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Case
study
Fan
Bearing Failure
3000 rpm Fan
Abstract:
A vibration increase was noted in a 3000 rpm fan. On shutdown, one bearing
was found to have serious defects.
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.
Background:
The
aptGroup carries out routine surveys on cooling fans at a manufacturing
plant in Sydney's west. These fans are critical to the operation of the
plant. Due to their high operating speed, any failure tends to be
catastrophic, so condition monitoring is vital.
Method:
The SKF Microlog and accelerometer/SEE sensor were used
to take the vibration measurements. The accelerometer was attached to the
machine using a magnet, with a small amount of grease on the
machine/magnet interface.
Measurements
of velocity (0-600 Hz), acceleration (0-5000 Hz) acceleration enveloping
using the Microlog’s built-in filters (5 - 40 kHz), and SEE
technology (0 - 1000 Hz) were made.
Results:
The fan showed an increase in vibration at a survey in August 1999. At
this point an alert was raised, indicating that a problem was present.
The
subsequent survey in September showed a dramatic increase in vibration
(fig 1), with a very noisy spectrum (fig 2). The vibration spectrum does
not show distinct bearing defect frequencies, but rather an increase in
broadband noise is seen.
Shortly
after this survey, the fan was shut down and the bearings replaced. It was
found that one row of the twin-row ball bearing was completely missing.
Figure
1: Vibration Trend, showing levels before and after
Figure
2:
Acceleration Spectrum from Damaged Bearing
Conclusion:
Vibration monitoring prevented a catastrophic failure in this machine.
There are two other observations to make:
>
The rapid rate of deterioration. The machine went from having slightly
raised levels to dangerous levels in a few weeks, while a lower speed
machine may have taken months.
>
The lack of bearing defect frequencies. The spectra never did
show the classic bearing defect frequencies, even though the bearing was
seriously damaged.
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