938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier Hydraulic System Pilot Valve Caterpillar


Pilot Valve
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938G Series II Wheel Loader and IT38G Series II <BR>Integrated Toolcarrier Hydraulic System [RENR6077]
HYDRAULIC SYSTEM
VALVE GP-PILOT
938G Series II Wheel Loader and IT38G Series II Integrated Toolcarrier Hydraulic System Pilot Valve
1.1. Pilot Valve HOLD Position
2.1. Pilot Valve That Is Shifted
3.1. Pilot Valve In Detent

The pilot valve controls the pilot pressure to the main control valve. The signal that is sent by the pilot valve will control the amount of movement and direction of the stem within the main control valve.




Illustration 1g00930378

Control Group (Pilot Valve)

(1) Pilot valve. (2) Tilt control lever. (3) Lift control lever. (4) Switch panel. (5) Implement lockout lever.

In order to move an implement, the flow of high pressure oil that is flowing into the main control valve must be sent to an actuator. Pilot valve (1) controls the movement. Pilot valve (1) is located in the cab on the right side of the operator's seat. Implement lockout lever (5) must be in the UNLOCKED position, In order to control the hydraulic system. The position of the lever is shown in illustration 1. In the UNLOCKED position, the pilot oil will flow through the ball valve to pilot supply (14) of the pilot valve. When implement lockout lever (5) is in the VERTICAL position, the pilot oil through the system is blocked.

The pilot valve has two control levers. Lever (2) is for the tilt function and lever (3) is for the lift function. If the machine is equipped with a third function, a pilot valve with three levers will be installed.

The pilot valve section for the tilt function will have one detent coil. The coil is in the TILT BACK position. The pilot valve section for the lift function has two detent coils. One coil is for the LOWER position and the other coil is for the RAISE position.

If the machine is equipped with a joystick control, the lift control section and the tilt control section are combined into one lever. The lever can control both functions at one time. The operation of tilt valve, lift valve, and auxiliary valve (if equipped) are similar. Only the operation of the lift valve section will be discussed.

Pilot Valve HOLD Position

When the pilot valve is in the HOLD position, pilot oil from the selector and pressure control valve flows into the pilot valve at pilot supply (15). The pilot oil is blocked by the spools as the pilot oil enters each cartridge assembly. Return oil from the control valve is open to the hydraulic tank through passages in the cartridge assembly and the pilot valve body in the inlet port.




Illustration 2g00930319

Pilot Valve in the HOLD Position

(6) Lever. (7) Detent plunger. (8) Plate. (9) Adjustment pin. (10) Plunger. (11) Retainer. (12) Detent coil. (13) Spool. (14) Pilot supply. (15) Orifice.

The pilot oil to the control valve and from the control valve is controlled by the position of spool (14). Spool (14) is part of a cartridge assembly.

Illustration 3 describes the movement of the components of the cartridge assembly in the pilot valve.




Illustration 3g00950334

Cartridge Assembly

(10) Plunger. (14) Spool. (16) Orifice. (17) Spring. (18) Ball. (19) Retainer. (20) Spring. (21) Hole. (22) Tank port. (23) Control port. (24) Inlet port. (25) Spring. (26) Cavity.

When the lift pilot valve is in the HOLD position, springs (17) keep the lever assembly (6) in the HOLD position. The oil from pilot supply (15) is blocked. At the same time, the return oil from the main control valve flows through tank port (22). The oil then flows out of the pilot valve and back to the hydraulic tank.

When the cartridge assembly is in the HOLD position, spring (17), spring (20), and spring (25) keep the spools centered.

Pilot oil flows into the pilot valve through the port at the bottom of the valve to pilot supply (15).

Oil in tank port (22) will flow into hole (21) in spool (14). The pilot oil will flow through the center of spool (14). Then, cavity (26) will be filled with oil. The oil is held in cavity (26) until the pilot valve is shifted.

Pilot Valve That Is Shifted




Illustration 4g00930366

Pilot Valve in the SHIFTED Position

(6) Lever. (7) Detent plunger. (8) Plate. (9) Adjustment pin. (10) Plunger. (11) Retainer. (14) Detent coil. (14) Spool. (15) Pilot supply. (16) Orifice.

When lift lever (2) is moved to the RAISE position, lever assembly (6) moves adjustment pin (9) against the plunger (10). Plunger (10) is forced in the downward direction.




