| 摘要 |
PCT No. PCT/JP89/00140 Sec. 371 Date Sep. 7, 1989 Sec. 102(e) Date Sep. 7, 1989 PCT Filed Feb. 13, 1989 PCT Pub. No. WO89/08190 PCT Pub. Date Sep. 8, 1989.Disclosed is a hydraulic control system mounted in a hydraulic circuit of a hydraulic excavator and adapted to increase the relief hydraulic pressure and the flow rate. In accordance with one aspect of the invention, a hydraulic control system for a hydraulic excavator having power constant control and cut-off control comprises: a variable relief valve (60) for increasing the pressure upon receipt of a pilot signal (Pc7), a solenoid valve (80) for connecting or disconnecting the pilot signal (Pc7), a variable cut-off control valve 10A which, upon receipt of a pilot signal (Pc6), cancels the cut-off control, a solenoid valve (70) for connecting or disconnecting the pilot signal (Pc6) and an electric circuit (X01) in which a switch (90) for opening or closing the solenoid valves (70, 80) is provided. In accordance with a second aspect of the invention, a timer is provided in the electric circuit (X02) used in the first aspect of the invention.
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