basic vibration analysis

8/10/2019 . Basic Vibration Analysis

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Creating better aviation maintenance solutions...

BasicVibration

Analysis

.


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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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What Is Vibration?

Vibration is the physical movement or

oscillation 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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TerminologyPrior to any discussion of vibration, it is

important to first understand the

common terms used for vibrationanalysis and their applications.


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Terminology

AmplitudeAmplitude 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

VelocityVelocity 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 @ 10000rpm.


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Terminology

AccelerationAcceleration is the rate of change of velocity and

is the measurement of the force being produced.

Acceleration is expressed in gravitational forcesor Gs, (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

DisplacementDisplacement is a measure of the actual distance

an object is moving from a reference point.

Displacement is expressed in mils 1 mil = .001inch

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

FrequenciesThe 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 VibrationVibration can be classified into one or

more of the following categories:

Periodic

Random

Resonant

Harmonic


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Terminology - Types of Vibration

PeriodicRepeats 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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RandomDo not repeat themselves

Not related to a fundamental frequency.

An example - the shock that is felt as a resultof driving down the road and hitting a pothole


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ResonantThe 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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HarmonicExact 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 bandwidthis 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

ResolutionThe resolution of a spectrum is the number of

lines or points used to plot the spectrum.

The higher the number of lines, the more dataacquired.


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EquipmentSensor

A transducer that converts mechanical motion

into electronic signals.Three categories:

Displacement

Velocity

Accelerometer


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Sensor Type

DisplacementMeasures 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

VelocityMeasures 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

AccelerometerMeasures 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 SelectionThe first consideration is manufacturers

recommendations. If none exist, then:

Frequency Range

Environmental conditions


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Sensor InstallationVaries depending upon the application.

Most manufactures provide the specific location

for mounting and this should be strictly adheredto. 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 AnalyzedTime Domain - Vibration vs. Time.

A vibration signal is presented as a sin wave form

with all frequencies and amplitudes combining togive one overall 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 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



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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 AnalyzedFrequency Domain

By applying the FFT (Fast Fourier Transform)

algorithm to a Time Domain signal, it is convertedto the 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 thefrequency (RPM) and Amplitude (IPS).


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Types of Vibration SurveysOverall Vibration

Steady State

Transient

Synchronous

Peak Hold

All have a very specific application.


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Overall VibrationOutputs 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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Steady StateUsed 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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TransientData 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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Peak HoldThe maximum

amplitude value

measured iscaptured and

held.


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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 signalinput plus or minus the

filter value.

Used to determine the

amplitude and phase (clock)angle of an imbalance

condition.


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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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Using a CursorModern 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 oneadditional 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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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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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


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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.


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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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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.


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The process is

repeated while the

analyzer averagesout errors caused by

momentary vibration

events outside the

running average.


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Predictive MaintenanceDefine interval

Define requirements

Select equipment that meets requirements

Implement the program

Evaluate the program


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Define IntervalHow often do we acquire data?

Inspections/Hourly

Define Requirements

What components do we have interest

in?


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Select equipment that meets requirementsFrequency 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



Basic Balancing



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Creating better aviation maintenance solutions

Contact

www.acessystems.com

1-865-671-2003

[email protected]

basic vibration analysis

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