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This work compares the friction between two contacting discs with the friction between gear teeth, using a mixed film lubrication model developed for the evaluation of contact pressure and shear stress fields [5]. Such model is used to analyse twin-disc contacts with smooth surfaces and gear teeth contacts with rough surfaces, both lubricated with an additive-free ISO VG 150 mineral oil.Experimental lubricant traction curves [4], measured in a twin-disc machine under full film elastohydrodynamic (EHD) lubrication ( > 2), were used to evaluate the lubricant rheological parameters, taking into account the surface roughness of the contacting discs. Experimental power loss measurements, made during gear efficiency tests performed in a Forschungsstelle für Zahnräder und Getriebebau (FZG) test rig [4], were used to evaluate the average friction coefficient between gear teeth along the meshing line, considering the surface roughness of the tooth flanks.The experimental friction coefficients between gear teeth were compared with predictions obtained using three different approaches: (a) values directly extrapolated from the experimental lubricant traction curves, (b) values calculated using a full film EHD lubrication model, considering surface roughness, and (c) values from a mixed film lubrication model. These analysis were performed at three different scales, from the roughness peaks (inside contact) to the entire gear meshing line, passing through the contact dimension.The results obtained show that the mixed film lubrication model is able to evaluate accurately the friction between gear teeth in power transmission equipment whatever the lubrication regime, boundary, mixed or full film.conditions and composite surface roughness of the contacting discs, and, consequently, the corresponding lubricant traction curves and rheological parameters cannot be used in boundary or mixed film lubrication conditions. In fact, the discs normally have a very good surface finishing, corresponding to a composite root mean square (r.m.s.) roughness σ ≈ 0.1, and the specific film thickness is greater than 2 (full film condition). Under these conditions, the correlations between experimental and numerical friction forces are very good, even if a smooth surface lubrication model is used, and the lubricant rheological parameters are almost independent of the discs surface roughness.However, in the case of gears, even with very good surface finishing (e.g. FZG type C gears [2]), the JET257

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Three different lubricating greases and their bleed and base oils were compared in terms of film thickness in a ball-on-disc test rig through optical interferometry. The theoretical values calculated according to Hamrock's equation are in close agreement with the base oil film thickness measurements, which validates the selected experimental methodology.The grease and bleed oil film thickness under fully flooded lubrication conditions presented quite similar behaviour and levels. Therefore, the grease film thickness under full film conditions might be predicted using their bleed oil properties, namely the viscosity and pressure-viscosity coefficient. The base and bleed oil lubricant parameter LP are proportional to the measured film thickness.A relationship between grease and the corresponding bleed oil film thickness was evidenced.