SKF - Rolling Bearings

Oil lubrication
Lubricating oils
Straight mineral oils are generally the favoured
lubricant for lubricating rolling bearings.
Oils containing extreme pressure (EP) or
anti-wear (AW) additives to improve lubricant
properties are generally used only in special
cases. The information covering EP and AW
additives in grease († Load carrying ability,
page 248) also apply to these additives in oils.
Synthetic versions of many of the popular
lubricant classes are available. Synthetic oils
are generally only considered for bearing lubrication
in extreme cases, e.g. at very low or
very high operating temperatures. The term,
synthetic oil, covers a wide assortment of different
base stocks. The main ones are polyalphaolefins
(PAO), esters and polyalkylene
glycols (PAG). These synthetic oils have different
properties than mineral oils († table 6).
The thickness of the hydrodynamic film,
which prevents metal-to-metal contact in a
bearing, plays a major role in bearing fatigue
life. The thickness of the hydrodynamic film is
determined, in part, by the viscosity index (VI)
and the pressure-viscosity coefficient. For
most mineral oil based lubricants, the pressure-viscosity
coefficient is similar, and generic
values obtained from literature can be
used. However, for synthetic oils, the effect on
viscosity to increasing pressure is determined
by the chemical structure of its base stock. As
a result, there is considerable variation in
pressure-viscosity coefficients for different
types of synthetic base stocks. Due to the differences
in the viscosity index and pressureviscosity
coefficient, it should be remembered
that the formation of a hydrodynamic lubricant
film, when using a synthetic oil, may differ
from that of a mineral oil with the same
viscosity. For additional information about
synthetic oils, contact the lubricant supplier.
In addition, additives play a role in the formation
of a hydrodynamic film. Due to differences
in solubility, different types of additives
are used in synthetic oils that are not included
in mineral oil based lubricants.
Properties of lubricating oil types
Properties
Base oil type
Mineral PAO Ester PAG
Table 6
G
Pour point [°C] –30 .. 0 –50 .. –40 –60 .. –40 appr. –30
[°F] –20 .. 30 –60 .. –40 –75 .. –40 appr. –20
Viscosity index low moderate high high
Pressure-viscosity
coefficient
high moderate low to moderate moderate
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