Why is monitoring vibration important

Why is Monitoring Vibration Important?
Monitoring machine vibration and using the information you obtain saves money!
How is this possible?
We will answer this question in the following pages.
After reading this article you will:
 Understand the term ‘machine vibration’
 Cite some common causes of machine vibration
 Explain the reasons for monitoring machine vibration
 Understand how monitoring machine vibration saves money
What is Machine Vibration?
Most of us are familiar with vibration; a vibrating object moves to and fro, back and forth. A vibrating object
oscillates.
We experience many examples of vibration in our daily lives. A pendulum set in motion vibrates. A plucked guitar
string vibrates. Vehicles driven on rough terrain vibrate, and geological activity can cause massive vibrations in
the form of earthquakes.
There are various ways we can tell that something is vibrating. We can touch a vibrating object and feel the
vibration. We may also see the back-and-forth movement of a vibrating object. Sometimes vibration creates
sounds that we can hear or heat that we can sense. To observe how vibration can create sound and heat, rub your
feet back and forth on a carpet.

In industrial plants there is the kind of vibration we are concerned about: machine vibration.
What is machine vibration? Machine vibration is simply the back and forth movement of machines or machine
components. Any component that moves back and forth or oscillates is vibrating.
Machine vibration can take various forms. A machine component may vibrate over large or small distances, quickly
or slowly, and with or without perceptible sound or heat. Machine vibration can often be intentionally designed and
so have a functional purpose. (Not all kinds of machine vibration are undesirable. For example, vibratory feeders,
conveyors, hoppers, sieves, surface finishers and compactors are often used in industry.)
At other times machine vibration can be unintended and lead to machine damage. Most times machine vibration is
unintended and undesirable. This article is about the monitoring of undesirable machine vibration.
Shown below are some examples of undesirable machine vibration.
What Causes Machine Vibration?
Almost all machine vibration is due to one or more of these causes:
(a) Repeating forces
(b) Looseness
(c) Resonance

(a) Repeating Forces
Imagine a boat anchored in a bay. Waves are slapping the sides of the boat, and as long as the waves continue to
act on the boat we would expect the boat to rock.
The boat would be rocking because the waves would be exerting a repeating force on the boat - a force of a
pattern repeated over and over again.
Most machine vibration is due to repeating forces similar to those causing the boat to rock. Repeating forces such
as these act on machine components and cause the machine to vibrate. Where do the repeating forces that cause
machine vibration come from?
Repeating forces in machines are mostly due to the rotation of imbalanced, misaligned, worn, or improperly driven
machine components. Examples of these four types of repeating forces are shown below.

(b) Looseness
Looseness of machine parts causes a machine to vibrate. If parts become loose, vibration that is normally of
tolerable levels may become unrestrained and excessive.
(c) Resonance
Imagine a child swinging freely on a swing, that is, without the child propelling himself or anyone pushing him. If
we observe the motion closely we will see the child swinging at a particular rate. For example, we may see that it
consistently takes him three seconds to complete one cycle of swinging.
The rate of the child’s free-swinging is in fact a physical property of the child-swing system - much as the weight
of the child is a physical property of the child. It is the rate at which the child will tend to swing while seated on
that particular swing. It is the child’s most natural swinging rate on the swing, and the only way he can change it
is to interfere with the natural swinging by propelling himself with his feet, changing his posture, rubbing his feet
on the ground and so on.

