Failure of Thin Film Lubrication—The Effect of Running-In on the Load Carrying Capacity of Thin-Film Lubricated Concentrated Contacts

The effect of running-in under low speed - high load conditions on the load carrying capacity at high speed of sliding crossed cylinders of steel AISI 52100, thin film lubricated with a marine diesel engine oil of 60°C, is described for three values of the initial composite roughness R_c, i.e. 0.14 μm, 0.42 μm and 0.71 μm. It is shown that—irrespective of the initial surface roughness—the surface of the (smaller) stationary cylinder becomes very smooth (R_a ≤ 0.1 μm), that of the (larger) rotating cylinder remaining virtually unaffected. As the local radius of curvature increases as a result of running-in, the load carrying capacity, expressed in terms of total force on the contact, increases considerably, i.e. 600 percent at R_c = 0.14 μm, 500 percent at R_c = 0.42 μm and 150 percent at R_c = 0.71 μm. This is not accompanied by a correspondingly large increase in Hertzian contact pressure at film collapse, p_Hc. In fact p_Hcincreases 45 percent at R_c = 0.14 μm and 15 percent at R_c = 0.42 μm and decreases 20 percent at R_c = 0.71 μm. It is further found that run-in surfaces show the phenomenon of delayed EHD-film collapse, meaning that the transition from the (partial) EHD to the scuffing regime may take from 1 to 40 s after application of the normal force. The test method should contribute significantly to the functional characterization of lubricants and—more in particular—running-in fluids, as candidates for use in concentrated contact situations.