BOOM CYLINDER HYDRAULIC CIRCUIT OF HYDRAULIC EXCAVATOR

摘要

PROBLEM TO BE SOLVED: To have a boom cylinder hydraulic circuit with simple construction excellent in economic efficiency. SOLUTION: A boom cylinder hydraulic circuit is equipped with a directional selector valve 3 for supplying discharge oil to a boom cylinder 4 from a hydraulic pump 2, a relief valve 5 capable of switching a relief set pressure connected to a rod side hydraulic circuit 14 for connecting a rod side oil chamber 41 of the boom cylinder to the directional selector valve into high pressure and low pressure and a switching controlling device capable of being switched to the high pressure side in the case an engine is stopped and being switched to the high pressure side or the low pressure side in the case the engine is driven, In that case, it is equipped with a set pressure variable section 15 switched to the high pressure side in the case hydraulic oil higher than certain pressure of a rod side hydraulic circuit is supplied to the switching controlling device and switched to the low pressure side in the case the hydraulic oil is discharged, a selector valve 16 provided between the rod side hydraulic circuit and set pressure variable section and switching the supply and discharge of the hydraulic oil to the set pressure variable section and a control section 10 for outputting a switching signal to the selector valve.