Detailed two-dimensional EHL film thickness maps can be obtained experimentally by using a Ball&Disc Apparatus, optical interferometry and image analysis. It is possible to compute the deformation of the surfaces, from the measured film thickness map, since the undeformed contact geometry of the ball and disc is known. By assuming linear elastic deformation it is also possible to numerically determine the contact pressure distribution. This experimental/numerical approach has been called the Hybrid technique. The problem addressed here can be stated as follows: Can the very detailed film thickness map and its corresponding pressure distribution, be used for the determination of lubricant parameters such as bulk modulus and density? The answer is yes but the quality of the results are very much dependent on the accuracy of the film thickness measurements and the corresponding pressure calculation. The refractive index have to be known everywhere in the EHL contact, i.e. the relationship between pressure and refractive index is required if refractive index cannot be measured directly. The method was applied on a PAO oil and the bulk modulus was determined over a pressure range of 250 MPa to 500 MPa. By using dilatometry results for the pressure range from zero to 100 Mpa it was also possible to determine the density in the pressure range from zero up to 500 MPa. The results are promising and if the accuracy can be improved further the method described can be a useful tool for determining lubricant pressure-deformation relationships.
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