Hydraulic drive system

A hydraulic drive system comprising an engine (1), a variable displacement hydraulic pump (10) driven by said engine, a plurality of actuators (4a,4b) driven by a hydraulic fluid delivered from said hydraulic pump, a plurality of directional control valves (20a,20b;

• 20Ba,20Bb;
• 20Ca,20Cb) for controlling respective flow rates of the hydraulic fluid supplied from said hydraulic pump to said plurality of actuators, a plurality of pressure compensating valves (21a,21b;
• 21Ba,21Bb;
• 21Ca,21Cb) for controlling respective differential pressures across said plurality of directional control valves, pump control means (12;
• 12B) for performing load sensing control to hold a delivery pressure of said hydraulic pump higher than a maximum load pressure of said plurality of actuators by a target differential pressure, and a main relief valve (30) for restricting an upper limit of the delivery pressure of said hydraulic pump, a target compensated differential pressure (Pc) for each of said plurality of pressure compensating valves being set in accordance with a differential pressure (Ps−PLMAX) between the delivery pressure of said hydraulic pump and the maximum load pressure of said plurality of actuators, a target differential pressure (PGR) in said load sensing control being set as a variable value depending on a revolution speed of said engine, wherein:
• said hydraulic drive system further comprises target compensated differential pressure modifying means (32,33;
• 60) for setting, as the target compensated differential pressure (Pc) for each of said plurality of pressure compensating valves (21a,21b;
• 21Ba,21Bb;
• 21Ca,21Cb), a modification value (PGR−α;
• PGR) different from the differential pressure between the delivery pressure of said hydraulic pump and the maximum load pressure of said plurality of actuators (4a,4b), when the delivery pressure of said hydraulic pump (10) rises up to a setting pressure of said main relief valve (30).