the smooth cylindrical surface of the shaft 44 a of the toothed belt wheel 44 as a result the toothed belt wheel 44 is fixed stopped so that it cannot rotate for as long as voltage is applied to the plunger coil 54 in fig7 there is shown a stopping device for slider 36 a and corresponding lower roller iii a pneumatic cylinder 60 having a piston rod 61 is attached to the slider 36 a when subjected to pressure from the pneumatic cylinder 60 the piston rod 61 is moved out in the direction of the arrow p and comes to rest with a high degree of contact pressure against the machine frame 61 the slider 36 a is fixed stopped so that it cannot be displaced with respect to the bar 37 a for as long as compressed air is applied to the pneumatic cylinder 60 lower roller ii may be provided with an analogous arrangement in accordance with fig8 there is provided as the bridge 50 between the sliders 35 a and 36 a a flat piece of metal plate which is fastened in the region of one of its ends 50 a to the slider 36 a for example using bolts in its region 50 b facing the slider 35 a the flat piece of metal has an elongate hole 50 c through which a bolt 62 can engage in a threaded.