Fluid pressure control system

Claim 1

A fluid pressure control system comprising a variable displacement pump (36) of which amount of discharge is variable, a first selector valve (22) adapted to be selectively held at any one of a neutral position, a first operative position and a second operative position, a second selector valve (24) adapted to be selectively held at any one of a neutral position, a first operative position and a second operative position, a first actuator (12) of which action is to be controlled by the shifting operation of the first selector valve (22), a second actuator (8) of which action is to be controlled by the shifting operation of the second selector valve (24), a first flow control valve (56) for controlling a fluid to be supplied to the first actuator (12), a second flow control valve (58) for controlling a fluid to be supplied to the second actuator (8), a feed flow passage (38) connecting the variable displacement pump (36) to the first and second selector valves (22, 24), a return flow passage (40) connected to the first and second selector valves (22, 24) a first and a second flow passage (26, 28) connect­ing the first selector valve (22) to the first actuator (12), a third and a fourth flow passage (30, 32) connect­ing the second selector valve (24) and the second actuator (8), and a main load-detecting flow passage (61) for controlling the amount of discharge of the variable displacement pump (36), the first selector valve (22) at the first operative position permitting communication of the feed passage (38) with the first flow passage (26) and also the return flow passage (40) with the second flow passage (28), the first selector valve (22) at the second operative position permitting communication of the feed passage (38) with the second flow passage (28) and also the return flow passage (40) with the first flow passage (30), the first selector valve (22) at the neutral position shutting off communication of the feed flow passage (38) and the return flow passage (40) with the first flow passage (26) and the second flow passage (28), the second selector valve (24) at the first operative position permitting communication of the feed passage (38), with the third flow passage (30) and also the return flow passage (40) with the fourth flow passage (32), the second selector valve (24) at the second operative position permitting communication of the feed passage (38) with the fourth flow passage (32) and also the return flow passage (40) with the third flow passage (30), the second selector valve (24) at the neutral position shutting off communication of the feed flow passage (38) and the return flow passage (40) with the third flow passage (30) and the fourth flow passage (32), the first flow control valve (56) being adapted to control a fluid to be supplied to the first (26) or second flow passage (28) from the feed flow passage (38) when the first selector valve (22) is at the first or second operative position, and the second flow control valve (58) being adapted to control a fluid to be supplied to the third (30) or fourth flow passage (32) from the feed flow passage (38) when the second selector valve (24) is at the first or second operative position; wherein a pressure reducing valve (124) and a relief valve (126) are disposed in relation to the second flow rate controlling valve (58), the pressure reducing valve (124) reduces the pressure of the fluid supplied to the second actuator (8) through the second selector valve (24), and the pressure on the secondary side of the pressure re­ducing valve (124) is controlled by the action of the re­lief valve (126) which is controlled by an external pilot pressure.