Patent ID: 6397591
Filing Date: 2002-06-04
Classification: E02F,F15B

Abstract:
A hydraulic drive system comprising a hydraulic pump, a plurality of actuators, including a swing motor, which are driven by a hydraulic fluid delivered from said hydraulic pump, a plurality of directional control valves for controlling respective flow rates of the hydraulic fluid supplied from said hydraulic pump to said plurality of actuators, a plurality of pressure compensating valves for controlling respective differential pressures across said plurality of directional control valves, and pump control means for load sensing control to control a pump delivery rate such that a delivery pressure of said hydraulic pump is held a predetermined value higher than a maximum load pressure among said plurality of actuators, wherein said hydraulic drive system further comprises:first means provided respectively in those of said plurality of pressure compensating valves, which are not for a swing section associated with said swing motor, and setting, as a target compensation differential pressure, a differential pressure between the delivery pressure of said hydraulic pump and the maximum load pressure among said plurality of actuators, second means provided in the pressure compensating valve for the swing section and setting a target compensation differential pressure of the pressure compensating valve, third means provided in at least one of said plurality of pressure compensating valves, which is for the swing section, and reducing the target compensation differential pressure set by said second means when a load pressure of said swing motor rises, thereby giving a load dependent characteristic to the pressure compensating valve for the swing section, and fourth means provided in the pressure compensating valve for the swing section and setting a lower limit of the target compensation differential pressure that is set by said second means and modified by said third means.