Machines also tend to vibrate at certain oscillation rates. The oscillation rate at which a machine tends to vibrate is
called its natural oscillation rate. The natural oscillation rate of a machine is the vibration rate most natural to the
machine, that is, the rate at which the machine ‘prefers’ to vibrate.
A machine left to vibrate freely will tend to vibrate at its natural oscillation rate. Most machines have more than
one natural oscillation rate. For example, a machine comprising two substructures of different natural oscillation
rates will exhibit at least two natural oscillation rates. In general, the more complex the machine, the more natural
oscillation rates it has.
Now consider again the child on the swing. If we aided the swinging motion by repeatedly pushing the child, we
would expect the child to swing higher and higher over time.
We would however only cause the child to swing higher and higher if we pushed with the right rhythm. If our
pushing rhythm is such that he is sometimes pushed down while he is ascending, we would not expect him to
swing properly. To make him swing higher and higher, our pushing rhythm would in fact need to be in harmony
with his natural oscillation rate.
For example, we could push him every time - or every alternate time - he reaches his highest point. Only by
pushing the child at a rate which is in harmony with his natural or preferred oscillation rate can we cause him to
quickly swing higher and higher.
What happens if a machine is ‘pushed’ by a repeating force with a rhythm matching the natural oscillation rate of
the machine? A similar situation will arise - the machine will vibrate more and more strongly due to the repeating
force encouraging the machine to vibrate at a rate it is most natural with. The machine will vibrate vigorously and
excessively, not only because it is doing so at a rate it ‘prefers’ but also because it is receiving external aid to do
so. A machine vibrating in such a manner is said to be experiencing resonance.
A repeating force causing resonance may be small and may originate from the motion of a good machine
component. Such a mild repeating force would not be a problem until it begins to cause resonance. Resonance,
however, should always be avoided as it causes rapid and severe damage. For example, whole bridges have
collapsed due to their natural oscillation rates being excited by the mere rhythm of soldiers marching in unison
across the bridges.
Why Monitor Machine Vibration?
To do a good job of monitoring machine vibration and to fully reap the benefits, we must understand the answers
to the above question. Monitoring the vibration characteristics of a machine gives us an understanding of the
‘health’ condition of the machine. We can use this information to detect problems that might be developing.

Why be concerned about the condition of a machine? Why not just continue to run the machine until it breaks
down and then repair it? Operating a machine until it breaks down might be acceptable if the machine were a
‘disposable’ one. Most machines, however, are not ‘disposable’ due to their cost.
If we regularly monitor the conditions of machines we will find any problems that might be developing, therefore
we can correct the problems even as they arise.
In contrast, if we do not monitor machines to detect unwanted vibration the machines are more likely to be
operated until they break down.
Because machine vibration monitoring finds potentially damaging vibration, we can prevent problems arising and
this saves a lot of time, money, and frustration. How? Below we discuss some common problems that can be
avoided by monitoring machine vibration. These problems are worth avoiding as the costs of dealing with them are
large and far exceed the cost of reasonably priced machine vibration monitoring programs.
(a) Severe Machine Damage
Machine vibration that is not detected early enough will often lead to severe machine damage requiring costly
repairs or even total machine replacement. However, if the condition of a machine is monitored regularly, potential
problems can be detected - and corrected - at an early stage when the repair required is simpler, faster, and
cheaper. This is similar to our own health. Regular visits to a doctor help us to de tect problems early and so avoid
the large costs of remedying severe health damage.
(b) High Power Consumption
A machine that is vibrating consumes more power. As well as the power required for the machine to perform its
intended function, additional power is also required to sustain the vibration. We can minimize this problem by
regularly monitoring and maintaining the machine.

(c) Machine Unavailability
Because an unmonitored machine is more likely to break down, it is more often out of action. Ho wever, the cost of
procuring and operating a machine is normally justified by its availability to process goods efficiently, or by its
availability to convert raw material into cash. A machine should be consistently available to generate the money to
justify its investment. Regular monitoring helps ensure that a machine is always available to generate money.
(d) Delayed Shipments
Because an unmonitored machine is more likely to break down, it is also more likely to cause delays in the
shipping of goods. Customers have to wait and their payment is delayed. Customers could also cancel their order
and stop doing business with us.
(e) Accumulation of Unfinished Goods
Because an unmonitored machine is prone to breaking down it is often unavailable. Goods still in the making tend
to get stuck at the input point of the machine. This leads to unnecessary wastage - the waiting goods run the risk
of spoiling, occupy floor space, and tie up money.

