TUBE AS 1299970 - Caterpillar

Illustration 1 g01445076
Pilot Relief ValveThe pilot relief valve is fastened to the inside of the right rear frame. The pilot relief valve is located below the hydraulic oil filter.
Illustration 2 g01445078
Pilot Relief Valve (1) Inlet (2) Chamber (3) Chamber (4) Pilot valve (5) Spring (6) Outlet (tank) (7) Dump valve (8) Spring (9) LocknutThe pilot relief valve maintains the oil pressure in the pilot system at 2400 150 kPa (350 22 psi) while the engine is at low idle speed.When pilot valve (4) is closed against the seat, the pressures in chambers (2) and (3) are equal. Dump valve (7) is held closed by the force of spring (8). The opening pressure of dump valve (7) is controlled by the force of springs (5) and (8) .ReferenceFor information about the adjustment of the pilot relief valve, refer to the Service Manual module Testing and Adjusting, "Pilot Relief Valve - Adjust" of the machine that is being serviced.When the control lever is in the HOLD position, the pressure in the pilot system will increase to the setting of the pilot relief valve. The increase in oil pressure causes pilot valve (4) to overcome the force of spring (5). This will cause the orifice at the seat of the pilot valve to open slightly. Oil will flow into chamber (2) and through outlet (6) to the hydraulic oil tank.The orifice in the seat of pilot valve (4) is larger than the orifice in dump valve (7). As pilot valve (4) moves against the force of spring (5), the opening in the seat of the valve becomes larger.When the opening in the seat of pilot valve (4) has opened the orifice in the dump valve enough, the pressure in chamber (3) becomes lower than the pressure in chamber (2). Dump valve (7) will open when the difference between the two pressures becomes larger than the force of spring (8) .The oil flows from chamber (2) through outlet (6) to the lift circuit and the tilt circuit. There is now no pressure increase in the pilot circuit.