Hydraulic drive system for hydraulic working machines

摘要

PCT No. PCT/JP93/01188 Sec. 371 Date Mar. 28, 1994 Sec. 102(e) Date Mar. 28, 1994 PCT Filed Aug. 25, 1993 PCT Pub. No. WO94/04828 PCT Pub. Date Mar. 3, 1994.A hydraulic drive system for hydraulic working machines has a variable displacement hydraulic pump, at least one actuator driven by the hydraulic fluid, a directional control valve of center bypass type for controlling the flow of the hydraulic fluid, a center bypass line for connecting the center bypass passage of the control valve to a reservoir and a fixed restrictor disposed in the center bypass line. A first control signal that determines a first target displacement of the hydraulic pump is determined by using the pressure generated by the fixed restrictor. A second control signal is generated for determining a second target displacement of a hydraulic pump. A third control signal, which is the larger one of the first and second control signals, provides a target displacement that is applied to a pump regulator that controls the displacement of the hydraulic pump. A metering characteristic as good as the conventional one can be obtained in the case of a light load, and a satisfactory metering characteristic can be achieved in the case of a heavy load, as a result.