125 STEERING SYSTEM

The 735/740 trucks use a load sensing, pressure compensated steering system. Minimal horsepower is used by the steering system when the truck is traveling in a straight path. Steering hydraulic horsepower requirements depend on the amount of steering pressure and flow required by the steering cylinders.

These illustrations show some of the main components in the steering system: - HMU (1)

- Steering cylinders (2)

- Steering and fan hydraulic tank (3)

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The steering pump is a variable displacement piston pump and is located under the cab on the right side of the machine. The top illustration shows the steering pump (1) on the 735 truck and the bottom illustration shows the steering pump (2) on the 740 truck.

The pump control valve (3) contains a flow compensator spool and and a pressure compensator spool. 126 127 1 3 3 2

The hydraulic tank (1) is located on the right side of the machine near the cab. The hydraulic tank holds oil for the steering and fan hydraulic systems.

The hydraulic tank contains a return filter that is accessed by removing a cover (2). Also located on the top of the tank is the tank breather (3) and fill cap (4). The oil level sight gauge (not visible) is located on the right side of the machine in front of the right tire.

The bottom illustration shows the steering check and combiner valve (5), which is mounted to the left side of the steering and fan hydraulic tank. The steering check and combiner valve includes two check valves that block oil flow from the primary steering pump into the

secondary steering pump circuit and also block oil flow from the secondary steering pump into the primary steering pump circuit.

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A pressure switch (6) on the steering check and combiner valve is used to monitor the status of the primary steering system pressure. When primary steering pressure is too low, the pressure switch sends a signal to the Transmission/Chassis ECM. The Transmission/Chassis ECM will activate the secondary steering relay, which activates the secondary steering pump.

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The Hand Metering Unit (HMU) (arrow) is mounted to the front of the cab.

The metering unit meters oil from the steering pump to the steering cylinders. The volume of oil that is metered to the steering cylinders depends on the rotational speed of the steering wheel.

The metering unit contains two crossover relief valves to dampen the shock on the steering cylinders. The crossover relief valves are not adjustable.

The steering cylinders are located at the oscillating hitch between the front and rear frames. This illustration shows the right steering cylinder (arrow). The head end of the cylinder is connected to the truck and the rod end of the cylinder is connected to the trailer.

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This illustration shows the steering hydraulic system in the HOLD position.

Oil from the steering pump flows through the check valve in the steering check and combiner valve and to the HMU. A second check valve in the steering check and combiner valve blocks oil from flowing to the secondary steering pump.

In the HOLD position, the spool in the HMU blocks the steering pump oil from flowing to the steering cylinders.

Steering supply oil also flows into a steering signal network through an orifice and the load sense check valve. Oil flows to the HMU and the margin spool in the pump control valve. Signal oil plus the force of the margin spring will determine the pump output.

Signal oil is continually metered through the HMU to the tank in the HOLD position. Steering signal network oil is metered to the tank to keep the pump slightly upstroked in the low

pressure standby condition. The slightly upstroked pump ensures a rapid response to a steering input.

Signal oil from the HMU flows back to the hydraulic tank through the tank return check valve. The tank return check valve maintains a slight back pressure in the signal network.

Maintaining the back pressure will ensure a rapid response from the HMU to a steering input.

M Transmission Pump Drive Steering Pump

Steering And Fan Hydraulic Tank

HMU Steering Pump

Steering Cylinders Crossover Relief Valves Steering Combiner Manifold Steering Pressure Switch Secondary Steering Pump Margin Spool Cutoff Spool Tractor Trailer STEERING SYSTEM HOLD

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This schematic shows the steering system in a RIGHT TURN position.

The steering pump supplies oil to the HMU. The gerotor pump in the HMU rotates with the steering wheel as the operator makes a turn. The gerotor pump transfers oil to the steering cylinders as the pump rotates. Return oil flows from the steering cylinders to the hydraulic tank.

The HMU has an internal crossover valve and an internal check valve for each steering cylinder. The crossover valves and check valves will direct oil from one side of the steering circuit to the other in the event of a pressure spike.

Load sensing pressure is transmitted to the load sensing line through an orifice in the

directional spool of the HMU. The load sensing pressure is proportional to the rotational speed of the steering wheel and the resistance to turn the machine. The load sensing pressure causes the output of the piston pump to increase to match the requirements of the steering system.

M Transmission Pump Drive Steering Pump

Steering and Fan Hydraulic Tank

HMU Steering Pump

Steering Cylinders Crossover Relief Valves Steering Combiner Manifold Steering Pressure Switch Secondary Steering Pump Flow Compensator Spool Cutoff Spool STEERING SYSTEM RIGHT TURN

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Secondary Steering System

The secondary steering pump (1) is located on the right side of the machine next to the engine and is driven by an electric motor (2).

The secondary steering system has three states: - OFF

- Armed - ON

The secondary steering system will be OFF after the machine is powered up. The

Transmission/Chassis ECM will arm the secondary steering system whenever engine speed is over 500 rpm or machine speed is over 8 km/h (5 mph). The Transmission/Chassis ECM will activate the secondary steering relay, which turns on the pump, only after the system is armed and the ECM has detected the loss of primary steering pressure.

The Transmission/Chassis ECM will log a level 3 event and turn on the secondary steering system if the primary steering pressure falls below 700 kPa (100 psi).

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The secondary steering electric drive pump supplies oil to the HMU in the event of a loss in primary steering pressure.

Oil flows from the secondary steering pump through a directional control valve in the pump and a check valve in the steering check and combiner valve to the HMU. A second check valve in the steering check and combiner valve blocks oil from flowing to the primary steering pump. In the HOLD position, the spool in the HMU blocks the secondary steering pump oil from flowing to the steering cylinders.

M Transmission Pump Drive Steering Pump

Steering And Fan Hydraulic Tank

HMU Steering Pump

Steering Cylinders Crossover Relief Valves Steering Combiner Manifold Steering Pressure Switch