124
This illustration shows the components in the right side implement control valve. The control valve is accessed from the articulation hitch. The following is a list of the components.
- Inlet port for the right fixed displacement implement pump (1) - Tilt rod end port (2)
- Tilt load check valve (3) - Check valve (4)
- Tilt head end port (5) - Check valve (6) - Lift rod end port (7) - Lift head end port (8) - Check valve (9)
- Makeup and vent valve (10) - Lift load check valve (11) - Hydraulic tank port (12)
2 3 4 5 6 7 8 9 10 11 12
125
This illustration shows the back side of the right implement control valve. Port (1) is the inlet connection for the center fixed displacement piston pump.
126
The 994F Wheel Loader main implement system is made up of two control valves, the right control valve and the left control valve. The above illustration shows the right side control valve in the float position including the makeup and vent valve. The makeup and vent valve is shown in both the makeup and float operation.
In the makeup operation, the pressure in the hydraulic tank exceeds the pressure in the rod end of the lift cylinder. Lowering the bucket faster than the pump can fill the rod end of the lift cylinder the piston displacement causes a vacuum in the rod end of the lift cylinders. The makeup valve allows oil from the tank line to flow into the rod end of the lift cylinders and fill the void.
In the float operation, the makeup and vent valve allows the oil that is holding the vent valve against the seat to flow through the float sequence valve (not shown) to the hydraulic tank. The small orifices in the base of the vent valve (in the left control vent valve) allows lift cylinder rod end pressure, along the with the spring force, to keep the makeup and vent valves for both the right and left control valves to remain seated. Both the left and right control valve makeup and vent valve spring chambers are connected. Once the oil that is behind the float sequence is opened to tank, the oil flows through the float sequence valve (not shown). The pressure in the spring chamber drops to tank pressure.
From Head End cylinder To Rod End Cylinder To Hydraulic Tank To Tank From Rod End Cylinder From Tank MAKEUP To Rod End Cylinder To Float Sequence Valve FLOAT A1 A3 A2
From Center Fixed Displacement Pump
The pressure of the supply oil that is flowing to the rod end of the lift cylinders acts against the vent valve and the force overrides the spring and the valve opens. When the vent valve shifts to the OPEN position and the supply oil flows to the hydraulic tank. At that time, both ends of the lift cylinders will be open to the tank. This will allow the bucket to follow the contour of the ground.
With the differential areas of the makeup and vent valve, the force that is produced by the lift rod end pressure is enough to move the valve off the seat. When the vent valve moves off the seat, the oil from the implement pumps flows past the vent valve to the hydraulic tank. Both ends of the lift cylinders are open to the hydraulic tank allowing the bucket to float along the contour of the ground.
With the make up and vent valve used in the float position, the following explanation describes how the forces with in the make up and vent valve effect the float operation.
A1 = The Tank Pressure Effective Area, A2 = The Lift Rod End Effective Area, and A3 = The Spring Chamber Effective Area.
When the float sequence valve (not shown) is blocked: (Tank Pressure)*A1 + (Lift Rod End Pressure from the right control valve)* A2 is less than the Spring Force + (Lift Rod End Pressure connected from the left control valve)*A3. The vent valve stays seated.
When the float sequence valve (not shown) is in the OPEN position: (Tank Pressure)*A1 + (Lift Rod End Pressure from the right control valve)* A2 is greater than the Spring Force + (Tank Pressure opened through the float valve) *A3. The vent valve moves off the seat and opens the lift rod end pressure to the implement hydraulic tank.
127
This illustration shows the components in the left side implement control valve. The control valve is accessed from the articulation hitch. The following is a list of the components.
- Inlet port for the left fixed displacement implement pump (1) - Check valve (2)
- Lift head end drain port (3) - Tilt load check valve (4) - Relief and makeup valve (5) - Tilt head end port (6)
- Tilt rod end port (7)
- Relief and makeup valve (8) - Lift rod end port (9)
- Lift head end port (10) - Makeup and vent valve (11) - Relief and makeup valve (12) - Lift load check valve (13) - Hydraulic tank port (14)
5 6 7 8 9 10 11 12 13 14
128
This illustration shows the back side of the left implement control valve. Port (1) is the inlet connection for the center variable displacement piston pump.
