Pressure-limiting hydraulic valve

A pressure-limiting valve for hydraulic fluids subjected to high pressure, which valve comprises

• (a) a valve housing having a stepped bore therein providing a first bore portion of smaller diameter leading directly into a second bore portion of larger diameter with an annular step between the two portions;
• (b) a stepped switching piston arranged for axial displacement in said bore between a valve-open position and a valve-closed position, said piston having a first portion of smaller diameter for movement in said smaller-diameter bore portion and a second portion of larger diameter, divided by an annular step from said first piston portion, for movement in said larger-diameter bore portion;
• (c) an inflow duct leading to said larger-diameter bore portion whereby an end of said larger-diameter portion of said piston is acted on by pressurised fluid supplied through said inflow duct;
• (d) a hard piston-closing spring arranged to act on a closed end of the smaller-diameter portion of said piston at all times to oppose the pressure of the pressurised fluid on said end of the larger-diameter portion of said piston, said spring thereby determining the setting pressure of the valve and being arranged to yield only when said piston is displaced;
• (e) an O-ring seal located in a circumferential annular groove in the inner wall of the smaller-diameter portion of said housing bore so as to seal off the smaller-diameter portion of said piston in the valve-closed position thereof;
• (f) a blind-end axial bore formed in said piston and having an inlet end and an outlet end, said outlet end having a larger cross-sectional area than said inlet end, first transverse bores extending from said outlet end in a star-like formation to the outer surface of the smaller-diameter portion of the piston below the O-ring seal whereby displacement of the piston, when the setting pressure is exceeded, into the valve-open position thereof causes said first transverse bores to moves beyond the O-ring seal and to communicate with a fluid discharge duct, the combined cross-sectional areas of said first transverse bores being at least equal to the cross-sectional area of said outlet end;
• and
• (g) second radially-extending transverse bores spaced axially from the first transverse bores and opening into an annular groove formed circumferentially in the larger-diameter portion of the piston adjacent the step thereof, the second radial bores being arranged to discharge fluid from said axial bore in said piston into a pressure chamber forming part of the housing bore and defined by the larger-diameter portion of the housing bore and the larger-diameter piston portion guided tightly therein, the cross-sectional area of the inflow duct being less than that of the larger-diameter portion of the housing bore and greater than that of the outlet end of said axial bore.