FLOORPLATE AS 2584225 - Caterpillar

The pilot operated relief valve (illustration 3) is often used in systems that require a large volume of oil and a small differential between the cracking pressure and the full flow pressure.In the pilot operated relief valve, a pilot valve (simple relief valve) is used to control the unloading valve (main valve).The pilot valve is much smaller. The pilot valve does not handle large volume oil flow. Therefore, the spring in the pilot valve is smaller allowing for a more precise pressure control. The difference between the pilot valve cracking pressure and maximum pressure is held to a minimum. The unloading valve is large enough to handle the full pump flow at the designed maximum relief pressure. The unloading valve uses the system oil pressure and the spring to keep the valve closed. Therefore, the unloading valve spring does not need to be strong and heavy. This allows the unloading valve to have a more precise opening pressure.The system oil flows into the relief valve housing, through the unloading valve orifice, filling the unloading valve spring chamber. The oil in the unloading valve spring chamber comes in contact with a small area of the pilot valve. This design allows the pilot valve to use a small spring to control a high pressure.When the oil pressure increases in the system, the same pressure is in the unloading valve spring chamber. Therefore, the oil pressure is the same on both sides of the unloading valve. The combined force of the system oil pressure in the unloading valve spring chamber and the spring force on the top of the unloading valve is greater than the force of the system oil pressure against the bottom of the valve. The combined force in the spring chamber keeps the unloading valve closed.Pilot Operated Relief Valve, OPEN Position
Illustration 4 g01062800
When the system oil pressure exceeds the pilot valve spring setting (illustration 4), the pilot valve opens. The open pilot valve allows the oil in the unloading valve spring chamber to flow to the tank. The pilot valve opening (orifice) is larger than the unloading valve orifice. Therefore, the oil will flow past the pilot valve much faster than through the unloading valve orifice. This differential inflow allows the pressure to decrease in the unloading valve spring chamber. The force of the higher system oil pressure moves the unloading valve up against the spring. The excessive pump oil flows through the throttling holes in the unloading valve to the tank. The throttling holes allow the unloading valve to dump the volume of oil necessary to maintain the desired relief pressure.Relief Valve ISO Symbol CLOSED
Illustration 5 g01062802
The relief valve ISO graphic symbol that is shown in illustration 5 can represent either a simple relief valve or a pilot operated relief valve. Both types of relief valves can use the same ISO graphic symbol.The above relief valve ISO symbol shows a single valve envelope in the CLOSED position. The system pressure is sensed through the pilot line at the top