Basic Vibration Analysis ACES Systems/TEC Aviation Division 06/06/2011 #1

QWhat is Vibration? QTerminology QEquipment QHow Vibration Is Analyzed QTypes of Vibration Surveys QInterpreting a Vibration Survey QBasic Balancing QPredictive Maintenance Overview 06/06/2011 #2

Vibration is the physical movement or oscillation of a mechanical part about a reference position. What is Vibration? 06/06/2011 #3

Vibration is: QWasted energy QA major cause of premature component failure QCause of aircraft noise which contributes to crew and passenger discomfort 06/06/2011 #4 What Is Vibration? Why do we care?

Terminology Prior to any discussion of vibration, it is important to first understand the common terms used for vibration analysis and their applications. 06/06/2011 #5

Terminology Frequencies QThe rate of mechanical oscillation in a period of time. Frequency can be expressed in one of the following units: QRPM - Revolutions per Minute QCPM - Cycles per Minute QCPS - Cycles per Second QHz - Hertz, 1 Hz - 1 Cycle per Second (to convert from Hz to RPM or CPM, apply the following formula: Hz * 60 = RPM. 06/06/2011 #6

Terminology Amplitude QAmplitude is an indicator of the severity of a vibration. Amplitude can be expressed as one of the following engineering units: QVelocity QAcceleration QDisplacement 06/06/2011 #7

Terminology Velocity QVelocity is the rate of change in position QTypical velocity units are: IPS (Inches Per Second), mm/sec (millimeters per second) QVelocity is the most accurate measure of vibration because it is not frequency related rpm is the same as rpm. 06/06/2011 #8

Terminology Acceleration QAcceleration is the rate of change of velocity and is the measurement of the force being produced. QAcceleration is expressed in gravitational forces or “G’s”, (1G = ft/sec/sec) QAcceleration is frequency related, in that rpm is not the same as rpm. 06/06/2011 #9

Terminology Acceleration 1000 RPM = 16.7 Hz QIPS = (61.44 x 1.0) / 16.7 QIPS = RPM = Hz QIPS = (61.44 x 1.0) / QIPS = /06/2011 #10

Terminology Displacement QDisplacement is a measure of the actual distance an object is moving from a reference point. QDisplacement is expressed in “mils” 1 mil =.001 inch QDisplacement is also frequency related, in that rpm is not the same as rpm. 06/06/2011 #11

Terminology Displacement Q RPM = 16.7 Hz QIPS = (3.14 x 16.7) x QIPS = 0.52 Q RPM = QIPS = (3.14 x 166.7) x QIPS = /06/2011 #12

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. Terminology Unit Modifiers 06/06/2011 #13

06/06/2011 #14 Unit Modifiers: 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) of peak. Root Mean Square (RMS) of peak.

If I’m testing an engine whose limit is 1.0 IPS Pk-Pk and my analyzer shows a measurement of 0.6 IPS Pk, does the engine pass or fail? Terminology Unit Modifiers 06/06/2011 #15 Fail. Why?Fail. 0.6 IPS Pk * 2 = 1.2 IPS Pk-Pk

Types of Vibration QVibration can be classified into one or more of the following categories: QPeriodic QRandom QResonant QHarmonic 06/06/2011 #16

Terminology - Types of Vibration Periodic QRepeats itself once every time period QResult of a mass imbalance in a component or disc. QAs the component rotates, it produces a “bump” every rotation which is referred to a the once-per-revolution or “1P” vibration. QThis vibration is usually correctable by balancing. 06/06/2011 #17

Terminology - Types of Vibration Random QDo not repeat themselves QNot related to a fundamental frequency. QAn example - the shock that is felt as a result of driving down the road and hitting a pothole 06/06/2011 #18

Terminology - Types of Vibration Resonant QThe natural frequency at which an airframe or mechanical system is inclined to vibrate. All things have one or more resonant frequencies. QResonant vibrations are the result of a response in a mechanical system to a periodic driving force. 06/06/2011 #19

Terminology - Types of Vibration Harmonic QExact multiples of a fundamental frequency QClassified in terms as 1st, 2nd, 3rd….. 06/06/2011 #20

Terminology Bandwidth QUpper 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. QSetting the frequency bandwidth is a way of eliminating vibration data or noise that is of no interest for your particular application. QIn the survey above, the frequency bandwidth is 0 CPM to 3000 CPM 06/06/2011 #21

Terminology Resolution QThe resolution of a spectrum is the number of lines or points used to plot the spectrum. QThe higher the number of lines, the more data acquired. 06/06/2011 #22

Equipment QSensor QA transducer that converts mechanical motion into electronic signals. QThree categories: QDisplacement QVelocity QAccelerometer 06/06/2011 #23

Equipment Sensor Construction 06/06/2011 #24

Sensor Type Displacement QMeasures the distance an object is moving from a reference position. This distance is typically reported in mils. QMost accurate in frequencies below 10 Hz, or 600 RPM 06/06/2011 #25

