Modelo de Tioacetamida (TAA)

Orifice Spring

Valve Spool Center Passage To

LESS THAN MAXIMUM TURN

MAXIMUM TURN

139

Steering System Schematic

When the engine is running and the steering system is in HOLD, oil is drawn from the

hydraulic tank by the steering pump. The oil flows past the secondary steering diverter valve to the back-up relief valve and the directional spool in the steering control valve. Also, the oil flows through the flow control orifice to the hand metering unit. At this time, the oil is blocked from flowing through the HMU. With the HMU in the center position, a small amount of oil then flows through the orifice and back to the hydraulic tank.

The HMU and the steering pump are connected by a signal line. A sense of change in the signal pressure at the HMU will send a reflected change in signal pressure to the pump control valve demanding a change in the output flow of the steering pump. If the pressure of the signal oil decreases, the steering pump will destroke. If the pressure of the signal oil increases, the steering pump will upstroke.

In the HOLD position, the flow of pressure oil from the steering pump to the steering cylinders is blocked at the directional spool in the steering control valve.

In the HOLD condition, there is no signal pressure sensed at the compensator valve on the steering pump. The steering pump goes to the LOW PRESSURE STANDBY position.

Secondary

compensate for any system leakage and to maintain sufficient system pressure to provide instantaneous response when the steering wheel is turned.

140

Steering System - Gradual Left Turn

During a GRADUAL LEFT TURN with the engine running, the steering pump sends supply oil past the secondary steering diverter valve to the steering control valve and the hand metering unit.

When the steering wheel is turned counterclockwise to make a LEFT TURN, pilot oil from the hand metering unit flows past the left neutralizer valve to the steering control valve to the selector spool. The selector spool shifts down, and oil flows to the end of the directional spool.

The directional spool shifts down against the force of the centering spring.

When the directional spool moves down, main steering pump oil flows through the directional spool to the rod end of the left steering cylinder and the head end of the right steering cylinder.

At the same time that oil flows into the two steering cylinders, return oil flows from the head end of the left steering cylinder and the rod end of the right steering cylinder through the directional spool and back to the hydraulic tank.

The machine articulates to the left for a left turn.

Secondary

141

Steering System - Full Left Turn - Steering Neutralized

During a FULL LEFT TURN with the engine running, the left striker contacts the left neutralizer valve. The neutralizer valve moves to the closed position, and oil flow from the hand metering unit to the steering control valve is blocked at the left neutralizer valve.

The steering selector spool and the steering directional spool return to the center position. Flow to the steering cylinders is blocked at the directional spool in the steering control valve, and the machine stops turning.

The neutralizer valves prevent the machine loader frame from contacting the machine rear frame when articulating fully to the right or left.

Secondary

FULL LEFT TURN (STEERING NEUTRALIZED)

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If the steering pump or the engine fails, the primary pressure switch will sense the low pressure in the steering system. The primary steering pressure switch closes and sends a signal to the monitoring system which causes a Category 3 Warning to occur. After a one second delay, the Power Train ECM energizes the intermediate relay for the secondary steering pump and the electric motor actuates. At the same time, the secondary steering indicator on the monitoring system display illuminates.

The secondary steering pump draws oil from the hydraulic tank. The oil then flows to the secondary steering valve, which causes the check valve in the hydraulic line from the steering pump to close and the check valve in the hydraulic line from the secondary steering pump to open. The closed check valve prevents pressure oil from flowing to the steering pump.

Oil from the secondary steering pump flows past the secondary steering valve to the steering control valve and hand metering unit.

Pilot oil from the hand metering unit flows past the left neutralizer valve and moves the steering selector spool. The oil then flows through the steering selector spool and moves the directional spool, allowing the secondary steering oil to flow to the rod end of the left steering cylinder and to the head end of the right steering cylinder. Secondary steering provides a method to steer the machine to a safe location if a failure occurs in the primary steering pump or in the engine.

Secondary

GRADUALLEFT TURN / SECONDARY STEERING

143

Secondary Steering

This illustration shows the location of the secondary steering components in the rear frame (1).

If the steering pump or the engine fails, the primary pressure switch will sense the low pressure in the steering system. The primary steering pressure switch closes and sends a signal to the monitoring system which causes a Category 3 Warning to occur. After a one second delay, the transmission electronic control module energizes the intermediate relay (4) for the secondary steering pump and the electric motor actuates. At the same time, the secondary steering indicator on the monitoring system display turns ON.

The secondary steering pump (3) draws oil from the hydraulic tank. The oil then flows to the secondary steering diverter valve (2), which causes the check valve in the hydraulic line from the steering pump to close and the check valve in the hydraulic line from the secondary steering pump to open. The closed check valve prevents pressure oil from flowing to the steering pump.

The secondary steering motor will be de-energized when either the ground speed is 0 rpm or a faulted TOS sensor signal for more than five seconds.

2

3 4

5

NOTE: The secondary steering pump does not produce the same amount of flow as the main steering system pump. Secondary steering operations are reduced compared to normal operation. Secondary steering provides a method to steer the machine to a safe location if a failure occurs in the primary steering system or in the engine.

144

The secondary steering diverter (1) valve directs oil from the secondary steering oil to the steering control valve when the primary pressure switch (3) senses a loss of oil pressure in the primary steering system and the ground speed is greater than 0 rpm. The primary pressure switch sends a signal to the Power Train ECM and the ECM enables the secondary steering pump motor.

When the engine is running and the steering pump is supplying oil to the steering system, oil flows into the diverter valve through the line (6) and oil also flows over the check valve (5) through the line (8) to the steering control valve (not shown). At this time, the oil pressure in the primary steering system holds the check valve (4) against the seat.

When the primary steering pressure switch senses a loss of oil pressure in the primary system, the secondary system is initiated. At this time, the primary pressure switch is closed and illuminates the primary steering warning LED. Oil flows into the diverter valve through the line (7), over the check valve (4), and out to the steering control valve through the line (8). At this time, the check valve (5) is seated. When the Power Train ECM enables the secondary steering pump motor, the secondary pressure switch (2) measures the oil pressure in the

secondary steering system and sends a signal to illuminate the secondary steering warning LED.

1 2

4 5

7

8

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This illustration shows the location of the components for the optional CCS steering system for the 966H and the 972H Wheel Loaders. The hydraulic tank is common to all hydraulic systems on the machine.

INSTRUCTOR NOTE: The color codes used for hydraulic oil throughout this presentation are:

Red - System or high pressure

Red and White Stripes - Reduced pressure

Orange - Pilot pressure

Blue - Blocked oil

Green - Tank or return oil

Hydraulic Tank Steering Pilot Valve