(f) Unnecessary Maintenance
To constantly ensure proper machine condition, some companies stop running machines according to
predetermined schedules to adjust and replace parts regardless of whether or not the machines are
malfunctioning. As a result, machines are often stopped unnecessarily to replace parts that are still good and to
correct problems that do not exist. We can avoid such waste if the machines are regularly monitored and repaired
only when necessary.
(g) Quality Problems
Sometimes a machine can be running into trouble even though it appears to be functioning normally. This is a
dangerous situation. If not caught early, the problem could lead to poor quality products being made, large yield
losses, rework costs, or worse still, warranty returns by irate customers. A machine that is regularly monitored is
less likely to lead to such problems.
(h) Bad Company Image
We noted above that machines that are not regularly monitored can lead to shipment delays and produce goods of
poor quality. Just one incident of shipment delinquency or product defect is often enough to seriously strain or
even end relationships with customers. A bad company image associated with shipment delays and poor quality is
something to be avoided. For a relatively small cost, machine vibration monitoring can protect customer relations
and thus profitability.

(i) Occupational Hazards
Due to the noise and shaking they create, vibrating machines can cause occupational hazards and human
discomfort. Human discomfort results in a loss to the company as workers who feel unwell will not be fully
productive. Also, unexpected machine breakdowns leave workers with no work, and production planners with
frustration.
Summary
In this article we described machine vibration and discussed the benefits of monitoring machines on a regular
basis.
Machine vibration is simply the back-and-forth movement of any machine or machine component, and is generally
caused by repeating forces acting on the machine, loose machine parts, or resonance (a repeating force with a
rhythm matching the natural oscillation rate of the machine).
We identified the reasons for monitoring machine vibration regularly, as well as the consequences of not doing so.
By regularly monitoring the vibration characteristics of a machine we can detect and correct machine problems as
they arise. By correcting machine problems early we avoid many unpleasant and costly problems, some of which
involve customers. The cost of failing to monitor machine vibration far exceeds the cost of implementing a
vibration monitoring program.
To find out how to set up your own machine vibration monitoring program, contact Commtest Instruments Ltd or
one of our representatives for a demonstration of a vbSeries vibration monitoring system. For the address of your
nearest representative please visit our website at www.commtest.com

How is Vibration Measured?
In the last section we identified the most important vibration analysis tool as the spectrum. When we measure
machine vibration we usually measure vibration spectra, since the spectrum of a vibrating component tells us a
great deal about the condition of the component as well as about the cause of vibration. Naturally it is vital that
the spectrum, which gives such valuable information, be obtained accurately.
What guidelines must be observed to ensure measurements are accurate? How should measurements be taken
and for which machines should they be taken?
In this section we will answer these questions.After reading this section you will be able to:
 Recognize which machines should be monitored
 Understand how vibration sensors should be mounted
 Determine how measurement parameters should be set
 Take measurements in a systematic way
Which Machines Need Monitoring?
When deciding which machines to monitor, critical machines should be given priority over other machines. This is
much the same as monitoring the health of people. It is inappropriate to closely monitor the health of perfectly
healthy people, and then to forsake the monitoring of others who genuinely need it. The same applies when
monitoring the condition of machines.
In general, the following critical types of machines should be monitored on a regular basis in order to avoid
unexpected and costly problems:
(a) Machines that require expensive, lengthy, or difficult repairs if broken down
(b) Machines that are critical to production or general plant operations
(c) Machines that are known to frequently suffer damage
(d) Machines that are being evaluated for their reliability
(e) Machines that affect human or environmental safety

