Pressure-relief valve devices

A valve device for pressure relief of hydraulic pressure fluid and especially for pressure relief of hydraulic props of mine equipment operated with an oil-water emulsion as pressure fluid at a pressure in the range of 200-600 bars, said device comprising:

• a main housing defining an internal chamber closed at one end;
• a guide located in the main housing, said guide having an end face at the opposite end of said chamber and a first axial bore;
• a valve member having a head with an annular stop face opposed to said end face and a stem received in said first axial bore, a second axial bore in said stem which communicates via radial borings in said stem with pressure fluid outlets in the external peripheral surface of said stem, said peripheral surface having a smoothness with a maximum peak-to-valley depth of about 1 .mu.m and being slidable within said bore,
• a groove in the wall of said first bore, said groove having a maximum diameter which is less than the outer diameter of said stop face,
• a sealing ring located in said groove to engage with its internal diameter in sealing relationship with said peripheral stem surface, the sealing ring having its internal diameter slightly smaller than the diameter of said peripheral stem surface, the axial distance between said groove and said end face being not greater than the cord diameter of said sealing ring,
• a spring located in said chamber to apply force to said valve member to urge said stop face against said end face and to displace said stem to a position at which communication between said pressure fluid outlets and said chamber is blocked by said sealing ring,
• a main inlet for pressure fluid communicating with said first and second axial bores, a main outlet for pressure fluid leading from said chamber, whereby said valve member can be displaced by excess fluid pressure against the force of said spring to axially displace said stem to a position at which said fluid outlets are between said sealing ring and said chamber, thereby permitting pressure fluid to discharge via said second axial bore, said radial borings and said pressure fluid outlets into said chamber, and from said chamber through said main outlet.