Note: The pilot valve in illustration 1 is for machines with an auxiliary function (side dump bucket, multipurpose bucket, etc). Machines without an auxiliary function will not have auxiliary valve (2).
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| Illustration 1 | g00931344 |
In the pilot hydraulic system, oil pressure is generated in the piston pump for the steering and pilot systems. The oil is directed to both the steering system and the pilot system. The oil that is directed to the pilot system will flow through a manifold and a pressure reducing valve. Next, the hydraulic oil flows through the solenoid valve (hydraulic lockout) for the pilot valve. The hydraulic oil flows through an inlet filter screen in the pilot valve.
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| Illustration 2 | g00931349 |
Note: The pilot valve in Illustration 2 is for machines with an auxiliary function. Machines without an auxiliary function will not have auxiliary valve (10) .
The pilot oil enters the implement joystick at the pilot pressure inlet (6). Illustration 2 shows the position of the pilot pressure inlet, the connector for the return line, and the connectors of the lines which go to the end caps of the main relief valve.
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| Illustration 3 | g00930998 |
The joystick controls the pilot oil. The pilot oil controls the movement of the spools in the main control valve. When the hand lever is moved, lever assembly (13) will rotate. Adjustment pin (14) will push down on the upper plunger (17).
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| Illustration 4 | g00931000 |
The joystick control is equipped with detent coils. When you start the engine, the detent coils are energized by electricity. Four positions can have detent coils.
- TILTBACK
- RAISE
- FLOAT
- AUXILIARY
You must pull the FLOAT and AUXILIARY levers out of the detent position by hand.
When the joystick control is rotated to 90 percent of lever travel, retainer (15) contacts detent coil (20). Detent coil (20) will hold retainer (15). The joystick will be held in position until the kickout switch interrupts the flow of current to the detent coil. The detent coil de-energizes and lever assembly (13) can return to the HOLD position. Plunger (12) initiates a force against cup (16) in order to assist the spring in pulling retainer (15) from detent coil (20) .
On the 924Gz Wheel Loader, when you push the joystick control into the FLOAT position, the pilot oil pressure increases from about 1700 kPa (250.0 psi) to about 2750 kPa (400.0 psi). The increase in pilot oil pressure causes a pressure switch to close. When the pressure switch closes, the float solenoid valve is energized by electricity. Hydraulic oil in the lift circuit drains to the tank. The lift arms lower to the ground.
The flow of pilot oil through the system is controlled by the cartridge assembly. The movement of each spool in the main control valve is determined by the individual cartridge assembly. The location of the cartridge assembly is shown in Illustration 3. The following sections explain the operation of the cartridge assembly.
The Cartridge in the HOLD Position
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| Illustration 5 | g00931358 |
When the cartridge assembly is in the HOLD position, centering spring (18), pressure control return spring (24), and spring (28) keep lever assembly (13) in illustration 3 in the HOLD position.
Pilot pressure oil flows into the joystick control through the port at the bottom of the valve. The pilot oil flows through pilot oil passage (31) in illustration 3. The passage takes the oil to port (29) of the cartridge assembly.
The position of spool (23) blocks port (29). The pilot oil is prevented from flowing into ports (25) and (26). The internal passages are open to return port (25). This drains the pilot lines from the main control valve to return port (25). Return port (25) provides a path back to the hydraulic oil tank. Illustration 5 shows the flow of pilot oil through the cartridge assembly in the HOLD position.
Spring (28) will help to push up the spool. In the HOLD position, oil flows through hole (32) and through the center of spool (23). The oil flows into chamber (33). The oil is held in the chamber until the spool is moved.
The Cartridge with an Activated Spool
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| Illustration 6 | g00931372 |
In order to move a work tool, the flow of oil in the main control valve must be routed to a cylinder. This is accomplished by sending pilot oil from the joystick to the end cap of the implement control valve. When the joystick is moved, lever assembly (13) in illustration 3 causes plunger (17) to move downward. As plunger (17) moves downward, the force on retainer (21) overcomes spring (24) .
As spring (24) compresses, spool (23) moves against spring (28). This moves spool (23) which closes port (25) to the hydraulic oil tank. At the same time, pilot oil supply port (29) opens. The pilot oil flows from pilot oil supply port (29) around spool (23) and into port (26). The oil pressure at port (26) to the control port also increases.
As plunger (17) continues to move down, pressure at port (26) to the control port increases. The spool in the implement control valve starts to move. High pressure oil is sent to the end of the appropriate cylinder.
Once the pilot control lever is shifted, the pilot valve becomes a pressure reducing valve. The pilot valve will maintain a downstream pressure. The pressure is equal to the spring forces above the spool.
At the same time as spool (23) is moving in the downward direction, pressure of the oil in chamber (33) increases. The trapped oil is forced through orifice (27). This helps to dampen the valve. The combination of the oil in chamber (33) and spring (28) increases the stability of the valve.
Cartridge in Ball Lock
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| Illustration 7 | g00931389 |
At approximately 85 percent of lever travel, ball (19) contacts both plunger (17) and spool (23). The opposing force of the spring (24) will no longer have an effect on the pilot pressure. The spring is removed from the force equation. This will equalize the oil pressure between port (28) and pilot supply passage (29).