| 摘要 |
A differential pressure between a delivery pressure of a fixed displacement hydraulic pump 2 and a maximum load pressure among a plurality of actuators 3a to 3c is maintained at a target differential pressure by an unloading valve 5. A setting pressure of the unloading valve 5 is changed depending on an engine revolution speed by introducing a differential pressure DeltaPp across a throttle 50, which is disposed in a delivery line of a fixed pump 30, to a pressure bearing sector 5d of the unloading valve. With such an arrangement, in a hydraulic drive system including an LS system, it is possible to ensure fine operability based on setting of the engine revolution speed, to perform flow rate control at a good response, and to realize superior operability.
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