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Using Vibration Monitoring Equipment For OTHER
Functions |
by:
Steve Goldman, P.E. - Author of Vibration
Spectrum Analysis
Originally published at MaintenanceResources.com
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Introduction: |
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In recent years, it has become rather
common for large and medium sized facilities to
begin predictive maintenance programs employing
narrow band FFT- based vibration analysis
equipment. The cost savings involved in this
sort of quality predictive maintenance program
easily justifies the ten to twenty thousand
dollar cost of the data gathering box and
related software. |
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The popularity of these new data gathering
boxes is such that one is more likely to find
one of these compact devices in the maintenance
department of a given plant than to find a fully
equipped dual channel spectrum analyzer in the
engineering department. |
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Thus, a strange thing has happened over the
years: the Maintenance Department has become
better equipped to handle some engineering
problems than the Engineering Department. While
the predictive maintenance technician may well
gloat over his superior capabilities in the area
of vibration measurement, he should recognize a
higher responsibility to his employer by
learning some of the non-P/M applications of his
FFT-based box. With some additional training,
the P/M technician can assist Engineering in
solving some of the engineering problems which
may cause his company the loss of future sales
or the damaging of their reputation for quality,
causing harm to the Company Profit and Loss
statement. |
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The following pages will discuss some of
the areas in which the user of a hand-held FFT
monitoring device may be pressed into service to
help solve engineering problems for the Quality
Control, Manufacturing Methods, Engineering, and
the Service departments of a manufacturing
plant. Be warned at the outset, however, that a
hand-held device is no match for a fully
equipped dual channel spectrum analyzer. We are
talking about making do with what you have, not
about what could be done with the proper
equipment. |
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Quality Control:Demonstrating compliance to a
specification |
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As the popularity of predictive maintenance
has grown, it has become more common for the
buyer of new machinery to insist that the
supplied machinery meet certain vibration and/or
noise specifications before accepting delivery.
It has long been the practice of governmental
agencies, such as the Navy, to require that
equipment meet certain sound and vibration
specifications. When one supplies critical
machinery to a nuclear facility such as a power
plant, it is often required that the equipment
manufacturer submit proof that his machinery
will perform satisfactorily during earthquake
conditions. |
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Since the P/M technician is capable of
obtaining a narrow band FFT data on manufactured
equipment, he is in a position to help the
quality control department verify the ability of
the plant's products to meet these various
customer specifications. |
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Customer Sound and Vibration Specifications: |
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Customer vibration limits are often
included in the original purchase order for the
equipment. Sometimes, due to an unfamiliarity
with vibration analysis, the added cost of
verifying compliance to a vibration spec. is not
added to the price of the machinery. A vibration
test is never thought about until the customer
either notices the lack of the test amongst the
papers transmitted from the vendor or, worse,
when the machine is mounted in place and shaking
violently. |
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The P/M technician is well qualified to
deal with the problem of verification of
acceptable vibration limits during the
operational tests of the unit. All that is
necessary is that the machine be rigidly mounted
in an area of low background vibration. The test
should be run at normal operating conditions,
after the machine has reached thermal
equilibrium. The P/M data gathering box should
the be used to gather FFT vibration data at each
bearing of the machine in each of three mutually
perpendicular directions. |
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During the above impromptu shop acceptance
tests, care must be made to ensure proper
alignment of the motor, proper operating
conditions, and adequate mounting. If the test
is run using a dynamometer rather than the motor
to be shipped with the machine, it is important
to determine the forcing frequencies of the
Dynamometer. If these frequencies have
sufficient amplitude to exceed the vibration
limits of the specification, some negotiations
with the purchaser will have to be made before
shipment. It is always best to be honest before
shipment than to face back charges and/or
litigation when all remedies must be taken at
the job site. |
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Government Acceptance Tests: |
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Governmental agencies such as the Navy
typically require that machinery meet certain
Octave Band sound levels and 1/3 Octave Band
vibration limits as spelled out in MIL STD 740.
