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Acceleration
enveloping > FAQs
1. What does
enveloping technology detect?
Enveloping
technology detects rotational defect signals. It enhances repetitive
phenomena such as:
>
Bearing Defects
>
Gearbox defects
>
Cavitation.
Since enveloping
enhances repetitive signals of the impact type, it allows early warning
of developing damage.
2. What type
of bearing damage generates an enveloped signal?
Bearing problems
such as:
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Holes
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Cracks
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Spalls
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Dirt
Other defects in:
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Belts
>
Felts
>
Felt Rollers
>
Chains
>
Chipped teeth in gears.
3. Can
enveloped readings be trended?
Yes, keeping in mind
that amplitude readings may decrease as damage becomes severe due to
smoothing of crater edges with a corresponding improvement in
lubrication flow. At the early stages of damage, however, readings
should be trendable.
4. Are
envelope readings quantitative or qualitative?
Quantitative. A larger
enveloped reading recognizes a stronger "hammering force" than
a smaller reading. Again, keep in mind that amplitude readings may
decrease as damage becomes severe due to smoothing of crater edges with
a corresponding improvement in lubrication flow.
5. On what
applications should enveloping measurements be performed and on which ones
can it not be done?
Enveloping measurements
are most significant on repetitive bearing defects of the impact type -
especially with low-force impacts. Enveloping measurements do not
reflect significant readings in applications which look at other things
besides impact defects, such as misalignment, gear backlash, and
applications on very slow speed machinery.
6. In which
applications does an enveloped signal give sufficient information about
rolling element bearing condition and in which applications is enveloping
not enough?
Since enveloping
measurements are vibration measurements and since enveloping enhances
repetitive impact phenomena, enveloping gives excellent information in
the early warning stages of bearing defects. It is not sufficient with
very large (stiff) machinery which give too small vibration readings.
(Use a SEE
sensor for early warning on these types
of machines.)
7. How
should enveloping measurements with a fixed sensor be performed?
Select your sensor with
the proper frequency response and the proper bandwidth relative to the
filter setting of your enveloper. Place the sensor in the load zone of
the bearing under test and in the proper direction. Consider the
stiffness of the machine so as to select the best axis for the
measurement. 'Me sensor should be installed in an approved, workmanlike
manner (solidly and to a clean surface).
8. How
should enveloping measurements with a hand-held sensor be performed?
Enveloping measurements
with a hand-held sensor must be consistently applied to achieve
consistent readings. You need a consistent pressure (2.2 Ibs or 1 kg
force) of the sensor against the machine, a consistent angle of attack
(90º ± 10º), consistent type of probe (magnetic vs non-magnetic),
clean measurement area (surface dirt and paint can be a problem), and at
the same place on the machine each time.
9. Is there
a difference between hand-held and fixed sensor enveloping readings?
Yes. Because the
efficiency of the coupling interface for hand-held and fixed sensors
differ, their amplitude readings may differ by a factor of 2 or 3.
However, absolute amplitudes are not as significant as changes in
amplitude measured over time by the same sensor (trend).
10. What
training and knowledge is required to take measurements and to interpret
envelope readings?
A user must be trained
to be careful and consistent in setting up and taking measurements:
location of points on the machine to be measured, direction to hold the
sensor, pressure of the sensor against the machine, and so on.
Experience and practice will improve consistency and accuracy.
You could plan to
attend a formal training
session by APT Technology where you
will learn basic bearing knowledge and the fundamentals of the tool,
such as signals in which frequency content is important, how to be
consistent, what envelope readings mean, trending, limitations of the
technology, and so on.
Read your user manuals
(QuickStart, Training Guide, and Reference Manual) to find out all the
things your tool can tell you. Subscribe to, and read, as many industry
publications as you can. Attend as many industry sponsored conferences
and seminars as you can.
Your return on the use
of this tool is directly proportional to your efforts to learn about it.
11. How do
envelope readings correlate with normal vibration measurements and SEE
measurements?
Because they have
different goals, they measure different things. You should use both
technologies in your predictive maintenance program to maximise your
ability to predict problems and to maximise your cost savings. If your
normal vibration measurements correlate very strongly all the time with
your enveloping readings, you should be suspicious of your envelope
filter setting. The low-end filter setting should be above10X rotational
speed. The high-end filter setting should be 6 orders of the outer race
frequency (about 200X rotational speed).
12. What
bearing defects generate only low-frequency signals?
Non-impact type bearing
signals, such as pure tone/waviness types, generate only low-frequency
signals and are not candidates for enveloping technology.
13. What
sensitivity is required for an enveloping sensor?
The recommended sensor
sensitivity to optimize the signal to noise ratio lies in the range from
10 to 500 mv/G (centered around 50-100 mv/G) based on the application.
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