Patent ID: 9200431
Filing Date: 2015-12-01
Classification: E02F,F15B

Abstract:
1. A hydraulic drive system for a construction machine, comprising: an engine; a variable displacement main pump driven by the engine; a plurality of actuators including traveling hydraulic motors, each of the traveling hydraulic motors being driven by hydraulic fluid delivered from the main pump; a plurality of flow control valves including traveling flow control valves, the flow control valves connected to control flow rates of the hydraulic fluid fed to the plurality of actuators from the main pump; a pump control device connected to the main pump to exercise load sensing control on a displacement volume of the main pump so that a delivery pressure of the main pump is higher than a maximum load pressure of the plurality of actuators by a target differential pressure; a plurality of pressure compensating valves connected to the plurality of flow control valves, where the plurality of pressure compensating valves each control to keep a differential pressure across a corresponding one of the flow control valves at a differential pressure between the delivery pressure of the main pump and the maximum load pressure of the plurality of actuators; a plurality of shuttle valves to detect whether or not an operation mode is a traveling operation in which the traveling hydraulic motors are to be driven; and a setting changing device connected with the pump control device to set the target differential pressure of the load sensing control at a first value when the operation mode is not the traveling operation, and to set the target differential pressure of the load sensing control at a second value lower than the first value when the operation mode is the traveling operation, the setting changing device including a signal pressure production device to produce a signal pressure at a first absolute pressure corresponding to the first value when the operation mode is not the traveling operation, and to produce the signal pressure at a second absolute pressure corresponding to the second value when the operation mode is the traveling operation, wherein the pump control device sets the target differential pressure of the load sensing control as the signal pressure output by the signal pressure production device and controls the displacement volume of the main pump, wherein the signal pressure production device includes: a differential pressure reducing valve to produce the first absolute pressure according to a revolution speed of the engine driving the main pump; a pressure reducing valve to reduce a pressure of a pilot hydraulic fluid source and produce the second absolute pressure from the reduced pressure of the pilot hydraulic fluid source; and a direction control valve to switch between the first absolute pressure as the signal pressure when the operation mode is not the traveling operation and the second absolute pressure as the signal pressure when the operation mode is the traveling operation.