Abstract: A pair of supply/drain pipes continuous with an actuator are respectively communicable with a pair of oil sumps respectively through an intermediation of a pair of extended paths. The pair of oil sumps is formed in a switch-spool insertion hole and enter a neutral state or a state of being selectively communicated with any one of respective pressure-oil paths and respective oil-return paths. A non-leak valve is provided over respective connection portions and between the pair of supply/drain pipes and the pair of extended paths. The non-leak valve is opened when an operation of a pump is started, is maintained to be opened during the operation of the pump, and is closed when the pump is out of operation.

Abstract: A pair of supply/drain pipes (Y and Z) continuous with an actuator (2) are respectively communicable with a pair of oil sumps (A, B, C) and (F, G, H) each other respectively through an intermediation of a pair of extended paths (a and b), the pair of oil sumps (A, B, C) and (F, G, H) being formed in a switch-spool insertion hole (7) and entering a neutral state or a state of being selectively communicated with any one of respective pressure-oil paths (S and V) and respective oil-return paths (Q and W). A non-leak valve (12) is provided over respective connection portions (I and J) and (K and L) between the pair of supply/drain pipes (Y and Z) and the pair of extended paths (a and b), the non-leak valve (12) being opened when an operation of a pump (P) is started, being maintained to be opened during the operation of the pump (P), and being closed when the pump (P) is out of operation.

Abstract: The hydraulic circuit of the invention includes first and second directional control valves 13, 15, each shiftable between three positions. Hydraulic circuit extending from the second directional control valve 15 and a second cylinder 7 to a first cylinder 11 is so configured that, when the second directional control valve 15 is shifted to the first position, with the first directional control valve 13 being assigned to the N-position, hydraulic oil from the second cylinder 7 is directed to the first cylinder 11, so as to cause the attachment 9 to be tilted in the forward direction, and when the second directional control valve 15 is shifted to the second position, hydraulic oil expelled from the second cylinder 7 is directed to the first cylinder 11, so as to cause the attachment 9 to be tilted in the rearward direction.

Abstract: A speedup device or valve means comprises a check-valve disposed in an oil passage connected with the rod-end chamber of a double-acting cylinder for preventing working oil from returning toward a tank only, a circulation valve provided in the upstream of said check valve, said circulation valve being usually urged by a spring to block communication between the oil passages connected with the rod-end and the piston-end chambers, respectively, said circulation valve being designed to move against the spring so as to connect the oil passages with each other in response to the pressure rise in the return oil from the rod-end chamber when said double-acting cylinder is in its rod-extending motion, causing the return oil from the rod-end chamber to shift into the piston-end chamber through a variable restriction, and a circuit set up in a portion of said circulation valve for allowing a portion of the return oil from the rod-end chamber to return to the tank through a variable restriction and a restriction passage

Abstract: A hydraulic control circuit system comprises first and second double-acting cylinders, a hydraulic pump for supplying the double-acting cylinders with pressure oil, and directional control valves respectively associated with the cylinders for controlling the supply of pressure oil to the latter. The hydraulic control circuit system has a circuit for independently operating the individual cylinders and also a circuit for causing the rod advancing and retracting actions of the first cylinder to be followed by the rod advancing and retracting actions of the second cylinder. The system is applied to the hydraulic control circuit of, e.g., a front loader for controlling a hydraulic cylinder used for lifting and lowering the loader arm and another hydraulic cylinder used for turning the bucket pivotally connected to the free end of the arm.