Hydraulic drive system with pressure compensating valve

A hydraulic drive system for a construction machine comprising a hydraulic pump (1a);

• a plurality of hydraulic actuators (5a, 5b) driven by a hydraulic fluid delivered from said hydraulic fluid;
• a plurality of flow control valves (6a, 6b) for controlling respective flow rates of the hydraulic fluid supplied from said hydraulic pump to said hydraulic actuators dependent upon input amounts of manipulation means (50, 51);
• a plurality of distribution compensating valves (7a, 7b) for controlling respective differential pressures across said plurality of flow control valves, said distribution compensating valves (7a, 7b) respectively having first pressure bearing chambers (52a, 52b) subjected to pressures upstream of the associated flow control valves for acting in a valve-closing direction, second pressure bearing chambers (53a, 53b) subjected to pressures downstream of the associated flow control valves for acting in a valve-opening direction, and third pressure bearing chambers (54a, 54b) subjected to first control pressures (P.sub.C1, P.sub.C2) for acting in the valve-closing direction to reduce target values of the differential pressures across the associated flow control valves;
• differential pressure sensor means (8) for detecting a differential pressure between a pressure of the hydraulic fluid delivered from said hydraulic pump and a maximum load pressure among said plurality of hydraulic actuators;
• first proportional control valve means (9a, 9b) for producing said first control pressures (P.sub.C1, P.sub.C2) dependent upon first control currents (I.sub.C1, I.sub.C2);
• and first computing control means (26, 203, 218) for calculating at least one target reducing value (.DELTA.P.sub.C1, .DELTA.P.sub.C2) to reduce the target values of the differential pressures across said plurality of flow control valves based on a detected value (.DELTA.P.sub.LS) of said differential pressure sensor means, and outputting the corresponding first control currents to said first proportional control valve means, wherein the hydraulic drive system further comprises:
• (a) a fourth pressure bearing chamber (55a, 55b) provided in at least one of said plurality of distribution compensating valves (7a, 7b) and subjected to a second control pressure (P.sub.CT) for acting in the valve-opening direction to set a target value (.DELTA.P.sub.T) of the differential pressure across the associated flow control valve (6a, 6b);
• (b) second proportional control valve mean (24) for producing said second control pressure (P.sub.CT) dependent upon a second control current (I.sub.T);
• (c) signal generating means (25, 20-23) for outputting a signal (F, a.sub.1, a.sub.2, b.sub.1, b.sub.2) relating to the target value (.DELTA.P.sub.T) of the differential pressure across the associated flow control valve (6a, 6b);
• and
• (d) second computing control means (26, 204-218) for calculating the target value (.DELTA.P.sub.T) of the differential pressure across said associated flow control valve dependent upon the signal from said signal generating means, and outputting the corresponding second control current (I.sub.T) to said second proportional control valve means (24).