938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier Hydraulic System Relief Valve (Main) Caterpillar


Relief Valve (Main)
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1.1. Operation of the Main Relief Valve



Illustration 1g00928577

Main Control Valve for the 938G Series II Wheel Loader

(1) Main control Valve. (2) Main relief valve.




Illustration 2g00928574

Main Control Valve for the IT38G Series II Integrated Toolcarrier

(1) Main control Valve. (2) Main relief valve.

Main relief valve (2) is located on main control valve (1). Main relief valve (2) limits the maximum pressure for the main hydraulic system.

Operation of the Main Relief Valve




Illustration 3g00938311

Sectional View of a Main Relief Valve

(3) Locking nut. (4) Spring. (5) Poppet. (6) Seat. (7) Inlet. (8) Adjustment screw. (9) Washer. (10) Tank passage.

When the oil pressure at the inlet of the main control valve is less than the relief valve setting, the force of spring (4) holds poppet (5) onto seat (6). Hydraulic oil will not flow to the hydraulic tank. Illustration 3 shows main relief valve (2) with the oil pressure below the adjusted pressure.




Illustration 4g00928553

Sectional View of a Main Relief Valve

(3) Locking nut. (4) Spring. (5) Poppet. (6) Seat. (7) Inlet. (8) Adjustment screw. (9) Washer. (10) Tank passage.

As the oil pressure at inlet (7) exceeds the relief valve setting, the oil pressure overcomes the force of spring (4). Poppet (5) will move off seat (6). The oil then flows through tank passage (10) and into the hydraulic tank. Illustration 4 shows the line relief valve with the hydraulic oil pressure at inlet (7) higher than the adjusted pressure.

In order to adjust the main relief valve, loosen locking nut (3). In order to raise the pressure adjustment, turn adjustment screw (8) clockwise. In order to lower the pressure adjustment, turn adjustment screw (10) counterclockwise.

Reference: For more information about the main relief valve, refer to the Specifications, RENR6076, "Main Relief Valve" for the machine that is being serviced.

Reference: For information about testing and adjusting the line relief valves, refer to the Service Manual module for Testing and Adjusting, "Relief Valve (Main) - Test and Adjust" for the machine that is being serviced.

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