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

Abstract

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 Rc, 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 (Ra ≤ 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 Rc = 0.14 μm, 500 percent at Rc = 0.42 μm and 150 percent at Rc = 0.71 μm. This is not accompanied by a correspondingly large increase in Hertzian contact pressure at film collapse, pHc. In fact pHcincreases 45 percent at Rc = 0.14 μm and 15 percent at Rc = 0.42 μm and decreases 20 percent at Rc = 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.