CW12 Pneumatic Compactor Machine Systems Caterpillar

Illustration 1	g06266457
Hydraulic Schematic (HOLD) (1) Counterbalance valve (2) Check valve (3) Steering cylinder (4) Shuttle valve (5) Steering control valve (6) Compensator spool (7) Warmup valve (8) Steering pump (9) Steering shunt valve (10) Margin spool (11) Main relief valve (12) Shuttle valve (13) Steering manifold

When the engine is running, steering pump (9) sends flow to port"P" of steering manifold (15). Supply oil passes through a filter and is directed to warmup valve (10), margin spool (12), and compensator spool (6). If differential pressure across the filter exceeds 700 kPa (102 psi), a bypass check valve opens.

When the steering system is enabled, steering shunt valve (9) is energized and closed. In this case, all supply oil is directed to compensator spool (6) and warmup valve (7). When the parking brake is applied, the steering system is disabled. In this condition, the shunt valve is not energized, and all oil flow is directed to the hydraulic tank.

When hydraulic oil temperature is less than 20 °C (68 °F), warmup valve (7) is energized. This valve allows the steering system to warm up the hydraulic oil. The warmup valve opens and directs supply oil to the demand side of margin spool (10). The margin spool closes and supply pressure increases to the setting of main relief valve (11). The relief valve opens and directs supply oil to the hydraulic tank. In this case, the relief valve serves to warm up the hydraulic oil.

When hydraulic oil temperature is greater than 25 °C (77 °F), warmup valve (7) de-energizes.

Margin spool (10) is a load-sensing valve. This valve compares the hydraulic pressure supplied to the steering system with the highest pressure demand being placed on the system. When the operator is not steering the machine, steering control valve (5) is not energized, and the demand pressure is at hydraulic tank pressure. In this case, the margin spool shifts and directs the supply oil to the hydraulic tank.

If the steering system experiences a shock load, check valve (1) in the low-pressure side of steering cylinder (3) opens. This action allows oil to enter the low-pressure side of the hydraulic cylinder, preventing cylinder cavitation. Counterbalance valves (1) limit the maximum pressure in the steering cylinder to 20700 (3000 psi).

Main relief valve (11) limits the maximum pressure in the steering system to 18350 ± 2661 kPa (2661 ± 386 psi).

Illustration 2	g06266615
Hydraulic Schematic (RIGHT TURN) (1) Counterbalance valve (2) Check valve (3) Steering cylinder (4) Shuttle valve (5) Steering control valve (6) Compensator spool (7) Warmup valve (8) Steering pump (9) Steering shunt valve (10) Margin spool (11) Main relief valve (12) Shuttle valve (13) Steering manifold

When the operator turns the steering wheel, the machine ECM sends signals to the solenoids in steering control valve (5). The steering control valve has two solenoids. One solenoid extends steering cylinder (3). When this happens, the machine turns right. The other solenoid retracts the steer cylinder. When this happens, the machine turns left. When neither solenoid is energized, the steering valve is spring-returned to the center position. In the schematic shown, the machine is turning right.

Compensator spool (6) senses the pressure on the inlet to steering control valve (5). The spool also senses the highest pressure on the outlet of the steering control valve. The load sensing signal acts against the compensator spool with a spring. The compensator spool controls the flow of oil to maintain a constant pressure drop of 1380 kPa (200 psi) across the steering control valve. This assures the steering speed will remain constant for a given steering valve command in the presence of changing pressure in the system.

When the front clockwise solenoid in steering control valve (5) is energized, the spool of the control valve will shift. This shift directs supply oil to the check valve at the head end of steering cylinder (3). The steering cylinder retracts and pressure builds in the head end of the cylinder. At the same time, the steering control valve directs supply oil to counterbalance valve (1) on the rod end of the steering cylinder. This supply oil acts with the pressure created in the head end of the cylinder to open the relief valve. The relief valve opens and the rod end of the steering cylinder is opened to the hydraulic tank.

When hydraulic oil temperature is greater than 25 °C (77 °F), warmup valve (7) de-energizes.

The load signal from the outlet of steering control valve (5) is sent to margin spool (10) through the series of shuttle valves. The load signal pushes against the spring in the margin spool to close the margin spool. This action increases the system pressure to the level required to move the steering cylinder.