Pressure Differential Valve
In Illustration 1 and Illustration 2, the spring exerts a 50 pound force on the 1 square inch valve spool. The supply oil pressure must exceed 345 kPa (50 psi) in order to overcome the spring force and be able to move the valve spool.
Pump Start-up
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| Illustration 1 | g01062813 |
Illustration 1 shows a pressure differential valve. The pressure differential valve maintains a specified difference in pressure between two circuits.
At pump start-up, also when the pressure in the primary circuit is less than 345 kPa (50 psi), the spring force holds the valve spool to the right. The oil flow is blocked to the secondary circuit. Any change in the primary circuit pressure is sensed at the valve spool.
Normal Operating Condition
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| Illustration 2 | g01062815 |
When the primary circuit is filled, pressure begins to increase. When the primary circuit pressure increases to more than 345 kPa (50 psi), the primary pressure overcomes the 345 kPa (50 psi) differential valve spring force. This will move the differential valve to the left. Supply oil flows to the secondary circuit. Supply oil also flows through the passage to the differential valve spring chamber.
When the secondary circuit is filled, the pressure begins to increase. The same pressure increase is sensed in the differential valve spring chamber. The combined oil pressure and spring force move the valve spool to the right and attempts to shut off the flow of oil to the secondary circuit. However, the increase in pressure in the primary circuit keeps the valve open. The pressure increases in both the primary circuits and secondary circuits until the relief valve (not shown ) opens sending the excess pump flow back to the tank.
The pressure differential valve establishes a position that constantly maintains a 345 kPa (50 psi) pressure difference between the primary circuits and the secondary circuits at all pressures above 345 kPa (50 psi).
Note: Pressure differential valves are used extensively in power shift transmissions on many machines.
Pressure Differential Valve ISO Symbol
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| Illustration 3 | g01062816 |
The pressure differential valve ISO symbol (Illustration 3) uses both inlet side pressure and outlet side pressure with a spring to control the differential between the two.
The pressure from the inlet side is sensed by the valve and works against the spring force as in the pressure relief valve. The outlet pressure is sensed by the valve. Then, the outlet pressure works with the spring force. The difference between the inlet and outlet pressures is always equal to the valve spool spring force pressure regardless of changes in pressure at the inlet port. An example is, a spring force pressure of 345 kPa (50 psi) will produce a pressure differential between the inlet and outlet pressure of 345 kPa (50 psi).
The spring is replaced in order to meet any required changes in the differential pressure. Normally, shims are not used to change the pressure requirements.