Abstract: A plurality of directional control valves (5, 6) respectively disposed between a hydraulic pump (50) and a plurality of actuators (3, 4) each comprises a pump port (9), a pressure chamber (10), a feeder passage (11), actuator ports (12a, 12b), a reservoir port (13), first meter-in variable restrictors (15a, 15b) disposed between the pump port and the pressure chamber, and a pressure compensating valve (16) disposed between the pressure chamber and the feeder passage and having a pair of opposite ends, one of which is subjected to a pressure in the pressure chamber and the other of which is subjected to a maximum load pressure among the plurality of actuators. The hydraulic pump (1) includes a pump flow control device (2) for controlling a delivery rate of the hydraulic pump so that a delivery pressure of the hydraulic pump is held higher by a predetermined value than the maximum pressure obtained, as a load sensing pressure, from load pressures of the plurality of actuators.

Abstract: A hydraulic circuit for a hydraulic power shovel of the type having a swingable boom, swingable arm and a bucket attached to the end of the arm. The hydraulic circuit includes a levelling cylinder adapted to extend and contract in response to the swinging of the arm, and a switching valve device movable between a first position where it provides a communication between the levelling cylinder and a boom cylinder and a second position where it provides a communication between the levelling cylinder and an arm cylinder. When the switching valve device takes the first position, the levelling cylinder is communicated with the boom cylinder so that the boom is swung downwardly in response to the upward swinging of the arm, thereby to move the bucket horizontally. On the other hand, when the switching valve device takes the second position, the levelling cylinder and the boom cylinder are disconnected hydraulically so that the arm is allowed to swing independently of the boom.