Patent ID: 7284371

Claim:
A hydraulic control system for a working machine, comprising: hydraulic actuators; a variable capacity hydraulic pump as an oil pressure source for said hydraulic actuators; a pump regulator for controlling a discharge amount of said hydraulic pump; an electromagnetic proportional regulator control valve for controlling an operation of said pump regulator; a controller adapted to send a control signal for changing the discharge amount of said hydraulic pump to said regulator control valve in accordance with an operation amount of an operating means, wherein a secondary pressure in said regulator control valve being fed as a controlling oil pressure to said pump regulator in accordance with said control signal; and a fail-safe valve disposed in a pump control line joining said pump regulator and said regulator control valve, said fail-safe valve having an ordinary position for sending the secondary pressure in said regulator control valve as a controlling oil pressure to said pump regulator and a fail-safe position for sending an oil pressure from a path other than said regulator control valve as a pump controlling oil pressure to said pump regulator in such a manner that a predetermined pump discharge amount is ensured, said fail-safe valve being configured so as to switch from said ordinary position to said fail-safe position in accordance with a failure signal indicative of failure in operation of said regulator control valve; wherein said fail-safe valve is configured as a hydraulic pilot switching valve with a hydraulic pilot port adapted to switch between said ordinary position and said fail-safe position on the basis of a pilot pressure introduced into said hydraulic pilot port and a spring force resisting said pilot pressure, the secondary pressure in said regulator control valve is introduced into the hydraulic pilot port of said fail-safe valve, a signal is issued as the failure signal when said secondary pressure drops below a preset value, and, in accordance with said signal, the fail-safe valve switches from said ordinary position to said fail-safe position by said spring force.