992G Wheel Loader and 854G Wheel Dozer Steering System Steering Control Valve Caterpillar


Steering Control Valve
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1.1. NEUTRAL Position
2.1. LEFT TURN Position
3.1. RIGHT TURN Position

NEUTRAL Position




Illustration 1g01074799

Steering Control Valve in the NEUTRAL Position

(1) Pressure reducing valve. (2) Inlet from the pump. (3) Outlet to the hydraulic tank for the steering system and for the braking system. (4) Pilot oil port. (5) Spool. (6) Steering cylinder crossover relief valve. (7) Shuttle valve. (8) Springs. (9) Retainer. (10) Pilot oil port. (11) Return oil passage. (12) Port for a right turn. (13) Passage for the supply oil. (14) Port for a left turn. (15) Return oil passage. (16) Relief valve.




Illustration 2g01074806

Location of the Steering Control Valve

The steering control valve is located on the left inner frame beneath the cab.

Note: Standard steering control valves and standard secondary steering control valves consist of the same basic components. Additionally, standard steering control valves and standard secondary steering control valves have the same basic function, unless there is a loss of primary pump flow. For information about the operation of secondary steering control valves, refer to Systems Operation, "Steering Control Valve (Secondary Steering)".

While the engine is running, oil flows from the hydraulic tank to the steering piston pump. The oil then flows to the steering control valve through port (2) .

When the STIC steering control lever is in the CENTER position, the flow of pilot oil to either end of spool (5) stops. Similarly, if a neutralizer valve for a left turn or a neutralizer valve for a right turn is actuated, the flow of pilot oil to either end of spool (5) stops.

When no pilot oil is acting on either end of spool (5), centering springs (8) return spool (5) to the NEUTRAL position. As the spool returns to the NEUTRAL position, the pilot oil that had been acting on an end of spool (5) is forced across an orifice and back to the hydraulic tank.

When spool (5) is in the CENTER position, oil in port (2) is blocked at oil passage (13). This causes the oil in the steering cylinder lines to be blocked in port (12) and in port (14) .

The shuttle valve in port (7) senses the load on the cylinder. The shuttle valve then routes the signal oil back to the steering piston pump. This causes the pump to destroke. When this occurs, there is only enough oil in order to compensate for leakage.

Note: If the machine is moving and the machine is turned at the same time, the machine will continue to move in the same direction as the last selected travel position of the STIC steering control lever. When you release the STIC steering control lever, the machine will not return to the STRAIGHT AHEAD travel position.

If the steering pilot gear pump fails, pressure oil from the steering piston pump is routed through the pressure reducing valve in the steering control valve. The oil then flows through a pilot check valve and to the steering pilot control valve. Pressure reducing valve (1) reduces pump pressure to pilot pressure.

LEFT TURN Position




Illustration 3g01074809

Steering Control Valve in the LEFT TURN Position

(1) Pressure reducing valve. (2) Inlet from pump. (3) Outlet to the hydraulic tank for the steering system and for the braking system. (4) Pilot port. (5) Spool. (6) Steering cylinder crossover relief valve. (7) Shuttle valve. (8) Springs. (9) Retainer. (10) Pilot port. (11) Return passage. (12) Port for right turns. (13) Passage for the supply oil. (14) Port for left turns. (15) Return oil passage. (16) Relief valve.

Note: Standard steering control valves and standard secondary steering control valves consist of the same basic components. Additionally, standard steering control valves and standard secondary steering control valves have the same basic function, unless there is a loss of primary pump flow. For information about the operation of secondary steering control valves, refer to Systems Operation, "Steering Control Valve (Secondary Steering)".

When the operator moves the STIC steering control lever in order to make a left turn, pilot oil flows from the pilot control valve through the right neutralizer valve. The oil then flows through the neutralizer check valves and to port (10). The pilot oil acts against retainer (9). The force of the oil overcomes springs (8). This causes spool (5) to move to the left.

When the spool moves to the left, oil in passage (13) flows past spool (5). The oil then flows to port (14) for left turns. The oil then flows through the lines to the rod end of the left steering cylinder. The oil in port (14) then flows through the lines to the head end of the right steering cylinder.

Oil from the head end of the left steering cylinder flows into port (12) for right turns. The oil from the rod end of the right steering cylinder then flows into port (12). The oil then flows to the hydraulic tank.

This causes the machine to articulate to the left. When the STIC steering control lever is released, pilot oil flow to pilot oil port (10) stops. Oil that is acting against retainer (9) must be removed before spool (5) can return to the NEUTRAL position.

Because pilot oil flow is blocked, springs (8) cause spool (5) to move back to the CENTER position. As spool (5) moves to the right, oil in the chamber of springs (8) flows through pilot oil port (10). The oil then flows back to the hydraulic tank.

When the STIC steering control lever is in the NEUTRAL position, valve spool (5) blocks the flow of oil to both steering cylinders. This keeps the machine in the LEFT TURN position.

The pressure of the oil in port (12) and in port (14) is sensed in the port for steering cylinder crossover relief valve (6) .

If an outside force tries to turn the machine, and if valve spool (5) is in the NEUTRAL position, an increase in pressure will occur. The increase in pressure will open steering cylinder crossover relief valve (6) .

RIGHT TURN Position

When the steering control valve is in the RIGHT TURN position, the steering control valve operates in a similar manner as the LEFT TURN position.

For a right turn, pilot oil enters port (4). This moves spool (5) to the right. Pressure oil from inlet (2) flows through passage (13). The oil then flows past spool (5) and into port (12) for right turns.

The pressure oil then flows to the rod end of the right steering cylinder. The oil then flows to the head end of the left steering cylinder. This causes the machine to articulate to the right.

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