Coefficient of friction in mixed film lubrication: gears versus twin-discs

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 (Lambda > 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 ffir Zahnrader 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.