Importance

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Abstract

An object to achieve favorable operability, in a combined operation that involves a specific actuator and involves a great difference in load pressure and when an operation of an operating unit for the specific actuator is a fine operation, by reducing energy consumption arising from wasted restricting pressure loss of a pressure compensating valve and by flexibly varying a flow rate of a hydraulic fluid supplied to the specific actuator depending on the load pressure. To achieve this object, a boom cylinder (3a) is provided with an open center type flow control valve (6a) that controls a hydraulic fluid from a main pump (202) and a closed center type flow control valve (6i) that controls a hydraulic fluid from a main pump (102). The main pump (102) is subject to load sensing control. The flow control valve (6a) is opened to control a supply flow rate up to an intermediate zone of an operating range of an operating unit for the boom cylinder (3a). Both the flow control valves (6a) and (6i) are opened to control the supply flow rate after the intermediate zone.

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First Claim