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 Use
of a Stroboscope for Coupling Inspection
An excellent way to inspect a coupling without
stopping the machine is to use a stroboscope (hereinafter referred to as a
strobe). First you must be able to visibly see the coupling; then, using a
strobe, adjust the flash rate to match the RPM of the rotating shaft. (This
will cause the shaft to appear to be stopped. Remember that it only appears
to be stopped!) Thereafter, by increasing or decreasing the flash rate very
slightly the shaft will appear to rotate slowly and allow the user to
visibly inspect for abnormal conditions such as insert wear, broken or bent
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Using
Cross Channel Phase for Soft foot Determination
The use of phase, either Absolute (tach w/fixed
reference point) or Relative (cross channel measurement using two waveforms)
is a very effective way to check for a Soft or Loose foot condition by
comparing the phase readings between the machine foot, base, soleplate,
foundation, and even the slab. If the phase shows a significant shift
(greater than 20 degrees) you probably have a soft foot or looseness between
the two surfaces.
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VIBRATION
TRENDS
Always set up trend parameters in your
vibration database. Most vibration analysis software will allow the overall
equipment vibration and specific frequency ranges to be trended over time.
Trending this information will help identify problems as they occur in your
equipment. Also, this will help determine the severity and repair urgency of
the problems identified. For example, if the vibration trend is increasing
slowly, then the failure may not be progressing rapidly. However, if the
vibration trend increases sharply between readings, then the problem may be
progressing more rapidly than previously thought, and failure may be more
imminent.
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WATERFALL
PLOTS
Vibration waterfall plots are a very good
analysis tool to identify changes in equipment condition. Waterfall plots
can reveal the development and progression of specific equipment failures.
Also, waterfall plots can be used to show management the progression and
severity of identified faults.
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Multi-Mode
vs. Balancing Readings
Have you ever wondered why your vibration
readings may be significantly different when taking the readings in
Multi-mode (off-route) as compared to the same readings taken in Balancing
mode?
Commonly it is thought that taking the same
type of reading (displacement or velocity) throughout the same frequency
range should give exactly the same result. So, why doesn’t this happen?
When taking the reading in Multi-mode, the
reading incorporates the total energy throughout the frequency range. When
taking the reading in the balancing program, the reading eliminates all
non-synchronous energy. Therefore, it is common for the balancing mode
reading to be smaller than your multi-mode (off route) reading.
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KEEP
THINGS SIMPLE
Many vibration programs fail because they
become too complicated. Too much data can sometimes become more confusing
than too little data. Many potential machinery problems can be eliminated
with the analysis if one keeps in mind several simple concepts
1. Vibration units such as acceleration are
more sensitive to high frequencies than low frequencies.
2. Vibration units such as displacement are
more sensitive to low frequencies that high.
3. Velocity units are evenly sensitive
between about 60 CPM to 60,000 CPM.
4. High frequency vibration does not travel
far and degrades rapidly through metal seams.
5. In general the closer your measurement is
to the source of the vibration the higher the amplitude will be.
These differences can be used to zero in on
machine faults.
Example Take a generic 100 HP motor. If an
outboard rolling element bearing begins to fail because of a lack of
lubrication the first indicator is high frequency ringing from the bearing.
This is characterized by a large increase in acceleration amplitude and
small to no increase in velocity or displacement. Now you have identified
that there is a high frequency problem and not a low frequency mechanical
problem. You can eliminate low frequency sources such as looseness, balance,
or misalignment. What is the most likely source of high frequency vibration
on the back end of a motor? Probably a bearing or shaft or rotor rub.
Now you can apply a simple test. Grease the
bearing and see if the acceleration returns to normal. If it does, you have
nailed the problem without knowing the bearing frequencies or even taking a
spectrum. Come back the next day and see if the acceleration is back up. If
it is, you either have a lubrication problem with contamination or a loss of
grease, a damaged bearing or both.
While not perfect, understanding the behavior
of vibration units combined with a mechanical understanding of machinery can
help you quickly identify machinery problems.
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VISUAL
INSPECTION
Always conduct a visual inspection of
equipment when acquiring vibration data or any technology data in the field.
