Pressure-control valve

A pressure-control valve, especially for use in an hydraulic, walking, mine roof support, comprising a valve housing an hydraulically-loadable control piston in said housing, an operating spring acting on said control piston against the force of the hydraulic load, a valve-closure member in said valve housing arranged to be displaceable through a predetermined displacement distance, and a valve-closure spring acting on said valve closure member to determine the opening pressure of the valve, said valve-closure member being adapted to seal off a pressure chamber of a connected hydraulic working cylinder from a low-pressure conduit, wherein the valve-closure member is securable in the closure position by the hydraulically-loaded control piston with the operating force of the operating spring being adjustable in such a manner that the control piston, in the case of a drop in the pressure of the pressurised fluid loading it below a predetermined minimum pressure, is returnable into a position in which the valve-closure member is displaceable out of the closure position into an opening position by the pressure present in the pressure chamber of the working cylinder during operation of the latter.