Hydraulic drive system for construction machine

A hydraulic drive system for a construction machine, comprising:

• a prime mover;
• a variable displacement first hydraulic pump driven by the prime mover;
• a variable displacement second hydraulic pump driven by the prime mover;
• a plurality of actuators driven by hydraulic fluids delivered by the first and second hydraulic pumps;
• a plurality of flow control valves that control flow rates of hydraulic fluids supplied from the first and second hydraulic pumps to the plurality of actuators;
• a plurality of pressure compensating valves that control differential pressures across the plurality of flow control valves;
• a first pump control unit that controls a delivery flow rate of the first hydraulic pump;
• and
• a second pump control unit that controls a delivery flow rate of the second hydraulic pump,
• the first pump control unit including
• a first torque control section that, when at least one of delivery pressure and capacity of the first hydraulic pump increases and absorption torque of the first hydraulic pump increases, controls the capacity of the first hydraulic pump such that the absorption torque of the first hydraulic pump does not exceed a first maximum torque,
• the second pump control unit including
• a second torque control section that, when at least one of delivery pressure and capacity of the second hydraulic pump increases and absorption torque of the second hydraulic pump increases, controls the capacity of the second hydraulic pump such that the absorption torque of the second hydraulic pump does not exceed a second maximum torque, and
• a load sensing control section that, when the absorption torque of the second hydraulic pump is lower than the second maximum torque, controls the capacity of the second hydraulic pump such that the delivery pressure of the second hydraulic pump becomes higher by a target differential pressure than a maximum load pressure of the actuators driven by a hydraulic fluid delivered by the second hydraulic pump,
• wherein the first torque control section includes a first torque control actuator that receives the delivery pressure of the first hydraulic pump and, when the delivery pressure rises, controls the capacity of the first hydraulic pump to decrease the capacity of the second hydraulic pump and decrease the absorption torque thereof, and first biasing means that sets the first maximum torque,
• the second torque control section includes a second torque actuator that receives the delivery pressure of the second hydraulic pump and, when the delivery pressure rises, controls the capacity of the second hydraulic pump to decrease the capacity of the second hydraulic pump and decrease the absorption torque thereof, and second biasing means that sets the second maximum torque,
• the load sensing control section includes
• a control valve that varies a load sensing drive pressure such that the load sensing drive pressure is lowered as a differential pressure between the delivery pressure of the second hydraulic pump and the maximum load pressure becomes smaller than the target differential pressure, and a load sensing control actuator that controls the capacity of the second hydraulic pump to increase the capacity of the second hydraulic pump and increase the delivery flow rate as the load sensing drive pressure becomes lower,
• the first pump control unit further includes
• a torque feedback circuit that receives the delivery pressure of the second hydraulic pump and the load sensing drive pressure and modifies the delivery pressure of the second hydraulic pump based on the delivery pressure of the second hydraulic pump and the load sensing drive pressure to provide a characteristic simulating the absorption torque of the second hydraulic pump both in the cases of when the second hydraulic pump is limited by control of the second torque control section and operates at the second maximum torque and when the second hydraulic pump is not limited by control of the second torque control section and the load sensing control section controls the capacity of the second hydraulic pump, and then outputs the modified delivery pressure as a torque control pressure, and
• a third torque control actuator that receives the torque control pressure and controls the capacity of the first hydraulic pump to decrease the capacity of the first hydraulic pump and decrease the first maximum torque as the torque control pressure becomes higher,
• the torque feedback circuit includes
• a fixed restrictor that receives the delivery pressure of the second hydraulic pump,
• a variable restrictor valve located in a downstream side of the fixed restrictor and connected to a tank in the downstream side thereof, and
• a pressure limiting valve connected to a hydraulic line between the fixed restrictor and the variable restrictor valve to control the pressure in the hydraulic line such that the pressure does not increase beyond a pressure that initiates the control of the second torque control section,
• the variable restrictor valve is configured such that the variable restrictor valve is fully closed when the load sensing drive pressure is at a lowest pressure and that the opening area of the variable restrictor valve increases as the load sensing drive pressure rises, and
• the torque feedback circuit generates the torque control pressure based on the pressure in the hydraulic line between the fixed restrictor and the variable restrictor valve, the torque control pressure being introduced to the third torque control actuator.