How Does the Instrument Work?
Before taking a vibration measurement, you need to attach a sensor that can detect vibration behavior to the
machine that is being measured. Various types of vibration sensors are available, but a type called accelerometer
is normally used as it offers advantages over other sensors. An accelerometer is a sensor that produces an
electrical signal that is proportional to the acceleration of the vibrating component to which the accelerometer is
attached.
What is the acceleration of a vibrating component? It is a measure of how quickly the velocity of the component is
changing.
The acceleration signal produced by the accelerometer is passed on to the instrument that in turn converts the
signal to a velocity signal. Depending on the user’s choice, the signal can be displayed as either a velocity
waveform or a velocity spectrum. A velocity spectrum is derived from a velocity waveform by means of a
mathematical calculation known as the Fast Fourier Transform or FFT.
The diagram below is a very simplistic explanation of how vibration data is acquired. You may wish to consult
other literature such as the vbSeries Instrument Reference Guide for further information.
How is the Accelerometer Mounted?
Most machines involve rotary mechanisms. Motors, pumps, compressors, fans, belt conveyors, gearboxes, all
involve rotary mechanisms and are frequently used in machines.
Most rotary mechanisms in turn have bearings that support the weight of rotating parts and bear the forces
associated with rotary motion and vibration. In general, large amounts of force are borne by bearings. It is not
surprising that bearings are often the place where damage occurs and where symptoms first develop.
Vibration measurements are thus usually taken at the bearings of machines, with accelerometers mounted at or
near the bearings.
Since conclusions regarding machine condition - and hence whether or not money and human safety are risked -
depend on the accuracy of measurements, we must be very careful how measurements are taken. It is important
to always remember that the way in which we mount the accelerometer very much determines the accuracy of
measurements.

How should accelerometers be mounted to ensure measurements are accurate and how can we do so safely? Here
are some guidelines:
(a) Mount as Close as Possible to the Bearing
Imagine a doctor who listened to your heart through thick clothing and placed the stethoscope closer to your
kidney than to your heart. You would likely doubt his diagnosis as he would be basing it on sounds distorted by
undue obstruction and noise from other organs.
When measuring vibration we must always attach the accelerometer as close as possible to the bearing. More
specifically, we must attach it as close as possible to the centerline of the bearing to avoid picking up distorted
signals.
(b) Make Sure the Accelerometer is Firmly Attached
For the accelerometer to detect true vibration behavior, it needs to undergo exactly the same vibratory movement
as the vibrating component. An accelerometer must therefore be attached firmly to the vibrating component so
that it does not rock or move independently of the component. A loosely mounted accelerometer produces signals
distorted by its own independent movements and therefore gives the wrong message.
Various mounting methods exist, but mounting by means of a magnet is the most popular as it offers a balance
between measurement reliability and convenience to the user. The magnetic mounting supplied in the Commtest
vb kit can be attached very firmly5 , while allowing the user to measure multiple machines using the same
accelerometer, with minimum time spent on attaching and detaching the accelerometer.
To ensure that the accelerometer is firmly attached, it must be stuck to a magnetic mounting surface this is even.
The magnetic mount must sit securely on the surface with the accelerometer positioned in the prescribed
orientation.

For the surface to be even, it must be free of debris, rust, and flaking paint.
The mounting surface must be truly magnetic (iron, nickel, or cobalt alloys). The magnetic mounting must not, for
example, be attached to an aluminum surface by virtue of iron beneath the aluminum surface.
To avoid loss of magnetism, the magnetic mounting must not be dropped or heated. Care must also be taken not
to strip the screw thread on the accelerometer and magnetic mounting.
(c) Make Sure the Accelerometer is Oriented Correctly
Different situations require the accelerometer to be oriented differently. For example, to detect parallel
misalignment the accelerometer is usually mounted in the radial direction of the bearings, but to detect angular
misalignment the accelerometer needs to be mounted in the axial direction.
The signal produced by the accelerometer is dependent on the orientation in which the accelerometer is mounted,
since the amplitude (amount) of vibration varies in different directions.

(d) Mount the Same Accelerometer in the Same Location
For a particular measurement point it is important to always mount the accelerometer at the same location to
minimize measurement inconsistencies that may lead to wrong conclusions. Where possible, always use the same
accelerometer for a particular measurement point.
(e) Mount the Accelerometer on Something Substantial
The accelerometer must never be mounted on a very flexible part of the machine as the spectrum will be distorted
by the flapping of the flexible part.
The accelerometer must never be used on structures that are very light as the weight of the accelerometer and
magnetic mounting will distort the vibration behavior of the structure. In general, the combined weight of the
accelerometer and magnetic mounting should be less than 10% of the weight of the vibrating structure.

