A winding motor drives a shaft (7), which may be the shaft of the winding motor itself, to which a lifting part (8) with a helicoidal sliding stage is fastened. The actuating side of a control lever (5) which rotates about a pivot slides on this sliding stage due to the force of a spring (6) and is rotated by the sliding stage through a certain angle in order to adjust the braking parts. The lifting part (8) co-operates through a disk-shaped sliding clutch with a rotatable control part (9) mounted coaxially on the shaft. In both end positions - brake on, brake off - the actuating side (B) of the control lever (5) lies on the cylindrical surface of the control part (9), which has two different height stages.