HYDRAULIC DRIVE FOR CONSTRUCTION MACHINE

摘要

Provided is a hydraulic drive for a construction machine which can detect a state wherein a hydraulic cylinder for a boom requires a driving pressure during a boom descent operation and automatically switch the operating characteristics of a boom direction switching valve. The hydraulic drive is provided with a control unit (42) which controls an electromagnetic switching valve (40) so that when the rod-side pressure of a hydraulic cylinder (20) for a boom as detected by a pressure sensor (41) is less than a threshold value, a pilot oil passage (38a) having a pressure relief valve (39)is selected and the limit position of a boom descent-side stroke of a boom direction switching valve (31) is at an intermediate position (L1), and when the rod-side pressure of the hydraulic cylinder (20) for the boom as detected by the pressure sensor (41) is not less than a threshold value, the pilot oil passage (38a) is selected and the limit position of a boom descent-side stroke of the boom direction switching valve (31) is at the maximum position (L2).