129
The 994F Wheel Loader implement system is made up of two control valves, the right control valve and the left control valve. The above illustration shows the left side control valve in the float position including the makeup and vent valve. The makeup and vent valve is shown in both the makeup and float operation.
Lowering the bucket faster than the implement pump can fill the rod end of the lift cylinder causes a void in the rod end of the lift cylinders. In the makeup operation, the pressure in the hydraulic tank side of the vent valve exceeds the pressure in the rod end of the lift cylinder. The vent valve moves upward and allows oil from the tank line to flow into the rod end of the lift cylinders and fill the void.
In the float operation, the oil that is holding the vent valve against the seat is allowed to flow through the float sequence valve (not shown) back to the hydraulic tank. The small orifices in the base of the vent valve allows lift cylinder rod end pressure, along with the spring force, to keep the makeup and vent valves for both the right and left control valves to remain seated. Both the left and right control valve makeup and vent valve spring chambers are connected. Once the oil behind the float sequence valve (not shown) is open to the hydraulic tank, the pressure in the spring chamber drops to tank pressure. With the differential areas of the makeup and vent valve, the force that is produced by the lift rod end pressure is enough to move the valve off the seat.
From Center Variable Displacement Pump
From Head End cylinder To Rod End Cylinder To Tank From Rod End Cylinder From Tank MAKEUP To Rod End Cylinder To Float Sequence Valve FLOAT Additional Stem Lift Stem
Makeup and Vent Valve A1
A3
When the vent valve moves off the seat, the oil from the implement pumps flows past the vent valve to the hydraulic tank. Both ends of the lift cylinders are open to the hydraulic tank allowing the bucket to float along the contour of the ground.
With the make up and vent valve used in the float position, the following explanation describes how the forces within the makeup and vent valve effect the float operation.
A1 = The Tank Pressure Effective Area, A2 = The Lift Rod End Effective Area, and A3 = The Spring Chamber Effective Area.
When the float sequence valve (not shown) is blocked: (Tank Pressure)*A1 + (Lift Rod End Pressure from the left control valve)* A2 is less than the Spring Force + (Lift Rod End Pressure connected through the orifice and connected to the right control valve vent valve spring
chamber)*A3. The vent valve stays seated.
When the float sequence valve (not shown) is in the OPEN position: (Tank Pressure)*A1 + (Lift Rod End Pressure from the right control valve)* A2 is greater than the Spring Force + (Tank Pressure open through the float valve) *A3. The vent valve moves off the seat and opens lift rod end pressure to the implement hydraulic tank.
130
The left control valve on the 994F Wheel Loader is now equipped with an additional stem that uses pilot oil pressure from the lower pilot valve to control when the stem will be shifted. When the lower pilot oil reaches the required pressure, the force that is developed by the pilot oil pressure will override the springs on the right side of the additional stem. The additional stem will start to shift to the right. Return oil from the head end of the lift cylinders will be allowed to flow around the additional stem through the passage in the valve to the hydraulic tank. This will help to decrease the lower cycle time by approximately 10%.
From Center Variable Displacement Pump
From Head End cylinder To Rod End
Cylinder Additional Stem
131
This schematic shows the hydraulic flow with the control levers in the HOLD position. When the engine is running, pilot oil that flows from the pilot pump through the pilot relief valve and is blocked at the pilot control valve. At the same time, oil from the pilot pump flows through the tilt selector valve and lift selector valve. With no signal pressure at either end of the selector valves, the selector valves are in the CENTER position. The pilot oil that is
flowing out of the selector valves passes through the orifices. Then, the oil flows back through the pilot control valves to the hydraulic tank.
The three fixed displacement piston pumps and the variable displacement piston pump draw oil from the hydraulic tank. Each implement pump directs system oil through the individual high pressure screens, past the individual relief valve, over the individual check valve and supply hydraulic oil to the main control valve. The main control valve is an open-centered valve. With no pilot control valve movement, the open-center valve directs the supply oil through the main control valve and returns the oil to the hydraulic tank. The solenoid valve for the variable displacement piston pump is de-energized and the signal oil to the pump is blocked. At this time, the variable displacement piston pump is upstroked and supplying oil to the hydraulic system. Pilot Oil Filter Pilot Pump Pilot Relief Valve Check Valve Check Valve Selector And Pressure Control Valve Tilt Selector Valve Pilot Control Valve
Lower Kickout Cushion Solenoid Valve Float Sequence Valve