Some vibration software will allow the technician to document visual
observations in the field. This data can be used to report extraneous
equipment and safety problems for repair. In addition, this information can
be used by the analyst to verify specific equipment defects that have been
identified in the vibration data collected.
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AMPLITUDE
Why aren’t my amplitudes the same on my
data collector as on the contractor’s data collector?
When an overall vibration reading is taken on
two different manufacturer’s data collectors, the readings may be
different. This often prompts the user to question the accuracy of their
data collector.
The explanation for this is most often quite
simple Some data collectors, like the VibXpert, measure the true peak value
whereas other data collectors display a calculated peak.
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Predictive Maintenance
Always document "Lost
Opportunities" in your predictive maintenance program. "Lost
Opportunities" occur when equipment goes down due to an unpredicted
failure. This documentation allows the predictive maintenance program to
evaluate that the correct technologies and setup are being applied to
correctly determine the failure modes on equipment. Also, this documentation
can be used as an example to management for identification of additional
technologies or additional resources that may be required to identify the
equipment failures in the future.
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Predictive Maintenance
Always make a routine backup of your
vibration database and any other technology database files used in your
predictive maintenance program. It is a good idea to store your database
files on a network drive when possible. This will allow your site IT group
to create a routine backup of your data. Otherwise, periodically copy your
technology database files to a network drive or a back-up device.
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Peel-Offs
for Your Data Collector
Your data collector is a high-tech piece of
equipment with a viewing screen that is critical to its ability to be used
in the field. Some working environments pose a hazard to your equipment that
may render it unusable due to the scratches on the screen. This is
especially the case in cement plants, for instance. To clear the cement dust
which collects on the plastic LCD lens, users simply rub the screen off with
their hand. This scratches the screen and eventually requires a complete LCD
replacement, which is generally expensive.
At any of the office supply chains like
Staples or Office Depot, Mylar PDA screen covers are available. It is a
simple process to cut these to size and apply them to your data collector
screen. Some of the newer multi-channel analyzers have a large screen. If
your data collector screen is too large for a standard PDA cover, then look
to the analyzer’s manufacturer for a Mylar cover kit. Many of these have
adhesive backing.
To apply adhesive coated covers, do this
1) Make sure your data collector is
waterproof-approved for a wash down.
2) First clean the screen surface with a mild
dish soap / water solution. You want the solution to be only slightly soapy.
3) Once clean, wet the screen with the dish
soap / water solution.
4) Rinse the screen until all bubbles are
gone but the surface is still soapy.
5) Bend or arch the plastic so that the
adhesive is bowed toward the screen and centered directly over it.
6) Gently place the cover on the screen. The
soap will allow some movement to center the screen but if things get out of
hand remove the Mylar and return to step 2.
7) Use an old credit card or similar piece of
plastic as a squeegee to push the soap / water solution out from under the
cover. Work from the center of the cover to the edges and use a paper towel
to absorb the solution from the edges of the Mylar. No credit cards? You can
use your thumb starting with small circles in the center and work your way
to the edges.
This should allow you to use your data
collector in adverse dust and dirt conditions while protecting the screen.
When the cover gets scratched simply peel it off and replace with a fresh
cover.
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Considerations
when setting up measurements
When setting up measurement in a vibration program the
analyst needs to consider several factors:
1. What type of equipment is going to be monitored?
2. Is the machine constant speed or variable speed?
3. What are the primary frequencies of interest?
4. What kind of resolution will be required to
distinguish these frequencies?
Download
this M-Tip (PDF file)
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AMPLITUDE
When checking the vibration levels on a fan,
look at the amplitude corresponding to the blade pass frequency. Does the
damper setting change these amplitudes? By simply changing the damper
setting, the vibration might be dramatically reduced or increased.
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LOADED
VS UNLOADED
It is considered better practice to evaluate
the vibration of a motor while it is coupled to the driven machinery.
Sometimes the characteristics of an electrical problem don’t show when the
equipment is unloaded.
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AMPLITUDE
If there is a dramatic change in the
amplitude of your vibration readings, a rotor rub could be the suspect. A
rubbing rotor may change the stiffness of the rotor, thereby changing the
natural frequency. The equipment may now be running in a resonant condition
due to this change in natural frequency.
