34 position and from the 34 on 34 position to the 34 off 34 position the hydraulic reservoir 3 is connected via a first pipe a to a portion 2a of drive cylinder 2 the portion 2a is in turn connected to the switch over control valve 5 via a second pipe b a third pipe c connects the switch over control valve 5 to another portion 2b of the drive cylinder 2 a fourth pipe d connects the switch over control valve 5 to the hydraulic medium container 4 the control signal organ 1 for switching on the switch 8 includes a central space 1a which is continuously connected to portion 2a of the drive cylinder 2 via the second pipe b and a fifth pipe f in which a first restrictive orifice 6 is arranged the central space 1a is also selectively connected to the hydraulic medium container 4 via a switchable control signal part for switching on the switch 8 the switchable control signal part is in the form of a ball valve 1b which in the open position of the ball valve 1b connects the space 1a to the hydraulic medium container 4 via a sixth pipe g and the fourth pipe d or in the closed position of.