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| Illustration 1 | g00347318 |
Schematic of Main Control Valve in TILT RIGHT Position (1) Tilt relief valve for pilot system. (2) Dump valve for tilt relief valve. (3) Load check valve for tilt circuit. (4) Tilt valve spool. (5) Pilot valve for relief of blade lift. (6) Dump valve. (7) Load check valve for lift circuit. (8) Makeup valve for rod end of lift cylinders. (9) Makeup valve for head end of lift cylinders. (10) Detents for lift valve spool. (11) Lift valve spool. (12) Shuttle valve. (13) Shuttle stem. (A) Line to dual tilt valve. (B) Line to tilt pilot valve. (C) Line to tilt pilot valve. (D) Inlet for oil from small pump section. (E) Passage to head end of left tilt cylinder. (F) Inlet for oil from large pump section. | |
The main control valve has two spools. One of the spools in the main control valve is for lifting and lowering the blade. The other spool in the main control valve is for tilting and tipping the blade.
Lift valve spool (11) has four manually operated positions. The four positions are RAISE, HOLD, LOWER and FLOAT.
Only the FLOAT position has detents. The lift valve spool must be manually moved from the FLOAT position.
Tilt valve spool (4) is operated by pressure oil from the pilot valve. Tilt valve spool (4) has three positions. The three positions are TILT RIGHT, HOLD and TILT LEFT. None of these positions have a detent.
The tilt circuit receives oil from the small pump section through passage (D). When the circuit is not active, the oil from the small pump section adds to the oil flow of the large pump section. Oil then flows to the lift circuit.
If neither circuit is active, dump valve (6) sends the oil flow to the hydraulic oil tank. The lift circuits and tilt circuits have load check valves and relief valves.
The lift circuit has makeup valves for both the rod ends and head ends of the lift cylinders.
Tilt and Tip Circuit
Tilt valve spool (4) has three positions. The three positions are TILT RIGHT, HOLD and TILT LEFT.
With the help of the dual tilt valve, each position of the tilt spool can perform three blade functions. The three functions of each position include the following items: single tilt, dual tilt and tip .
Reference: For additional information on operation of these functions, refer to the Service Manual module Systems Operation, "Pilot Hydraulic System" for the machine that is being serviced.
Relief Valve for the Tilt and Tip Circuit
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| Illustration 2 | g00362521 |
Tilt Relief Valve (14) Spring. (15) Pilot relief valve. (16) Chamber. (2) Dump valve for tilt relief valve. (17) Spring. (18) Outlet to Hydraulic oil tank. (19) Orifice. (20) Inlet from small section of pump. | |
The relief valve prevents high pressures from damaging components in the tilt circuit and in the tip circuit. The pressure relief valve is part of the main control valve.
Oil from the small section of the pump flows into chamber (20). The oil then flows from chamber (20) through orifice (19) into chamber (16). Dump valve (2) is normally held closed by the force of spring (17) .
When the pressure of the small pump section reaches 24560 ± 50 kPa (3560 ± 10 psi), relief valve (15) opens. The oil in chamber (16) then flows to the hydraulic oil tank through outlet (18) .
When the tilt relief valve opens, the oil pressure in chamber (16) decreases. Now, the oil pressure in chamber (20) is more than the oil pressure in chamber (16) and spring (17) .
Dump valve (2) moves against the force of spring (17) and dump valve (2) opens a passage from chamber (20) to outlet (18). The oil from the small section of the pump flows through this passage to the hydraulic oil tank.
When the oil pressure drops, relief valve (15) is closed by the force of spring (14) and dump valve (2) is closed by the force of spring (17) .
Lift Circuit
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| Illustration 3 | g00347320 |
Schematic of Main Control Valve in HOLD Position (5) Pilot valve for relief of blade lift. (6) Dump valve. (7) Load check valve for lift circuit. (8) Makeup valve for rod end of lift cylinders. (9) Makeup valve for head end of lift cylinders. (10) Detents for lift valve spool. (11) Lift valve spool. (12) Shuttle valve. (13) Shuttle stem. (21) Spring. (22) Spring. (23) Outlet to hydraulic oil tank. (24) Passage. (F) Inlet for oil from large pump section. (G) Chamber. (H) Passage to rod end of lift cylinders. (I) Passage to head end of lift cylinders. | |
Lift valve spool (11) has four positions. The four positions are RAISE, HOLD, LOWER and FLOAT.
