vibrations and measurments

8/7/2019 vibrations and measurments

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Creating better aviation maintenance solutions...

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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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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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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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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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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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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Peak Hold The maximum

amplitude value

measured iscaptured and

held.


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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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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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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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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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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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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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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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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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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



Basic Balancing



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Contact

www.acessystems.com

1-865-671-2003

[email protected]

vibrations and measurments

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