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| Illustration 1 | g00465999 |
Main relief valve for the boom and hook control valve (A) Inlet port (B) Relief port (1) Spring chamber (2) Adjustment screw (3) spring (4) pilot valve (5) spring (6) Dump valve | |
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| Illustration 2 | g00466126 |
Location of the main relief valve (7) Main relief valve | |
The main relief valve is attached to the boom and hook control valve which is located below the floorplate in the operator's compartment. The relief valve is a pilot-operated valve. This valve limits the system pressure to approximately 33100 kPa (4800 psi) .
Pump oil enters relief valve (7) through port (A) . The oil flows through an orifice in dump valve (6) . The oil fills spring chamber (1) . This pressure begins building against pilot valve (4) .
When the oil pressure is less than the relief valve setting, dump valve (6) is held closed by spring (5) and pilot valve (4) is held closed by spring (3) .
When the oil pressure reaches the relief valve setting, pilot valve (3) opens. The oil in chamber (1) is dumped to the tank at a rate that is faster than the oil that is passing through the orifice in dump valve (6) . This causes the pressure in chamber (1) to decrease. The pressure of the pump oil moves dump valve (6) upward against the force of spring (5) . This opens port (B) to the tank and pump flow is dumped to the tank.
The pressure setting of the relief valve can be changed by changing the force of spring (3) which keeps pilot valve (4) closed. This is done by turning adjustment screw (2) . Pressure is increased when the screw is turned clockwise. Pressure is decreased when the screw is turned counterclockwise. One full turn equals a pressure change of approximately 8050 kPa (1170 psi).