Vibration Monitoring-Transducers-trouble shooting

1. Maintenance engineering
Module III
Prepared By
Dhanesh S
Assistant Professor
Department of Mechanical Engineering
Ahalia School of Engineering and Technology
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2. Vibration Monitoring
 Vibration-motion of a machine or machine part which is back and
forth from its position of rest
 Heartbeat of all mechanical equipment
 Analysing the vibration signals produced during operation of
machineries-provide important information about condition of
machinery
 Purpose-to establish running condition of machinery in a fashion
which is objective and scientific
 Can analyse machine problems such as imbalance, misalignment,
mechanical looseness, bad bearings, gear damage etc
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3. Vibration Monitoring
 Means vibration measurement/collection and analysis programme
and it monitors response of the system of equipment/machine to
internal and external forces being applied
 Response measure by general purpose transducer at the pump and
motor bearings and other rotating/oscillating components and is
passed to an analyser for interpretation
 Results are downloaded to asset management software/technique for
analysis and recommendations
 Aims for early identification and correction of potential machinery
problems
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4. Vibration Monitoring
 It involves the following
 Vibration fundamentals and Vibration signatures
 Vibration collection and analysis
 Vibration transducers
 Causes of machine vibration
 Vibration signature measurement and display
 Vibration signal frequency analysis
 Machine vibration standards and acceptable limits
 Fault diagnosis based on forcing functions
 Various online/offline and automatic/semi-automation monitoring techniques
and instruments etc
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5. Vibration Monitoring
 Vibration Fundamental:
Cyclic motion of rotating and reciprocating machine which is back and
forth from its position of rest
Generally four forces act: Exciting forces such as unbalance and
misalignment, mass of the vibrating system, stiffness of vibrating
system, damping characteristics of vibrating system
It is the most prominent secondary signal in most machines
Is considered in displacement mode and frequency mode and also in
acceleration mode
Machine vibration is very complex-consisting of many frequencies etc
due to vibration of a number of components
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6. Vibration Monitoring
 Overall level of vibration is only an indication of operating condition of
the machine
 Difficult to locate any specific fault in this as amplitude is not occurring at
just one frequency
 For fault location/identification-essential to first determine individual
amplitudes and frequencies
 A plot of amplitudes and frequencies of all individual vibrating
components of a machine is known as vibration signature
 These signatures are characteristics of a machine
 Change in such signature-indication of impending failure and its location
 Signature-also called vibration spectrum
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7. Vibration Monitoring
 Vibration Collection and Analysis:
Equipment as basic as metallic rod and a pencil and paper to advances
in technology enable computer assisted data collection which provides
much more detail in a fraction of time
Either out-sourcing or self
Recording can be continuous or periodic
Portable data collection instruments-used to collect vibration,
temperature , electric current signatures and other condition monitoring
data for assessing the mechanical and electrical health of plant
operating assets
DI-225D Vibration Data Collector, DI-1100 Single Channel Vibration
Data Collector etc are examples
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8. Vibration Monitoring
 Vibration Analysis-Effective method for detecting machine faults and
diagnosing the nature and severity of fault
 Problems such as rotor imbalance and misalignment make up a great
proportion of mechanical deficiencies and can be identified and rectified
 Other problems can be related to bearing wear and qualified as to severity
of wear
 Common vibration analysis methods are:
 Spectral Analysis including RPM Spectral Maps or Cascade Diagrams and
Campbell Diagrams etc
 Statistical Analysis and Kurtosis Method
 Envelope Analysis
 Spike Energy Method etc
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9. Vibration Monitoring
 Following analysis methods are more commonly used:
 Natural Frequency and crucial damping
 Harmonic forces and concept of resonance
 Transient and impulse forces
 Vibration isolation and vibration absorption
 Motor Current Analysis
 Real Time Analysis for vibration etc
 Vibration Analysis carried out in following three levels:
 Overall vibration level measurement to detect that the problem exists
 Spectral of frequency analysis to locate where the problem is in the machine
 Special techniques at more detailed level which can indicate what the
problem is
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10. Vibration Monitoring
 Following categories
Single Value Methods
Hand Held Meters
SPM Units
Acoustic Emission Units
Vibration Pens
4-20mA sensors
Time Frequency Methods
Hand Held Data-collectors
On line systems
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11. Vibration Monitoring
 Single Value Measurements:
Method Single Value Method
Summary
Applications
ISO Filtered Velocity 2Hz-1kHz filtered Velocity Works as a general condition
indicator
SPM Carpet and Peak related
to demodulation of a
sensor resonance around
30kHz
One of the better single value
bearing indicator methods. Some
problems on larger bearings and
gear units
Acoustic Emission Distress and dB,
demodulates a 100kHz
carrier which is sensitive to
stress waves
Better general indicator than velocity,
without the ISO comfort zone
