Patent ID: 8650778

Claim:
A hydraulic drive system for a hydraulic working machine, said hydraulic drive system being provided with a first hydraulic pump and second hydraulic pump for delivering pressure oil to drive a working element which is movable up and down, a double-acting hydraulic cylinder operable by pressure oil, which has been delivered from the first hydraulic pump and second hydraulic pump, to drive the working element, a first directional control valve for the working element, said first directional control valve being for controlling a flow of pressure oil to be fed from the first hydraulic pump to a bottom chamber or rod chamber of the hydraulic cylinder, a second directional control valve for the working element, said second directional control valve being for controlling a flow of pressure oil to be fed from the second hydraulic pump to the bottom chamber or rod chamber of the hydraulic cylinder, a control device for switchingly controlling the first directional control valve for the working element and the second directional control valve for the working element, and a jack-up selector valve to be switched when a pressure in the bottom chamber of the hydraulic cylinder has reached a predetermined pressure, wherein: the hydraulic drive system is further provided with a third directional control valve for the working element to control a flow of pressure oil to be fed to the bottom chamber or rod chamber of the hydraulic cylinder, said third directional control valve for the working element being switchable by a manipulation of the control device, and also with a third hydraulic pump for feeding pressure oil to the third directional control valve for the working element; and the jack-up selector valve has: a first select position where, when a pressure in the bottom chamber of the hydraulic cylinder is higher than the predetermined pressure, the second directional control valve for the working element and the third directional control valve for the working element are held in neutral positions, respectively, to hold permissible the switching of the first directional control valve for the working element by a manipulation of the control device, and a second select position where, when a pressure in the bottom chamber of the hydraulic cylinder is not higher than the predetermined pressure, the feeding of pressure oil, which is to be delivered from the second hydraulic pump and third hydraulic pump, to the rod chamber of the hydraulic cylinder is held permissible in association with the switching of the second directional control valve for the working element and the third directional control valve for the working element by a manipulation of the control device.