SKF - Rolling Bearings

Lubrication
Protection against corrosion
Grease should also protect the bearing against
corrosion and should not be washed out of the
bearing arrangement if water is present. The
type of thickener solely determines the ability
of the grease to resist water washout. Thickeners
with very good washout resistance include
lithium complex, calcium complex, and
polyurea greases. The ability of a grease to inhibit
corrosion is determined by the properties
of the rust inhibitor additive for that grease.
For very slow-speed applications, filling the
housing up to 90% with grease can improve
corrosion protection and help prevent the
ingress of water.
Load carrying ability
If the thickness of the hydrodynamic lubricant
film is not sufficient to prevent metal-to-metal
contact, bearing service life can be extended
significantly by using a lubricant containing
extreme pressure (EP) or anti-wear (AW)
additives.
In applications where k > 1 († Viscosity
ratio k, page 241), SKF does not generally
recommend using EP or AW additives. However,
there are circumstances where EP/AW
additives may be beneficial such as if excessive
sliding between the rolling elements and raceways
is expected.
Extreme pressure additives
EP (extreme pressure) additives are an option
to overcome metal-to-metal contact of the
asperities on the contact surfaces. Elevated
temperatures, induced by local asperity contact,
activate these additives, which promote
mild chemical wear at the points of contact.
The result is a smoother surface, lower contact
stresses and extended bearing service life.
Some modern EP additives contain sulphurphosphorus,
which may become chemically
active even without asperity contact. In these
cases, high operating temperatures and/or
contact stresses become the catalyst. The resulting
chemical activity may not be restricted
to the asperity contacts and can have a detrimental
effect on the strength of the bearing
steel matrix. This can promote corrosion/diffusion
mechanisms in the contacts and may
lead to accelerated bearing damage, usually
initiated by micro spalls.
Therefore, SKF recommends using EP
additives that are less reactive in applications
with operating temperatures > 80 °C (175 °F)
and < 100 °C (210 °F). For very slow speeds,
solid lubricant additives such as graphite and
molyb denum disulfide (MoS 2 ) can be used to
enhance the EP effect. These additives should
have a high purity level and a very small particle
size. Otherwise, dents due to overrolling of the
particles might reduce bearing service life.
For additional information about EP additives,
contact the SKF application engineering
service.
Anti-wear additives
AW (anti-wear) additives, like EP additives,
prevent direct metal-to-metal contact. However,
the way they work is very different. The
main difference is that AW additives build a
protective layer that adheres to the surface of
the metal. The asperities can then slide over
each other, avoiding direct contact. The surface
finish is therefore not affected by mild
wear as is the case with EP additives. Note
that AW additives, like EP additives, may contain
elements that can weaken the steel structure
close to the surface.
Certain thickeners (e.g. calcium sulphonate
complex) also provide an EP/AW effect without
chemical activity and the resulting effect on
bearing fatigue life. The temperature limits for
these thickeners are well within the temperature
limits of most greases and oils.
For additional information about AW additives,
contact the SKF application engineering
service.
Miscibility
If it becomes necessary to change from one
grease to another, the miscibility or the ability
to mix greases without adverse effects should
be considered. If incompatible greases are
mixed, the resulting consistency can change
so dramatically that bearing damage due to
severe leakage could result.
Greases with the same thickener and similar
base oils can generally be mixed without any
detrimental consequences. For example,
a lithium thickener / mineral oil grease can
generally be mixed with another lithium
thickener / mineral oil grease. Also, some
greases with different thickeners, such as
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