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Case
study
Identifying
the stages of bearing damage
The early
identification of troublesome conditions such as
inadequate lubrication or misalignment enables an analyst
to apply proactive corrective measures to extend bearing
service life. The onset of bearing deterioration often
occurs very early on, as raceways begin to wear,
developing micropitting in the load zone. Micropitting
at this very early point does not necessarily reduce
operational life, but is often a good indication that
progression to stage one is imminent.
Stage
One is still a
"good" bearing. However, after a substantial
portion of the bearing life has passed, micropitting
results in the degeneration of the bearing to the point
where very small craters develop in the raceways.
These
small defects are not always impacted with enough force
to generate measurable vibration signals in the velocity
domain. The aptGroup
supply, use and recommend the
patented SEE Technology from SKF. SEE (Spectral Emitted
Energy) Technology frequently provides a detectable
signal which relates to periodic surface contact (For
more information, go to the SEE Technology page).
As the bearing degrades and the rolling elements impact
the fault, harmonics of the damage frequency will begin
to show in the FFT spectrum. Stage Two
is a bearing with some wear as noted by the harmonics.
There is no reason to change the bearing at this point.
In fact, bearings have been pulled at this stage and the
only damage apparent is pinpoint spalling in the
raceways. As the harmonics increase in amplitude it may
be prudent to increase the frequency of data collection.
Bearing degradation is usually linear for a period of
time and can be trended, but as service life is
shortened, it becomes non-linear.
In
Stage
Three the bearing is becoming
terminal. The FFT spectra show the fundamental defect
frequency and the harmonics will often begin to indicate
sidebands of bearing shaft rotating speed. This is
particularly true of the BPFI (Ball Pass Frequency Inner
Race) where the defect rotates through the bearing load
zone. The vibration increases as the defect goes through
the load zone and the signal is modulated producing the
rotational sidebands. The BPFO (Ball Pass Frequency Outer
Race) signal generally has constant bearing loading until
bearing looseness, imbalance, misalignment or bent shaft
amplitude modulates the defect signal often resulting in
rotational sidebands.
For
example using a BPFO of 107.6 Hz on a shaft turning 1800
RPM or 30 Hz, the sidebands will be at 77.6 Hz and 137.6
Hz (107.6 plus and minus 30) and the second harmonic will
have sidebands at 185.2 Hz and 245.2 Hz (215.2 plus and
minus 30). Further progression of the damage will
generate additional sidebands at 2X running speed (47.6
and 167.6). When the sidebands overlap, the spectra
become more difficult to analyse. But beware! The bearing
is in the last days of its life and should be changed as
soon as possible. During Stage 3, in addition to the
spectrum information, the overall amplitudes provide a
clue to the condition of the bearing.
Stage Four
The service life at this point is extremely short and
requires immediate corrective action. It is often
characterised in the velocity or acceleration spectral
domain as "haystack" amplitudes (broadband noise) in the bearing defect region. In acceleration
enveloping spectra there will appear high amplitude
defect frequency components as well as the 1X, 2X speed
sidebands (indicating looseness) about the BPFO and in
the extreme case, cage defect frequency components will
appear. Bearing
Damage > Examples
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