Hydraulic Drive System for Construction Machine

A hydraulic drive system for a construction machine, comprising:

• a variable displacement hydraulic pump;
• a plurality of actuators driven by hydraulic fluid delivered from the hydraulic pump;
• a control valve device that distributes and supplies the hydraulic fluid delivered from the hydraulic pump to the plurality of actuators;
• a plurality of operation lever devices that instruct driving directions and speeds of the plurality of actuators;
• a pump regulation device that controls a delivery flow rate of the hydraulic pump so as to deliver a flow rate according to input amounts of operation levers of the plurality of operation lever devices;
• an unloading valve that discharges the hydraulic fluid of a hydraulic fluid supply line of the hydraulic pump to a tank when a pressure of the hydraulic fluid supply line increases and exceeds a set pressure equal to a sum of a highest load pressure of the plurality of actuators and at least a target differential pressure;
• and
• a controller that controls the control valve device, wherein
• the control valve device includes:
• a plurality of directional control valves that are individually shifted by the plurality of operation lever devices and associated with the plurality of actuators to adjust driving directions and speeds of the respective actuators, and
• a plurality of pressure compensating valves arranged in downstream sides of the plurality of directional control valves for controlling pressures in downstream sides of meter-in openings of the plurality of directional control valves such that the pressures in the downstream sides of the meter-in openings of the plurality of directional control valves becomes equal to the highest load pressure, and
• the controller is configured to:
• calculate demanded flow rates for the plurality of actuators and meter-in opening areas of the plurality of directional control valves based on input amounts of the operation levers of the plurality of operation lever devices, calculate a meter-in pressure loss of a particular directional control valve among the plurality of directional control valves based on the meter-in opening areas and the demanded flow rates, and output the pressure loss as the target differential pressure to control the set pressure of the unloading valve.