Although it is possible to simulate Octave and
1/3 Octave Band data by taking two or three sets
of 400 line FFT data and combining the energy in
each of the 400 filters in certain ways via
computer, don't bother. |
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Firstly, the MIL STD requires certain
levels of accuracy which your hand-held FFT is
unlikely to meet. Further, all of the test
equipment used must be calibrated to certain
Government standards every six months. As this
is quite a costly matter, it is probably less
expensive to perform the final tests at the
facilities of an outside vendor who specializes
in Governmental testing. |
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All is not lost, however. The Government's
Octave and 1/3 Octave Band tests are throw-backs
to the days before narrow band FFT analyzers. It
is quite difficult to find the cause of a
problem in these wide band filtered sets of
data. Therefore, the machinery to be tested
should undergo a narrow band analysis of its
vibration and sound characteristics to avoid the
embarrassment and cost of flunking the official
test. Note that a flunk will occur before the
very eyes of the government DCAS officer who
will witness all of your future qualifying
tests. |
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The preliminary vibration tests can be run
in the same manner as any predictive maintenance
test. The sound tests require replacing the
accelerometer with a microphone and power supply
designed to fit the particular FFT instrument.
Calibration for sound can be approximated by
following the instructions of the microphone
vendor. By going to the trouble of performing
the suggested tests, your company can proceed to
the official Government tests with confidence of
passing on the first try. |
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Nuclear Specifications: |
The nuclear regulatory agencies of various
countries (including our own) insist that
various pieces of critical machinery be capable
of withstanding operation during an earthquake.
One of the "standard" earthquakes is shown in
Figure 1. The verification of this
capability can be achieved either through a long
series of analytical calculations (whose
underlying assumptions can be thrown out at any
time by the inspector assigned to review) or via
an actual test on a shaker. |
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A given piece of well designed machinery
can usually pass the static g loading of the
earthquake. The reason for failure is more
likely due to the existence of a natural
frequency in the region of frequencies where the
earthquake has high energy. This has the effect
of amplifying the earthquake at that frequency
by a large factor. Thus, a machine without a
natural frequency at, say 1.0 Hz. will see a
velocity excitation of 100 in/sec. (see
Figure 1). A machine with a lightly damped
natural frequency at 1.0 Hz. might think that
the same test is exciting it at 2,000 in/sec. at
100 Hz. This machine will probably fail. |
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Although a completely correct test for
natural frequencies requires a dual channel
spectrum analyzer, an approximation of what may
occur on the shaker test stand can often be
achieved using a hand held FFT analyzer. An
accelerometer is mounted on the machine and the
machine is struck with soft objects such as a
2X4 (to elicit a low frequency excitation
spectra). A high amplitude response at some
frequency, as seen by the FFT device, is an
indication of a possible natural frequency. This
test is not conclusive, but as stated at the
outset of this paper, is better than nothing in
terms of locating possible problems before the
official shaker test. |
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Reducing Production Costs: |
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The labor costs in manufacturing components
for large pieces of machinery is directly
proportional to the speed at which the parts can
be manufactured in existing lathes, boring
mills, etc. There have been tremendous advances
in tool bit cooling techniques in recent years
which have allowed manufacturers to increase the
speeds and feeds of many machining operations.
At some point, however, the maximum allowable
speeds and feeds are limited by tool bit
chatter. The hand held FFT data gathering box
can be used to investigate these problems, often
helping arrive at a solution. |
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Tool chatter and excessive noise are caused
by a phenomena called sticktion. The tool bit,
which normally cuts through the metal being
machined, adheres to the metal surface due to
the force on the tip of the bit. The tool bit is
pulled down with the motion of the rotating work
piece until the force on the tip is enough to
overcome static friction. Since the coefficient
of static friction is greater than the
coefficient of dynamic friction, the tool bit
bounces back up until the force of cutting is
sufficient to pull it down again, due to static
friction. The resultant motion of the tip of the
tool bit is similar to the motion of a spring
mass system excited by a series of impacts. The
frequency of the chatter is equal to the natural
frequency of the tool bit/support system. |
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When sticktion develops, it is usually
necessary to reduce the speed and/or the feed of
the cut. This increases machining time and
reduces profit. |
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Two other solutions to the problem exist:
One can increase the natural frequency of the
tool bit/support system by increasing the
stiffness. This would move the chatter frequency
to a point where it takes higher speeds and
feeds to excite it. An accelerometer of
sufficiently small mass to avoid mass loading
the tool bit can be used with a hand held FFT
box to investigate the problem in a manner
similar to that discussed above for finding
seismic natural frequencies for nuclear tests.
