122 and the recesses 123 racks 110 and 115 are provided along edges of the shells facing each other one shell forming the gearbox part 109 is provided with the inner bead 108 which engages the circumferential recess 107 at the end of the central tubular part 105 and carries the rack 110 the other shell is axially displaceable in the tubular element 120 and forms the rack 115 at its outer end projecting from the gearbox the shell carrying the rack 115 is provided with a flange 140 which is positioned in a cut out 141 in the end of the tubular element 120 carrying the button 119 so that this button and the tubular element 120 can be moved so far inward in the device that the engagement of the teeth 132 and 133 can be released before the button 119 abuts the flange 140 a tubular connection element 112 connects the threaded piston rod 104 with the gearbox at its end engaging the piston rod 104 the connection element has a nut 113 with an internal thread mating the external thread of the piston rod at its end engaging the gear box the connection element is provided with two pins 111 projecting perpendicular to the longitudinal axis of the.