Patent ID: 9482214
Filing Date: 2016-11-01
CPC Classification: E02F,F04B,F15B

Claim Text:
1. A hydraulic circuit for controlling a boom of a construction machine, comprising: a hydraulic pump connected to an engine; a boom cylinder connected to the hydraulic pump through a first path and a second path; a boom control valve shiftably installed in a flow path provided between the hydraulic pump a holding valve including a holding poppet installed between the boom control valve and the first flow path of the boom cylinder and a drain valve configured to supply or discharge a hydraulic fluid to or from a back pressure chamber of the holding poppet so that the natural descending movement of the boom is prevented when the boom control valve is in a neutral state; a port relief valve installed in the first path at the downstream side of the holding poppet and configured to relieve the hydraulic fluid when an overload occurs in the first path; and an orifice valve installed at the downstream side of the port relief valve and configured such that when the boom control valve is in a neutral state, the hydraulic fluid passing through the port relief valve passes through an orifice to relieve the hydraulic fluid, and when the boom control valve is shifted by a boom-up pilot signal pressure that is applied thereto and a part of the boom-up pilot signal pressure is applied to a signal pressure-receiving portion of the orifice valve, the hydraulic fluid passing through the port relief valve is discharged to a hydraulic tank.