COMPONENT OPERTATION / Control Valve HYDRAULIC CIRCUIT Main Circuit Pressure oils from pump 1 and pump 2 flow to the 4-spool section and 5-spool section of the control valve respectively. The parallel circuit is provided in both right and left main circuits and makes the combined operation pos- sible. The flow combining circuit is provided in both boom and arm circuits so that pressure oils from pump 1 and pump 2 are combined during a single operation. The main relief valve is provided in the main circuit (between pump and actuator).
The main relief valve works so that pressure in main circuit does not ex- ceed the set pressure when the spool is in operation (or when the control lever is in operation). The overload relief valve is provided in the actuator circuits (between control valve and actuator) of boom, arm, bucket and auxiliary. The overload relief valve prevents surge pressure caused by external force in the actuator circuit does not exceed the set pressure when the spool is in neutral (with the control lever in neutral). The boom overload relief control valve is provided in the boom lower circuit of boom 2.
The boom overload relief valve decreases relief set pressure of the over- load relief valve and reduces vibration of machine during boom lowering operation. T3-3-16