Film thickness measurements for the characterization of EHL contacts

Abstract: This licentiate thesis describes experimental techniques related to the investigation of elastohydrodynamic lubrication of point contacts as well as measurements and evaluation of results from laboratory tests. Elastohydrodynamic lubrication is one of the most severe forms of lubrication when a medium (often a liquid) is expected to separate two surfaces in a mechanical device under extremely high pressures and shear strain rates as well as with very thin lubricant films. These criteria set high demands on both the surfaces themselves and the lubricant. In order to simulate elastohydrodynamic lubrication, a ball and disc apparatus, of which there are many different types and configurations, can be used. These produce images (interferograms) of the actual conjunction by employing the phenomena of optical interference. In the work presented in this thesis, a computer controlled ball and disc apparatus has been used to investigate two different types of interferogram evaluation methods; the HSI and the Multi Channel method. Lubricant behaviour at low rolling speeds and hence very thin lubricating films, was also studied. It was seen that some lubricants create very thin boundary layers preventing direct contact between the ball and disc. It was also concluded that the measurement of very thin lubricant films requires interferogram evaluation methods which do not depend upon model based calibrations. At high slide/roll ratios, and often with lubricants possessing a high pressure-viscosity coefficient, dimples may form within the normally flat central contact region. The cause of this phenomena has been disputed, but studies suggest that differences in the thermal properties of the contacting bodies may be the cause. Experiments which caused such dimples were used as the basis for evaluating the hybrid film thickness evaluation scheme which takes account of the influence of pressure on the lubricant's refractive index. This proved useful since dimple formation causes abnormally high pressures which would, with other techniques, reduce the accuracy of the film thickness measurements. The shear rates which occur in a sliding contact affects the rheological and thermal properties of the lubricant within the conjunction. This results in a reduction in film thickness from that predicted by isothermal assumptions. It was shown that this reduction is not only affected by operating conditions, but is also lubricant type dependent. Experiments indicated that an ester base fluid possessed characteristics which gave less film thickness reduction as compared to a mineral and a Poly-a-olefin base fluid.