HYDRAULIC DRIVE SYSTEM FOR CONSTRUCTION MACHINE

摘要

A hydraulic drive system for a construction machine allows automatic changes of the operational characteristics of a boom directional control valve by judging whether or not a hydraulic boom cylinder needs driving pressure at the time of a boom lowering operation. A solenoid switch valve is controlled such that a pilot oil passage with a pressure reducing valve is selected to set the limit of a boom-lowering spool stroke of a boom directional control valve to a middle position L1 when the rod-side pressure of a hydraulic boom cylinder detected by a pressure sensor is less than a threshold value and such that a pilot oil passage is selected to set the limit of the boom-lowering spool stroke of the boom directional control valve to a maximum stroke position L2 when the rod-side pressure is equal to or greater than the threshold value.