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DATA
COLLECTION
Sometimes the simplest things are overlooked.
When taking vibration data, remember to always prepare the surface of your
machinery before mounting the accelerometer. It is best to clean the
surface; this may include sanding or wire brushing the surface to remove any
paint that may interrupt the transmission of the vibration data. If the
surface is curved, it is not recommended to attach an accelerometer with a
flat magnet, try a curve surface magnet that has two legs (poles) that
contact the surface.
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PHASE
DATA
When analyzing phase data to check for force
unbalance, the horizontal inboard and outboard readings will most likely be
very similar to the vertical inboard and outboard readings. But if couple
unbalance is the culprit, the horizontal readings on the inboard and
outboard readings will be approximately 180 degrees out of phase.
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DATA
COLLECTION
When using a portable vibration data
collector in hard to reach areas it is highly recommended to install
permanently mounted accelerometers on those points and run cable to a switch
box. It will make data collection easier and safer.
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REPORTING
To help justify your vibration analysis
program, try providing your machinery’s health in a report. Print out a
statistical report showing how many pieces of equipment are within
acceptable limits versus how many are outside acceptable limits. Often, you
think management only wants to see the bad equipment, but one of the major
benefits is showing a problem in its infancy, before the failure.
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ONLINE
DATA COLLECTION
When selecting an online vibration analysis
system, be sure to purchase a system which will allow the user to select a
variety of sensors such as current line drive (CLD), ICP, 4..20 mA,
proximity probes, thermocouples,etc. When you choose a system that has this
much flexibility, you're maximizing the capabilities of acquiring data on
most machinery.
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ONLINE
DATA COLLECTION
Vibration data collection on critical
equipment, which is located at remote sites such as wind turbines,
pipelines, & draglines, are very difficult to monitor. Install a
continuously online system that provides trending, analysis, alarm, and
notification capabilities.
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WINDOW
FACTOR
Remember to adjust the window factor on your
analyzer. Different window factors work better for different tests. For
instance, if you're going to conduct an impact test, switch to a uniform
window. This offers good frequency certainty and is ideal for transient
equipment.
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productivity
Remember that some of the data collectors on
the market allow users to document and log other parameters, not just
vibration readings. Some products offer collection of visual data, such as
oil levels or foundation condition. When collecting data the user can pick
from a list selection showing the gauge is empty, half-full or full. Users
can keep track of all the oil levels with their data collector without the
need for running a separate route. Other readings, like manual entry allow
the user to add numerical entries corresponding to a condition, like
pressure. Maximize the productivity of your systems and personnel.
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adaptive
trigger
To save time while collecting routine
vibration data, try using a feature called adaptive trigger. This is a
feature offered on some data collectors that only collects spectral data if
another criteria is met. For instance, if an overall vibration reading is
above a certain level, say above the ISO alarm level, then and only then is
a spectrum taken. For closely monitored equipment this is a great time
saver.
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sensors
Have your portable sensors calibrated every
two years to ensure their performance. Heavy impacts (as in magnetic
mounting applications), exposure to high temperatures above their
specifications and exposure within a high DC field can cause degradation in
their performance. It is a good idea to have your data collector checked and
re-linearized at the same time.
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CROSS
CHANNEL PHASE
Always pick two vibration sensors of the same
model when measuring Cross Channel Phase. Otherwise, differences in the
sensors’ high pass & low pass can influence the phase readings.
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Clean
that Magnet
Taking accurate vibration measurements is critical to
the success of any vibration program. The weak link in this process is often
the connection between the accelerometer and the machine being measured.
Following several simple procedures will enhance the accuracy and
repeatability of your measurements.
1. Avoid taking vibration measurements through paint.
Direct metal-to-metal contact on a flat spot is vastly superior for
transmitting vibration to your accelerometer. This may require spot facing
and possibly gluing on a metal slug using a high metal-content epoxy potting
compound similar to Belzona Super Metal.
2. Do not slap a magnet-mounted accelerometer onto the
measurement surface. This can cause the magnet to ring for several seconds,
which can adversely affect your measurements. Set the magnet down softly on
a corner or edge of the magnet then rock it flat making sure it is seated
solidly.
