COMPONENT OPERTATION / Control Valve BOOM OVERLOAD RELIEF CONTROL VALVE The boom overload relief control valve is provided in the boom lower circuit of boom 2. The boom overload relief control valve is shifted by pilot pressure from solenoid valve unit (SC) and decreases the set pres- sure in overload relief valve of boom cylinder rod side circuit. Therefore, during boom lower operation, vibration of the machine is reduced. (Refer to the Control System group / SYSTEM.) Operation 1. In normal, pressure oil in port HP flows to cham- ber C through piston (6) and acts on main poppet (5). Thereby, main poppet (5) is closed and the set pressure in overload relief valve is kept high. 2.
When pilot pressure from solenoid valve unit (SC) acts on piston (1), piston (1) opens poppet (2). When poppet (2) opens, chambers a, b are con- nected to port LP through the external circumfer- ence of sleeve (7). 3. When pressure in chamber c reaches the set pressure of spring (3), pilot poppet (4) opens. When pilot poppet (4) opens, pressure in cham- ber c decreases, and piston (6) and main poppet (5) move to the left. 4. As main poppet (5) opens, pressure oil flows to port LP from port HP and the set pressure of overload relief valve decreases. 5. Consequently, as force in the boom cylinder rod side decreases, vibration of the machine during boom lower operation is reduced. T3-3-24