Hydraulic drive system for construction machines

A hydraulic drive system for a construction machine comprising a hydraulic pump, at least first and second hydraulic actuators driven by a hydraulic fluid supplied from said hydraulic pump, first and second flow control valves for controlling flows of the hydraulic fluid supplied to said first and second actuators, respectively, first and second distribution compensating valves for controlling first differential pressures produced between inlets and outlets of said first and second flow control valves, respectively, and discharge control means responsive to a second differential pressure between a discharge pressure of said hydraulic pump and a maximum load pressure out of said first and second actuators for controlling a flow rate of the hydraulic fluid discharged from said hydraulic pump, said first and second distribution compensating valves having respective drive means for applying control forces in accordance with said second differential pressure to the associated distribution compensating valves, to thereby set target values of said first differential pressures, comprising:

• first means for detecting said second differential pressure from the discharge pressure of said hydraulic pump and the maximum load pressure out of said first and second actuators;
• second means for calculating individual values, as values of said control forces applied from the respective drive means of said first and second distribution compensating valves, in accordance with at least the second differential pressure detected by said first means;
• and
• first and second control pressure generator means provided in association with said first and second distribution compensating valves, respectively, said first and second control pressure generator means producing control pressures dependent on the individual values obtained by said second means and outputting said control pressures to the respective drive means of said first and second distribution compensating valves.