The spool valves in the main control valve are shifted by pilot pressure. When the operator moves the joystick control, pilot oil is directed to the valve for the implement function that was selected. When the spool valve for an implement is shifted, high pressure oil from the implement pump is allowed to flow to the implement function. The main control valves are equipped with a main relief valve. The main relief valve keeps the pressure from exceeding 24132 kPa (3500 psi).
The spool for the lift function includes a FLOAT position. The float function allows the blade to move along the contour of the ground. The oil in the lift cylinder is not under pressure when the spool valve is in the float position.
824H Wheel Dozer
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| Illustration 1 | g01126187 |
824H Wheel Dozer Main Control Valve (1) Outlet port. (2) Inlet port. (3) Raise. (4) Tilt right. (5) Tip forward. (6) Main relief valve. (7) Lower. (8) Tilt left. (9) Tip back. | |
The 824H Wheel Dozer is equipped with a main control valve with three functions as standard equipment. The functions include a lift circuit, a tilt circuit, and a tip circuit.
825H Soil Compactor
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| Illustration 2 | g01126198 |
825H Soil Compactor Main Control Valve (1) Outlet port. (2) Inlet port. (3) Raise. (4) Tilt right. (6) Main relief valve. (7) Lower. (8) Tilt left. | |
The 825H Soil Compactor is equipped with a main control valve with two functions as standard equipment. The functions include a lift circuit and a tilt circuit. The left side of the soil compactor is equipped with a stabilizer bar.
826H Landfill Compactor
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| Illustration 3 | g01126208 |
826H Landfill Compactor Main Control Valve (1) Outlet port. (2) Inlet port. (3) Raise. (6) Main relief valve. (7) Lower valve. | |
The 826H Landfill Compactor is equipped with a main control valve with a single function as standard equipment. This circuit is the lift circuit.
Control Valve
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| Illustration 4 | g00874787 |
Housing for the Main Control Valve (1) Outlet port. (2) Inlet port. (BB) Return oil. (CC) High pressure oil. | |
High pressure oil from the implement pump flows into the main control valve through inlet port (2). The low pressure oil that has circulated through the main control valve will return to the hydraulic tank through outlet port (1). The housing is used on the main control valves for the 824H Wheel Dozer, the 825H Soil Compactor, and the 826H Landfill Compactor.
Hold Position
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| Illustration 5 | g01126219 |
Control Valve Spool in the HOLD position (10) Work port. (11) Work port. (12) Parallel port. (13) Neutral port. (14) Spool. (15) Cavities. (16) Tank passage. | |
Each section of the main control valve is a control valve. Each valve controls one function of the blade. The functions could be the lift, the tilt, or the tip. The control valve for the lift function operates in a similar manner to the control valve for the tip or the tilt. The lift valve is equipped with the float function.
When the joystick control is in the HOLD position, the high pressure oil flows in the input port of the housing. The oil is directed to the valve section through parallel passage (12).
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| Illustration 6 | g00874799 |
Control Valve Spool in the HOLD Position (12) Parallel port. (13) Neutral passage. (14) Spool. (15) Cavities. (16) Passage to the next control valve. | |
Parallel passage (12) is ported within the valve to neutral passage (13). The high pressure oil flows around the center section of spool (14) and into cavities (15). Oil from cavities (15) will flow together. The oil flows through the passage on the opposite side of the bank valve. There will be a passage in line with neutral passage (12). This passage will be the pressure inlet to the next control valve. If the main control valve has only one bank valve the oil is diverted to tank passage (16) within the housing of the valve. The oil will flow through the outlet port to the hydraulic tank.
Shifted Position
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| Illustration 7 | g01126229 |
Control Valve Spool that is shifted to the Left (10) Work port. (11) Work port. (12) Parallel port. (14) Spool. (16) Tank passage. (17) Passage to the work ports. (18) Load check valve. (19) Pilot oil. | |
When the joystick control is moved from the HOLD position, the solenoid in the pilot valve is energized. Pilot oil (19) is allowed to flow to the end cap. The pilot oil pushes the end of spool (14). The spool shifts to the left. The implement oil that is free flowing through the main control valve and back to the hydraulic tank is blocked.
Implement oil enters the control valve through parallel passage (12). The oil can not flow through the neutral passage. The implement oil is ported behind load check valve (18). The high pressure oil opens the load check valve and the oil flows into passage (17). The high pressure oil follows the path of the arrows to work port (11). The high pressure oil will flow out of work port (11) to the actuator. Illustration 7 shows the flow of oil in the control valve when the spool is shifted.
As the oil flows into the actuator, the actuator moves. The oil that is in the opposite side of the actuator must flow back to the main control valve. The return oil flows into work port (10) around the spool and out through passage (16) to the hydraulic tank.
When the joystick control is placed in the HOLD position, the solenoid control valve shifts and pilot oil (19) flows back to the hydraulic tank. The high pressure oil is allowed to circulate through the main control valve and back to the hydraulic tank. The high pressure oil will continue to flow until the joystick control is moved from the HOLD position.
When the joystick control is placed in the HOLD position, the springs on each side of the spool will center the spool. The high pressure oil will circulate through the main control valve and back to the hydraulic tank.
Float Position
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| Illustration 8 | g01126630 |
Control valve Spool that is shifted to the right (10) Work port. (11) Work port. (14) Spool. (16) Tank passage. (20) Pilot oil. | |
This function is only on the bank valve for the lift. The joystick control is moved from the HOLD position. The solenoid valve in the pilot valve is energized. Pilot oil (20) is directed to the end cap. The pilot oil pushes the end of spool (14). The spool shifts to the right. The spool will be in position for lowering the blade.
The joystick control is moved to the full LOWER position. The spool will shift to the FLOAT position. The increase of pilot pressure at the end cap forces the spool to move past the LOWER position and into the FLOAT position. Illustration 8 shows the flow of oil in the FLOAT position. Work port (10) and work port (11) now has a path to the hydraulic tank through passage (16). The implement oil that is flowing from the implement pump through the main control valve to the hydraulic tank is blocked. The weight of the blade and the lift arms hold the blade to the contour of the ground. The up movement and down movement of the blade is not controlled by the implement hydraulics.