Patent attributes
To permit using hold-side pressure oil in a first hydraulic cylinder for the acceleration of a second hydraulic cylinder upon performing a combined operation of the first and second hydraulic cylinders, a hydraulic drive system is provided with a main hydraulic pump 21, a boom cylinder 6 and an arm cylinder 7, a directional control valve 23 for a boom and a directional control valve 24 for an arm, and a boom control device 25 and an arm control device 26. The hydraulic drive system is also provided with a pressure oil feed means for feeding pressure oil in a rod chamber 6b of the boom cylinder 6 to an upstream side of the directional control valve 24 for the arm when a bottom pressure of the arm cylinder 7 has increased to a high pressure equal to or higher than a predetermined pressure. This pressure oil feed means includes a flow combiner valve 44 arranged in a reservoir line 42 which can communicate to the rod chamber 6b of the boom cylinder 6. When the bottom pressure of the arm cylinder 7 is the predetermined pressure or higher, the flow combiner valve 44 maintains a communication line 40, which communicates the reservoir line 42 and the upstream side of the directional control valve 24 for the arm with each other, in a state that pressure oil can be fed to the upstream side of the directional control valve 24 for the arm.