Lift valve spool (11) is manually operated by the control lever. Movement of the control lever to the rear sends pressure oil to the rod end of the lift cylinders and the blade raises.
When the control lever is moved forward, pressurized oil is sent to the head end of the lift cylinders. This causes the blade to lower.
When the operator pushes the lever past the LOWER position, the lever will go into the FLOAT position. The FLOAT position is the only position with a detent.
In the FLOAT position, the blade is free to move up or down according to the outside forces.
Oil from the large section of the pump flows through inlet (F). The oil then flows into chamber (G). When the tilt circuit is not used, the oil mixes with the oil from the tilt circuit.
HOLD Position
When the valve spool is in the HOLD position, the pump oil pressures for the small pump section and the large pump section are kept at approximately 700 kPa (100 psi).
The force of spring (22) keeps dump valve (6) closed. With lift valve spool (11) in HOLD position, the pump oil in chamber (G) can not open load check valve (7). This causes the pressure of the pump oil to increases.
When the pressure of the pump oil in passage (24) reaches approximately 700 kPa (100 psi), the pump oil has more force than spring (22) and the pump oil moves dump valve (6) .
When the pump oil moves dump valve (6), the pump oil flows through holes in the valve. Next, the oil flows into outlet (23). The oil then flows into the hydraulic oil tank.
When the engine is running and lift valve spool (11) is in the HOLD position, dump valve (6) keeps the pressure of the pump oil in chamber (G) at approximately 700 kPa (100 psi).
The pump oil can move dump valve (6) because any oil in the chamber for spring (22) can flow through a line into shuttle stem (13) .
The oil flows from the shuttle stem through passage (24), shuttle valve (12), and grooves in lift spool (11). The oil then flows into outlet (23) to the hydraulic oil tank.
RAISE Position
When lift valve spool (11) is moved to the RAISE position, the pressure of the pump oil in chamber (G) increases and opens load check valve (7) .
When load check valve (7) opens, the spring in dump valve (6) has more force than the pump oil in chamber (G) .
The spring moves dump valve (6) which closes the holes in the dump valve. Now, the pump oil can not flow through the dump valve to the hydraulic oil tank.
The pump oil flows through load check valve (7). Next, the oil flows around lift valve spool (11) and into passage (H). The oil then flows to the rod end of the lift cylinders.
The pump oil in passage (H) also flows through shuttle valve (12) and shuttle stem (13). The oil then flows through a line into the spring chamber of dump valve (6) .
Now, the pump oil pressure is in dump valve (6) and against pilot valve (5). Dump valve (6) and pilot valve (5) act as the relief valve for the lift circuit.
The rod end of the lift cylinders are connected to the blade. The pump oil from passage (H) in the rod end of the lift cylinders moves the pistons and rods farther into the cylinders and the rods lift the blade.
The oil that is pushed from the head end of the lift cylinders flows into passage (I). The oil then flows into the main control valve, and through outlet (23) to the hydraulic oil tank.
LOWER Position
When lift valve spool (11) is moved to the LOWER position (no illustration), the pump oil in chamber (G) opens load check valve (7). The oil then flows into passage (I) to the head end of the lift cylinders.
The pump oil in passage (I) also flows to shuttle valve (12). This causes the ball to move so that the pump oil can flow to shuttle stem (13) .
The pump oil from the shuttle valve flows into the spring chamber of dump valve (6). Valves (5) and (6) are now the relief valve for the circuit.
The pump oil in the head end of the lift cylinders causes the pistons and rods that are connected to the blade to move farther out of the lift cylinders. This causes the blade to move downward.
The oil that is pushed from the rod end of the lift cylinders flows through passage (H). The oil then flows into outlet (23) to the hydraulic oil tank.
FLOAT Position
Lift valve spool (11) is held in the FLOAT position by the detents on the end of the spool (no illustration). This causes load check valve (7) to open.
The pump oil in chamber (G) flows through open load check valve (7) into passage (I). The pump oil from passage (I) then flows to the head end of the lift cylinders. The pump oil from passage (I) also flows into outlet (23) to the hydraulic oil tank.
