Experimental evidence is presented for the existence of three regimes of traction in concentrated contacts. Data were obtained in a rolling contact simulator by varying the lubricant, temperature, rolling speed, load and surface roughness at a fixed slide-to-roll ratio. In one regime, the traction is mainly governed by the limiting shear stress of the lubricant. At lower thicknesses (Λ < 1) the mixed regime is encountered and traction is increased due to asperity interaction. At higher film thicknesses, the lubricant pressure is distributed over a greater area than the Hertzian region resulting in a lower average pressure and reduced traction.

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