Hydraulic drive system for construction machines

A hydraulic drive system comprising a hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from said hydraulic pump, a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied to said actuators, respectively, and a plurality of distribution compensating valves for controlling differential pressures across said flow control valves, respectively, said plurality of actuators including a first actuator which operates under a relatively large load and a second actuator which operates under a smaller load than that of the first actuator, wherein:

• said hydraulic drive system further comprises distribution control means for controlling the distribution compensating valve associated with the second actuator such that a differential pressure across the flow control valve associated with the second actuator becomes larger than a differential pressure across the flow control valve associated with the first actuator when the first and second actuators are driven simultaneously;
• wherein the plurality of distribution compensating valves are disposed downstream of associated flow control valves, respectively;
• the distribution compensating valve associated with the first actuator includes piston means having a first pressure receiving portion subjected to a pressure on the downstream side of its associated flow control valve acting in the valve-opening direction, and a second pressure receiving portion subjected to a maximum load pressure among the plurality of actuators acting in the valve-closing direction;
• and the distribution compensating valve associated with the second actuator includes piston means having a third pressure receiving portion subjected to a pressure on the downstream side of its associated flow control valve acting in the valve-opening direction, and fourth and fifth pressure receiving portions subjected to the maximum load pressure acting in the valve-closing direction, the fourth and fifth pressure receiving portions having the total of their pressure receiving areas substantially equal to the pressure receiving area of the third pressure receiving portion;
• and
• wherein said distribution control means includes pressure reducer means, responsive to drive of the first actuator, for cutting off communication of one of the fourth and fifth pressure receiving portions with the maximum load pressure.