ABSTRACT
The science of tribology is dedicated to understanding the fundamental physical and chemical factors that govern friction and wear. As energy conservation and equipment longevity continue to increase in importance, efforts to maximize unit fuel consumption while reducing wear-related failure are becoming more relevant. Lubricated contacts may be characterized by the ratio of the average lubri-
cant film thickness h and the composite root mean square surface roughness σ = (σ21 + σ
1/2 of both contacting surfaces in the concentrated contact region. The classical modes of lubrication are depicted schematically in Fig. 6.1. A
typical Stribeck curve is shown in Fig. 6.2, which describes wear regimes in terms of the effects of lubricant viscosity, sliding speed, and normal load on friction coefficient. When the lubricant film is sufficiently thick (h σ), the opposing surfaces never contact one another. The condition when such contact surfaces are almost non-deformed is known as hydrodynamic lubrication and is the predominant lubrication mode in lightly loaded journal bearings. Since normal loads are relatively small, there is negligible deformation of opposing contact surfaces. Lubrication dynamics are largely dictated by viscous properties of the lubricating film, and the coefficient of friction is directly proportional to viscosity when speed and load are fixed. As the film thickness h approaches 3-5 times σ the asperities of the opposing
surfaces are still not in contact (only rarely asperities brush against each other) [1, 2] normal forces become sufficiently large that elastic deformation of the contacting surfaces occur. This condition is known as elastohydrodynamic lubrication (EHL) because the lubrication regime is controlled by both viscous effects and elastic deformation of the contacting surfaces. The EHL regime is usually encountered in situations in which high loads act over relatively small contact areas such as in ball bearings, roller bearings, and gear teeth. For hard materials, higher loads are required to reach EHL than for softer, deformable materials. Asperity contact becomes more frequent as h approaches σ. In the mixed
for Line and Point
FIGURE 6.1
Pictorial representation of lubrication regimes. (after Kudish and Covitch [3]). Reprinted with permission from CRC Press.
