creating better aviation maintenance solutions... basic vibration analysis
TRANSCRIPT
Creating better aviation maintenance solutions...
What Is Vibration?
Terminology
Equipment
How Vibration Is Analyzed
Types of Vibration Surveys
Interpreting a Vibration Survey
Basic Balancing
Predictive Maintenance
Creating better aviation maintenance solutions...
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 and passenger discomfort
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Terminology
Prior to any discussion of vibration, it is important to first understand the common terms used for vibration analysis 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:VelocityAccelerationDisplacement
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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.
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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 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.
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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 = .001
inchDisplacement 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 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:
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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 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
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.
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Terminology - Types of Vibration
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
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Terminology - Types of Vibration
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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Terminology - Types of Vibration
HarmonicExact multiples of a fundamental frequencyClassified 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 frequency bandwidth 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 data
acquired.
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EquipmentSensor
A transducer that converts mechanical motion into electronic signals.
Three categories:DisplacementVelocityAccelerometer
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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 manufacturer’s
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 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.
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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 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
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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 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).
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Types of Vibration SurveysOverall VibrationSteady StateTransientSynchronousPeak 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” type survey, whereby if the overall indication is above a specified value, a more detailed survey is performed to identify the possible cause.
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Types of Vibration Surveys
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.
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Types of Vibration Surveys
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.
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Types of Vibration Surveys
Peak HoldThe maximum
amplitude value measured is captured and held.
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Types of Vibration Surveys
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.
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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, typically 100 %.
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Interpreting a Vibration Survey
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 >
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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.
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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 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.
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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 the analyzer.
The Reflective tape triggers a response as it passes the Phototach, which then sends an electrical signal 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-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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Fundamentals of Balancing
Data Collection and Processing
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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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 the exact 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 averages out errors caused by momentary vibration events outside the running average.
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Predictive MaintenanceDefine intervalDefine requirementsSelect equipment that meets requirementsImplement the programEvaluate the program
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Predictive Maintenance
Define IntervalHow often do we acquire data?
Inspections/Hourly
Define Requirements
What components do we have interest in?
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Predictive MaintenanceSelect 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
Creating better aviation maintenance solutions...
Contact
www.acessystems.com
1-865-671-2003