shows that to the driving shaft 40 of motor drive 11 there is fixed a pinion 41 of a bevel gearing e g with spiral bevel gears whose plate 42 is fixed to a shaft 43 of the tensioning roller 10 the bevel gearing 41 42 could also be replaced by another mitre gear while the radial forces on pinion 41 are absorbed by a radial roller bearing 44 the axial forces are absorbed by a first axial roller bearing 45 which is supported via a plate cam 46 fig4 and 5 on a second axial roller bearing 47 the latter being supported on a fixed housing part 15 due to the axial force on pinion 41 a friction force proportional to the axial force on it is produced on the first roller bearing 45 and this rotates the plate cam 46 counter to the tension of a compression spring 48 the spring 48 is supported at one end against the housing part 49 and at the other against a spring plate 50 of a bolt 52 mounted in movable manner in a housing bore 51 bolt 52 has a tooth 53 which engages in a slot 54 of plate cam 46 so that on rotation of the cam 46 bolt 52 is moved and consequently the tension of spring 50 is changed opposite the bolt 52 there is provided a control member constructed as a lifting rod 55.