The valve piston (2) guided within the valve body and cover (1) is adjusted by turning the handwheel (4) axially via the thread (3). By turning it clockwise, the valve is closed pressing the valve piston (2) into the valve seat (5). The thread (3) of the valve piston has a self-locking action; the hydraulic axial forces at the valve piston result in frictional forces within the thread (3) and on the piston seal (6) fixing the valve piston within the position. In addition, the valve piston can be fixed via the locking screw (7). The proximity switch (8) is arranged in the bottom position and monitors the closed valve position. In order to keep the actuation forces at the valve to a minimum, for this valve the preferred direction of flow is to be selected from the B-channel to the A-channel; depending on the valve size this specification may vary, see Valve data. To enable adjustment under pressure for larger nominal value and thus larger hydraulic surfaces, these valves are additionally equipped with a bearing.