950G Series II Wheel Loader Caterpillar


Pressure Reducing Valve

Usage:

950G II AXR

Pressure Reducing Valve

The pressure reducing valve allows two circuits of different pressures to be supplied by the same pump. The maximum supply oil pressure is controlled by the system relief valve. The pressure reducing valve controls the maximum pressure in the controlled oil circuit.

Pump Start-up




Illustration 1g01062810

Illustration 1 shows the pressure reducing valve in the normally open position.

At pump start-up, the valve spring force holds the valve spool and the piston to the right. The supply oil flows around the pressure reducing valve spool to the controlled oil circuit (downstream side of the valve). The supply oil also flows through the oil passage to the piston chamber at the right of the valve spool. Any change in the controlled oil circuit pressure is sensed in the piston chamber. At pump start-up, the supply oil pressure is the same as the controlled oil pressure.

Normal Operating Condition




Illustration 2g01062811

Illustration 2 shows the pressure reducing valve in the normal operating condition.

When the pressure increases in the controlled oil circuit, the increase is sensed in the piston chamber. The increasing pressure moves the piston to the left against the valve spool and the force of the spring. When the valve spool moves to the left, the valve spool restricts the supply oil flow through the valve. This causes a reduction in the controlled oil circuit pressure.

The moving valve spool creates a variable orifice between the supply oil and the controlled oil circuit. The variable orifice allows the oil flow to increase and decrease as needed in order to control the pressure in the controlled oil circuit.

The oil in the spring chamber must be drained to the tank. Any increase in the spring chamber oil pressure will cause an increase in the valve setting.

Note: The pressure reducing valve is used in many systems to create a lower supply oil pressure for pilot control systems, steering clutch, brake valves, and brake systems.

Pressure Reducing Valve ISO Symbol




Illustration 3g01062812

Illustration 3 shows the ISO symbol for the pressure reducing valve.

The ISO symbol uses a single envelope to represent the infinite positioning or metering capability of the pressure reducing valve.

The ISO symbol shows pump oil flow through the NORMALLY OPEN valve to the controlled oil circuit. The controlled oil circuit pressure is sensed through the pilot line and moves the valve (arrow) against the spring. When controlled oil circuit pressure overcomes the spring force, the valve shifts downward causing a restriction of the oil flow to the controlled oil circuit. The upstream pressure may continue to increase. However, the downstream pressure will not increase beyond the pressure reducing valve setting.

When the controlled oil circuit pressure decreases, the spring force will shift the arrow upward to the open position. The valve constantly meters the oil flow in order to maintain the controlled oil circuit pressure.

Caterpillar Information System:

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