connection to one or several contact points 90 which are arranged on an inner intermediate and outer circular conducting path 91 92 93 accordingly in switch position 1 current is supplied to the inner conducting path 91 which is connected via a cable 94 to a solenoid 95 of the valve 24 illustrated schematically in fig1 the free end of the solenoid 95 in turn is connected to ground as evident from the schematic illustration of the valves 24 25 26 in fig1 the valves 24 25 26 are provided with a pressure release 96 formed as a bore at the free end of each valve body the switch combination resulting from the switch position 1 of the selector switch 83 makes possible that the valve 2 is actuated while the valves 25 26 are inoperative finally in this switch position the inner cancel 64 is supplied with compressed air due to the construction and the cooperation of the individual elements thus also the group g1 associated with the inner canal 64 is supplied with compressed air and actuated by means of two pins 69 finally the actuated pins 69 come into contact with the tensioning disc 76 while the groups g2 and g3 are not actuated via solenoids 96 of.