CONTROL CIRCUIT OF CONSTRUCTION MACHINE

摘要

A control circuit of a construction machine capable of securing a same operability as in a case where a hydraulic shovel having an open center circuit is tuned by using a standard bucket even when the bucket weight of the hydraulic shovel is changed. Center bypass lines (Cb) are installed for a boom operation valve and a stick operation valve (251) controlling an oil supplied from a hydraulic pump (28) to a boom cylinder and a stick cylinder (25). A stick-in meter-out load pressure compensating valve (42) is installed in a rod side return line (55) from the rod side (25r) of the stick cylinder (25) to a tank (29). A pressure sensor (81) detecting the pressure of the hydraulic oil supplied to the rod side (25r) is installed on the rod side (25r) of the stick cylinder (25). An electromagnetic proportional relief valve (85) controlling a center bypass line pressure on the downstream side of the stick operation valve (251) in an increasing direction according to the rise of the pressure detected by the pressure sensor (81) is installed in the center bypass line (Cb).