(57) To provide a way to ensure the supply of a necessary amount of hydraulic fluid to
the established actuator circuits even if the pressure in the independent attachment
circuit becomes excessively high. Hydraulic fluid fed from hydraulic pumps 2,3 is
controlled by means of established actuator circuits 5,6 to operate established actuators
of a construction machine. A flow dividing control valve 10 causes a part of the hydraulic
fluid fed to the established actuator circuits 5,6 to branch off and be fed to the
independent attachment circuit 7 with its amount controlled by variable throttles
39,40, thereby driving the attachment of the construction machine. Pressure compensation
valves 11,12 respectively control the variable throttles 39,40 of the flow dividing
control valve 10 to maintain a constant differential pressure between the inlet and
the outlet of each variable throttle. When a controller 53 detects by a pressure sensor
51 that the circuit pressure in the independent attachment circuit 7 has increased
to an excessively high level, the controller 53 controls an electromagnetic proportional
control valve 57 regardless of operation signals from an attachment operation lever
54 so as to reduce the aperture areas of the variable throttles 39,40 of the flow
dividing control valve 10.
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