Hydraulic drive system for construction machine

A hydraulic drive system for a construction machine, comprising:

• a first pump device having first and second delivery ports;
• a second pump device having third and fourth delivery ports;
• and
• a plurality of actuators which are driven by hydraulic fluid delivered from the first and second delivery ports of the first pump device and hydraulic fluid delivered from the third and fourth delivery ports of the second pump device, wherein:
• the first pump device includes a first pump controller which is provided for the first and second delivery ports as a common controller, and
• the second pump device includes a second pump controller which is provided for the third and fourth delivery ports as a common controller, and
• the first pump controller includes a first load sensing control unit which controls displacement of the first hydraulic pump device so that delivery pressures of the first and second delivery ports of the first hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the first and second delivery ports by a prescribed pressure and a first torque control unit which performs limiting control of the displacement of the first hydraulic pump device so that absorption torque of the first hydraulic pump device does not exceed a prescribed value, and
• the second pump controller includes a second load sensing control unit which controls displacement of the second hydraulic pump device so that delivery pressures of the third and fourth delivery ports of the second hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the third and fourth delivery ports by a prescribed pressure and a second torque control unit which performs limiting control of the displacement of the second hydraulic pump device so that absorption torque of the second hydraulic pump device does not exceed a prescribed value, and
• the plurality of actuators include first and second actuators which are driven at the same time in a certain combined operation of the construction machine while producing a relatively large supply flow rate difference therebetween, and
• the first actuator is connected so that hydraulic fluids delivered from the first and second delivery ports of the first pump device are merged and supplied to the first actuator, and
• the second actuator is connected so that hydraulic fluids delivered from the third and fourth delivery ports of the second pump device are merged and supplied to the second actuator, and
• the first pump torque control unit of the first pump device controls the displacement of the first hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the first and second delivery ports of the first hydraulic pump device related to itself but also the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device, and
• the second pump torque control unit of the second pump device controls the displacement of the second hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device related to itself but also the delivery pressures of the first and second delivery ports of the first hydraulic pump device.