detecting bearing faults, part 2
TRANSCRIPT
7/29/2019 Detecting Bearing Faults, Part 2
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58 april/may11
How Much of a Risk AreYou Willing to Take?
What are your goals? Do youwant to know that a bearing mayail just days beore it is likely toail, with no prior warning? Orwould you like to know that abearing has been poorly lubri-cated, or has a minor deect thatwill develop into a major ault?With the techniques described inthis article you could learn thesethings months (certainly weeks)beore the bearing is likely to ail.With that extra time you couldchange the lubrication, orderparts, orga-
nize the labor, and look or the best op-portunity to perorm the bearing replace-ment. The result is a saer plant with lessdowntime, less stress, and higher prots.
Brief RecapIn the previous article, a ew important
points were made that are pertinent tothis article:
1. As bearings begin to ail, the vibra-tion is very low in amplitude, and therequency is very high (beyond your ability to hear, evenwith the best screw driver).
2. Simple spectrum analysis will not reveal the ault until ithas developed to stage three, unless you take special pre-cautions (listed later in the article).
3. To measure high requency vibration you must mount thesensor correctly.
Beyond Your Hearing: UltrasoundThe ultrasound technique is very easy to implement. The
measurement tool listens or very high requency vibration andprovides an indication o amplitude. It also amplies the vibra-
tion and shits (heterodynes) the requency so that you canhear it through headphones. Thereore, you can listen or thetelltale sounds o poor lubrication and bearing distress. Whenused correctly and appropriately, ultrasound instruments canbe very complimentary to the other techniques described inthis article.
What Are “Stress Waves,”and Why Should I Care?Beore describing the next two techniques it is important to
briey introduce the concept o the stress wave (also known asshock pulse). Metal-to-metal contact sets o a ripple eect: astress wave races through the metal components, causing thecomponents to vibrate due to resonance. The stress wave isa very short-duration, low-amplitude, high-requency wave.Every time the rolling elements roll over the damaged area onthe inner and/or outer race (or as the damaged areas on the
rolling ele-ments contactthe raceways),a stress wavewill be gener-ated. We canseek to detectthat wavewith tech-niques such asShock Pulse,PeakVue, and
SWAN (Stress Wave ANalysis, not discussed urther in this brie article). The vibration that results can also be detected via theenvelope method, and as the ault develops urther, via thetime waveorm and spectrum.
There is one very key point you must be aware o: we are talk-
ing about very high requencies, and as such the vibration sensormust be mounted correctly . Unless specically designed or thepurpose (e.g., Shock Pulse), a handheld probe is horribly inad-equate. Even a two-pole magnet mounted directly to the ma-chine surace is not adequate! All o the analyzer vendors will tellyou, you must properly prepare the surace and use an attach-ment pad (or stud mount) in order to achieve the best results.
It is also important to note that there are other deects that will gener-ate stress waves and high-requency vibration, including looseness, gearwear, and cavitation. That can help us to detect those conditions, but itcan conuse our attempts to detect bearing and lubrication aults.
Shock PulseThe vibration sensors provided by SPM and PRÜFTECHNIK are de-
signed to ampliy (through resonance) high-requency vibration (at ap-
proximately 35 kHz). As noted earlier, lubrication and physical deects(including wear/spalls) will generate vibration around this requency.The vibration can be displayed as an amplitude to be trended, or a spec-trum can be displayed in order to better understand the specics o thedeect: inner race, outer race, etc.
Spike EnergyThe Spike Energy (units o gSE) technique aims to utilize the acceler-
ometer’s mounted resonance to ampliy the high requency vibration.However, in more recent years, the accelerometers provided have notbeen manuactured to have a repeatable resonance characteristic. Whatthat means is that when you change your accelerometer, the amplitudeswill change.
PeakVueThe PeakVue technique, developed by Emerson Process Management
(CSi Division), is also designed to detect the stress wave; however, it isperormed in a dierent way. The signal rom the accelerometer is digi-tally sampled (converted rom analog voltages to digital numbers) at avery high rate so that the very short duration stress waves can be de-tected and quantied. The PeakVue waveorm and spectrum providean indication o the bearing deect. As with all o the techniques, theaccelerometer must be mounted correctly, and the lter settings (usedto “tune in”to the bearing vibration) must be set correctly.
EnvelopingAlso known as “demodulation,”the enveloping technique, which is
used by a large number o vibration analyzer vendors, has been opti-mized to measure the low-amplitude, high-requency bearing vibration.
