Hydraulic drive system for construction machine

A hydraulic drive system for a construction machine, the hydraulic drive system comprising:

• a plurality of hydraulic pumps including variable-displacement first and second hydraulic pumps driven by a prime mover;
• a plurality of actuators driven by hydraulic fluids delivered from the plurality of hydraulic pumps;
• a first regulator to which a delivery pressure of the first hydraulic pump is introduced and that controls a displacement volume of the first hydraulic pump such that a torque consumed by the first hydraulic pump does not exceed a first allowable torque;
• a second regulator to which a delivery pressure of the second hydraulic pump is introduced and that controls a displacement volume of the second hydraulic pump such that a torque consumed by the second hydraulic pump does not exceed a second allowable torque;
• and
• a first valve device that generates a first output pressure to feed back the torque consumed by the second hydraulic pump to the first regulator based on the delivery pressure of the second hydraulic pump,
• wherein the first regulator includes a first operation drive section to which the first output pressure is introduced and with the first operation drive section, the first regulator corrects a horsepower control starting pressure for securing the first allowable torque so as to be smaller by the first output pressure thereby to control the displacement volume of the first hydraulic pump such that a sum of the torques consumed by the first and second hydraulic pumps does not exceed a predetermined value, and
• the plurality of actuators include a boom cylinder for driving a boom of a front work implement and a swing motor for driving an upper swing structure, the boom cylinder being driven by a hydraulic fluid delivered by the first hydraulic pump, and the swing motor being driven by a hydraulic fluid delivered by the second hydraulic pump, and
• wherein the hydraulic drive system further comprises:
• a controller that, when the swing motor and the boom cylinder are driven simultaneously, calculates a correction value for the horsepower control starting pressure for reducing the second allowable torque of the second hydraulic pump so as to be smaller than a maximum allowable torque at a time when the swing motor is driven independently;
• a second valve device for generating a second output pressure corresponding to the correction value calculated by the controller;
• a second operation drive section included in the second regulator and to which the second output pressure is introduced for correcting the horsepower control starting pressure for securing the second allowable torque so as to be smaller by the second output pressure;
• and
• an output pressure corrector for limiting the first output pressure of the first valve device such that the first output pressure of the first valve device does not exceed the horsepower control starting pressure for securing the second allowable torque corrected by the second operation drive section.