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Definition of EHD lubrication. The
lubrication principles applied to rolling bodies, such as
ball or roller bearings, is known as elastohydrodynamic (EHD)
lubrication.
Rolling body lubrication. Although
lubrication of rolling objects operates on a considerably
different principle than sliding objects, the principles of
hydrodynamic lubrication can be applied, within limits, to
explain lubrication of rolling elements. An oil wedge,
similar to that which occurs in hydrodynamic lubrication,
exists at the lower leading edge of the bearing. Adhesion of
oil to the sliding element and the supporting surface
increases pressure and creates a film between the two
bodies. Because the area of contact is extremely small in a
roller and ball bearing, the force per unit area, or load
pressure, is extremely high. Roller bearing load pressures
may reach 34,450 kPa (5000 lb/sq in) and ball bearing load
pressures may reach 689,000 kPa (1,000,000 lb/sq in). Under
these pressures, it would appear that the oil would be
entirely squeezed from between the wearing surfaces.
However, viscosity increases that occur under extremely high
pressure prevent the oil from being entirely squeezed out.
Consequently, a thin film of oil is maintained.
Effect of film thickness and roughness.
The roughness of the wearing surfaces is an
important consideration in EHD lubrication. Roughness is
defined as the arithmetic average of the distance between
the high and low points of a surface, and is sometimes
called the centerline average (CLA).
As film thickness increases in relation to
roughness fewer asperities make contact. Engineers use the
ratio of film thickness to surface roughness to estimate the
life expectancy of a bearing system. The relation of bearing
life to this ratio is very complex and not always
predictable. In general, life expectancy is extended as the
ratio increases. Full film thickness is considered to exist
when the value of this ratio is between 2 and 4. When this
condition prevails, fatigue failure is due entirely to
subsurface stress. However, in most industrial applications,
a ratio between 1 and 2 is achieved. At these values surface
stresses occur, and asperities undergo stress and contribute
to fatigue as a major source of failure in antifriction
bearings.
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