ENGINE OFF

132

Low Pressure Standby

This illustration shows the steering pump in LOW PRESSURE STANDBY. When there is no demand for steering system oil, the pump goes to LOW PRESSURE STANDBY. At that position, the pump produces a sufficient amount of oil flow to compensate for internal leakage and maintain sufficient pressure to ensure instantaneous response when the signal from the HMU commands steering oil flow.

At LOW PRESSURE STANDBY, no load sensing pressure signal is detected at the flow compensator spool. Pump supply oil pushes the flow compensator spool up. System pressure is then directed into the large actuator piston. The large actuator piston moves the swashplate towards minimum angle until the cross-drilled hole in the large actuator piston opens to case drain. At this point, the pressure inside the piston decreases and the pump stops destroking.

The pump will supply sufficient flow to maintain the standby pressure and to compensate for internal leakage.

NOTE: LOW PRESSURE STANDBY is more than margin pressure because of the higher back pressure the blocked oil the closed-center valves create when all of the control valves are in NEUTRAL. The pump supply oil pushes the margin spool up to further compress the margin spring. More supply oil goes to the large actuator piston and flows through the cross-drilled hole in the spool to the pump case.

LOW PRESSURE STANDBY

Small Actuator and Bias Spring Swashplate

Piston and Barrel Assembly Spring

133

Pump Upstroke

When a demand for increased oil flow in the steering system occurs, a load sensing signal is sent to the pump control valve. The load sensing signal is equal to the steering system pressure.

The load sensing signal is directed to the spring chamber of the flow compensator spool. The spring force plus the load sensing signal from the HMU shifts the flow compensator spool downward.

The flow compensator spool blocks oil between the pump discharge of the large actuator. The oil in the large actuator piston flows around the pressure compensator spool and the flow compensator spool to case drain. Pump system pressure plus spring force on the small actuator piston moves the pump swashplate toward maximum angle to increase pump flow.

As pump flow increases, system pressure will also increase. When system pressure increases to 2400 kPa (350 psi) more than the load sensing signal from the control valve, the flow

compensator spool starts to move upward. The center land on the margin spool reaches a balance point where flow is metered to and from the large actuator piston. At this point, flow from the pump remains constant until there is a change in the load sensing signal pressure from the control valve.

UPSTROKE

Pump Output

Large Actuator

Small Actuator and Bias Spring Swashplate

Piston and Barrel Assembly

STEERING PUMP AND PUMP CONTROL VALVE

Spring

134

Pump Destroke

This illustration shows the pump and pump control valve in the DESTROKE position. When demand for oil flow in the steering system is decreased, the signal from the HMU is decreased allowing the flow compensator spool to move upward. Oil is allowed to flow around the flow compensator spool and into the large actuator. The large actuator moves to the right and forces the swashplate toward a minimum angle.

DESTROKE

Pump Output

Large Actuator

Small Actuator and Bias Spring Swashplate

Drive Shaft

Pressure Compensator Flow

Compensator Signal From HMU

Piston and Barrel Assembly Spring

135

High Pressure Stall

This illustration shows the pump and pump control valve at HIGH PRESSURE STALL. When steering system pressure reaches the pressure setting of the pressure compensator, the force on the bottom of the cutoff spool will shift the cutoff spool upward. System pressure is then directed into the large actuator piston. The large actuator piston moves the swashplate towards minimum angle until the cross-drilled hole in the large actuator piston opens to case drain.

When system pressure decreases to less than the pressure setting of the pressure compensator, spring force pushes the cutoff spool down and the margin spool again controls flow from the pump.

HIGH PRESSURE STALL

Spring

Large Actuator Piston Pump

Output

Large Actuator

Small Actuator and Bias Spring Swashplate

Drive Shaft

Pressure Compensator Flow

Compensator Signal From HMU

Piston and Barrel Assembly

STEERING PUMP AND PUMP CONTROL VALVE

136

Steering Control Valve

Shown is the steering control valve. This valve is mounted above the transmission between the operator's station and the engine.

This illustration shows the location of the following components:

- Right neutralizer valve (1) - Left neutralizer valve (2) - Right return to tank orifice(3) - Left return to tank orifice (4) - Steering control valve (5) - Crossover relief valve (6) - Back-up relief valve (7) - System pressure port (8) - Return flow port (9)

1

2 3

4

5 6

7

8 9

137

Steering Neutralizer Valves

The steering neutralizers are a plunger-type valve. The neutralizer valve is used to block the pilot oil that is flowing from the HMU to both the pilot control spool and the main steering control spool. As the machine is articulating to the left and the neutralizer valve (1) meets the striker (2), the the pusher will move inward and block the pilot oil flow to the steering control valve. The left articulation will stop. As the machine is articulating to the right and the neutralizer valve (3) meets the striker (4), the the pusher will move inward and block the pilot oil flow to the steering control valve. The right articulation will stop.

1 2

3 4

138

Steering Neutralizer Valve

This illustration shows a sectional view of the neutralizer valve.

During a less that maximum turn, oil from the HMU flows through the valve to the pilot end of the spool in the steering control valve.

When the striker comes in contact with the neutralizer valve, the valve spool shifts and oil flow to the steering control valve is blocked. Steering pilot oil flows back through the orifice in center passage in the spool to drain.

Pilot oil from the neutralizer valve to the steering control valve (not shown) is blocked. The centering springs in the steering control valve return the spool to the HOLD position and steering supply oil to the steering cylinders is blocked. The machine will stop articulating until the steering wheel is turned in the opposite direction.

To Tank

Orifice Spring