Patent ID: 7272928

Claim:
A hydraulic circuit for a construction machine, said hydraulic circuit having an engine, a first hydraulic pump of a variable displacement type, second hydraulic pump of the variable displacement type and third hydraulic pump, all of which are drivable by said engine, capacity control means for controlling displacements of said first hydraulic pump and second hydraulic pump, plural actuators drivable by pressure oil fed from said first, second and third hydraulic pumps, and plural directional control valves for controlling flows of pressure oil to be fed to said actuators, wherein one of said actuators, which is driven by pressure oil fed from said third hydraulic pump, is a specific actuator to which a larger load pressure is applied during driving thereof than those applied to remaining ones of said actuators, which are driven by said first and second hydraulic pumps, respectively, said third hydraulic pump is a hydraulic pump of the variable displacement type, said hydraulic circuit is provided with capacity control means for said third hydraulic pump to control a displacement of said third hydraulic pump and also with first, second and third state quantity detection means for detecting quantities of states associated with respective torque consumptions by said first, second and third hydraulic pumps, said capacity control means for said first and second hydraulic pumps controls displacements of said first and second hydraulic pumps on a basis of said quantities of states detected by said first, second and third state quantity detection means, said capacity control means for said third hydraulic pump controls a displacement of said third hydraulic pump on a basis of said quantity of state detected by said third state quantity detection means, and wherein said first and second hydraulic pumps are controlled under torque characteristics obtained by subtracting an input torque, which has been preset for said third hydraulic pump, from a maximum input torque to said first and second hydraulic pumps.