Lesson 4/Learning Event 1
The relief valve is usually a spring-loaded ball or plunger type. Oil under pressure is delivered through the main
oil line to the parts that are pressure lubricated. The oil exerts pressure on the plunger in the pressure relief
valve. When the oil pressure exceeds the plunger spring pressure, the plunger is forced off its seat and oil can
flow by the plunger into the oil outlet and from there to the sump. The plunger is held off its seat by the oil
pressure in the main oil line until the pressure drops to the point at which the spring can seat the plunger. The
spring must have just the right amount of strength. The oil pressure from the pump is controlled by the
strength of the plunger spring. The oil pressure in the system can be increased by increasing the spring tension
or it can be reduced by decreasing the spring tension.
FIGURE 60. OIL PUMP WITH RELIEF VALVE.
This is a cutaway view of a gear-type oil pump with the relief valve in the discharge side of the pump. It is
common practice by many engine manufacturers to place the relief valve in the pump.
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