Vibration
Meters/pens
Combine velocity,
bearings and acceleration
techniques
Look for ISO Velocity, envelope and
high frequency acceleration for best
performance
4-20mA sensors Filtered data converted to
DCS/PLC compatible
signal
ISO velocity version available,
envelope version still awaited. Can
be used to home in on specific
problems by special order
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12. Vibration Monitoring
 Time Frequency Measurements:
Involves detection and display of specific components of a time history
sensor output
Use of specific frequency components lends itself to detection of faults
down to a single mechanical component
May measure the following: Displacement(low frequency),
velocity(mid frequency), acceleration ( high frequency)
Two main types of envelope mesaurment:
Band pass filtered enveloping
High pass filtered enveloping
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13. Vibration Transducers
 A device that accepts an input of energy in one form and produces an
output of energy in some other from with a known, fixed relationship
between input and output
 Output may be either in same form as input or in other from
 Vibration transducers-convert vibration input of the machine to electrical
signal outputs in various forms giving required information
 Accelerometer, piezoresistive actuators, FFT analyzers, low resonance
geophone, vibration holographs etc are some of types of vibration
transducers/sensors
 Different types respond differently
 For example-proximity probes are sensitive to displacement, velocity
probes to velocity etc
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14. Vibration Transducers
 Accelerometer:
 A transducer whose output is directly proportional to acceleration forces
 Output is generally produced by a piezoelectric crystal which generates a
current proportional to applied forces
 Current is then amplified and displayed at a time waveform or processed by a
Fourier transform to produce a frequency display
 Single integration of acceleration signal will produce a velocity display and
double integration will produce a displacement display
 Generally consist of a piezoelectric crystal and mass normally enclosed in a
protective metal case
 Some have variable capacitance or work as combination
capacitive/piezoelectric sensors
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15. Vibration Transducers
 ST703-C(capacitance type),ST703-P(piezoelectric type),5300, 6000
series etc are few examples
 Velocity Transducer:
It is an electrical/mechanical transducer whose output is directly
proportional to velocity of the measured unit
Velocity pickup is a very popular transducer or sensor for monitoring
the vibration of rotating machinery
This type of vibration transducer installs easily on machines and
generally costs less than other sensors
This type of transducer is ideal for general purpose machine
applications
Have been used as vibration transducer for a very long time
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16. Vibration Transducers
 Consist of a magnet suspended on a coil, surrounded by a conductive coil
 Movement of transducer induces movement in suspended magnet
 Movement inside conductive coil generates and electrical current
proportional to velocity of movement
 A time waveform or a Fourier transform of current will result in a velocity
measurement
 Signal can also be integrated to produce a displacement measurement
 Also called as velocity transmitter and velocity monitor and are
compatible with peizo-voltage output accelerometer
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17. Vibration Transducer
 PZT(lead zirconate titanate)Actuators:
 Typical piezoelectric material
 Change dimension when an electric field is applied
 Dimensional change is moderately proportional to applied filed, piezoelectric
materials or piezoactuators are ideal for positioning applications where
nanometer sensitivity is required
 Often used for vibration suppression/cancellation/damping in many
engineering components
 One type operate at low frequency and are typically used for positioning and
vibration control applications
 Second type operate at resonance and are used in a multitude of ultrasonic
applications
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18. Vibration Transducer
 FFT(Fast Fourier Transform) Analyzer:
 A device that uses FFT algorithm to calculate a spectrum rom a time domain signal
and most common type of spectrum analyser
 Stores an iput signal waverform as data by digitally sampling it and then
determines Fourier coefficients in short time using FFT
 Also called frequency analyser or spectrum analyser
 Vibration Shakers:
 Generate vibration for structural research and modal testing
 Produces dynamic force operating on mass reaction principle
 Heavy mechanical component of shaker is caused to oscillate by drive signal from
power amplifier
 Reaction force is generated which excites structure under test
 Two types-low frequency electromagnetic shakers and high frequency
piezoelectric shakers
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19. Vibration Holographs
 Non-contacting vibration monitoring
 Used to measure vibration with help of laser holography
 Fully a non-contacting type if optical excitation is used
 When laser strikes a solid, localised heating will take place at the point of
focus
 Produces a strain thereby modulating vibration waves which is similar to
laser light modulation waves
 Enhances one vibration mode over another
 Single point detection can be used to form an image of vibration over
surface of plate
 Output beam intensity is measured directly by suitable camera
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21. Machinery Vibration Trouble shooting
 Certain vibrations may be good and some others may be no
 Vibration may be essential in certain tasks
 Eg: vibratory feeders use vibration to move materials
