Displacement of variable displacement pump controlled by load sensing device having two settings for low and high speed operation of an actuator

A hydraulic drive system having a hydraulic pump of the variable displacement type having displacement volume varying means, at least one hydraulic actuator driven by a hydraulic fluid discharged from said hydraulic pump, a directional control valve for controlling a flow of the hydraulic fluid supplied from said hydraulic pump to said actuator, and discharge control means for controlling a flow rate of the hydraulic fluid discharged from said hydraulic pump, said discharge control means including drive means for driving said displacement volume varying means and load-sensing control means for controlling operation of said drive means responsive to a differential pressure between a discharge pressure of said hydraulic pump and a load pressure of said actuator for holding said differential pressure at a setting value, said hydraulic drive system comprising:

• instruction means operated exteriorly for instructing a change in the differential pressure between the discharge pressure of said hydraulic pump and the load pressure of said actuator;
• differential pressure setting means for changing the setting value of said differential pressure in response to an instruction from said instruction means, said load-sensing control means including detector means for detecting the differential pressure between the discharge pressure of said hydraulic pump and the load pressure of said actuator, a controller for outputting control signals when the differential pressure detected by said detector means is different from a setting value, and valve means driven by said control signals, wherein
• said controller includes a storage means for storing a plurality of differential pressure target values, and arithmetic means for selecting one of said plurality of stored differential pressure target values in response to the instruction from said instruction means and then defines the selected value as a setting value of said differential pressure, and said differential pressure setting means includes said storage means and said arithmetic means of said controller.