(f) Take Care of the Accelerometer
If the accelerometer is treated roughly it may produce unreliable signals. Because of the strength of the magnetic
mount, you must take care when attaching the accelerometer to a mounting surface. You can achieve this by
approaching the mounting surface with the magnetic mounting tilted at an angle. When detaching the magnetic
mounting, you must not use the accelerometer as a lever for breaking contact. Instead, the magnetic mounting
should be gripped tightly and then tilted sideways to break the contact.
The accelerometer cable should never be twisted acutely, but must be anchored in a manner that prevents it from
being damaged. Twisted or freely swinging cables can distort the measured spectrum.
(g) Take Care of Personal Safety
You must manage hazards at all times. When taking vibration measurements, three kinds of hazards stand out in
likelihood or severity: injury by moving parts, electrical shock, and magnet- induced damage.
Firstly, when mounting the accelerometer, you must take care to prevent the cable from becoming entangled with
moving machinery. While the quick-release connector minimizes this danger it should not be relied on as a
substitute for correct installation.
Other things that could tangle with moving machinery include loose clothing, long hair, data transfer cables and
straps.
Secondly, you must never attach the accelerometer to any high voltage surface as this may cause electrical shock.
Thirdly, you must never bring the magnetic mounting near any magnet-sensitive objects such as pacemakers,
credit cards, floppy disks, video tapes, cassette tapes and watches since these items can be damaged by magnetic
fields.
There are other possible hazards. You should read and thoroughly understand the vbSeries Instrument Reference
Guide before using the instrument or its accompanying accessories.
How are Parameters Set?

What are measurement parameters?
Measurement parameters are the details that specify how a measurement is to be taken. By specifying
measurement parameters, we specify how data is to be collected and processed before it is presented to us.
Before taking a vibration measurement we need to specify what parameters will be used.
The parameters for vibration measurement may be likened to the ‘what and how’ details that a doctor must
specify before a medical test is performed.
We will now look at how measurement parameters are set when we measure a spectrum. For the rest of this
section, we will use the Commtest vb instrument as an example vibration monitoring instrument for our
discussions, as it is a particularly simple instrument to use. For example, the default measurement parameter
values (except for the default Fmax value) are suitable for taking most vibration measurements, so that in most
situations few or none of the default parameter values requires adjusting. These parameters are those displayed in
the Set Parameters screen of the vb instrument, with ‘domain’ set to ‘frequency’.
What are some of these measurement parameter values and what do they mean?The parameters used for
measuring vibration spectra may be divided into four classes; namely, parameters that determine:
(a) How data is collected
(b) How much or how fast data is collected
(c) How data is processed
(d) How data is displayed
(a) How Data is Collected
The parameters that determine how data is collected are ‘Trigger type’ and the parameters listed under ‘Sensor
setup’.
‘Trigger type’ is the parameter that tells the instrument how to begin measuring. If set to ‘Free run’, the
instrument will take measurements continuously. If set to ‘Single’, only one measurement cycle will take place. In
most cases the instrument can be set to ‘Free run’.
The parameters under ‘Sensor setup’ inform the instrument what type of accelerometer is being used to take
measurements. If the ICP®-type accelerometer supplied in the vb kit is used, the ‘Drive current’ needs to be
turned ‘ON’, and the ‘Sensitivity’ of the accelerometer needs to match that specified on the vb Quality Assurance
Card. ‘Settling time’ is the time required for the accelerometer and instrument to settle before measurements can
be taken accurately. You should use the default ‘Settling time’ value (which varies with the Fmax value) to ensure
measurement accuracy.
(b) How Much or How Fast Data is Collected
The parameters that determine how much or how fast data is collected are the parameters ‘Fmax’, ‘Spectral lines’,
and ‘Overlap percentage’.

