An increase in spring force on the valve as it opens , flattens the belle velle washer so there is no frictional force between spring housing upper valve cap. Further spring force balls to move down in a ramp imparting a rotational motion to the valves spindle.
Since very small needle lift is obtained in the test equipment result in an unequal pressure distribution in the cut off shaft resulting in a hard contact in a small area along with high frequency oscillations, low lubricity of test oil will increase the chance of seizure. Full lift and high lubricity of heavy oil eliminates this risk of seizure during normal operation.( Always open and clean slide type valves before testing as cold sticky heavy oil along with small clearances can cause restriction in the movement of the spindle).
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