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A pressure-limiting valve comprises a cylindrical actuator piston guided displaceably in a bore and held under the pressure of a spring in the closure position of the valve. Longitudinally-directed throughflow passages are formed in the actuator piston on the pressure-loaded end and are arranged con

A pressure-limiting valve comprises a cylindrical actuator piston guided displaceably in a bore and held under the pressure of a spring in the closure position of the valve. Longitudinally-directed throughflow passages are formed in the actuator piston on the pressure-loaded end and are arranged concentrically with respect to the central axis and open into an annular groove extending around and in the circumferential surface of the actuator piston. This groove is sealed off by a gasket mounted in the bore wall. When the set pressure predetermined by the closure spring is exceeded, the actuator piston travels with the gasket over the annular groove so that the pressure medium can flow away through the outlet opening.

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A hydraulic pressure-limiting valve for an hydraulic longwall mine roof support system comprising: a valve housing with a longitudinal bore therein, a fluid entry opening and a fluid discharge opening in the valve housing, a cylindrical actuator piston arranged for guided axial displacement within s

A hydraulic pressure-limiting valve for an hydraulic longwall mine roof support system comprising: a valve housing with a longitudinal bore therein, a fluid entry opening and a fluid discharge opening in the valve housing, a cylindrical actuator piston arranged for guided axial displacement within said bore, a spring within said bore serving to urge said piston towards one end of the bore, a plurality of blind bores in said piston disposed in a circle concentrically of the longitudinal axis of the piston and leading from that end of the piston remote from the spring, the said piston end being subjected to the direct pressure of hydraulic fluid entering the valve housing through said fluid entry opening therein, an annular groove extending around in the circumferential surface of the actuator piston and breaking into the blind end regions of said blind bores, an annular fluid-sealing gasket located in an annular groove in the bore of the valve housing and disposed between said entry and discharge openings, said gasket surrounding the actuator piston and sealing off the annular groove therein from the fluid discharge opening in the valve housing when the piston is in a first position nearer the said one end of the housing bore under the pressure of the spring, said piston annular groove being narrower axially of the piston than the width of the gasket axially of the housing bore, said gasket being uniformly pressed into the annular groove in the bore of the valve housing by the pressure of fluid in said piston annular groove when said piston moves into a second position away from said one end of the housing bore.