BULK MODULUS OF SOLIDIFIED OIL AT HIGH-PRESSURE AS PREDOMINANT FACTORAFFECTING LIFE OF THRUST BALL-BEARINGS

In a previous paper, it was pointed out that the solidification behavi or of mineral oils at high pressures exceeding glass transition points plays an important role on bearing life. Further detailed investigati ons have been carried out based upon actual observations of solidifica tion phenomena of traction oils and mineral oils. Traction oils charac terized by large pressure-viscosity coefficients and high friction res ult in shorter bearing life in spite of their excellent separating fil m formation ability, in particular, under solidified conditions. The g eneral tendency of mineral oils shows that bearing life increases with the film parameter. However, the beneficial effect of high film param eters of traction oils is considered to be disturbed by high friction which is found to increase linearly with the bulk modulus of solidifie d oil. In view of this fact that brittleness of solidified oils is est imated by the temperature difference from the viscoelastic solid trans ition to the initiation of cracks under lowering temperature condition . Since this is inversely proportional to the bulk modulus, the high m odulus is considered to promote lowering of the bearing life. In parti cular, thrust ball bearings operate with ball spinning, which acts to cause brittle failure of solidified films. Both factors, i.e. expectat ion of the beneficial effect to improve bearing life and the detriment al one to increase friction and temperature rises, are in conflict wit h each other, but are both characterized by a high bulk modulus under the elastic-plastic condition. The present investigation was carried o ut in order to clarify merits or demerits of the high bulk modulus of supplied oil concerning bearing life. Conversely, it is noted that min eral oils and polyalphaolefins show shorter life at high temperatures exceeding the glass transition points.