What Are The Different Types Of
Lubrication?
Five distinct forms of lubrication may
be identified:
1 Hydrodynamic
2 Hydrostatic
3 Elastohydrodynamic
4 Boundary
5 Solid film
Hydrodynamic lubrication means that the
load-carrying surfaces of the bearing are separated by a relatively
thick film of lubricant, so as to prevent metal-to-metal contact, and
that the stability thus obtained can be explained by the laws of
fluid mechanics.
Hydrodynamic lubrication does not
depend upon the introduction of the lubricant under pressure, though
that may occur; but it does require the existence of an adequate
supply at all times.
The film pressure is created by the
moving surface itself pulling the lubricant into a wedge-shaped zone
at a velocity sufficiently high to create the pressure necessary to
separate the surfaces against the load on the bearing. Hydrodynamic
lubrication is also called full-film, or fluid, lubrication.
Hydrostatic lubrication is obtained by
introducing the lubricant, which is sometimes air or water, into the
load-bearing area at a pressure high enough to separate the surfaces
with a relatively thick film of lubricant. So, unlike hydrodynamic
lubrication, this kind of lubrication does not require motion of one
surface relative to another.
We shall not deal with hydrostatic
lubrication in this book, but the subject should be considered in
designing bearings where the velocities are small or zero and where
the frictional resistance is to be an absolute minimum.
Elastohydrodynamic lubrication is the
phenomenon that occurs when a lubricant is introduced between
surfaces that are in rolling contact, such as mating gears or rolling
bearings. The mathematical explanation requires the Hertzian theory
of contact stress and fluid mechanics.
Insufficient surface area, a drop in
the velocity of the moving surface, a lessening in the quantity of
lubricant delivered to a bearing, an increase in the bearing load, or
an increase in lubricant temperature resulting in a decrease in
viscosity—any one of these—may prevent the buildup of a film
thick enough for full-film lubrication.
When this happens, the highest
asperities may be separated by lubricant films only several molecular
dimensions in thickness. This is called boundary lubrication. The
change from hydrodynamic to boundary lubrication is not at all a
sudden or abrupt one.
It is probable that a mixed
hydrodynamic- and boundary-type lubrication occurs first, and as the
surfaces move closer together, the boundary-type lubrication becomes
predominant. The viscosity of the lubricant is not of as much
importance with boundary lubrication as is the chemical composition.
When bearings must be operated at
extreme temperatures, a solid-film lubricant such as graphite or
molybdenum disulfide must be used because the ordinary mineral oils
are not satisfactory. Much research is currently being carried out in
an effort, too, to find composite bearing materials with low wear
rates as well as small frictional coefficients.
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