Proper Lubricant Selection for Rolling Bearing Applications0 pages

PROPER LUBRICANT SELECTION FOR BEARING APPLICATIONS
RKB TECHNICAL REVIEW – DECEMBER 2013

Proper Lubricant Selection for Rolling Bearing Applications
Lucian TUDOSE, Cristina TUDOSE

RKB Bearing Industries – Advanced Software Engineering Unit

Abstract: Proper selection of lubricant is a decisive task in achieving the maximum possible
bearing life for a certain application. In this paper a theoretical part concerning bearing
lubrication and the RKB software dedicated to the proper lubricant selection are presented.
Key words: lubricant selection, reference/actual kinematic viscosity, viscosity index

1 INTRODUCTION
It is well known that at least 60% of premature bearing failures are due to incorrect lubrication. Even high quality
bearings can exhibit best performance only when they are lubricated correctly. Rolling bearings must be lubricated
to prevent the harmful metal-to-metal contact between the rolling elements, raceways and cage and to protect
the bearing from corrosion and wear as well. In addition, bearing lubricant has to ensure dissipation of heat,
elimination of contaminants, flushing away of wear debris, lubrication of the seal lips and filling of labyrinth seal gaps.
It is worth mentioning here that the function of lubricant in seals is different from that in bearing itself and therefore it is
better to lubricate bearings and seals separately, choosing the best lubricant for each, even if, unfortunately, this
solution is rarely implemented, because of high costs, possibility of making mistakes by substituting one lubricant with
another, etc.
The general principles of hydrodynamic lubrication can be applied, within certain limits, to explain lubrication of
rolling bearing elements. However, in the case of rolling bearings, the contact area is extremely small and,
consequently, the load pressures may reach high values, in the range of 1–3 GPa. Under such extremely high
pressures, an elastic deformation of the contacting surfaces occurs, the load-bearing area increases and the oil is
totally squeezed from between mating surfaces. Consequently, the oil pressure increases by several orders of
magnitude and, therefore, a thin viscoelastic lubricant film, which becomes capable of supporting the external load,
is maintained between the surfaces. This special lubrication regime is known as elastohydrodynamic lubrication in
which the friction and film thickness between the two bodies in relative motion are determined by the geometry and
the elastic properties of the bodies, by the relative speed, and by the viscosity of the lubricant at the actual pressure,
temperature and rate of shear.
In the particular case of rolling bearings, the thickness and load carrying capacity of the lubricant film mainly
depend on the viscosity and other oil properties and on the bearing internal geometry, speed, and size. When the
bearing starts to run, the contact between mating surfaces of rolling elements and raceways is mostly metal-tometal, because the oil film thickness is small, in relation to surface roughness, hence a large number of metallic
asperities come into contact. As speed increases, the lubricant film thickness also increases and fewer asperities
make contact. In order to reach the desirable lubricant full film, bearing geometry, application conditions (speeds
and loads), and lubricant properties have to combine to form an oil layer so thick that even the highest peaks do not
penetrate the film and do not come into contact with each other. If this situation is achieved during operation, the
subsurface stress will be the only responsible for the bearing fatigue failure and a long bearing life is expected. In this
context the role of the temperature cannot be excluded. The most appropriate working temperature for a rolling
bearing is the temperature that requires the minimum lubricant quantity necessary for an optimal lubrication.

2 OIL OR GREASE LUBRICATION?
Among other important factors (especially those connected to operating and environmental conditions), bearing
loads and speeds are of crucial importance when selecting the appropriate type of lubricant for a certain
application. As a basic orientation in using grease lubrication, equation (1) can be used. If the mentioned condition
is not fulfilled, oil lubrication can be chosen.

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