Introduction

Steering System Schematic
(1) Steering cylinders. (2) Steering control unit (SCU). (3) Steering pump. (4) Strainer. (5) Hydraulic oil tank. (6) Left turn line. (7) Right turn line. (8) Hydraulic oil filter. (9) Hydraulic oil cooler. (10) Charge oil line to propulsion and vibration systems. (11) Check valve. (12) Return manifold.

The main components of the steering system are steering cylinders (1), steering control unit (SCU) (2), steering pump (3), hydraulic oil tank (5), hydraulic oil filter (8), hydraulic oil cooler (9) and return manifold (12).

Hydraulic Oil Tank Location
(5) Hydraulic oil tank.

Hydraulic oil tank (5) provides hydraulic oil for the steering, propulsion and vibration systems. The hydraulic oil tank is located behind the operator's platform.

Under Platform
(3) Steering pump.

Steering pump (3) pulls oil from hydraulic tank (5). The steering pump is driven off of the back of the tandem propulsion pumps. The propulsion pumps are driven by the pump drive group on the rear of the engine. The steering pump is located under the operator's platform.

Steering Control Unit Location
(2) Steering control unit (SCU). (13) Console.

The oil flows from steering pump (3) to steering control unit (SCU) (2). The steering control unit is located at the bottom of the steering column just under console (13). The steering control unit is closed center and load sensing.

Steering Cylinder Location
(1) Steering cylinder.

When the steering wheel is turned, two double acting steering cylinders (1), receive oil from steering control unit (2) so that the compactor can make the desired turn. When the steering wheel is not being turned, oil goes through the steering pump to maintain pressure at the steering control unit. There is a small amount of oil required to make up for losses through the bleed orifice in the steering pump. With the steering control unit in the spring centered position, no oil is ported to the steer cylinders.

Hydraulic Oil Filter Location
(8) Hydraulic Oil Filter.

If hydraulic oil filter (8) becomes plugged a pressure drop is created across the hydraulic oil filter. When the pressure drop reaches 350 kPa (50 psi), the filter bypass valve opens and oil flows through the filter bypass valve to spring loaded check valve (11), which is part of the parking brake and speed shift valve. When the pressure reaches 1220 kPa (175 psi) in the passage between the bypass and check valves, the spring loaded check valve opens and the oil flows to the hydraulic tank. The spring loaded check valve prevents the brake pressure from ever dropping below 1550 kPa (225 psi). A pressure drop of 172 kPa (25 psi) shows red on the visual bypass indicator, which means the hydraulic oil filter should be changed.

Steering Pump

Steering Pump Cross Section

Steering Pump Cross Section
(1) Priority valve spool. (2) Pilot load sensing signal port. (3) Bleed orifice. (4) Charge outlet port. (5) Unloading relief valve. (6) Steering pump gears.

The steering pump is a fixed displacement gear pump driven off of the back of the tandem propulsion pumps. The steering pump supplies oil to the steering, propel and vibratory systems. Within the pump is a priority valve which is a pressure compensated flow divider. The priority valve divides the flow between the steering circuit and the charge circuit. The steering circuit has priority, and the return flow from the steering circuit goes back into the charge circuit.

The steering pump causes the relief pressure to go into the charge circuit. There is an unloading relief valve in the steering circuit that empties into the inlet of the steering pump. The unloading relief valve is the pressure relief for the steering cylinders.

Pressure directly from steering pump gears (6) acts against the lower end of priority valve spool (1) going through bleed orifice (3) and the passage in the priority valve spool. The oil pressure pushes the priority valve spool up, to allow the oil to flow to charge outlet port (4).

Pilot load sensing signal port (2) from the steering control unit acts against the upper end of the priority valve spool in conjunction with the spring pressure. The pilot load sensing signal also acts against unloading relief valve (5). The unloading relief valve empties into the pump inlet and is the pressure relief valve for the steering cylinders.

