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


Relief Valve (Line)
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1.1. Direct Acting Poppet Type Valve
2.1. Pilot Operated Relief Valve
3.2. Relief Valve
4.2. Makeup Valve

The line relief valves are located on the main control valve. The main control valve is mounted in the front frame. For the 938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier, line relief valves are used for both the tilt function and the auxiliary function. If the IT38G Series II Integrated Toolcarrier is equipped with a fourth function, the fourth function will have two line relief valves. Access to line relief valves is gained by removing the loader tower access panel.

Direct Acting Poppet Type Valve




Illustration 1g00928979

(1) Cavity. (2) Spring. (3) Poppet. (4) Tank oil. (5) Passage. (6) Seat. (7) Inlet oil.

The line relief valve is located in the main control valve. In the 938G Series II Wheel Loader and the IT38G Series II Integrated Toolcarrier, a direct acting poppet type valve is installed for the tilt back operation.

Access to the line relief valves can be gained by removing the loader tower access panel.




Illustration 2g00937900

(1) Cavity. (2) Spring. (3) Poppet. (4) Tank. (5) Passage. (6) Seat. (7) Inlet.

Illustration 2 shows a direct acting relief valve with a higher pressure at the inlet. Since the setting of the main relief valve is above all of the operating pressures, the valve serves as a backup valve. When the line relief valve senses a pressure spike or an outside force on the work tool, poppet (3) moves off seat (6). Pump oil flows into the line relief valve through the hole at inlet (7). When the force that is developed by the area of the hole times the pressure is greater than the force of spring (2), the spring will be compressed. The excess oil will flow through the line relief valve by way of tank passage (4). As poppet (3) is moving to the left, oil in cavity (1) must be relieved. The oil inside cavity (1) flows through passage (5). Then, the oil flows out of the relief valve and into the hydraulic tank.

Pilot Operated Relief Valve




Illustration 3g00929617

(8) Spring. (9) Passage. (10) Spring. (11) Passage. (12) Tank passage. (13) Seat. (14) Cavity. (15) Spring. (16) Seat. (17) Seat. (18) Poppet. (19) Makeup spool. (20) Inlet oil. (21) Dump spool.

The line relief valve is located in the main control valve. In the 938G Series II Wheel Loader and the IT38G Series II Integrated Toolcarrier, a pilot operated relief valve is used for the dump operation in the tilt section and the auxiliary valve. The pilot operated relief valve can be used as either a relief valve or a makeup valve. The following paragraphs describe the operations of the pilot operated relief valve in the hydraulic system. Illustration 3 shows the pilot operated relief valve with the inlet pressure less than the adjusted pressure.




Illustration 4g00929582

(9) Spring. (10) Passage. (11) Spring. (12) Passage. (13) Tank passage. (14) Seat. (15) Cavity. (17) Seat. (18) Seat. (19) Poppet. (20) Poppet. (21) Inlet oil. (21) Dump spool.

Relief Valve

When the pilot operated relief valve is used as a relief valve, the valve is regulating pressure spikes from either the pump or any unusual outside forces on the work tool.

When the pilot operated relief valve operates as a relief valve, the hydraulic oil flows in inlet (20). From the inlet, the oil passes through passage (12) that is in the center of seat (16). As the pressure rises above the adjusted pressure of the relief valve, the poppet (18) will move off seat (13). The oil will flow into cavity (15). Then, the oil will exit out through passage (10) to the hydraulic tank.

The force on the end of seat (16) will override the force of spring (10). Seat (16) moves to the left and contacts contact poppet (18). At that time, dump spool (21) will move off seat (17). Oil will flow from inlet (20) around dump spool (21) and out of the valve through tank passage (12). The relief valve will be open until the pressure at inlet (20) is less than the adjusted pressure of the relief valve.

Makeup Valve

The pilot operated relief valve is also used as a makeup valve. The relief valve will assist in filling a component from another component within the system. The oil pressure in the tank passage of the pilot operated relief valve will force makeup spool (19) from the seat that is built into the body of the control valve. Tank oil will flow through tank passage (12), around makeup spool (19), and to inlet (20).




Illustration 5g00929863

(12) Tank passage. (15) Spring. (19) Makeup spool. (22) Area.

In the hydraulic system, the makeup valve will allow oil to fill a void in a hydraulic cylinder or on a hydraulic motor. The oil from tank passage (12) flows against area (22) on makeup spool (19). When the pressure of the tank oil on area (22) is greater than the force of spring (15), makeup spool (19) will override spring (15). Makeup spool (19) will move to the left. Makeup spool (19) comes off the seat. The tank oil will flow around makeup spool (19) to the inlet (20). Then, the flow of oil will fill the void in the actuator.

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