3. And finally, clean that magnet, and clean the
surface onto which you will place it. Small metal shavings, metal dust and
greasy dirt tend to stick to the magnet. Wipe off the magnet with a rag
before placing it on a measurement location. This will help ensure a solid
interface with the machine and further enhance the consistency of your
measurements.
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PHASE
DATA
Do you need phase data to better analyze your
equipment but don’t have the time to set up a tachometer at each
measurement location? Try using cross-channel phase readings. This is a
feature offered by some vibration analyzers. This reading uses two
accelerometers to take vibration data and the difference is phase is
calculated between both of them. Often times a coherence value will also be
displayed, which verifies the validity of the readings.
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Spectrum vs. Cepstrum
When a spectrum contains several sets of
sidebands or harmonic series, it can be confusing due to the overlap. Try
using a Cepstrum. A cepstrum is a spectrum of a spectrum. In the cepstrum,
they will be separated in a way similar to the way in which the spectrum
separates repetitive time patterns in the waveform. Try using a cepstrum on
your Gearboxes and rolling element bearings.
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Looseness & Vibration
If you are having trouble determining if
looseness is truly causing the vibration in your machinery, take a look at
each component individually. For example, take vibration readings on the
machine foot, baseplate and the base, then compare the amplitude and phase
readings. If the equipment is not experiencing any looseness, the phase
readings should be relatively close. If looseness is actually the issue, the
phase and amplitudes of each component will be considerably different.
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Improving vibration sensor performance
For magnetically mounted accelerometers, keep
your magnets clean and free from metallic chips with the use of the sticky
side of Duct Tape or packaging tape. Removal of this debris will improve
sensor performance. Occasionally, rub your sensor magnet on 400 to 600 grit
sandpaper to remove any high spots, this will maximize the contact area,
thus improving the sensor performance.
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Spectrum Analysis
Sometimes when analyzing a spectrum, it is
difficult to differentiate between unbalance, looseness and misalignment.
Their signatures all have a 1 times RPM peak and multiples of running speed
peaks, excluding pure unbalance. One way to quickly decipher what is causing
excessive vibration is to look at the time waveform. For instance, unbalance
will display a sinusoidal pattern, whereas looseness will show random
impacting.
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Selecting a vibration sensor
When purchasing a new vibration sensor there
are many factors to take into consideration. Make sure the desired machinery
measurement parameters fit within the sensor Fmin & Fmax capabilities.
If you’re trying to capture 3x gear mesh frequency information, you will
need a sensor with a frequency response that is greater than this value.
Ensure the sensor is suitable for the environmental conditions. If it will
be used in an electrically noisy area, an electrically isolated sensor may
be necessary. Does the sensor meet the temperature requirements? One aspect
that is often overlooked is the physical size of the sensor. If there are
limited clearances due to piping or protective covers, will the sensor fit?
Remember, cost isn’t the only consideration when choosing a sensor.
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Magnetic
Center
When performing an alignment on a machine train with a
motor fitted with a sleeve bearing, it is important to account for the
magnetic center of the motor. Failure to do so can cause excessive vibration
and premature failure of motor components and the shaft coupling.
If the motor has recently been rebuilt is should come
from the motor shop with a magnetic centerline scribed on the shaft. To
properly set the shaft coupling gap do the following:
- Determine the correct coupling gap based on the
manufacturer’s recommendation. (Note, we refer here to the proper
installation gap size and its tolerance, not the alignment gap
tolerances for angularity.)
- Identify the correct scribe mark on the shaft that
represents magnetic center.
- Measure the distance between the scribed mark and
the outside bearing housing lip. In the case that the magnetic center
scribe mark falls inside the motor housing while at rest, scribe a mark
in the rest position. While the machine is un-coupled run the motor and
estimate the difference between the newly scribed mark and the magnetic
center mark.
- This is the distance that will need to be
compensated for when setting the coupling gap.
- Set the coupling gap according to the manufacturer’s
recommendation minus the distance measured for the magnet center
correction if the mark is outside the bearing housing. Add the
difference if the mark is inside the bearing housing.