See Figure 1.The envelope spectrum is then checked or signs o the ault condi-
tion. Similar to the spectrum that results in the Shock Pulse, Spike En-ergy, and PeakVue systems, we are looking or peaks, sidebands, andharmonics that are related to the our characteristic bearing requencies:Ball Pass Frequency Outer race (BPFO), Ball Pass Frequency Inner race(BPFI), Ball (or roller) Spin Frequency (BSF), and Fundamental Train (orcage) Frequency (FTF). See Figure 2 or a summary o the progressionwe expect to see.
Spectrum AnalysisI we do not use one o these techniques and simply view a spectrum,
then we may have limited success unless we take precautions:
1. Acceleration is most sensitive to high-requency vibration, so i we view the spectrum in units o acceleration (Gs or mm/s2) andhave a high Fmax (70X or higher) and, better yet, we view thespectrum in logarithmic ormat, then we will achieve the best re-sults (with a spectrum alone).
2. I we view the spectrum in units o velocity (in/sec or mm/s), thenwe may need to wait until the bearing is at stage three until wesee positive signs o the ault. Increasing the Fmax and viewingthe spectrum in logarithmic ormat will help signicantly.
When viewing the velocity or acceleration spectrum (or any spectrumrom PeakVue, enveloping, etc.) there are a ew techniques that help toachieve the best results:
1. Look or peaks at requencies that are non-integer multiples o the shat speed (e.g., 3.09X, 4.65X, 7.89X, etc.).
Detecting
BearingFaults
vibration
ViB
c o n d i t i o n
m o n i t o r i n g
Fig
Descripton
In this example, we have a
smooth vibraon signal with
the weak bearing signal over-
laid. In reality, the vibraon
would not be as smooth, and
the bearing vibraon would
not be visible.
First, the relavely high-
amplitude, lower-frequency
vibraon must be removed
from the accelerometer signal.
This is typically achieved with
a high-pass or band-pass
lter. The me between
these “pulses” is equal (in this
example) to the me between
balls rolling past the damaged
area on the outer race, which
represents the “ball pass
frequency” for the outer race:
BPFO.
The signal is then reced (all
of the negave-going vibraon
is turned into posive-going
vibraon).
The signal is then “enveloped”
(thus the name) via a low-pass
lter. There is a lot that could
be said about this process, and
there are dierences from one
vendor to the next, but the
end result is a low-frequency
me waveform that, whentransformed to a spectrum
(FFT), will indicate that a fault
exists on the outer race (in this
example).
Metal-to-metal contact setso a ripple eect: a stress waveraces through the metal compo-nents, causing the componentsto vibrate due to resonance.
P a r t 2
Jason Tranter
This article is the second in a
series o our. The rst article
provided a summary o how
the vibration patterns change
as the bearing ails. Ater very
briefy recapping the basics,
this article will discuss the
detection and analysis tools
that can be used to determine
the nature and severity o the
bearing ault: ultrasound,
Shock Pulse, PeakVue, envel-
oping, and spectrum and time
waveorm analysis.
7/29/2019 Detecting Bearing Faults, Part 2
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60 april/may11
2. There should be harmonics o those requencies (e.g., peaks at3.09X, 6.18X, 9.27X, etc.).
3. Check or sidebands o the turning speed o the shat. I they exist,then suspect a ault on the inner race. I there are no sidebands,suspect an outer race ault.
4. Check or sidebands o the undamental train requency (slightlyless than hal the turning speed o the shat). I t hey exist, thensuspect a ault on the rollers/balls.
Time waveform analysisIt is typically possible to view the time waveorm rom the Shock Pulse,
PeakVue, and envelope process, but I’ll ocus on t he raw waveorm romthe accelerometer. In the early stages o the ault condition it will be verydifcult to detect the ault with a time waveorm. However, as the aultdevelops, an acceleration waveorm can reveal the ault, especially whentaken rom low-speed machinery. As the ault develops, the waveormwill have characteristic “pulses”and patterns that indicate the conditiono the bearing ault. In the later stages o the ault, a waveorm in velocityunits can display the deect quite clearly.
Characteristic “modulated”pattern inthe accelerationwaveorm
(otencalledthe “angelfsh”pattern).
“Spikes”in the velocity waveormin
ConclusionI hope this article has helped to pro
techniques. They have all been used bearing aults at a very early stage. rectly, choose the correct settings, an
Figure 2
Descripton Vsal
If the lubricaon is
OK and the bearing
is faultless, then
the envelope spec-
trum will contain
“noise” – no peaks
to speak of.
As a fault develops,
peaks will rise up
out of the noise
oor at frequencies
described in the
spectrum analysis
secon.
When the peaks
are clearly above
the noise oor the
bearing should be
replaced.
As the fault
develops further,
the noise oor will
begin to li.
When the noise
oor swallows the
peaks, the bearing
is in late stage four
and is about to fail.
The surface of the
bearing is severelydamaged; there is
no longer period
vibraon.
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