Illustration 5g00950360

Cartridge Assembly

(10) Plunger. (14) Spool. (16) Orifice. (17) Spring. (18) Ball. (19) Retainer. (20) Spring. (21) Hole. (22) Tank port. (23) Control port. (24) Inlet port. (25) Spring. (26) Cavity.

As plunger (10) is moved downward, plunger (10) places a downward force on retainer (19). Moving plunger (10) will increase the force of spring (20). As the force of spring (20) increases, spring (20) starts pushing spool (14) in the downward direction. A path will be opened between inlet port (24) and port (23). Then, a flow of oil will be sent to the main control valve in order to shift the stem. At the same time, spool (14) will close the path to tank port (22).

Once the path to the tank is closed, the oil pressure in port (23) and at the stem begins to increase.

As the control lever is shifted, the further plunger (10) and spring (17) will move spool (14) in the downward direction. This will increase the amount of pilot oil flow to the stem of the main control valve. If the working pressure of the stem against the spring is increased, the movement of the stem will be increased.

At the same time as spool (14) is moving in the downward direction, the pressure of the oil in cavity (26) increases. The trapped oil in cavity (26) is forced upward through orifice (16). The flow of oil through orifice (16) will dampen the movement of the piston in the cartridge assembly. The combination of the oil in cavity (26) and spring (25) increases the stability of the valve.

Once the operator stops moving the lever, the pressure in port (23) is sensed at spool (14). Spool (14) moves up against spring (20). The pilot oil flow is now metered by spool (14) to the stem in the control valve. The desired stem position will be maintained.

The operation of the spool (14) and the spring (20) acts as a pressure reducing valve.




Illustration 6g00950361

Cartridge Assembly

(10) Plunger. (14) Spool. (16) Orifice. (17) Spring. (18) Ball. (19) Retainer. (20) Spring. (21) Hole. (22) Tank port. (23) Control port. (24) Inlet port. (25) Spring. (26) Cavity.

If the operator continues to shift the lever to a detent position, at approximately 85 percent lever travel of plunger (10), ball (18) contacts both plunger (10) and spool (14). This creates a hard joint between plunger (10) and spool (14). The displacement of plunger (15) is equal to the displacement of spool (14). The passage between inlet port (23) and control port (24) will be open. This will cause the oil pressure in control port (23) to increase. The oil pressure at control port (23) will be equal to the oil pressure at inlet port (24). There will be direct movement between plunger (10) and spool (14) .

Pilot Valve In Detent




Illustration 7g00930374

Pilot Valve in the DETENT Position

(6) Lever. (7) Detent plunger. (8) Plate. (9) Adjustment pin. (10) Plunger. (11) Retainer. (12) Detent coil. (14) Spool. (15) Pilot supply. (16) Orifice.

For the 938G Series II Wheel Loader, the pilot valve is equipped with detent coils (13). Detent coils (13) are energized when the key start switch is placed in the ON position. For the IT38G Series II Integrated Toolcarrier, the pilot valve is equipped with detent coils (13). Detent coils (13) are energized when the key start switch is placed in the ON position. The kickout switch must also be activated. When the above conditions are met, detent coils (13) are energized.

When detent coils (13) are energized, the detent coils become electromagnets. When lift control lever (2) is placed at approximately 92 percent of lever travel, retainer (11) is drawn to detent coil (13) by the electromagnetic force. Retainer (11) will be held in place by the electromagnet. When the pilot valve is in the DETENT position, the cartridge assembly will be at equal pressure between inlet port (23) and control port (22). The lift stem in the main control valve will be fully shifted.

Four functions of the pilot valve can have detent coils.

  • TILTBACK

  • RAISE

  • FLOAT

  • AUXILIARY

Both the tilt back and the raise are held in the DETENT position until the control group for the kickouts interrupts the flow of current to the detent coil. The detent coil de-energizes and lever assembly can return to the HOLD position. Detent plunger (7) initiates a force against cup (12) in order to assist the spring in pulling retainer (11) from detent coil (13). Approximately 10 percent lever travel may also be used in order to manually remove the pilot valve from the DETENT position.

Manual control is also used in order to pull lift control lever (3) from the FLOAT position or the auxiliary lever out of the DETENT position.

Reference: For information about the kickouts, refer to the Systems Operations and Testing and Adjusting, "Kickout System" for the machine that is being serviced.

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