Thin film lubrication of real surfaces

When rolling bearings and gears are lubricated with a clean lubricant the initial metal to metal contact through the oil film will normally stop in less 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 decreased, metallic contact through the oil film will start again and continue until the surfaces have run in again at the new running conditions. The running-in continues as long as some asperities reach each other through the oil film, and the run-in state is characterized by the minimum separation between the highest roughness tops being very close to zero. rface structure 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 functionality of the surfaces. If the lubricant contains particles, and these are large and hard enough, the running-in of the surfaces can be destroyed and they will never become smooth enough to be separated by an oil film. 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 contaminant particles. For some levels of contamination the surfaces can first wear and then spall when the load distribution over the contact area has been changed by the wear. ure that the surfaces can work in a stable mode without wear or spalling, the running conditions have to be ideal for each point on the lubricated surface all the time.