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| Illustration 1 | g00558766 |
Control valve (steering) in NEUTRAL position (1) Steering column (2) Centering springs (3) Steering valve body (4) Check valve (5) Spool (6) Sleeve (7) Drive shaft (8) Metering pump (9) External gear (10) Internal gear (11) Line relief valve (12) Line relief valve (13) Main relief valve (14) Port "T" (return oil) (15) Port "P" (pressure oil) (16) Port "R" (right turn) (17) Port "L" (left turn) (18) Makeup valves (19) Check valve | |
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| Illustration 2 | g00558472 |
Typical Example (3) Steering valve (hand metering unit) (14) Port "T" (return oil) (15) Port "P" (pressure oil) | |
Note: Main relief valves (13) are not adjustable on the following machines: 6TL410-UP, 6WL1-UP, 7ML860-UP, 8AL1020-UP, 6ES229-UP, 8SS1-UP and 9PS1-UP.
The steering valve controls the direction of travel by sending oil to the steering cylinder. This type of steering eliminates the need for a mechanical coupling between the steering wheel and tie rods. Oil flow from the steering cooling pump to the steering valve is through port "P" (15). Return oil goes through port "T" (14) to the hydraulic tank.
Oil from the steering valve enters the bottom swivel in order to control the steering cylinder.
The steering valve consists of the following components:
Metering pump (8) - Pump that consists of external gear (9) and internal gear (10). The pump is capable of rotation in either direction.
Sleeve (6) - The sleeve is moved axially.
Two line relief valves (secondary pressure) - The relief valves relieve oil pressure in the lines that connect ports "L" and "R" to the steering cylinder. The valves are set at 22000 kPa (3200 psi).
First check valve - When the machine is steered and the engine stopped, oil flows through this valve from the return line to the metering pump.
Second check valve in the inlet passage - This valve is set to 1200 kPa (175 psi). The check valve prevents air intake if a pressure line breaks.
Two makeup valves - The makeup valves prevent cavitation in the steering system.
Main relief valve (primary pressure) - This valve limits the oil pressure that is coming from the steering pump to 15000 kPa (2200 psi).
Check valve with a pressure relief valve - This valve prevents the wheels from automatically returning to the original position when the sleeve is deflected. The valve also reduces the pressure spikes inside the steering cylinder.
Steering Valve In Neutral Position
When the steering wheel is released, sleeve (6) returns to the NEUTRAL position. This allows the oil flow from the steering cooling pump to go through the check valve, through the return oil passages and to the hydraulic tank. The oil in the other passages of the metering pump does not move. Control passages for left and right turn ports are blocked. Line relief valves (11) and (12) prevent high pressure peaks in the steering system. The pressure peaks can occur during travel.
Steering Valve In Operating Position
When the steering wheel is turned, the steering column (1) is moved. The steering column is splined with drive shaft (7) and connected to metering pump (8). When the steering column rotates, metering pump (8) produces flow through the rotation of internal gear (10) inside external gear (9). The oil pressure in the steering cylinder slows the rotation of the metering pump.
Steering Valve Operation (Left Turn)
When the steering wheel is turned to the left, sleeve (6) inside the steering housing moves in order to allow oil passages in the valve to open. The oil flow goes from the steering cooling pump through port "P" (15) to the internal oil passages. Three oil passages of the sleeve are always connected to the three oil passages of the valve body (3) that lead to the intake area of the metering pump (8) .
Steering Valve Operation (Right Turn)
When the steering wheel is turned to the right, sleeve (6) inside the steering housing moves in order to connect port "P" to the internal oil passages. Oil from the steering cooling pump flows to the intake end of the metering pump. The internal oil passages are connected to port "R". Oil in the metering pump flows in the steering cylinder through port "R". The wheels move in order to make a right turn. Oil in the port "L" passage is connected to port "T". The oil that is forced out of the steering cylinder enters the metering pump through port "L". Oil goes through the return oil passage to port "T" and to the hydraulic tank.
Manual Steering
When the machine is steered manually, the operator must generate the oil pressure that is required to change direction via the metering pump. The metering pump is driven by the rotation of the steering wheel. This creates enough pressure in order to move the steering cylinder. In this instance, a greater effort is required.
Metering Pump
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| Illustration 3 | g00558529 |
Metering pump (8) Metering pump (9) External gear (10) Internal gear | |
Internal gear (10) of metering pump (8) is driven by the steering column. The steering column shaft is attached to the steering wheel by the steering spindle. The internal gear has one tooth less than external gear (9), so that a gap results during rotation. There is a chamber on the opposite side of the gap. The tooth of the internal gear fills the tooth opening of the external gear. The intake areas of the other teeth and chambers are enlarged and the output areas are reduced when the pump rotates. The center of the internal gear carries out a planetary movement in the opposite direction to the rotation of the steering spindle.
Intake areas and output areas revolve at the same rpm as the planetary movement of the internal gear. This requires distribution control at the same time, which is done by the rotation of sleeve (6) .
Control Valve
The control valve is contained in the housing of the steering control valve. In the housing, sleeve (6) rotates and sleeve (6) moves axially. Radial grooves and axial grooves are on the surface of the sleeve. The grooves correspond to passages in the housing. Any rotation of the steering wheel will be transmitted by the steering column (1) through drive shaft (7) to internal gear (10) and external gear (9). The rotation between the steering column and the drive shaft moves the valve sleeve. The axial movement of the sleeve regulates the operating pressure and the movement diverts the flow of oil. The radial movement is affected by the distributor control of the metering pump.
Primary Pressure Relief Valve
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| Illustration 4 | g00558530 |
Primary pressure relief valve (20) Dump port (21) Pressure pin (22) Valve body (23) Spring plate (24) Piston (25) Spring (26) Dump port (27) Orifice | |
The primary pressure relief valve limits the required operating pressure of the steering system in the steering valve to the maximum pressure (18000 kPa (2600 psi)). Higher pressures can damage the system components. When the maximum pressure is reached, the relief valve opens the outlet to the oil line to the hydraulic tank. The relief valve consists of a primary valve and a pilot control valve.
Oil that enters the relief valve goes to the front of piston (24), through orifice (27) to the spring chamber and into valve body (22). The piston is now balanced for pressure and the piston is pushed against the seat by spring (25). Pressure pin (21) will also be pressed against the seat by spring pressure through spring plate (23). This closes dump port (20). When pressure on the larger surface area of the pressure pin increases, the pressure pin will move to the right and open dump port (20). The small amount of oil that is dumped causes the pressure in the large spring chamber to drop. There is a lower pressure in the spring chamber. Therefore, there is a pressure difference that is relative to pressure in the valve. The oil now pushes the piston against the spring. This movement opens dump port (26) to the hydraulic tank.