 In construction vibrators are used to help concrete settle into forms
and compact fill materials
 Unavoidable in operation of reciprocating pump, compressors etc
 Such vibration is called as Benign of normal vibration
 Benign vibration becomes cause of worry when these exceed the
normal levels and increase is not corresponding to load change
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22. Machinery Vibration Troubleshooting
 There are many other vibrations as well
 Vibrations to be worried about are the following and “Vibration
Troubleshooting” needed for those:
Benign vibrations above normal level, not corresponding to load
change
1x RPM amplitudes above balance limits
Shock pulses
Large shaking motions
Abnormal noise etc
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24. Machinery Vibration Troubleshooting
 Vibration Isolation:
Process of reducing the vibrations of machines and hence reducing the
transmitted force to foundation using vibration isolating materials
Used to reduce transmission of vibration to surrounding structures
Also prevent ambient vibration from reaching precision machines
May be obtained by placing materials, called vibration isolators such
as isolation pads,springs etc in between vibrating body and supporting
foundation or structure
Vibration troubleshooting –routinely applied to rotating and
reciprocating equipment to assess general mechanical health
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25. Machinery Vibration Troubleshooting
 Vibrations resulting from bearing, electric motors, gearbox, belt and
chain drives etc can be easily identified before it becomes severe
operation problem
 Root cause can be identified using vibration analysis and corrective
actions can be taken to avoid such problems
 Vibration analysis-therefor a powerful diagnostic tool and
troubleshooting of major process machinery would be unthinkable
without modern vibration analysis
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26. Causes of Vibration
 Causes can be mechanical, hydraulic causes and other causes
Mechanical Causes
Unbalanced rotating components
A bend of warped shaft
Pump and driver misalignment
Pipe strain, either by design or as a result of thermal growth
Mass of pump base is too small
Thermal growth of various components especially shafts
Worn, loose or bad rolling element bearings
Loose hold down bolts
Some part rubbing/others
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27. Causes of Vibration
Product attaching to a rotating component
Loose parts or damaged parts
Bad gears, not meshing properly or wornout
Improper design of base plate and /foundation
Operation at too low a capacity
 Hydraulic Causes of Vibration:
Pumps/hydraulic motors etc not operating at rated or best efficiency point
Vaporization of product
Impeller vane running too close to pump cutwater
Internal recirculation
Air getting into system through vortexing
Turbulence in system
Water hammer
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28. Causes of Vibration
Cavitation, clogged suction strainers or presence of air in hydraulic
fluid etc
Parallel operation of poorly matched pumps etc
Other Causes of Vibration
Harmonic vibration form nearby equipment
Operating pump at critical speed
Seal “slip-stick” at sea faces
A pump discharges recirculation line aimed at seal face
Electrical troubles
Dynamic loading of mechanical components
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31. Machinery Vibration Standard,
Severity Chart and Acceptable Limits
 Vibration standards are intended for following purposes:
To setup criteria for rating or classifying the performance of equipment
of material
To provide a basis for comparison of the maintenance qualities of
pieces of equipment of same type
To test equipment whose continuous operation is necessary for
industrial or public safety
To provide a basis for selection of equipment of material
To setup a procedure for calibration of equipment
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32. Machinery Vibration Standard, Severity
Chart and Acceptable Limits
 VDI 2056/1964 or its equivalent 4675, ISO2372 or ISO/IS3954 are used for
overall level measuring equipment's normally for vibration signals within the
frequency range of 10Hz to 1000Hz.
 Indian standard IS4729 is applicable to rotating electrical machines with power
ranging from 0.15KW to 1000KW or more
 Alarms levels can be based on:
 Fixed alarms based on above mentioned standards
 Percentage change alarm
 Trend based alarms
 Narrow band alarm
 Statistical by point, bearing , machine etc
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33. Machinery Vibration Standard, Severity
Chart and Acceptable Limits
 Vibration Severity Chart:
Difficult to develop a universal overall level Skipe Energy severity
chart for general machinery operations
This is due to fact that too many variables involved such as different
machine types, operating conditions, accelerometers, mounting
methods and ambient conditions
Possible to develop an overall Spike Energy severity chart based upon
empirical data for certain type of machines
General machinery vibration severity chart used by vibration analyst
for setting alarm level at different points of machine
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34. Machinery Vibration Standard, Severity
Chart and Acceptable Limits
 There is general guide for acceptable variation on many classes of
common machines
 Guidelines are general in that they are economically achievable and
vibration below these levels will allow the machine to survive a normal
life in service
 These levels may need to be reduced for product quality purposes or for
stealthiness in military applications
 Levels need to be allowed for float above these levels for flexible
mounting conditions such as springs or elevated platforms
 Higher levels may also need to be allowed when higher vibration sources
are nearby
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