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Basic
VibrationAnalysis
.
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What Is Vibration?
Vibration is the physical movement oroscillation of a mechanical part about a
reference position.
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What Is Vibration?Why do we care about vibration?
Vibration is:
Wasted energy
A major cause of premature component failure
Cause of aircraft noise which contributes to crew andpassenger discomfort
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Terminology
Amplitude Amplitude is an indicator of the severity of a vibration.
Amplitude can be expressed as one of the following
engineering units: Velocity
Acceleration
Displacement
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Terminology
Velocity Velocity is the rate of change in position
Typical velocity units are: IPS (Inches Per Second),
mm/sec (millimeters per second)
Velocity is the most accurate measure of vibration
because it is not frequency related. 0.5 IPS @ 1000
rpm is the same as 0.5 IPS @ 10000 rpm.
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Terminology
Acceleration Acceleration is the rate of change of velocity and is
the measurement of the force being produced.
Acceleration is expressed in gravitational forces orGs, (1G = 32.17 ft/sec/sec)
Acceleration is frequency related, in that 1 g @ 1000
rpm is not the same as 1 g @ 10000 rpm.
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Terminology
Displacement Displacement is a measure of the actual distance an
object is moving from a reference point.
Displacement is expressed in mils 1 mil = .001 inch
Displacement is also frequency related, in that 10
mils @ 1000 rpm is not the same as 10 mils @ 10000
rpm.
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Terminology - Continued
Unit Modifiers:
Since vibration is transmitted as an AC signal, there arefour Unit Modifiers that may be used to condition the
signal. These modifiers have a direct impact on the
measurement value. If the wrong modifier is used, the
measurement could be either too high, or too low, thuscausing possible maintenance action to be, or not to be,
accomplished erroneously.
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Peak to Peak-
the distance from the top of the positive peak tobottom of the negative peak.
Peak- the measurement from the zero line to the top of the positivepeak.
Average (AVG) - .637 of peak.
Root Mean Square (RMS) - .707 of peak.
Unit Modifiers:
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Terminology
Frequencies The rate of mechanical oscillation in a period of time.
Frequency can be expressed in one of the following
units: RPM - Revolutions per Minute
CPM - Cycles per Minute
CPS - Cycles per Second
Hz - Hertz, 1 Hz -1 Cycle per Second (to convert from Hz to RPM or CPM,
apply the following formula: Hz * 60 = RPM.
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Types of Vibration
Vibration can be classified into one or more
of the following categories:
Periodic
Random
Resonant Harmonic
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Terminology - Types of Vibration
Periodic Repeats itself once every time period
Result of a mass imbalance in a component or disc.
As the component rotates, it produces a bump
every rotation which is referred to a the once-per-
revolution or 1P vibration.
This vibration is usually correctable by balancing.
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Terminology - Types of Vibration
Random Do not repeat themselves
Not related to a fundamental frequency.
An example - the shock that is felt as a result of
driving down the road and hitting a pothole
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Terminology - Types of Vibration
Resonant The natural frequency at which an airframe or
mechanical system is inclined to vibrate. All things
have one or more resonant frequencies.
Resonant vibrations are the result of a response in a
mechanical system to a periodic driving force.
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Terminology - Types of Vibration
Harmonic Exact multiples of a fundamental frequency
Classified in terms as 1st, 2nd, 3rd..
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Terminology
Bandwidth Upper and lower frequency limits of the survey being acquired - either
hardware set (with the use of an external band pass filter) or software
controlled by the analyzer.
Setting the frequency bandwidth
is a way of eliminating vibration
data or noise that is of no interest
for your particular application.
In the survey above, the frequencybandwidth is 0 CPM to 3000 CPM
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Terminology
Resolution The resolution of a spectrum is the number of
lines or points used to plot the spectrum.
The higher the number of lines, the more data
acquired.
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Equipment Sensor
A transducer that converts mechanical motion into
electronic signals.
Three categories:
Displacement
Velocity
Accelerometer
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Sensor Type
Displacement Measures the distance an object is moving from a
reference position. This distance is typically
reported in mils.
Most accurate in frequencies below 10 Hz, or 600
RPM
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Sensor Type
Velocity Measures the rate of change of position an object
is moving, and is commonly reported in Inches
Per Second (IPS)
Best suited to measure vibrations between ~ 10
Hz and 1000 Hz, or 600 to 6000 RPM.
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Sensor Type
Accelerometer Measures the rate of change of velocity per time
period. Acceleration is reported in Gs
Most effective frequency range for an
accelerometer is above 1000 Hz, or 6000 RPM.
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Sensor Selection The first consideration is manufacturers
recommendations. If none exist, then:
Frequency Range
Environmental conditions
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Sensor Installation Varies depending upon the application.
Most manufactures provide the specific location for
mounting and this should be strictly adhered to. If
these recommendations are not followed, the
resulting measurements may be invalid.
Generally, mount in a location that provides theclosest proximity to the component of interest.
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How Vibration Is Analyzed Time Domain - Vibration vs. Time.
A vibration signal is presented as a sin wave form with
all frequencies and amplitudes combining to give oneoverall signal.
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Signals from four helicoptercomponent vibrations: Main
Rotor 1x, Main Rotor 2x, Tail
Rotor, and Tail Rotor Drive
combined by the vibrationsensor to produce one signal.