Sensor Type Velocity QMeasures the rate of change of position an object is moving, and is commonly reported in Inches Per Second (IPS) QBest suited to measure vibrations between ~ 10 Hz and 1000 Hz, or 600 to RPM. 06/06/2011 #26

Sensor Type Accelerometer QMeasures the rate of change of velocity per time period. Acceleration is reported in Gs QMost effective frequency range for an accelerometer is above 1000 Hz, or RPM. 06/06/2011 #27

Sensor Selection QThe first consideration is the manufacturer’s recommendation. If none exist, then: QFrequency Range QEnvironmental conditions 06/06/2011 #28

Sensor Installation QVaries depending upon the application. QMost manufacturers 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. QGenerally, mount in a location that provides the closest proximity to the component of interest. 06/06/2011 #29

How Vibration Is Analyzed QTime Domain - Vibration vs. Time. QA vibration signal is presented as a sin wave form with all frequencies and amplitudes combining to give one overall signal. 06/06/2011 #30

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 06/06/2011 #31

06/06/2011 #32 Separated, the four signals are distinguishable. To separate the signals, a conversion is required.

How Vibration Is Analyzed QFrequency Domain QBy applying the FFT (Fast Fourier Transform) algorithm to a Time Domain signal, it is converted to the Frequency Domain. QIn the Frequency Domain, each individual amplitude and frequency point are displayed. 06/06/2011 #33

06/06/2011 #34 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 Surveys QOverall Vibration QSteady State QTransient QSynchronous QPeak Hold All have a very specific application. 06/06/2011 #35

Types of Vibration Surveys Overall Vibration QOutputs the sum of all vibration measured within a specified frequency range. QUsed 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. 06/06/2011 #36

Types of Vibration Surveys Steady State QUsed to measure vibration at a constant engine/component operational frequency. QUsed to determine the speed / frequency at which balancing should be performed. It can also be used to identify critical operational conditions. 06/06/2011 #37

Types of Vibration Surveys Transient QData collected during a controlled change in the aircraft / component operational frequency. QOften used in trending vibration over time by comparing surveys taken at specified intervals. 06/06/2011 #38

Types of Vibration Surveys Peak Hold QThe maximum amplitude value measured is captured and held. 06/06/2011 #39

QUtilizes 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. QUsed to determine the amplitude and phase (clock) angle of an imbalance condition. Types of Vibration Surveys Synchronous 06/06/2011 #40

QDefine the frequency range QIdentify component frequency. QFrequency charts QMultiple components within a system such as a gearbox will have the ratio listed versus some operational speed of the assembly, typically 100 %. Interpreting a Vibration Survey 06/06/2011 #41

Interpreting a Vibration Survey Using a Normal Cursor QModern digital analysis equipment provides for identification of frequencies within a spectral plot with the use of a cursor. QWhen the cursor is placed over a peak in the plot, the specific frequency and amplitude values for that point are displayed RPM > Amplitude > 06/06/2011 #42

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. Interpreting a Vibration Survey Using a Harmonic Cursor 06/06/2011 #43

QMass Imbalance QAerodynamic Imbalance Basic Balancing 06/06/2011 #44

QThe vibration sensor is installed on the engine as near the front bearing as possible. QThe Phototach is mounted on the cowling, behind the propeller. QThe reflective tape is applied to the back side of the target propeller blade in line with the Phototach beam. QThe mass is located by the relative occurrence of tach trigger and mass passage at the radial sensor location. Fundamentals of Balancing Data Collection and Processing 06/06/2011 #45

QAs the heavy spot on the propeller passes the location of the vibration sensor, the sensor generates and sends an electrical pulse to the analyzer. QThe Reflective tape triggers a response as it passes the Phototach, which then sends an electrical signal to the analyzer. Fundamentals of Balancing Data Collection and Processing 06/06/2011 #46

QIn 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 Fundamentals of Balancing Data Collection and Processing 06/06/2011 #47

QThe 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. Fundamentals of Balancing Data Collection and Processing 06/06/2011 #48

QIn 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). Fundamentals of Balancing Data Collection and Processing 06/06/2011 #49

QThe tape and mass have both passed the 0 degree location. The unit now waits for the exact sequence to repeat for averaging. QSolution would be to add weight at 270 degrees. Fundamentals of Balancing Data Collection and Processing 06/06/2011 #50

QThe process is repeated while the analyzer averages out errors caused by momentary vibration events outside the running average. Fundamentals of Balancing Data Collection and Processing 06/06/2011 #51

Predictive Maintenance QDefine interval QDefine requirements QSelect equipment that meets requirements QImplement the program QEvaluate the program 06/06/2011 #52

Predictive Maintenance Define Interval QHow often do we acquire data? QInspections/Hourly Define Requirements QWhat components do we have interest in? 06/06/2011 #53

Predictive Maintenance Equipment Selection QFrequency Range QEnvironmental Conditions QSoftware QCost Implement the program Evaluate the program 06/06/2011 #54

QWhat is Vibration? QTerminology QEquipment QHow Vibration Is Analyzed QTypes of Vibration Surveys QInterpreting a Vibration Survey QBasic Balancing QPredictive Maintenance Overview 06/06/2011 #55