| 摘要 |
PROBLEM TO BE SOLVED: To provide constant driving speed according to an operation quantity of an operation lever device in a low load pressure side actuator when plural actuators are simultaneously driven. SOLUTION: Bypass passages 16 and 17 to connect the upstream side and the downstream side of pressure control valves 11 and 12 to valve devices 50 and 51 and auxiliary variable throttle valves 18 and 19 which are arranged in the bypass passages 16 and 17 and have a characteristic by which the opening area A becomes large when an operation quantity X of an operation lever device becomes large and the opening area A becomes large at a rate smaller than main variable throttle valves 9 and 10, are provided. As the opening area A of the auxiliary variable throttle valves 18 and 19 becomes large, and as longitudinal differential pressureΔPV between valve devices 51 and 52 becomes large, a bypass flow rate QB passing through the bypass passages 16 and 17 increases, and a flow rate of a reduced quantity of a passing flow rate Q by fluid force acting on the pressure control valves 11 and 12 is passed, and is merged into a downstream part of the pressure control valves 11 and 12, and is supplied to actuators 5 and 6.
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