creating better aviation maintenance solutions... basic vibration analysis

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

AmplitudeAmplitude is an indicator of the severity of a

vibration. Amplitude can be expressed as one of the following engineering units:VelocityAccelerationDisplacement


Terminology

VelocityVelocity is the rate of change in positionTypical 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

AccelerationAcceleration 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

DisplacementDisplacement is a measure of the actual distance

an object is moving from a reference point.Displacement is expressed in “mils” 1 mil = .001

inchDisplacement 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

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.


Types of Vibration

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


Terminology - Types of Vibration

PeriodicRepeats itself once every time periodResult 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.



RandomDo 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



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.



HarmonicExact multiples of a fundamental frequencyClassified 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

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 data

acquired.


EquipmentSensor

A transducer that converts mechanical motion into electronic signals.

Three categories:DisplacementVelocityAccelerometer


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


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.


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.


Sensor SelectionThe first consideration is manufacturer’s

recommendations. If none exist, then:

Frequency Range

Environmental conditions


Sensor InstallationVaries 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.


How Vibration Is AnalyzedTime 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.



How Vibration Is AnalyzedFrequency 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).


Types of Vibration SurveysOverall VibrationSteady StateTransientSynchronousPeak 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 StateUsed 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 HoldThe 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 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.

15300 RPM >

0.324 Amplitude >


Interpreting a Vibration Survey - continued

Harmonic CursorUsing 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.



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 the analyzer.

The Reflective tape triggers a response as it passes the Phototach, which then sends an electrical signal to the analyzer.



Data Collection and Processing

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.


Predictive MaintenanceDefine intervalDefine requirementsSelect equipment that meets requirementsImplement the programEvaluate the program



Define IntervalHow often do we acquire data?

Inspections/Hourly

Define Requirements

What components do we have interest in?


Predictive MaintenanceSelect equipment that meets requirements

Frequency Range

Environmental Conditions

Software

Cost

Implement the program

Evaluate the program


Review What Is Vibration?

Terminology

Equipment

How Vibration Is Analyzed



Basic Balancing



Contact

www.acessystems.com

1-865-671-2003

[email protected]

creating better aviation maintenance solutions... basic vibration analysis

Documents

z terminology z equipment

z types of vibration

energy z

terminology frequencies

second z velocity

ftsecsec z acceleration

inch z displacement

discussion of vibration