STEERING SYSTEM
This section of the presentation explains the operation of the steering system. As on other Caterpillar Off-highway Trucks, the steering system uses hydraulic force to change the direction of the front wheels. The system has no mechanical connection between the steering wheel and the steering cylinders.
• Steering tank
1. Upper sight gauge 2. Lower sight gauge
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The steering tank is located on the right platform. Two sight gauges are on the side of the tank. When the engine is shut off and the oil is cold, the oil should be visible between the FULL and ADD OIL markings of the upper sight gauge (l). When the engine is running and the accumulators are fully charged, the oil level should not be below the ENGINE
RUNNING marking of the lower sight gauge (2). If the ENGINE RUNNING level is not correct, check the nitrogen charge in each
accumulator. A low nitrogen charge will allow excess oil to be stored in the accumulators and will reduce the secondary steering capacity.
A combination vacuum breaker/pressure relief valve is used to limit the tank pressure. Before removing the fill cap, be sure that the engine was shut off with the key start switch and the oil has returned to the tank from the accumulators. Depress the pressure release button (3) on the breather to vent any remaining pressure from the tank.
Supply oil for the steering system is provided by a piston-type pump.
Case drain oil from the pump returns to the tank through the filter (4).
The remaining steering system oil returns to the tank through the main steering filter (5). Both filters are equipped with bypass valves to protect the system if the filters are plugged or during cold oil start-up.
4. Case drain oil filter 5. Main steering filter 3. Combination
vacuum breaker/
relief valve and pressure release button
If the steering pump fails or if the engine cannot be started, the
connector (6) is used to attach an Auxiliary Power Unit (APU). The APU will provide supply oil from the steering tank at the connector (6) to charge the steering accumulators. Steering capability is then available to tow the truck.
The steering oil temperature sensor (7) provides an input signal to the VIMS which informs the operator of the steering system oil temperature.
INSTRUCTOR NOTE: For more detailed information on servicing the steering accumulators, refer to the Service Manual Module "793C Off-highway Truck Steering System" (Form SENR1452) and the Special Instruction "Repair of 4T8719 Bladder Accumulator Group"
(Form SEHS8757). For more information on using the APU, refer to the Special Instructions "Using 1U5000 Auxiliary Power Unit (APU)"
(Form SEHS8715) and "Using the 1U5525 Attachment Group"
(Form SEHS8880).
6. APU supplemental steering connector
7. Steering oil
temperature sensor
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2 4
3
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The 793C is equipped with a load sensing, pressure compensated, piston-type pump (1). The steering pump is mounted to the pump drive.
The pump drive is located on the inside of the right frame rail near the torque converter.
The steering pump operates only when the engine is running and provides the necessary flow of oil to the accumulators for steering system
operation. The steering pump contains a load sensing controller (2) that works with an accumulator charging valve to monitor and control steering pump output.
The steering pump will produce flow at high pressure until the steering accumulators are charged with oil and the pressure increases to
21400 ± 345 kPa (3100 ± 50 psi) at LOW IDLE. This pressure is referred to as the CUT-OUT pressure. When the CUT-OUT pressure is reached, the accumulator charging valve reduces the load sensing signal pressure to the pump load sensing controller, and the pump will destroke to the LOW PRESSURE STANDBY position. During LOW PRESSURE STANDBY, the pressure should be between 2410 and 3445 kPa (350 and 500 psi).
The pump operates at minimum swashplate angle to supply oil for lubrication, leakage and Hand Metering Unit (HMU) "thermal bleed."
Because of the normal leakage in the steering system, the pressure in the accumulators will gradually decrease to 19200 ± 315 kPa (2785 ± 45 psi).
This pressure is referred to as the CUT-IN pressure.
1. Steering pump
• CUT-OUT pressure
• LOW PRESSURE STANDBY 2. Load sensing
controller
• CUT-IN pressure
When the pressure in the accumulators decreases to the CUT-IN pressure, the accumulator charging valve blocks the load sensing signal line to the load sensing controller from returning to the tank, and the pump will upstroke to maximum displacement (full flow).
A pressure tap (3) is located on the pump pressure switch manifold. If steering pump pressure is measured at this tap during LOW PRESSURE STANDBY, a gauge acceptable for testing maximum steering system pressure must be used to avoid damaging the gauge when the steering pump upstrokes to provide maximum oil flow.
Two pressure switches monitor the condition of the steering system on the 793C. One switch (4) monitors the output of the steering pump. The purpose of this switch is to monitor pump supply pressure during LOW PRESSURE STANDBY. The VIMS refers to this switch as the "low steering pressure" switch.
The other steering pressure switch is mounted on the solenoid and relief valve manifold, which is located on the front frame rail below the engine.
This switch monitors the steering system accumulator pressure. The VIMS refers to this switch as the "high steering pressure" switch.
Both steering pressure switches provide input signals to the VIMS which informs the operator of the condition of the steering system. A steering system warning is only displayed if the ground speed is above 8 km/h (5 mph).
4. Low steering pressure switch
4. High steering pressure switch
• Steering pressure warnings only above 8 km/h (5 mph) 3. LOW PRESSURE
STANDBY pressure tap
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5 4
6 2
1 3
Steering pump supply oil flows through a check valve (1) to the solenoid and relief valve manifold (2). The solenoid and relief valve manifold connects the steering pump to the accumulator charging valve (3), the accumulators and the steering directional valve (4). The solenoid and relief valve manifold also provides a path to drain for the steering oil.
When checking the steering system CUT-OUT and CUT-IN pressures, a gauge can be connected at the pressure tap (5).
Steering system oil samples can be taken at the steering system Scheduled Oil Sampling (S•O•S) tap (6).
1. Check valve 2. Solenoid and relief
valve manifold 3. Accumulator
charging valve 4. Steering directional
valve
5. Steering system pressure tap 6. Steering system
S•O•S tap
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2
1
Shown is a closer view of the accumulator charging valve (1). Steering system CUT-OUT pressure is adjusted at the upper valve (2). Steering system CUT-IN pressure is adjusted at the lower valve (3).
Steering pump supply pressure increases until the accumulator pressure acting on the accumulator charging valve shifts the cut-out and cut-in pressure valves. Together, the cut-out and cut-in pressure valves reduce the Load Sensing (LS) signal pressure (feedback pressure) to slightly above tank pressure. The pump is destroked to LOW PRESSURE STANDBY (CUT-OUT).
When the pressure in the accumulators decreases, the cut-in and cut-out pressure valves shift again and block the load sensing signal pressure from the tank. The pump load sensing signal pressure becomes equal to pump pressure, and the steering pump returns to the FULL FLOW position (CUT-IN).
1. Accumulator charging valve 2. CUT-OUT pressure
valve
3. CUT-IN pressure valve
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After the engine is started, pressure increases in the steering accumulators.
The pump load sensing controller is spring biased to vent the actuator piston pressure to drain. Venting pressure from the load sensing controller and the actuator piston positions the spring biased swashplate to
maximum displacement (full flow).
As pressure increases in the accumulators, pump supply pressure is sensed in the accumulator charging valve and on both ends of the flow
compensator. When pressure is present on both ends of the flow
compensator, the swashplate is kept at maximum angle by the force of the spring in the pump housing and pump discharge pressure on the
swashplate piston. The pistons travel in and out of the barrel and maximum flow is provided through the outlet port. Since the pump is driven by the engine, engine rpm also affects pump output.
NOTE: Because the signal lines are sensing pump supply pressure and not a "load" pressure, the steering system does not operate the same as other load sensing systems with a margin pressure.