Patent ID: 8336443

Claim:
A hydraulic working machine provided with: a double-acting hydraulic cylinder arranged for extension or contraction by pressure oil, which is delivered from a main pump, to drive a working element, a directional control valve for controlling a flow of pressure oil to be fed from said main pump to said hydraulic cylinder, and a control unit for performing a change-over control of said directional control valve, characterized in that said hydraulic working machine is provided with: a jack-up selector valve for being changed over in flow-line when a feed pressure to said hydraulic cylinder has reached a predetermined pressure, and a pilot-controlled flow-line changing means for changing a flow-line for pressure oil, which is to be fed from said main pump to a meter-in port of said directional control valve, from an open side to a closed side in response to a change-over control of said jack-up selector valve which generates a pilot hydraulic control signal from said jack-up selector valve to said pilot-controlled flow-line changing means, wherein, when a holding pressure on said hydraulic cylinder is equal to or higher than the predetermined pressure upon lowering said working element, said jack-up selector valve is changed over to a first selected position to change over said flow-line changing means to the closed side such that pressure oil delivered from said main pump is not fed to a non-holding pressure feeding side of said hydraulic cylinder, and when the holding pressure on said hydraulic cylinder is lower than the predetermined pressure upon lowering said working element, said jack-up selector valve is changed over to a second selected position to change over said flow-line changing means to the open side such that pressure oil delivered from said main pump is fed to a non-holding pressure side of said hydraulic cylinder via said directional control valve.