HYDRAULIC DRIVE DEVICE FOR HYDRAULIC WORKING MACHINE

摘要

Provided is a hydraulic drive system for a hydraulic working machine. The hydraulic drive system can realize a jack-up operation without arrangement of a flow rate control valve, which would otherwise be needed enable feeding of pressure oil to directional control valves for a working element, and a center bypass selector valve. The hydraulic drive system is provided with a first hydraulic pump (21) and second hydraulic pump (22), a first directional control valve (28) and second directional control valve (29) for controlling boom cylinders (9), and is further provided with a third directional control valve (30) for controlling the boom cylinders (9) and a third hydraulic pump (23) for feeding pressure oil to the third directional control valve (30). A jack-up selector valve (31) has a second select position (31b) where, when a pressure in bottom chambers (9a) of the boom cylinders (9) is not higher than a predetermined pressure, feeding of pressure oil, which is delivered from the first hydraulic pump (21) and second hydraulic pump (22), to rod chambers (9b) of the boom cylinders (9) is held permissible in association with switching of the second directional control valve (29) and third directional control valve (30) by a manipulation of a control device (32).