In Section 2, we noted that the higher the Fmax, the larger the frequency range over which informa tion6
may be
obtained from the spectrum.
Thus, if the Fmax value is high, data is displayed up to high vibration frequencies. To acquire information
regarding high vibration frequencies, the measurement frequency - or rate of sampling data - needs to be high as
well. As a result, the higher the Fmax, the faster the measurement will be.
The more spectral lines a spectrum has, the more information can be obtained from it. This means that the more
spectral lines there are, the more data needs to be collected to generate the additional information, and therefore
the longer the measurement will take.
What Fmax Value Should be Used?
The higher the operating speed of the machine, the higher its frequencies of vibration will be, and the higher the
Fmax will need to be in order to capture vibration behavior at those high frequencies.
For vibration that does not involve rotary fingers such as gear teeth, fan blades, pump vanes, and bearing
elements, an Fmax value equal to 10 times the operating speed is usually sufficient to capture all crucial
information.
For example, if the operating speed is 10 000 rpm, then an Fmax value of 100 000 cpm (100 kcpm) is most likely
sufficient.
For vibration that does involve fingered elements such as gears, fans, pumps, and roller bearings, an Fmax value
equal to 3 times the number of fingers multiplied by the operating speed is usually sufficient to capture all crucial
information.
For example, for a gear driven by a 12-toothed pinion rotating at 10 000 rpm, an Fmax value of 360 000 cpm
(360 kcpm) is most likely sufficient.
If the Fmax value required is very large the resolution of the spectrum will be low, and information pertaining to
low vibration frequencies may be lost. It may be necessary to take some low Fmax measurements in addition to
the high Fmax measurement.
How Many Spectral Lines Should be Used?

In most cases 400 lines of resolution are sufficient. However, if a large Fmax value is used, the lines will be spread
out over a large frequency range, leaving wide gaps between lines. Thus, for large Fmax values, more spectral
lines may be needed to avoid loss of detail.
It should, however, be noted that the more spectral lines are used the longer the measurement will take and the
more instrument memory space will be occupied. A high Fmax value or a high number of spectral lines should
therefore be used only where necessary.
How Much Overlap Should I use?
Overlapping data is a means of reusing a percentage of a previously measured waveform to calculate a new
spectrum. The higher the ‘Overlap percentage’, the less newly acquired data is needed to generate a spectrum,
and thus the faster the spectrum can be displayed. 50% overlap is ideal for most cases.
(c) How Data is Processed
The parameters that determine how data is processed are the parameters ‘Average type’, ‘Number of averages’,
and ‘Window type’.
Imagine you had to accurately measure the width of the pages in this book. Because the width may vary slightly
from page to page, you would probably measure not just the width of one page but rather that of a few pages and
then take the average.
Similarly, when vibration is measured several spectra are usually measured and then averaged to produce an
average spectrum. The average spectrum better represents vibration behavior as the averaging process minimizes
the effect of random variations or noise spikes that are inherent in machine vibration.

The parameter ‘Average type’ determines how spectra are averaged. ‘Linear’ averaging is recommended for most
cases. ‘Exponential’ averaging is usually used only if vibration behavior varies significantly over time. ‘Peak hold’
does not really involve averaging but causes the worst-case (largest) amplitude for each spectral line to be
displayed.
The parameter ‘Number of averages’ determines the number of consecutive spectra used for averaging. The larger
the number of spectra used for averaging, the more noise spikes are smoothed out and the more accurately true
spectral peaks are represented.
However, the larger the number of averages, the more data needs to be collected, and therefore the longer it
takes to obtain the ‘average spectrum’. A ‘Number of averages’ of 4 is sufficient for most cases.
The collected data is usually not directly used to generate a spectrum, but is often modified beforehand to cater
for certain limitations of the FFT process (the process that transforms the data into a spectrum). Data is usually
modified by multiplication with a correction window. This prevents spectral lines from ‘smearing’ or ‘leaking’ into
one another.
‘Window type’ is the parameter that determines the kind of window that is used. The ‘Hanning’ window is usually
used. If the ‘rectangular’ window is used, the data will effectively not be modified.
(d) How Data is Displayed
The parameters that determine how the spectrum is to be displayed are listed under ‘Display units’.
To specify how the spectrum is to be displayed, the scale of the spectrum needs to be specified. The scale of the
spectrum determines how easily spectral details can be seen, and is defined by the parameters ‘Amplitude scale’,
‘vdB reference’, ‘Log range’, and ‘Velocity max.

