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Case
study
Bearing
Fault in Large Motor
The
aptGroup
is an agent for machine condition monitoring and
vibration analysis consultancy services conducted on behalf of SKF
Australia.
The
following case study was taken from a regular survey of plant located at
the site one of our customers.
Introduction
A
faulty bearing in a 2800 kW motor was successfully detected using the SKF
Microlog's enveloping filters.
Outline
As
part of the initial survey at a gold mine in NSW, measurements were taken
on a semi-autogonous mill. When the measurements were analysed, the
spectrum from the non-drive end bearing showed the characteristic features
of an inner race fault. This bearing was then monitored over the next six
months before being replaced. When it was inspected, the inner race was
found to have been damaged as had been predicted from the vibration
analysis.
Method
The
Microlog and accelerometer/SEE sensor were used to
take the vibration measurements. The surface of the machine was prepared
by grinding a small flat spot on the side of the bearing housing. The
accelerometer was attached to the machine using a magnet, with a small
amount of grease on the machine/magnet interface.
Measurements
of velocity (0-500 Hz), acceleration (0-2000 Hz) acceleration enveloping
using two of the Microlog’s built-in filters (500 - 10000 Hz and 5 - 40
kHz), and SEE technology (0 - 1000 Hz) were made.
Results
At
the first survey, it was noted that both the enveloping and SEE
measurements showed spectra consistent with an inner race bearing fault
(fig 1,2), while the fault was not at all visible in either velocity or
acceleration spectra.
Figure
1: SEE Spectrum from Damaged Bearing
Figure
2: Spectrum from Envelope Filter 4
Measurements
were made monthly over the next four months, and the trend showed a rise
in vibration level, particularly in the SEE spectrum. Prior to the next
survey, some extra grease was applied to the bearing, which resulted in
the SEE level being reduced, as can be seen in figure 3. However, one
month later, the SEE level had risen again, as had the envelope levels, so
the decision was taken to replace the bearing. Upon removal and inspection
of the bearing, it was found to have inner race damage consistent with the
spectra (fig 4).
Figure
3: Trend of SEE
Figure
4: Damaged Bearing
Conclusion
Enveloping
and SEE measurements successfully detected this bearing fault, even though
no fault was indicated by the velocity spectrum.
The
customer stated that detecting the fault prior to failure justified the
cost of the condition monitoring program for the next two years.
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