Bva apr-03 (1)

Creating better aviation maintenance solutions...
Basic
Vibration
Analysis
.

What Is Vibration?
Terminology
Equipment
How Vibration Is Analyzed
Types of Vibration Surveys
Interpreting a Vibration Survey
Basic Balancing
Predictive Maintenance

What Is Vibration?
Vibration is the physical movement or
oscillation of a mechanical part about a
reference position.

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 and
passenger discomfort

Terminology
Prior to any discussion of vibration, it is
important to first understand the
common terms used for vibration
analysis and their applications.

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

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.

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
or “G’s”, (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.

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.

Terminology - Continued
Unit Modifiers:
Since vibration is transmitted as an AC signal, there are
four 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, thus
causing possible maintenance action to be, or not to be,
accomplished erroneously.

Peak to Peak - the distance from the top of the positive peak to
bottom of the negative peak.
Peak - the measurement from the zero line to the top of the positive
peak.
Average (AVG) - .637 of peak.
Root Mean Square (RMS) - .707 of peak.
Unit Modifiers:

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.

Types of Vibration
Vibration can be classified into one or
more of the following categories:
Periodic
Random
Resonant
Harmonic

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.

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

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.

Harmonic
Exact multiples of a fundamental frequency
Classified in terms as 1st, 2nd, 3rd…..

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 frequency
bandwidth is 0 CPM to 3000 CPM

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.

Equipment
Sensor
A transducer that converts mechanical motion
into electronic signals.
Three categories:
Displacement
Velocity
Accelerometer

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

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.

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.

Sensor Selection
The first consideration is manufacturer’s
recommendations. If none exist, then:
Frequency Range
Environmental conditions

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 the
closest proximity to the component of interest.

Time Domain - Vibration vs. Time.
A vibration signal is presented as a sin wave form
with all frequencies and amplitudes combining to
give one overall signal.

Signals from four helicopter
component vibrations: Main
Rotor 1x, Main Rotor 2x, Tail
Rotor, and Tail Rotor Drive
combined by the vibration
sensor 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

Separated, the four signals are distiguishable.
To separate the signals, a conversion is required.

Frequency Domain
By applying the FFT (Fast Fourier Transform)
algorithm to a Time Domain signal, it is converted
to the Frequency Domain.
In the Frequency Domain, each individual
amplitude and frequency point are displayed.

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).

Overall Vibration
Steady State
Transient
Synchronous
Peak Hold
All have a very specific application.

Overall Vibration
Outputs the sum of all vibration measured within a
specified frequency range.
Used as an initial “alarm” type
survey, whereby if the overall
indication is above a
specified value, a more
detailed survey is
performed to identify the
possible cause.

Steady State
Used to measure vibration at a constant
engine/component operational frequency.
Used to determine the
speed / frequency at
which balancing should
be performed. It can
also be used to identify
critical operational conditions.

Transient
Data collected during a controlled change in the aircraft /
component operational frequency.
Often used in trending
vibration over time by
comparing surveys
taken at specified
intervals.

Peak Hold
The maximum
amplitude value
measured is
captured and
held.

Synchronous
Utilizes a tachometer signal and a filter to track vibration of a
specific 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.

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, typically
100 %.

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.
15300 RPM >
0.324 Amplitude >

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.

Basic Balancing
Mass Imbalance
Aerodynamic Imbalance

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 the
cowling, 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.

As the heavy spot on the propeller
passes the location of the vibration
sensor, the sensor generates and
sends an electrical pulse to the
analyzer.
The Reflective tape triggers a
response as it passes the Phototach,
which then sends an electrical
signal to the analyzer.

In this illustration, the
vibration sensor and
Phototach beam are co-located
at the 12:00 or 0 degree
position. Rotation is clock-wise
from the viewers position.
This is our starting point,
elapsed time = 0

The speed is 1 RPM. Fifteen
seconds (90 degrees) of travel
has occurred. In this
sequence, the reflective tape
has just entered the Phototach
beam to trigger the tach event.
Elapsed time = 15 seconds.

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).

The tape and mass have both
passed the 0 degree location.
The unit now waits for the
exact sequence to repeat
for averaging.
Solution would be to add
weight at 270 degrees.

The process is
repeated while the
analyzer averages
out errors caused by
momentary vibration
events outside the
running average.

Define interval
Define requirements
Select equipment that meets requirements
Implement the program
Evaluate the program

Define Interval
How often do we acquire data?
Inspections/Hourly
Define Requirements
What components do we have interest
in?

Select equipment that meets requirements
Frequency Range
Environmental Conditions
Software
Cost
Implement the program
Evaluate the program

Review
What Is Vibration?
Terminology
Equipment
Basic Balancing

Contact
www.acessystems.com
1-865-671-2003
sales@acessystems.com

Bva apr-03 (1)

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