Effect of extreme-pressure additives on rolling bearing fatigue life

Rolling-bearing fatigue tests showed that under marginal film lubrication conditions (lambda, lambda = 1.2), a commercially available sulphur-phosphorus extreme-pressure (EP) additive package used in blended lubricating oil reduced the bearing fatigue life by at least a factor of 4.5 compared with the base oil alone. The Weibull slope was found to be significantly higher than normally observed for material tests made under identical conditions. Post-test examination showed a remarkable difference of topography of the raceway contact surfaces between those run with the same oil with or without the EP additive package. It is suggested that the reduction in bearing life is primarily due to the chemical reactivity of the EP additives in blended oil. The action of lubricant oxidation by the addition of EP additives is also discussed.