IT does not yet seem to be recognized fully that it is the local temperature at the surface of contact and not the local specific pressure that chiefly determines the occurrence of seizure under extreme-pressure-lubrication conditions. This local temperature is the result of the temperature level of the parts lubricated, considered as a whole (“bulk” temperature) and of a superimposed instantaneous temperature rise (temperature “flash”) which is localized in the surface of contact. It appears typical for extreme-pressure-lubrication conditions, as met in gear practice, that the temperature flash is much higher than the bulk temperature.
With existing conventional test methods for the determination of the protection against seizure afforded by EP lubricants, a considerable rise of the bulk temperature mostly occurs; as it cannot be controlled sufficiently; thus, leaving an unknown margin for the temperature flash, it renders impossible a reliable determination. With the “seizure-delay” method the effects of this variable bulk temperature are eliminated, so that only the effects of the temperature flash remain.
Seeing that the temperature is the main factor under so-called extreme-pressure lubrication (EPL), “extreme-temperature-and-pressure” lubrication (ETPL) would be a more rational term and, consequently, this term will be adopted throughout the following pages.