Abstract: A movable carriage of a support unit is guided via bearing surfaces along a base. A double acting piston and cylinder unit is formed within a bore located in the base. A piston divides the cylinder into front and rear chambers and a piston rod thereof extends through the rear end of the cylinder and is connected to the carriage for movement therewith parallel thereto. A first pressure fluid connection extends through the front end of the cylinder into the front chamber. The bore has a sleeve, the outer surface of which partly defines a channel extending along the exterior of the cylinder. A second fluid pressure connection is also arranged at the front end of the cylinder, but it communicates with the channel to communicate with the rear cylinder chamber.