Steering Control System
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| Illustration 1 | g00900665 |
(1) Accumulator charging valve (2) Line to braking system (3) Line to implement system (4) Check valve (5) Metering pump (steering) (6) Shock relief valves (7) Steering signal relief valve (8) Hydraulic tank (9) Steering selector valve (10) Rear steering cylinder (11) Rear axle (12) Front steering cylinder (13) Front axle (14) Main hydraulic pump (15) Priority valve (16) Signal relief valve (17) Regulator valve (18) Load sensing line | |
Hydraulic pump (14) is a fixed displacement pump. The pump supplies oil to accumulator charging valve (1) from hydraulic tank (8). Priority valve (15) in the accumulator charging valve directs the pump oil to metering pump (5), braking system (2) and implement system (3). Priority flow is given to the steering system and the braking system. Oil flow is delivered to the steering system and to the braking system with equal priority. Excess flow is directed to the implement system. When the steering wheel is not turning or when the brakes are not being applied, most of the oil is directed to the implement system.
When the brake accumulator has been fully charged, regulator valve (17) opens in order to allow a small quantity of oil to flow to metering pump (5) and hydraulic tank (8) through load sensing line (18). When the steering wheel is turned this oil also acts on the left side of priority valve spool (15) in response to a signal from the metering pump. The priority valve spool is kept to the right in order to allow full oil flow to the metering unit and the braking accumulator. The flow of oil through the load sensing line also allows the oil in the system to warm up, resulting in a quicker steering response time when the steering wheel is turned.
Check valve (4) prevents pressurized oil from flowing back into the pump line from the steer cylinders. Relief valve (7) protects the system against externally created pressure spikes by relieving the excess pressure to the hydraulic tank. The relief valve also limits the system pressure during operation. Shock valves (6) are installed in order to reduce the effect of shocks that are transmitted from the road wheels. The shock valves are direct acting valves that react very quickly.
Steering mode selector valve (9) is a three-position solenoid operated valve which controls the rear axle steering cylinder. A manifold block connects metering pump (5) to steering mode selector valve (9) .
Steering in Hold
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| Illustration 2 | g00832878 |
(1) Accumulator charging valve (2) Line to braking system (3) Line to implement system (4) Check valve (5) Metering pump (steering) (6) Shock relief valves (7) Steering signal relief valve (8) Hydraulic tank (9) Steering selector valve (10) Rear steering cylinder (11) Rear axle (12) Front steering cylinder (13) Front axle (14) Main hydraulic pump (15) Priority valve (16) Signal relief valve (17) Regulator valve (18) Load sensing line | |
When the steering is in the HOLD position, there is no load sensing signal from the metering unit. Pressure increases in the line from the metering pump to the accumulator charging valve. The pressure increase is felt on the right side of priority valve spool (15). The priority valve spool moves to the left and most of the oil flow from the main pump is directed to the implement system.
Right Turn in Two-Wheel Steer Mode
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| Illustration 3 | g00832912 |
(1) Accumulator charging valve (2) Line to braking system (3) Line to implement system (4) Check valve (5) Metering pump (steering) (6) Shock relief valves (7) Steering signal relief valve (8) Hydraulic tank (9) Steering selector valve (10) Rear steering cylinder (11) Rear axle (12) Front steering cylinder (13) Front axle (14) Main hydraulic pump (15) Priority valve (16) Signal relief valve (17) Regulator valve (18) Load sensing line | |
When the steering mode switch in the cab is turned fully to the right to the two-wheel steer mode, the solenoids on mode selector valve (9) are de-energized. The valve spool is then centered by spring force, and the ports that connect rear steering cylinder (10) are blocked. Oil that is locked into the cylinder by the valve spool holds the rear wheels in the straight ahead position and the rear axle operates as a rigid axle.
Note: In order to properly operate the machine in two-wheel steer mode, the rear wheels must be in the straight ahead position. If the steering mode switch is returned to two-wheel steer without first aligning the rear wheels back to the straight ahead position, then the rear wheels will remain locked in the turned position. If this situation occurs then the steering should be realigned before the machine is operated in two-wheel steer mode.
When a right turn is made in two-wheel steer mode, oil flows through metering pump (5) to the right side of front steering cylinder (12). Oil that enters the cylinder causes the tie rods to move to the left. This causes the wheels to move to the right. Oil in the left side of the steering cylinder flows to tank (8) through mode selector valve (9) and metering pump (5) .
Note: The wheels will turn faster when the steering wheel is turned faster.
If the steering cylinder reaches the end of the stroke and the operator continues to turn the steering wheel, pressure in the steering system will increase. The increase in pressure causes signal relief valve (7) in metering pump (5) to open and excess signal oil is directed to the tank in order to relieve the pressure.
Note: If the engine stops and the machine is in motion, the machine can still be steered. When the steering wheel is turned in this situation, the metering section of metering pump (5) acts as a pump.
Left Turn in Circle Steer Mode
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| Illustration 4 | g00832933 |
(1) Accumulator charging valve (2) Line to braking system (3) Line to implement system (4) Check valve (5) Metering pump (steering) (6) Shock relief valves (7) Steering signal relief valve (8) Hydraulic tank (9) Steering selector valve (10) Rear steering cylinder (11) Rear axle (12) Front steering cylinder (13) Front axle (14) Main hydraulic pump (15) Priority valve (16) Signal relief valve (17) Regulator valve (18) Load sensing line | |
When a left turn is made in circle steer mode, oil flows from metering pump (5) through mode selector valve (9) to the left side of rear steering cylinder (10). Oil that is displaced from the right side of the rear steering cylinder flows through steering selector valve (9) to the left side of front steering cylinder (12) .
Oil that is displaced from the right side of the front steering cylinder flows through metering pump (5) to tank (8). The front cylinder rods move to the right. This causes the front wheels to pivot to the left. The rear cylinder rods move to the right. This causes the rear wheels to pivot to the right.
Right Turn in Crab Steer Mode
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| Illustration 5 | g00832945 |
(1) Accumulator charging valve (2) Line to braking system (3) Line to implement system (4) Check valve (5) Metering pump (steering) (6) Shock relief valves (7) Steering signal relief valve (8) Hydraulic tank (9) Steering selector valve (10) Rear steering cylinder (11) Rear axle (12) Front steering cylinder (13) Front axle (14) Main hydraulic pump (15) Priority valve (16) Signal relief valve (17) Regulator valve (18) Load sensing line | |
Note: In crab steer, the front wheels and the rear wheels turn in the same direction. Crab steer allows the machine to move in a sideways manner for maneuverability in confined areas.
When the steering mode switch in the cab is turned fully to the left, and the crab steer mode is selected, the solenoid for the crab steer on steering selector valve (9) is energized. The selector valve spool is offset to the crab steer position.
When the steering wheel is turned clockwise in crab steer mode, oil flows through metering pump (5) to the right side of front steering cylinder (12). Oil that is displaced from the left side of the front steering cylinder flows through the mode selector valve to the left side of the rear cylinder (10) .
Return oil from the right side of the rear cylinder flows to tank (8) through mode selector valve (9) and metering pump (5). The front steer cylinder rods move to the left, and the rear cylinder rods move to the right. This causes the front wheels and the rear wheels to pivot to the right. All the wheels are now turned in the same direction and all wheels are steered.