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CAM MATERIALS AND LUBRICATION 273

9.5 LUBRICATION

9.5.1 Introduction (Buckley, 1997)

The purpose of lubrication is to separate surfaces in relative motion, by a material that has
a low shear resistance to friction and wear and also to minimize damage or prevent major
damage of the surfaces. The material can be of any variety, such as adsorbed gases, chem-
ical reaction products, films, liquids, or solid lubricants. The lubricating film (Stribeck,
1902) is classified first by the well-known Stribeck-Hersey curve (1902) in which
the coefficient of friction is plotted as a function of the ZN/P parameter where Z is the
viscosity of the medium, N is the velocity, P is the load, and h is the film thickness
(Fig. 9.10).
The fluid film regimes are: hydrodynamic, elastohydrodynamic, and mixed-lubrication
regimes as well as boundary lubrication. Hydrodynamic lubrication or fluid-film lubrica-
tion occurs at high speeds and low loads or high viscosities in which the surfaces are com-
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pletely separated by a thick film h > 10 in. This rheology of the lubricant determines the
friction in this hydrodynamic regime. Elastohydrodynamic lubrication (EHD) implies a
fluid-film regime where the loads of the surfaces and the pressure-viscosity of the lubri-
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cant occur at h ~ 10 to 10 in. In mixed-lubrication regime, which is thinner, surface
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asperity interactions occur with increased friction and fluid film effects, with h ~ 10 to
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10 in. Last, with low values of the ZN/P parameter h ~ 10 in is the realm of boundary
lubrication. The most important aspect of the boundary lubrication regime is the forma-

FIGURE 9.10. Coefficient of friction as a function of viscosity-velocity-
load parameter (Stribeck-Hersey curve). From Stribeck (1902).