Patent ID: 8919109
Filing Date: 2014-12-30
Classification: E02F,F01N,F02D,F04B,F15B

Abstract:
1. A hydraulic drive system for a construction machine, comprising: an engine; a hydraulic pump of a variable displacement type, the pump being driven by the engine; a plurality of actuators that are driven by hydraulic fluid delivered from the hydraulic pump; a plurality of flow rate/direction control valves that control flow rates of the hydraulic fluid supplied from the hydraulic pump to the actuators; a maximum load pressure detecting circuit that detects maximum load pressure of the actuators; a pump control device including a torque control unit that conducts constant absorption torque control for controlling absorption torque of the hydraulic pump not to exceed preset maximum torque by reducing displacement of the hydraulic pump with the increase in delivery pressure of the hydraulic pump, and a load sensing control unit that controls the delivery pressure of the hydraulic pump to be higher than the maximum load pressure of the actuators by target differential pressure; and an unload valve that is arranged in a line connecting the hydraulic pump to the plurality of flow rate/direction control valves and restricts the increase in the delivery pressure of the hydraulic pump by shifting to an open state and returning the delivered hydraulic fluid from the hydraulic pump to a tank when the delivery pressure of the hydraulic pump exceeds the sum total of the maximum load pressure and preset pressure, wherein the hydraulic drive system comprises: a first selector valve that selects between predetermined pressure and tank pressure, outputs the selected pressure, and supplies the output pressure to the maximum load pressure detecting circuit as dummy load pressure; a second selector valve that selects between enabling and disabling of load sensing control implemented by the load sensing control unit of the pump control device; an exhaust gas purification device that purifies exhaust gas from the engine; and a control device that actuates the first and second selector valves so that the first selector valve outputs the tank pressure as the dummy load pressure and the second selector valve enables the load sensing control implemented by the pump control device when the exhaust gas purification device does not need regeneration and so that the first selector valve outputs the predetermined pressure as the dummy load pressure and the second selector valve disables the load sensing control implemented by the pump control device when the exhaust gas purification device needs the regeneration.