in this case two hydraulic cylinders which are connected in parallel and are each connected to an external hydraulic supply 6 via a flexible pressure tight hydraulic line 6 a connected to a hydraulic connection 7 a piston 5 is arranged in each of the cylinders 1 so as to be movable in the longitudinal direction l said piston being sealed off in the direction of the cylinder 1 in the exemplary embodiment these are thus a lower piston 5 and an upper piston 5 which are moreover only movable jointly when the hydraulic supply 6 is activated the feeding of hydraulic pressure into the working spaces of the cylinders causes the pistons 5 arranged therein to be raised this may take place counter to the action of a compression spring 16 supported on the uppermost piston 5 the pistons 5 are connected rigidly to an exchange socket 10 arranged centrally therein and so the movement of the pistons 5 results in an identical movement of the exchange socket 10 the latter is designed to be exchangeable by means of suitable measures and can thus be exchanged for an exchange socket 10 of different geometry whereas the piston 5 or the pistons 5 are not exchanged for other pistons the exchange socket 10 is composed integrally of a lower coupling section and an upper shank section the coupling section is located inside the supporting tube 2 and has an internal thread which is screwable together with the external thread of the threaded bolt 3 the shank section of the exchange socket.