HYDRAULIC DRIVING APPARATUS

A hydraulic driving apparatus has at least one hydraulic pump, a plurality of hydraulic actuators driven by hydraulic fluid discharged from the hydraulic pump, a tank to which return fluid from the plurality of hydraulic actuators is discharged, and a flow control valve associated with each of the plurality of hydraulic actuators. The flow control valve has a first main variable restrictor for controlling flow rate of the hydraulic fluid supplied from the hydraulic pump to the hydraulic actuator, and a second main variable restrictor for controlling flow rate of the return fluid discharged from the hydraulic actuator to the tank. A pump control operative in response to the difference between the discharge pressure of the hydraulic pump and the maximum load pressure of the hydraulic actuators normally controls the discharge rate of the hydraulic pump so that the pump discharge pressure is raised more than the maximum load pressure by a predetermined value. A first pressure-compensating control operates with a valve determined by the difference between the pump discharge pressure and the maximum load pressure, the value acting as a compensating differential-pressure target value, and pressure-compensation-controls the first main variable restrictor of the flow control valve. A second pressure-compensating control is operative with a value determined by the pressure difference across the first main variable restrictor, the valve acting as a compensating differential-pressure target value, for controlling the second main variable restrictor of the flow control valve.