Rolling bearings in electric motors and generators

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1 Rolling bearings in electric machines
Design requirements
Vibration
In environments where machinery is
subjected to vibrations caused by an
external source, it is generally recommended
to use ball bearings when
possible. Ball bearings, especially when
preloaded with springs, are less sensitive
to the damage caused by external
vibrations. (➔ chapter 6 “Failure modes
and corrective actions”, starting on
page 91).
Quiet running
Motors and generators are expected to
run quietly. Therefore, it’s important to
select a bearing with the best combination
of cage material, lubricant and internal
clearance. SKF bearings already
have very low noise levels. However,
the levels can be further reduced by
preloading the bearing arrangement
with springs (➔ section “Preloading
with springs”, starting on page 47).
Bearing life
The rated life of a rolling bearing is
defined as the number of revolutions
(or the number of operating hours at
a given constant speed) the bearing
could endure, before the first sign of
fatigue (spalling or flaking) occurs on
one of its rings or rolling elements.
Laboratory tests and practical experience,
however, show that seemingly
identical bearings operating under
identical conditions have different lives.
The “service life” of a bearing depends,
to a large extent, on its operating
conditions. However, the procedures
used to mount and maintain the
bearing can also have a direct affect
on its service life. Despite all the precautions,
bearings can still fail prematurely.
When this happens, the bearing
should be examined carefully in order
to determine the root cause of the failure.
By doing so, corrective actions
can then be taken.
The “specification life” is the life
specified by the motor manufacturer
and is based on hypothetical load and
speed data. E.g. nominal life at maximum
allowable load is 20 000 hours
minimum.
Under specific operating conditions,
SKF bearings can attain a much longer
life than predicted by normal or traditional
life calculation methods, particularly
when loads are light. These
specific conditions prevail, when a
lubricant film effectively separates the
rolling surfaces (raceways and rolling
elements) and when surface damage
caused by contaminants is limited.
For appropriate calculation methods,
please refer to SKF calculation tools,
the SKF General Catalogue or the SKF
Interactive Engineering Catalogue on
CD-ROM or online at www.skf.com.
When selecting sealed for life bearings
in electric machines, the service life
most often will be limited by the grease
life (➔ chapter 4 “Lubrication and
sealing”, starting on page 59). Therefore,
bearing service life and grease
life need to be verified.
Lubrication: grease versus oil
The choice between grease lubrication
and oil lubrication is chiefly determined
by the following factors:
• Grease should be used in applications
where the following requirements
apply:
– Simplified maintenance
– Improved cleanliness (fewer leaks)
– Better protection against
contaminants
• Oil lubrication should be used in
applications where normal operating
temperatures are high as a result of
an external heat source or excess
heat generated by the machine or
its bearings at high speed.
Note: A temperature rise due to friction
in the bearing, is generally lower with
grease than with an oil bath, provided
that the appropriate type and amount
of grease is used and that it is supplied
to the bearing in a suitable manner.
Oil lubrication should be used when
the relubrication interval for grease is
too short (➔ chapter 4 “Lubrication
and sealing”, starting on page 59).
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