Hydraulic drive system for construction machine

A hydraulic drive system for a construction machine comprising at least first and second hydraulic pumps (25a, 25b), at least first and second actuators (19, 21) driven by a hydraulic fluid supplied from said first and second hydraulic pumps, first and second valve apparatus (50, 51) respectively disposed between said first and second hydraulic pumps and said first and second actuators for selectively controlling operation of said first and second actuators, and first and second pump control means (30a, 30b) for respectively controlling said first and second hydraulic pumps so that pump delivery pressures are held higher than higher one of load pressures of said first and second actuators, said first and second valve apparatus respectively including first and second flow control means (11a, 11b, 12a, 12b), first and second pressure control means (13a, 13b, 15b), and first and second directional control means arranged in this order, said hydraulic drive system further comprising a pressure signal transmitting line (52) for introducing, as a pressure signal, higher one of the load pressures of said first and second actuators to said first and second pressure control means, said first and second pressure control means being operated in response to said pressure signal to respectively control pressures downstream of said first and second flow control means, wherein:

• said first flow control means includes first and second flow control valves (11a, 11b) and first interlock means (54, 55) for interlocking said first and second flow control valves with said first directional control means (7), and said second flow control means includes third and fourth flow control valves (12a, 12b) and second interlock mechanism (56, 57) for interlocking said third and fourth flow control valves with said second directional control means (9),
• said first pressure control means includes at least first pressure control valve (13a) operated in response to said pressure signal in a valve-closing direction, and said second pressure control means includes only second pressure control valve (15b) operated in response to said pressure signal in a valve-closing direction, and
• said first hydraulic pump (25a) is connected to said first actuator (19) via said first flow control valve (11a), said first pressure control valve (13a) and said first directional control means (7), said second hydraulic pump (25b) is connected to said first actuator (19) via said second flow control valve (11b) and said first directional control means (7), said first hydraulic pump (25a) is also connected to said second actuator (21) in parallel to said first actuator (19) via said third flow control valve (12a) and said second directional control means (9) without passing any pressure control valve, and said second hydraulic pump (25b) is also connected to said second actuator (21) in parallel to said first actuator (19) via said fourth flow control valve (12b), said second pressure control valve (15b) and said second directional control means (9).