Overall vibration, severity levels
and crest factor plus
By Dr. George Zusman, Director of Product Development, PCB Piezotronics
and Glenn Gardner, Business Unit Manager, Fluke Corporation
White Paper
High
frequency
4,000 Hz to
20,000 Hz
Low frequency
10 Hz to
1,000 Hz
Temperature
-20 °C to
200 °C
01/09/2011 09:10 AM
Bearing
3
Overall Vibration
0.42
ID :FA N1:BEAR ING1
TY PE :Axial Fan
RPM :> 6 00
CF+
GOOD
in/s
(cal pk)
UNACCEPTABLE
Temperature
68.7˚F
Bearing
vibration
(CF+)
Overall
vibration
IR Temperature
Introduction
Mechanical Vibration is a notoriously difficult
subject matter to master. A quick glance at a
textbook in the field shows that the advanced
practitioner must master sophisticated concepts
in mathematics and physics in order to accurately
collect and interpret vibration data. This has
caused a major problem for industrial plants –
the benefits of a vibration testing program are
well known, but far too many plants choose to
avoid vibration testing because of the perceived
complexity.
Fluke aims to solve this dilemma by developing
easy to use vibration products that yield significant
benefits without requiring advanced training
by end-users. Fluke’s first vibration product,
released in 2010, was the 810 Vibration Tester
(see picture on next page). The 810 is a vibration
spectrum analyzer that includes an automated
diagnostic engine that identifies the four most
common root causes of abnormal machinery
vibration: Misalignment, Unbalance, Looseness,
and Bearing Faults. The 810 is dramatically easier
to use than existing spectrum analyzers because
the interpretation of frequency spectrum data is
automated. The user simply sees a severity scale
for each of the four faults listed above.
Severity Scale
GOOD
SATI SFACTO RY
UNS ATISFAC TORY
UNACCEPTABLE
No immedi ate repa ir is reco mmende d.
No immedi ate repa ir is required. Increase the
frequency or measurements and monitor the
condition of the machine.
Have kno wledg eable v ibration technician
conduc t more ad vance d testing at the earliest
opportunity. Consider taking maintenance
action during th e next planned downtown or
maintenance period.
Have kno wledg eable v ibration technician
conduc t more ad vance d testing a s soon as
possible. Consider immedia te shutdo wn of the
machine to m ake repairs and prevent failure.
Fluke’s second vibration product, the 805 Vibration
Meter, was released in 2012. The 805 is an
overall vibration meter, as compared to a spectrum
analyzer like the Fluke 810. The 805 provides
the user a simple overall vibration number for
both low frequency (10 – 1,000 Hz) and high
frequency (4,000 – 20,000 Hz) ranges. In addition
to the overall vibration value, the 805 provides a
severity scale for both frequency ranges. The low
frequency severity scale can be adjusted to reflect
the particular type of machine being tested (pump,
compressor, etc.), and is a great way to screen
for problems such as misalignment, unbalance,
looseness, bent rotors, etc. The high frequency
severity scale, known as the “Crest Factor
Plus” scale, uses a novel proprietary method of
processing time domain vibration signals to identify
bearing flaws. The 805 also provides the user a
non-contact temperature reading for all vibration
points tested. The 805 overall vibration meter is a
screening tool. Once an adverse vibration condition
has been identified by the 805, it is recommended
that the team perform more advanced testing to
validate the problem and identify the root cause of
the vibration.
In this white paper, we’ll discuss the following
topics:
• The difference between frequency spectrum
analysis and overall vibration analysis.
• Overall vibration analysis and severity scales
• Crest Factor Plus: High frequency overall
vibration analysis and severity scales
Vibration spectrum analysis vs.
overall vibration analysis
All vibration data is collected in the time domain.
When using a spectrum analyzer or overall vibration meter, this time domain data is typically
collected from a piezoelectric accelerometer.
The accelerometer is placed in rigid contact with
a vibrating machine. The machine vibration is
transferred to the accelerometer, which in turn
strains the piezoelectric element within and creates a voltage signal proportional to the vibration.
This voltage signal is captured by the spectrum
analyzer or overall vibration meter. A sample time
domain signal is shown on next page (Fig A).
