The thickness of the fluid film determines the lubrication
regime, or the type of lubrication. The basic regimes of fluid film
lubrication are: Hydrodynamic lubrication two surfaces are
separated by a fluid film, Elastohydrodynamic lubrication two
surfaces are separated by a very thin fluid film, Mixed lubrication
two surfaces are partly separated, partly in contact, and, Boundary
lubrication two surfaces mostly are in contact with each other even
though a fluid is present.
Boundary of LubricationBoundary lubrication occurs when the
lubricating film is about same thickness as the surface roughness
such that the high points (asperities) on the solid surfaces
contact.
Mixed Lubrication
Mixed lubrication occurs between boundary and hydrodynamic
lubrication, as the name would suggest. The fluid film thickness is
slightly greater than the surface roughness, so that there is very
little asperity (high point) contact, but the surfaces are still
close enough together to affect each other.
Elastohydrodynamic LubricationA thick fluid film, low friction
and no wear are the defining characteristics of hydrodynamic
lubrication, which generally occurs at conformal contacts. A
lubricated nonconformal contact will experience elastohydrodynamic
lubrication (EHD).
Hydrodynamic Lift
Consider two parallel plates with relative motion: if one
surface is angled where the entrance area is slightly larger than
the exit area, then a wedge shaped gap is created. This is a
converging gap, and is the geometry necessary to produce
hydrodynamic lift.
