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Over the last three years IRS has been monitoring motor bearings
displaying various stages of fatigue. Most of these motors
were taken out of service before a failure could occur but
had their life significantly reduced as a
consequence of the phenomena of bearing skidding.
Roller bearings appear to be particularly susceptible to
skidding, which is the result of a combination of a rolling
and a sliding motion that compromises the rolling integrity
of the bearing because the forward force at the inner ring
is smaller than the resistance to rotation of the roller/cage
set.
Skidding may cause smearing and streaks in the circumferential
direction of the rolling elements and raceways, which will
lead to high running noise and may cause premature bearing
failure due to excessive pitting formation.
Larger electric machines are equipped at the driven end with
cylindrical roller bearings and in order to obtain a load
carrying capacity as high as possible at the shaft end, the
design engineer frequently selects a bearing for a heavier
series load. A high load carrying capacity is however, actually
required only with belt or gear take-off but in practice 90
per cent of today’s electric motors feature a coupling
take-off. Thus the bearings are not subjected to high externally
applied loads and will skid as a result.
Skidding problems occur mainly during test runs with unloaded
bearings. In this case skidding is not only due to the low
loads but will also be influenced by the unfavourable marginal
conditions experienced during the test run. In a new bearing
somewhat more grease is packed than the bearing will require
for normal lubrication. This will affect the resistance to
which the bearing cage is subjected and enhance its tendency
to skid.
Bearing skidding can be determined relatively easily.
- Experience has shown that bearing noise created due to
skidding will disappear for a short time when the bearing
is relubricated.
- If the bearing cover can be removed skidding can be diagnosed
with a stroboscope that will highlight the sliding, as opposed
to rolling motion of the roller bearing element.
With critical skidding, it will be difficult to bring the
cage to an apparent stop with a stroboscope. The rotational
speed of the bearing cage will not be constant, but will be
influenced by the varying friction contact and/or
lubrication conditions in the contact area.
Skidding can be avoided in general by:
- Applying sufficient external load to force the bearing
elements to rotate.
- Replacing the cylindrical roller bearings with deep groove
ball bearings of identical dimensions. Deep groove ball
bearings are less sensitive to skidding as the rolling elements
have an additional degree of freedom and the bearing has
a lower radial load carrying capacity than the cylindrical
roller bearing of the same size.
- Reducing the grease charge applied to the bearing.
For more information related to bearing skidding contact
Mark Drayton at Integrated Reliability Solutions on
0438 000 624.
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