Hydraulic drive system for construction machine

摘要

A manually operated directional control valve 39 is interposed between attachment-drive lines 37a and 37b to establish connection between an attachment-drive actuator 22 and an attachment-drive control valve 33. The directional control valve 39 is switched to any one of switching positions X, Y, and Z, the first switching position X allowing an actuator port A and a valve port B to communicate with each other and to block a line port C and a tank port D, the switching position Y allowing the actuator port A and the tank port D to communicate with each other and to block the valve port B and the line port C, and the switching position Z allowing the line port C and the tank port D to communicate with each other and to block the actuator port A and the valve port B. Thus, depressurizing work for the attachment-drive lines can be done with a hydraulic circuit having a simple, compact structure.