are in engagement with the smooth reverse side of the toothed belt 47 the sliders 35 a and 36 a are rigidly.