This would be difficult at best to
use as a means of determining
vibration faults in mechanical
structure.
What a Vibration Sensor Sees
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Separated, the four signals are distiguishable.
To separate the signals, a conversion is required.
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How Vibration Is Analyzed Frequency Domain
By applying the FFT (Fast Fourier Transform)
algorithm to a Time Domain signal, it is converted tothe Frequency Domain.
In the Frequency Domain, each individual amplitude
and frequency point are displayed.
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The Frequency domain spectra shown here has separated all four
of the components listed earlier, Main Rotor 1x, 2x, Tail Rotor, and
Tail Rotor Drive, into their own individual points showing both the
frequency (RPM) and Amplitude (IPS).
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Types of Vibration Surveys Overall Vibration
Steady State
Transient
Synchronous
Peak Hold
All have a very specific application.
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Types of Vibration Surveys
Overall Vibration Outputs the sum of all vibration measured within a specified
frequency range.
Used as an initial alarm typesurvey, whereby if the overall
indication is above a
specified value, a more
detailed survey isperformed to identify the
possible cause.
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Types of Vibration Surveys
Steady State Used to measure vibration at a constant
engine/component operational frequency.
Used to determine thespeed / frequency at
which balancing should
be performed. It can
also be used to identifycritical operational conditions.
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Types of Vibration Surveys
Transient Data collected during a controlled change in the aircraft /
component operational frequency.
Often used in trendingvibration over time by
comparing surveys
taken at specified
intervals.
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Types of Vibration Surveys
Peak Hold The maximum
amplitude value
measured iscaptured and
held.
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Types of Vibration Surveys
Synchronous Utilizes a tachometer signal and a filter to track vibration of aspecific rotor or shaft. The
filter eliminates all
vibrations above and below
the tachometer signal
input plus or minus the
filter value.
Used to determine the
amplitude and phase (clock)angle of an imbalance
condition.
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Interpreting a Vibration Survey
Define the frequency range
Identify component frequency.
Frequency charts
Multiple components within a system such as a gearbox
will have the ratio listed versus some operational speed of
the assembly, typically100 %.
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Interpreting a Vibration Survey
Using a Cursor Modern digital analysis equipment provides for identification
of frequencies
within a spectral plot with the
use of a cursor.
When the cursor is placed over a
peak in the plot, the specific
frequency and amplitude values
for that point are displayed.
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Interpreting a Vibration Survey - continued
Harmonic Cursor
Using the same example as before,the harmonic multiples of the
primary peak identified can also be
identified by using the harmonic
option (if available). When the
harmonic function is pressed, the analyzer will position one
additional cursor at each of the multiples throughout the range.
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Basic Balancing Mass Imbalance
Aerodynamic Imbalance
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Fundamentals of Balancing
Data Collection and Processing The vibration sensor is installed on
the engine as near the front bearing
as possible.
The Phototach is mounted on thecowling, behind the propeller.
The reflective tape is applied to the
back side of the target propeller blade
in line with the Phototach beam.
The mass is located by the relative
occurrence of tach trigger and mass
passage at the radial sensor location.
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Fundamentals of Balancing
Data Collection and Processing As the heavy spot on the propeller
passes the location of the vibration
sensor, the sensor generates and
sends an electrical pulse to theanalyzer.
The Reflective tape triggers a
response as it passes the Phototach,
which then sends an electricalsignal to the analyzer.
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Fundamentals of Balancing
Data Collection and Processing In this illustration, the
vibration sensor and
Phototach beam are co-locatedat the 12:00 or 0 degree
position. Rotation is clock-wise
from the viewers position.
This is our starting point,elapsed time = 0
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Fundamentals of Balancing
Data Collection and Processing The speed is 1 RPM. Fifteen
seconds (90 degrees) of travel
has occurred. In thissequence, the reflective tape
has just entered the Phototach
beam to trigger the tach event.
Elapsed time = 15 seconds.
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Fundamentals of Balancing
Data Collection and Processing In this sequence, the mass
(heavy spot) is passing the
accelerometer position,15 seconds (90 degrees)
after the tape passed the
Phototach beam.
Elapsed time = 15 seconds(90 degrees of travel).
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Fundamentals of Balancing
Data Collection and Processing The tape and mass have both
passed the 0 degree location.
The unit now waits for theexact sequence to repeat
for averaging.
Solution would be to add
weight at 270 degrees.
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Fundamentals of Balancing
Data Collection and Processing The process is
repeated while the
analyzer averagesout errors caused by
momentary vibration
events outside therunning average.
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Predictive Maintenance Define interval
Define requirements
Select equipment that meets requirements
Implement the program
Evaluate the program
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Predictive Maintenance
Define Interval How often do we acquire data?
Inspections/Hourly
Define Requirements
What components do we have interest in?
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Predictive Maintenance
Select equipment that meets requirements Frequency Range
Environmental Conditions
Software
Cost
Implement the program
Evaluate the program
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Review
What Is Vibration?
Terminology
Equipment
How Vibration Is Analyzed
Types of Vibration Surveys
Interpreting a Vibration Survey
Basic Balancing
Predictive Maintenance
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Contact
www.acessystems.com
1-865-671-2003
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