The bleed orifice serves as a flushing passage for the oil and puts pressure on the bottom of the priority valve spool. The bleed orifice minimizes the priority valve spool being pushed against each stop which can occur during changes in the steering load. During operation, the priority valve spool will oscillate.

Operation (No Turn)

The steer load pilot signal from the steering control unit is basically charge pressure. The force on the bottom of the priority valve spool by the gear pump outlet pressure is sufficient to overcome the force exerted on the top of the priority valve spool by charge pressure and spring force. The priority valve spool moves up and will not allow oil to go to the steering cylinders.

Operation (While Turning)

In a turning condition, the load sensing pilot signal which was once charge pressure increases to a maximum of 18 950 kPa (2750 psi). The oil pressure and spring force exerted on the top of the priority valve spool pushes the spool down, and sends the porting oil to the steering pump outlet. The oil coming out of the steering pump outlet flows to the steering control unit and to the appropriate end of the steering cylinders.

Steering Control Unit (SCU)

Steering Control Unit
(1) Outlet port. (2) Check valve (not shown). (3) Inlet port. (4) Internal pump gear. (5) External pump gear. (6) Spool. (7) Centering springs. (8) Cross pin. (9) Left turn port. (10) Right turn port. (11) Sleeve. (12) Body. (13) Drive. (A) Control section. (B) Metering section.

The steering control unit (SCU) is a spring centered (no turn), load sensing type. The charge oil flows through the port of the steering control unit and out to the load sensing port.

The steering control unit (SCU) has two main sections. They are control section (A) and metering section (B). The two sections are connected inside the unit, both hydraulically and mechanically. These two sections work together to send oil to the steering cylinders.

Oil goes through inlet port (3) into control section (A). When the steering wheel is turned, the control section sends oil to from metering section (B). Metered oil from section (B) is directed by control section (A) to either left turn port (9) or right turn port (10).

Metering section (B) is a small hydraulic pump. It produces a specific amount (metered) of oil flow. This metered oil is then directed by control section (A) to the steering cylinders.

The direction and the speed of a turn are controlled by the steering control unit. As the steering wheel is turned faster, there is an increase in the flow of oil from the steering control unit to the steering cylinders. This increased flow causes the cylinders to move farther and faster, which turns the machine faster.

Oil Flow

Oil flows through inlet port (3). When the steering wheel is stationary (NEUTRAL), the oil is stopped by spool (6). The oil cannot flow through the steering control unit to the steering cylinders until the steering wheel is turned.

The steering wheel is connected to spool (6) by a shaft assembly and splines. When the steering wheel is turned, spool (6) turns a small amount until centering springs (7) are compressed. Then, sleeve (11) starts to turn. As long as the steering wheel is turned, the spool and sleeve both turn as a unit, but they turn a few degrees apart.

When the spool and sleeve are a few degrees apart, oil passages are opened between them. This allows oil to flow from inlet port (3) through passages in body (12) to the metering section.

Pump Gears In Metering Section
(4) Internal pump gear. (5) External pump gear. (8) Cross pin. (13) Drive.

When the steering wheel is turned, cross pin (8) turns with the sleeve and causes drive (13) to turn also. The drive causes a rotation of internal pump gear (4) inside external pump gear (5). This rotation of the gear sends a controlled (metered) flow of pilot oil back through body (12). This oil flows to right or left turn ports (9) or (10) and then to the steering cylinders. The right or left turn port that is not used for pressure oil to the steering cylinders, is used for return oil from the other ends of the steering cylinders.

If the steering wheel rotation is stopped, centering springs (7) will move sleeve (11) back in alignment with spool (6). This will close the passages between the metering section and control section, and the steering control unit will be in the neutral position.

When the engine is off, the steering control unit can be manually operated. The metering section will work as a pump. The oil that is returned from the steering cylinders is not returned to the hydraulic oil filter. The suction of the metering section will open check valve (2) and let return oil from the steering cylinders go to the inlet side of the metering section. During power operation, supply pressure keeps the check valve closed.