BUSHING 1990668 - Caterpillar

Illustration 1 g00385273
Location of the Pilot Relief Valve (1) Pilot relief valve. (2) Manifold.
Illustration 2 g00324084
Cross section of pilot relief valve (3) Inlet. (4) Chamber. (5) Chamber. (6) Pilot valve. (7) Spring. (8) Adjuster. (9) Cap. (10) Locknut. (11) Outlet (tank). (12) Chamber. (13) Spring. (14) Dump valve. (15) Outlet (cooling circuit).Pilot relief valve (1) is mounted in manifold (2). Manifold (2) is located between the front pump drive and the main control valve. The pilot relief valve maintains the oil pressure in the pilot system at 2400 40 kPa (350 6 psi).When pilot valve (6) is closed against the seat, the pressure in chambers (4) and (5) are equal. Dump valve (14) is held closed by the force of spring (13). The opening pressure of dump valve (14) is controlled by the force of springs (7) and (13). To adjust the pressure of the relief valve, remove cap (9) and loosen locknut (10). Turn adjuster (8) by a 1/4 turn for an approximate pressure change of 170 kPa (25 psi).When the controls for the pilot system are in the HOLD position, pressure in the circuit for the pilot system increases to the setting of the relief valve. The oil pressure now causes pilot valve (6) to overcome the force of spring (7). This will cause the orifice at the seat of the pilot valve to slightly open. Oil will flow into chamber (12) and through outlet (11) to the tank.The orifice in the seat of pilot valve (6) is larger than the orifice in dump valve (14). As pilot valve (6) moves against the force of spring (7), the opening in the seat of the valve becomes larger.When the opening in the seat of pilot valve (6) has opened the orifice in the dump valve enough, the pressure in chamber (5) becomes lower than the pressure in chamber (4). Dump valve (14) will open when the difference between the two pressures becomes larger than the force of spring (13) .The oil flows from chamber (4) through outlet (15) to the cooling circuit. There is now no pressure increase in the pilot circuit.