Section 8. Theory of Operation Steering System
Steering System System Components •
Steer Orbitrol – Hydraulically couples operator request (from steer tiller) to the steering axle.
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Pump Power Amplifier (PPA) – Controls the speed of the pump motor. Receives input from the brake switch (SW6) or accelerator potentiometer (P2), VM, and TPA via the CAN-Bus.
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AC Pump Motor – Drives the hydraulic pump.
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Priority Valve – Routes hydraulic fluid from the pump to the steering, lift, and auxiliary systems. Ensures the steering system receives priority over lift and aux functions.
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Hydraulic Pump – Provides fluid to the Steer Orbitrol via the Priority Valve.
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Steer Potentiometer (P1) – Provides steer wheel position feedback to the TPA Master. P1 also provides input to the steer wheel position indicator on the Operator Display.
NOTE: Steer Potentiometer (P1) is present on dual traction motor trucks only.
System Overview The steering system is a hydraulically activated system. A steer request is generated when the operator rotates the steering wheel in either a clockwise or counterclockwise direction. The steering system uses the same hydraulic fluid as the lift and auxiliary systems by means of a priority valve. Because steering takes priority over all other hydraulic functions, the priority valve ensures that sufficient oil flow to the steering system is always present.
Model 4700 Maintenance Manual System Components The pump motor starts when travel is requested, providing hydraulic flow to the steering system. Depressing the service brake pedal also causes the pump motor to run.
Functional Description A forward or reverse travel request is received by the VM and is encoded into a digital message that is sent to the PPA. The PPA generates the correct voltage to the pump motor to run the pump. When the Service Brake pedal is depressed, SW6 closes and GND is removed from the VM. The VM encodes this into a digital message that is sent to the PPA. The PPA generates the rpm request to the pump motor to run the pump. The priority valve ensures adequate hydraulic pressure to the steering system at all times, regardless of other hydraulic demands on the system. When the pump motor starts, fluid is delivered to the orbitrol through port CF of the priority valve. As pressure builds in the steering system, the pilot moves the priority valve against spring tension, dividing the fluid flow and delivering a restricted flow to the steering system (through CF) and an unrestricted flow to the lift and auxiliary functions (through EF). When the orbitrol is rotated, the pressure at LS increases, assisting the spring to move the priority valve toward the steering only position. Fluid is directed to the steer cylinder. Steer Position Feedback is provided to the TPA from the Steer Pot (P1) located on the right side steer axle.
Fluid capacity is checked by the Priority Valve. Excess oil is sent to the Control valve for other functions. When the steering wheel is turned, the orbitrol measures a volume of oil proportional to the movement of the steering wheel and sends it to the steer cylinder.
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Publication 1160093, Issued: 20 May 2013