Once the time domain signal has been captured,
Slight
Moderate
Serious
Extreme
No repair action is recommended. Retest the machine
and monitor condition after maintenance.
(Months, even up to a year) - No immediate repair action
required. Increase the frequency of measurements and
monitor the condition of the machine.
(Weeks) - Take maintenance action during the next
planned downtime or maintenance period.
(Days) - Immediate action is required. Consider shutting
down the equipment and taking repair action now to
avoid failure.
there are two common analysis methods that can
be performed: spectrum analysis or overall vibration analysis.
In spectrum analysis, the time domain signal
is transformed into a frequency domain signal
using an algorithm known as the Fast Fourier
Transform (FFT). A detailed description of the FFT
is beyond the scope of this white paper, but in
short it reconstructs the time domain signal using
a series of harmonic sine waves. The amplitude
of each of these sine waves is then plotted against
the frequency of the sine waves. The result is
known as a frequency spectrum plot. A sample
frequency spectrum plot is shown on next page
(Fig B). A properly trained vibration analyst can
use the frequency spectrum to determine which,
if any, machinery flaws exist. The analysis can be
somewhat complicated, but a significant benefit
is that the root cause of the vibration problem can
typically be identified.
2 Fluke Corporation Cali bration and documentation for process manufactu ring: Costs, benefits and feasibil ity
FFT
Figure A Figure B
Overall vibration analysis, by contrast, is much
simpler. In overall vibration analysis, the time
domain data is used to calculate one single overall
vibration value. This single value can be used as
an indicator of the overall health of the machine.
(Note: there are multiple methods available for
calculating the overall vibration value. They
are described in the appendix for the interested
reader). The benefit of overall vibration analysis is
its simplicity – there is only one number to examine. However, there are three limitations of using
overall vibration analysis that are worth noting:
• Different machines have different healthy levels
of overall vibration.
• Certain machinery flaws, particularly bear-
ing flaws, will not impact the overall vibration
value until significant damage has occurred.
• Overall vibration analysis does not identify the
root cause of vibration
Despite these limitations, overall vibration
meters are still very valuable as a screening tool.
Technicians can use them to trend values over
time, and if any adverse trends are identified a
more thorough analysis (such as spectrum analysis) should be performed to validate the problem
and identify the root cause.
It is noteworthy that Fluke has designed the 805
vibration meter to help mitigate these limitations,
as discussed on the next page.
Overall vibration analysis
and severity scales
As discussed in the previous section, there are
three key limitations of overall vibration analysis.
In this section we’ll explain a bit more about each
of these limitations, and then show how an overall
vibration meter can be an effective vibration
screening tool despite the limitations. We’ll
also show how the Fluke 805 vibration meter
specifically helps to overcome the three limitations
of a conventional vibration pen.
The three limitations of overall
vibration analysis
The first limitation of Overall Vibration Analysis
is that different machines have different healthy
levels of overall vibration. Rotary blowers, for
example, have very high overall vibration levels
even when they are in a healthy state. This is
caused by the basic mechanical design of the
machine, which consists of two parallel shafts
with interleaving lobes rotating in synchronization
to compress air. By comparison, single-end
suction centrifugal pumps (which have only one
rotating shaft and an impeller rotating inside of
a volute) tend to have very low overall vibration
levels in the healthy state. An overall vibration
level of 0.300 in/s on the centrifugal pump
is indicative of a problem with the machine,
whereas that same vibration level on a rotary lobe
compressor indicates a healthy machine.
The second limitation of overall vibration analysis
is that certain machinery flaws, particularly
bearing flaws, will not cause an alarm in the
overall vibration level until significant damage has
occurred. When a bearing is first starting to fail
due to a flaw on the outer race, inner race, or ball/
roller, very short duration spikes will appear in
the time domain each time the flaw makes contact
with an adjacent bearing component. These
spikes have very low energy, and therefore cause
very little change to the overall vibration value.
As the bearing damage progresses, the spikes
will continue to increase in energy along with the
baseline vibration of the machine, and ultimately
will cause the overall vibration value to increase
3 Fluke Corporation Overall vi bration, severit y level s and crest factor plus
Loading...