Bulk Modulus of Solidified Oil at High Pressure as Predominant Factor Affecting Life of Thrust Ball Bearings

In a previous paper, it was pointed out that the solidification behavior of mineral oils at high pressures exceeding glass transition points plays an important role on bearing life. Further detailed investigations have been carried out based upon actual observations of solidification phenomena of traction oils and mineral oils. Traction oils characterized by large pressure-viscosity coefficients and high friction result in shorter bearing life, in spite of their excellent separating film formation ability, in particular, under solidified conditions. The general tendency of mineral oils shows that bearing life increases with the film parameter. However, the beneficial effect of high film parameters of traction oils is considered to be disturbed by high friction, which is found to increase linearly with the bulk modulus of solidified oil. In view of this fact that brittleness of solidified oils is estimated by the temperature difference from the viscoelastic solid transition to the initiation of cracks under lowering temperature condition. Since this is inversely proportional to the bulk modulus, the high modulus is considered to promote lowering of the bearing life. In particular, thrust ball bearings operate with ball spinning which acts to cause brittle failure of solidified films. Both factors, i.e. expectation of the beneficial effect to improve bearing life and the detrimental one to increase friction and temperature rises, are in conflict with each other, but are both characterized by a high bulk modulus under the elastic-plastic condition. The present investigation was carried out in order to clarify merits or demerits of the high bulk modulus of supplied oil concerning bearing life. Conversely, it is noted that mineral oils and polyalphaolefins show shorter life at high temperatures exceeding the glass transition points. Presented at the 48th Annual Meeting in Calgary, Alberta, Canada May 17-20, 1993