Patent ID: 6772590
Filing Date: 2004-08-10
Classification: E02F,F04B,F15B

Abstract:
A hydraulic drive system comprising an engine (1), a first fixed displacement hydraulic pump (2) driven by said engine, a plurality of actuators (3a, 3b, 3c) driven by a hydraulic fluid delivered from said first hydraulic pump, a plurality of flow control valves (6a, 6b, 6c; 6Da, 6Db, 6Dc) for controlling flow rates of the hydraulic fluid supplied to the plurality of actuators from said first hydraulic pump, a plurality of pressure compensating valves (7a, 7b, 7c; 7Da, 7Db, 7Dc) for controlling respective differential pressures across said plurality of flow control valves, said plurality of pressure compensating valves having respective target differential pressures set in accordance with a differential pressure between a delivery pressure of said first hydraulic pump and a maximum load pressure among said plurality of actuators,wherein said hydraulic drive system further comprises an unloading valve (5) for controlling the delivery pressure of said first hydraulic pump (2) so that the differential pressure between the delivery pressure of said first hydraulic pump and the maximum load pressure among said plurality of actuators (3a, 3b, 3c) is maintained at a setting pressure, and variably setting means (20; 20A; 20E; 20F) for setting the setting pressure of said unloading valve as a variable value that varies depending on a revolution speed of said engine (1), said unloading valve (5) including a valve member (5a), a first pressure bearing sector (5b) acting upon said valve member to move in an opening direction, a second pressure bearing sector (5c) acting upon said valve member to move in a closing direction and a third pressure bearing sector (5d) acting upon said valve member to move in a closing direction, the delivery pressure of said first hydraulic pump (2) being introduced into said first pressure bearing sector and said maximum load pressure being introduced into said second pressure bearing sector; said variably setting means including throttle means (31, 50, 31F, 50E) which varies the differential pressure thereacross depending on said revolution speed of the engine (1), a differential pressure detecting valve (40) for changing said differential pressure across the throttle means to a signal pressure, and means (41) for introducing said signal pressure into said third pressure bearing sector (5d) of the unloading valve.