UBRICATION CAN GENERALLY be defined as the reduction of friction by the use of a
fluid lubricant. Although the concept is simple, in practice this procedure becomes
much more complex. In fact, lubrication by this method can be divided into four
basic types or regimes.
When a fluid lubricant is present between two rolling and / or sliding surfaces,
a thicker pressurized film can be generated by the movement of the surfaces (velocities).
The non-compressible nature of this film separates the surfaces resulting in no
metal-to-metal contact. The condition in which surfaces are completely separated
by a continuous film of lubricating fluid is commonly referred to as Hydrodynamic or
Full Fluid Film lubrication.
In this regime, the fluid immediately adjacent to each surface travels at the
same speed and direction of each surface. If you were to travel through the width
of the film, differential speed graduates and direction is reversed.
Hydrodynamic film thickness can be formed by wedging the lubricant through a
convergent gap with the tangential surface velocities. Known as wedging film action,
this is similar to a car tire hydroplaning on a wet road surface. Hydrodynamic
lubrication often occurs in components such as cylinders, gears, and plain bearings.
Although hydrodynamic lubrication is the ideal situation, in many instances it
cannot be maintained. Factors which affect hydrodynamic lubrication include lubricant
viscosity, rotation speed or RPM, oil supply pressure, and component loading.
An increase in speed or viscosity increases oil film thickness. An increase in
load decreases oil film thickness.
Boundary Lubrication (sometimes referred to as thin film lubrication)
is a condition in which the lubricant film become too thin to provide total separation.
This may be due to excessive loading, speeds, or a change in the fluid's characteristics.
In such a situation, contact between surface asperities (peaks and valleys) occurs.
Friction reduction and wear protection is then provided through chemical compounds
rather than the properties of the lubricating fluid. Boundary lubrication often occurs
during the startup and shutdown of equipment, or when loading becomes excessive.
This condition can commonly be observed in certian types of gear sets that need to
withstand sliding pressures and shock loading, such as hypoid gears found in automotive
Mixed Film Lubrication is a combination of both hydrodynamic and boundary
lubrication. In such a situation, only occasional asperity contact occurs. This
condition can be the result of lubricant breakdown or increased load placed upon
Elasatohydrodynamic Lubrication (EHD or EHL) occurs as pressure or load
increases to a level where the viscosity of the lubricant provides a higher shear
strength than the metal surface it supports. This regime can occur in roller
bearings or gears as the lubricant is carried into the convergent zone approaching
a contact area or the intersection of two asperities. As a result, the metal
surfaces deform elastically in preference to the highly pressurized lubricant
which increases the contact area and thus increases the effectiveness of the
A good understanding of these different types of lubrication can assist greatly
in the selection of the proper lubrication for a specific application and the
prevention of equipment failure.