Tribology of Mechanical Systems: A Guide to Present and Future Technologies
2 Elastohydrodynamic Lubrication
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Elastohydrodynamic lubrication (EHL) is the lubrication of contacts between non-conformal surfaces, i.e., surfaces that do not fit each other well. EHL is found in most common machine elements, such as rolling element bearings, gears and cam mechanisms, and it is characterized by concentrated forces, high contact pressure (1–3 GPa), thin lubricant films (1–1,000 nm) and elastically deformed surfaces. Understanding EHL means that we can predict the magnitude of contact pressures and surface stresses as well as how thick lubricant films are formed. We can then predict the risk of fatigue and adhesive wear failures.
During the past 30 years great efforts have been made to increase the understanding of EHL. Experimental techniques such as optical interferometry and thin film transducers have made it possible to measure the characteristic lubricant film thicknesses, contact pressures and temperatures associated with EHL. The rapid improvement of computers and computational methods has also made it possible to apply numerical simulation to EHL problems. We can now accurately model the lubrication between smooth non-conformal surfaces under steady and pure rolling conditions.
In real contacts, however, steady conditions are rare, and sliding/spinning motion is often superimposed. Real surfaces are not smooth and the surface roughness height is normally of the same order of magnitude as the lubricant film thickness. Sliding contacts cause heating of the lubricant and sliding also causes non-Newtonian behavior in most lubricants. In order to obtain full understanding of all the mechanisms of film formation in EHL, we have to learn more about film formation between rough surfaces and how the lubricant behaves under EHL conditions, i.e., high pressures and high shear strain rates.