Thin film lubrication of real surfaces

When rolling bearings and gears are lubricated with a clean lubricant the i nitial metal to metal contact through the oil film will normally stop in le ss than a million loading cycles if the surfaces are at all able to run-in. If, after the running-in, the temperature is increased or the speed is dec reased, metallic contact through the oil film will start again and continue until the surfaces have run in again at the new running conditions. The ru nning-in continues as long as some asperities reach each other through the oil him, and the run-in state is characterized by the minimum separation be tween the highest roughness tops being very close to zero. The surface stru cture is changed by running and if that change is slow and well controlled, the surfaces will run in until they become smooth enough to no longer wear . This running-in thus determines the running conditions and the functional ity of the surfaces. If the lubricant contains particles, and these are lar ge and hard enough, the running-in of the surfaces can be destroyed and the y will never become smooth enough to be separated by an oil him. This leads to wear and/or spalling failure of the surfaces. Whether the surfaces will wear or spall depends on the properties and the concentration of contamina nt particles. For some levels of contamination the surfaces can first wear and then spall when the load distribution over the contact area has been ch anged by the wear. To ensure that the surfaces can work in a stable mode wi thout wear or spalling, the running conditions have to be ideal for each po int on the lubricated surface all the time. (C) 2000 Elsevier Science Ltd. All rights reserved.