In most cases, the ‘Amplitude scale’ can be ‘Linear’. If a linear amplitude scale is used, then the parameters ‘vdB
reference’ and ‘Log range’ are of no consequence (and therefore need not be set).
In general, you should set ‘Velocity max’ to ‘Automatic’ to allow the instrument to automatically select an ideal
amplitude scale that allows spectral peaks to be clearly seen.
To specify how the spectrum is to be displayed, the ‘amplitude type’ to be used also needs to be specified. In
Section 2 (page 18), we defined two amplitude types - peak amplitude and rms amplitude.
If the ‘Ø-peak’ (or ‘peak’) amplitude is used, the spectrum will display the maximum speed achieved by the
vibrating component at the various vibration frequencies.
On the other hand, if the ‘rms’ amplitude is used, a quantity indicative of vibration energy at the various
frequencies will be displayed instead.
For vibration spectra, the peak amplitude at a particular frequency is exactly √2 times (roughly 1.4 times) the rms
amplitude at that frequency. Thus which amplitude type is used is not really important since amplitude
conversions7
may be readily done.
We recommend that you always use the same amplitude type for a particular measurement point to avoid
misinterpretations. A switchover from the rms amplitude to the peak amplitude causes an apparent rise in
vibration amplitude that might be mistakenly interpreted as machine deterioration. On the other hand, a
switchover from the peak amplitude to the rms amplitude might hide a genuine rise in vibration amplitude.
Finally, the amplitude and frequency units to be used in the spectrum also need to be specified. Which units
should be used is really a matter of personal choice, or more often, geographic location.

In North America, the velocity unit usually used (for linear velocity scales8
) is in/s, and a commonly used
frequency unit is kcpm (kilocycles per minute).
In other parts of the world, the velocity unit and frequency unit usually used are mm/s and Hz respectively. Shown
below are the relationships between the units9
:
5
When pulled perpendicularly away from the mounting surface, the vb accelerometer’s magnetic mounting resists
with a force of 22 kgf (48.4 lbf)
6
A higher Fmax does not cause more data to be collected but causes the data to span across a wider range of
frequencies.
7
For a spectrum, the peak amplitude is times the rms amplitude. This relationship is generally not valid fo r
waveforms.
8
Many vibration analysts prefer the logarithmic velocity unit vdB. However, discussion on logarithmic scales and
units is beyond the scope of this book.
9
We have rounded the in/s ø-peak, mm/s rms conversion to 18. The correct ratio is 17.96.
From the Beginner’s Guide to Machine Vibration, copyright © Commtest 1999, 2006.
Revised 28/06/06
To find out how to set up your own machine vibration monitoring program, contact Commtest Instruments Ltd or
one of our representatives for a demonstration of a vbSeries vibration monitoring system. For the address of your
nearest representative please visit our website athttp://www.commtest.com
http://reliabilityweb.com/index.php/articles/list
http://reliabilityweb.com/index.php/articles/how_is_vibration_measured/

When to use displacement, velocity or acceleration amplitude
units in vibration analysis?
A novice vibration analyst always has a question whether to use displacement, velocity or acceleration
amplitude unit? Here is a rule of thumb based on the frequency.
Displacement is a good measure at lower frequencies especially less than 5 Hz. The failure mode is
generally the “stress” causing due to the displacement. Velocity measures how often the displacement
is being applied in a given time period. It is related to the fatigue mode of failure. Velocity amplitude
unit is a good measure in the range of 5-2000 Hz frequency. Even at small displacement amplitude the
repeated motion can cause fatigue failure. Above the 2000 Hz the failure is normally force related.
Acceleration is measure of the likelihood of force being the mode of failure.
There are areas on chart where stress, fatigue and force related failures overlap. The proper selection
of amplitude unit will depend upon the application under study. Note that frequency, displacement,
velocity and acceleration are related. Knowing any 2 quantities, other variables can be easily
calculated.
Picture reference: www.vibrationschool.com
http://amitadeshpande.blogspot.com.br/2008/11/when-to-use-displacement-velocity-or.html

Why is monitoring vibration important

More Related Content

Viewers also liked (9)

Similar to Why is monitoring vibration important (14)

Recently uploaded (20)

Why is monitoring vibration important