Abstract: The invention relates to a relief valve and intends to provide a relief valve which can reduce the difference between the dynamic set pressure and static set pressure of the relief pressure and its variation.An internal circumferential surface of the pilot valve element 214 to which a support 227 is inserted is expanded to form a sub valve chest 231, a support and sub valve element 232 externally fitted to the sealing member 229 is arranged in this sub valve chest 231, the clearance between the inner circumferential surface of the sub valve element 232 and the support 227 is sealed with the sealing member 229, and at the same time the sub valve element 232 is pressed against the sub valve seat 238 formed in the inner circumferential surface of the pilot valve element 214 by oil pressure exerted on the sub valve element 232 from the pressure chamber 208 side to seal the clearance between the sub valve element 232 and the support 227.

Abstract: The invention intends to provide a relief valve which can prevent malfunction caused by generation of negative pressure lowering the minimum pressure to be originally maintained by a load circuit.Maloperation of a relief valve 401 is prevented by allowing a check valve 423 for preventing a backflow from a main valve element 410 and pilot valve element 412 side to a load port side 402 to be intervened at a pressure chamber 408 between a load port 406 connected to a load circuit, a main valve element 410, and a pilot valve element 412 and by applying a hydraulic lock when negative pressure lower than the minimum pressure to be originally maintained by a load circuit is generated. And by communicating a downstream chamber 425 downstream of the check valve 423 to a return port 407 with a reset oil passage 429 which is opened and closed by a reset valve 430, the hydraulic lock is designed to be released.

Abstract: An overload preventive device is capable of preventing elevation of preload pressure due to sticking action or break-through load of a hydraulic cylinder of a press machine. A delay valve for opening when the internal pressure of the load detecting circuit is lower than the minimum pressure to be maintained continuously over a specified time is interposed between a hydraulic pump for supplying pressurized oil of a specified pressure to a load detecting circuit.

Abstract: The invention relates to a relief valve and intends to provide a relief valve designed to improve response and operational stability. In an overload prevention safety device in which a main valve element including a pilot valve element in a valve case is arranged, a pilot valve is opened by oil pressure exerted on the pilot valve element from an oil passage formed in the main valve element while resisting to an energizing means for energizing the pilot valve element. By this operation oil pressure is exerted on the pilot valve element side of the main valve element to open the main valve. An oil passage is designed to be constructed in holes with different diameters with a small-diameter section, a taper section, and a large-diameter section continuously arranged in that order. A pilot valve seat is sealably in contact with the pilot valve element mounted at the connections between the small-diameter section and the taper section.

Abstract: In the preferred and illustrated embodiment of a reciprocating pump, a plunger for pumping a liquid, a piston for reciprocating the plunger and a selector valve for changing a pilot flow passage between a supply passage communicating a pressure chamber with a supply port and a discharge passage communicating the pressure chamber with a discharge port are disclosed. The selector valve is switched by a pilot valve means. A pilot valve body of the pilot valve means is so designed to engage with the piston for predetermined distances to and from the ends of the respective forward and backward strokes of the piston and to be moved to the switching position where the pilot valve body opens one of the pilot supply passage working against the selector valve and the pilot discharge passage and close the other of the pilot passages and the pilot valve body is released from engagement with the piston and held at the switching position for a predetermined distance after the stroke of the piston is reversed.

Abstract: In the preferred and illustrated embodiment of a reciprocating pump, a plunger for pumping a liquied, a piston for reciprocating the plunger and a selector valve for changing a pilot flow passage between a supply passage communicating a pressure chamber with a supply port and a discharge passage communicating the pressure chamber with a discharge port are disclosed. The selector valve is switched by a pilot valve means. A pilot valve body of the pilot valve means is so designed to engage with the piston for predetermined distances to and from the ends of the respective forward and backward strokes of the piston and to be moved to the switching position where the pilot valve body opens one of the pilot supply passage working against the selector valve and the pilot discharge passage and close the other of the pilot passages and the pilot valve body is released from engagement with the piston and held at the switching position for a predetermined distance after the stroke of the piston is reversed.