Note that the impact device, in this case, must
be hard in order to cause relatively high
frequency excitation. These tests will reduce
the cost of a trial and error solution. |
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A second solution to the problem is to
ensure that the tool bit always remains in
motion (so that it is seeing dynamic friction
rather then a static/dynamic/static friction
cycle). This can be done by mounting a small
exciter on the tool bit/support system such that
the tool is always moving at some high
frequency. The frequency of excitation must be
high enough that the displacement perturbations
of the tool bit are small enough to allow for
meeting the necessary surface finish
specifications. Again, the FFT box can help in
the development of the exciter system. |
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New Product Development: |
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The development of a new product is a
costly venture involving design problems, the
building of patterns for castings, the
fabricating of parts, and many hours of
prototype testing. By changing the input
transducer of the hand held FFT data device from
accelerometers to microphones to pressure
transducers, the P/M technician can find any
undesirable oscillatory characteristics of the
new product while still in the prototype stage.
This allows the engineer to make the necessary
changes in design before the final designs and
patterns have been finalized and imperfect
machines have been shipped. Again, the use of a
dual channel spectrum analyzer would be more
help in the process than a hand held box, but
the hand held box is better than ignoring
potential problems altogether. |
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Field Problems: |
No
matter how perfect a given piece of machinery is
when it leaves the manufacturers door, the odds
are good that the customer will misapply,
misalign, or misuse it, causing high levels of
vibration or noise. The finger of guilt
invariably points to the machine manufacturer.
It is in the best
interest
of the machine manufacturer to have the ability
to examine the operation of the machine under
actual on site conditions to determine whether
the problem is the fault of the customer or the
vendor. This ability insures that the liability
falls where it should. It is much easier to
assign fault to the "other guy" if correct
instrumentation is used to ascertain the facts.
Often, the P/M technician has the only piece of
instrumentation in the company able to do this.
An actual example of a simple problem which
could have cost the vendor tens of thousands of
dollars was resolved with a simple filtered
vibration measurement device. |
A new power plant had two
large pump packages mounted on a mezzanine as
shown in Figure 2. Unit #2 ran well, but
unit # 1 shook the entire mezzanine. The pump
manufacturer was being blamed. The threat of
multiple lawsuits filled the air. The power
company was refusing to accept delivery of the
plant until the problem was resolved. The
engineering company and construction company
were ready to sue each other as well as the pump
manufacturer. |
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A set of tests were run by the author using
a simple tunable filter vibration meter (Some of
up are old enough to pre-date FFT equipment).
Both pumps were shut down. A pneumatic impact
hammer was fastened to the mezzanine near pump #
2. The speed of the hammer was slowly adjusted
until everyone present agreed that their feet
tickled most - tunable filter analyzers are too
slow to run "real time" data as can the modern
FFT data gathering box. Vibration data taken on
the impact hammer showed that the predominant
frequency excited by the hammer was equal to the
blade frequency of the pump. |
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The conclusion was simple: The design of
the mezzanine was such that it had a natural
frequency equal to the blade frequency of the
pumps. Pump # 2 was located near a node of the
floor, making it almost impossible to excite the
structure at that frequency from that location.
Pump #1 was at an anti-node, making excitation
at that location very simple. The structural
design engineer was at fault. The pump
manufacturer, through the use of simple filtered
vibration equipment, was found to be innocent. |
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Summary: |
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An FFT based vibration monitoring device,
in the hands of a properly trained technician,
can do far more for the good of the company than
simply performing predictive maintenance
functions. It can be used by quality control for
compliance testing, to reduce manufacturing
costs, in new product design, and in dealing
with field service problems. The well trained
P/M technician, then, has the ability to widen
his horizons as far as is permissible by his
employer.
More Vibration
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