Relief and unloading valve

Claim 1

A valve for controlling relief of fluid under pressure in a hydraulic system, said valve comprising a casing having a bore therein defining a chamber, said chamber having a deformable bladder positioned therein, means to charge said bladder with gas under pressure, said casing bore having a reduced diameter portion extending axially from the inner end of said chamber and defining an opening in the inner end of said chamber, the periphery of said opening defining a valve seat, a valve member secured to said bladder and movable against said seat, said casing having a first transverse relief bore intersecting said reduced diameter bore portion, the outer end of said relief bore defining a pressure relief port, said casing also having a second transverse bore, the ends of which define a pressure outlet port and a pressure inlet port respectively, said reduced diameter bore portion extending axially between said transverse bores to provide communication therebetween and having a portion exposed in said first transverse bore and defining an additional valve seat, a stem operatively connected at one end to said bladder and extending axially from said first valve member through the reduced diameter portion of said casing bore, the free end of said stem defining an additional valve member normally retained on said additional valve seat when said bladder is expanded to cut off communication between said second transverse bore and said first transverse bore, said additional valve member being subjected to the pressure in said second transverse bore, whereby when the pressure in the hydraulic system connected to said second transverse bore exceeds a predetermined amount said additional valve member will be moved off its seat to provide communication between said pressure port and said relief port.