- This will provide the proper coupling gap under the
normal running condition.
Avoid excessive vibration problems with your sleeve bearing motors by
following these simple steps.
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Measurement Etiquette
The proper usage of the accelerometer is similar to
the way Doctors use their stethoscope.
1. Positioning for bearing fault frequencies - the
vibration pick-up (sensor) should be placed close to the load zone of the
bearing. For horizontal shaft positions rotating clockwise (CW) place the
sensor at the 730 o'clock position. Shafts rotating counterclockwise (CCW)
place the sensor at the 430 o'clock position.
2. To monitor internal bearings in a gearbox the
sensor should be mounted in-line with the webbing that comes in contact with
the bearing of interest. Assembly drawings of the gearbox help in
positioning.
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Sensors
When mounting a sensor, for non-permanent applications
use silicon grease or petroleum jelly between contact surfaces and for
permanent applications use an approved adhesive. This will fill any voids in
the contact area and improve the sensors performance.
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SOFT FOOT
Vibration analysis can be used to determine if a soft
foot condition exists on a machine that is running. If you cannot shut the
machine down to check for soft foot, using this simple procedure will help
determine if soft foot is your problem:
1. Mount your Accelerometer on the suspected soft
foot.
2. Use the live monitoring mode of your vibration
analyzer to monitor the 1x rotation speed vibration.
a) Use enough resolution to distinguish the desired
peak, but not so much that it requires long collection time between
averages.
3. While monitoring the 1x vibration, loosen the
mounting bolt to about hand-tight.
4. If the vibration at 1x rotational speed reduces
significantly in amplitude it is very likely that you have a soft foot
condition.
5. Re-tighten the mounting bolt and schedule the
appropriate corrections.
When corrections are not made to eliminate a soft foot
condition, the foot will deflect when tightened. This will affect the
alignment and the motor air gap, and can cause significant vibration at
rotational speed. By loosening the foot while running, the force deflecting
the foot is removed.
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BEARING DEFECT FREQUENCIES
When learning vibration analysis we are taught that
determining bearing defect frequencies is as simple as a mathematical
formula. Simply input some numbers and presto! While this is the fundamental
technique needed to determine these frequencies, there are some other
important considerations that need to be made. If you find that you have a
frequency that you believe is a defective bearing but you just cannot get
the frequencies to line up with your fault frequency overlays here are some
things you need to consider
1. Check the true running speed of the machine. You
may have the machine identified as an 1800 rpm machine that is actually
running 1780 rpm.
2. Ensure the bearing characteristics match those in
your vibration database. Most vibration software comes with a database of
bearings. You may find your bearing number in that database, however the
actual bearing may be slightly different. The bearing may have one more or
one less ball than the selected bearing.
3. Determine if axial loading is occurring. High axial
loading will change the roller path and thus effect the frequency
calculations. As the axial load increases the rollers have less distance to
travel because they are being forced on the raceway shoulder.
4. Determine if wear is an issue. As a bearing wears
over time, the distance the rollers have to travel will increase as a
result. The rollers will actually decrease in diameter as wear occurs.
5. Determine if you have identified the right bearing.
Occasionally when equipment is rebuilt, the original bearings are replaced
with a similar bearing of a different brand. This can lead to differences in
bearing defect frequencies.
6. Ensure you have enough resolution to separate
defect frequencies from running speed harmonics.
If you follow these simple guidelines for determining
the proper bearing defect frequencies, you will find that your fault overlay
will match your suspected bearing defect frequencies a much higher
percentage of the time.
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Proper Shaft Key Length
Proper shaft key length is often overlooked
when performing repairs or troubleshooting higher than normal vibration. An
improperly sized key will cause an unbalance condition and increase the
vibration levels at 1x rotational speed. To ensure you have the proper key
length use the following formula.
Proper key length = Total shaft key slot
length + coupling key slot length DIVIDED BY 2
This will produce a key that
will fill the entire coupling slot, to provide proper power transmission,
and provide enough exposed key to compensate for the removed shaft material
of the unused slot length.
A properly sized key should fill
approximately half of the exposed shaft key slot.
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