Compression spring with adjustable length and seat with such a spring

A piston (12) attached to a piston rod (8) can be axially moved inside a housing (1) dividing the inner space into two pressure chambers (15, 16) connected when a first valve (20) is opened. The spring is locked when the valve (20) is closed. An additional throttle valve (36) facilitates an overflow connection to fully open only when the throttle element (36) is moved into a release position. An uncontrolled moving of the piston rod (12) is prevented.