Hydraulic drive system for construction machine

Abstract

It is an object of the present invention to accurately detect the absorption torque of the other of two hydraulic pumps by a purely hydraulic structure and feed the absorption torque to one of the two hydraulic pumps, thereby to accurately perform a total torque control, effectively utilize a rated output torque of a prime mover, and enhance mountability. To achieve the object, there are provided: a torque feedback circuitto which the delivery pressure of a first hydraulic pumpand a load sensing drive pressure are introduced, which modifies the delivery pressure of a second hydraulic pumpto provide a characteristic simulating the absorption torque of the second hydraulic pump, and which outputs the modified pressure; and torque feedback pistons,to which the output pressure of the torque feedback circuitis introduced, and which control the capacity of the first hydraulic pumpto decrease the capacity of the first hydraulic pumpand decrease a maximum torque Tmax as the output pressure becomes higher. The torque feedback circuitincludes pressure dividing restrictor parts,, pressure dividing valves,, and relief valves,.