The weight of the blade causes the blade to move down.
The oil from the rod end of the lift cylinders flows through passage (H) into outlet (23). The oil then flows to the hydraulic oil tank.
When an outside force moves the blade upward, the pressure of the oil in the rod end of the lift cylinders decreases. This causes the rods to move into the lift cylinders.
The pressure of the oil in the rod end circuit and in passage (H) is less than the pressure of the oil in outlet (23). The higher pressure of the oil in outlet (23) causes makeup valve (8) to open. Makeup valve (8) opens in order to fill the rod end circuit. Makeup valve (8) only opens when oil is needed.
The control lever must be moved away from the FLOAT position in order to move lift valve spool (11) .
Dump Valve
Dump valve (6) has four functions:
- Valve (6) dumps the pump flow when the control valve spools are in the HOLD position.
- Valve (6) acts as a pressure compensator valve for the implement pump.
- Valve (6) provides control of oil flow.
- Valve (6) acts as a relief valve for the main hydraulic system.
When the lift valve spool is in the HOLD position, there is pressure behind dump valve (6) due to the force of spring (22). The pressure that exists behind dump valve (6) equals the pressure in the hydraulic oil tank plus 700 kPa (100 psi).
When the blade is raised or lowered, the pressure behind dump valve (6) will be equal to the pressure that is in the rod end or the head end of the lift cylinders. The pressure exists in the end of the cylinder that is receiving pump flow at the time.
The pressure is felt through shuttle valve (12) and shuttle stem (13). When the cylinder pressure plus the spring force is behind dump valve (6), the pump pressure is raised to about 700 kPa (100 psi) above the cylinder pressure (pressure compensation).
Flow control and fine modulation is possible because of the constant 700 kPa (100 psi) pressure behind dump valve (6) .
When lift valve spool (11) is in either the RAISE or the LOWER position, the pump oil flows through passages to shuttle valve (12) .
The pump oil that flows through the shuttle valve flows through shuttle stem (13) and into the spring chamber of dump valve (6) .
When the pump pressure in the chambers is equal at both ends of dump valve (6), the pressure of the pump oil is less than the pressure setting of relief valve (5) and dump valve (6) does not move.
Note: Dump valve (6) can not be adjusted.
When the pump oil is at approximately 22750 kPa (3300 psi), the force of the pump oil is greater than the force of the spring in relief valve (5) .
The high pressure of the pump oil in the spring chamber opens pilot valve (5) and flows into outlet (23) to the hydraulic oil tank.
When the oil pressure in the spring chamber of dump valve (6) is less than the pressure of the pump oil, dump valve (6) will open.
The pump oil in chamber (G) flows through the open dump valve into outlet (23) to the hydraulic oil tank. The pressure of the pump oil can not increase to more than 22750 kPa (3300 psi).
Makeup Valves
If the pressure of the pump oil becomes less than the pressure of the hydraulic oil tank, makeup valves (8) and (9) will function.
Pump oil from inlet (F) opens load check valve (7) and flow to lift valve spool (11). When lift valve spool (11) is out of the HOLD position, pump oil flows through passage (H) or (I). The oil then flows into the lift cylinders.
When the pressure of the pump oil is less than the pressure of the hydraulic oil tank, makeup valve (9) will open so that the pressure of the hydraulic oil tank can add to the pressure of the pump oil.
This pressure prevents cavitation in the main circuit. Makeup valve (8) is used when pump oil is in passage (H). Makeup valve (9) is used when pump oil is in passage (I) .
Load Check Valves
The load check valves in the main control valve prevent reverse oil flow in the cylinders, which can cause cylinder drift. Load check valve (3) is for the tilt cylinder. Load check valve (7) is for the lift cylinder.
For example, when lift valve spool (11) is in the LIFT position, pump oil opens load check valve (7). The oil then flows through passage (H) to the rod end of the lift cylinders.
If the pump pressure decreases, the pressure of the oil in passage (H) and the pressure of the spring for the load check valve closes load check valve (7) .
Load check valve (7) holds the oil in the rod end of the cylinders. This prevents any movement of the lift cylinders.
Note: Load check valves (3) and (7) have the same operation.