Liquid/Solid Transition Lubrication in Hertzian Contacts

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(1) The observed minimum film thickness at a rolling contact is plotted in the form of the Greenwood diagram. In the solidification ranges VE (Viscoelastic) and EP (Elastic-Plastic) larger thickness is formed. The upper parts of these thickness curves are limited by the boundary of the PR (Piezoviscous rigid) range defined by Houpert.(2) In the case of traction oils bearing life is hardly related with the film parameter in contrast to mineral oils, which show a general feature that bearing life increases with increasing film parameter.(3) Traction oils having larger pressure-viscosity coefficients and higher coefficients of traction are easily solidified under the operating conditions of bearings.(4) In the case of thrust ball bearings the merit of the solidified films is rather little recognized. They shorten bearing life, increase friction and operating temperature. These facts are related with bulk moduli of solidified films.(5) Demerits of traction oils are attributed to severe operating conditions of thrust ball bearings, which are strongly affected by the presence of spin moment.(6) The solidified oil films of traction oils easily cause breakdown owing to their brittle nature. The brittleness shows a relation reversed to the bulk modulus. It is estimated by measuring the temperature difference between solidified and failure temperature at lowering temperatures.(7) By raising operating temperatures by external heating bearing life is rapidly shortened as the product of the pressure-viscosity coefficient and the average contact pressure is reduced considerably lower than the critical value 25, i.e. necessary to maintain the elastic plastic state.