of a spring 55 for this purpose the spring 55 is disposed between the bracket 51 and the ratchet 52 the ratchet 52 is swivellable in the direction of movement 54 furthermore the braking device has a cylinder piston unit 57 the cylinder piston unit is capable of being acted upon via a pressure connection 60 by a pressure medium preferably compressed air the piston 58 has a piston shaft 58 1 which is guided in the cylinder fastened on the cylinder shaft 58 1 is the piston rod 58 2 the piston rod 58 2 which emerges from the cylinder has a piston cap 58 3 at the opposite end formed between the piston shaft 58 1 and the piston cap 58 3 is an undercut 64 the piston cap 58 3 is executed conically with the cone 62 the cone 62 is formed here in such a manner that when the piston is extended i e in the event of pressure loading of the piston it slides with a facet 63 along the ratchet 52 disposed between the piston shaft 58 1 and the cylinder in the intermediate space to the piston rod 58 2 is a spring 61 so that the piston has to extend against the direction of force of the spring 61 the cylinder piston unit 57 is connected to a control device 4 the control device 4 is also connected to the pressure source 1 and the control.