Technical Associate I Metric Textbook

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Ho w To Im p lem en t An Effec tive Co n d itio n M o n ito rin g Pro g ram

Using Vib ration Analysis

Authored By: Mr. J ames E. Berry P.E.

of Technical Associates of Charlotte, Inc.

ANALYSIS I

Technical Associates Of Charlotte, P.C.347 North Caswell Road Charlotte, NC 28284, U.S.A. TELEPHONE: (704) 333-9011 FAX: (704) 333-1728

SPECIALISTS IN PREDICTIVE MAINTENANCE, MACHINERY DIAGNOSTICS, AND VIBRATION REDUCTION

M ETRIC VERSION


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i© Copyright 1997 Technical Associates Of Charlotte, P.C.

Technic al Assoc iates of Charlotte L evel I

TABLE OF CONTENTS AND SEMINAR AGENDA

PREDICTIVE MAINTENANCE AND VIBRATION SIGNATURE ANALYSIS I

SECTION SUBJ ECT PAGE

1. ANALYSIS I SEMINAR OVERVIEW...........................................................................................1-1

2. WHAT IS VIBRATION AND HOW CAN IT BE USED TO EVALUATE MACHINERY CONDITION?.......................................................................................................................... 2-1

2.0 Introduction....................................................................................................................... 2-12.01 What is Vibration Frequency and How Does it Relate to a Time Waveform?................. 2-32.02 What is Vibration Displacement?.............................................................................. 2-32.03 What is Vibration Velocity?.......................................................................................2-42.04 What is Vibration Acceleration?................................................................................ 2-42.05 What is Vibration Phase?..........................................................................................2-5

2.1 What is a Vibration Spectrum (Also Called an "FFT" or "Signature")?....................................... 2-72.2 Difference Between RMS, Peak and Peak-To-Peak Vibration Amplitude?................................ 2-112.3 When to Use Displacement, Velocity, or Acceleration?......................................................... 2-132.4 How Much is Too Much Vibration?...................................................................................... 2-142.5 Understanding a Vibration Spectrum................................................................................... 2-22

2.51 Effect on Frequency Accuracy of #FFT Lines Used................................................... 2-222.52 Effect on Frequency Accuracy of Frequency Span Used............................................ 2-24

2.6 What is Overall Vibration (Digital and Analog Overall Level)?................................................. 2-332.7 Understanding Phase and Its Applications........................................................................... 2-37

2.71 Definition of Phase.................................................................................................. 2-372.72 How to Make Phase Measurements.......................................................................... 2-372.73 Using Phase Analysis in Vibration Diagnostics........................................................... 2-39

2.731 Evaluating Axial Motion of a Bearing Housing to Reveal aPossible Cocked Bearing or Bent Shaft........................................................ 2-39

2.732 Phase Behavior Due to Unbalance................................................................ 2-402.733 Phase Behavior Due to Looseness/Weakness................................................ 2-422.734 Phase Behavior Due to Misalignment............................................................ 2-422.735 Using Phase Analysis to Find the Operating Deflection Shape of a

Machine and Its Support Frame................................................................... 2-44

3. OVERVIEW OF THE STRENGTHS AND WEAKNESSES OF TYPICAL VIBRATIONINSTRUMENTS....................................................................................................................... 3-1

3.0 Introduction....................................................................................................................... 3-13.1 Instrument Comparisons..................................................................................................... 3-13.2 General Capabilities of Each Vibration Instrument Type......................................................... 3-5

3.21 Overall Level Vibration Meters.................................................................................. 3-53.22 Swept-Filter Analyzers............................................................................................. 3-63.23 FFT Programmable Data Collectors......................................................................... 3-63.24 Real-Time Spectrum Analyzers................................................................................ 3-73.25 Instrument Quality Tape Recorders.......................................................................... 3-8

4. OVERVIEW OF VARIOUS VIBRATION TRANSDUCERS AND HOW TO PROPERLY SELECT THEM........................................................................................................................4-1


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4.0 Introduction....................................................................................................................... 4-14.1 Types of Vibration Transducers and Their Optimum Applications........................................... 4-1

4.11 Accelerometers....................................................................................................... 4-44.12 Velocity Pickups..................................................................................................... 4-9

4.13 Non-Contact Eddy Current Displacement Probes...................................................... 4-144.14 Shaft Contact Displacement Probes......................................................................... 4-184.141 Shaft Sticks................................................................................................ 4-184.142 Shaft Riders................................................................................................ 4-20

4.2 Selection Criteria For Transducers....................................................................................... 4-214.3 Mounting of Transducers.....................................................................................................4-23

4.31 Transducer Mounting Applications............................................................................ 4-23Appendix - Specifications for Various Transducers from a Variety of Manufacturers.........................4-26

5. ROLE OF SPIKE ENERGY, HFD AND SHOCK PULSE (SPM) AND SPECIFICATION OFTHEIR ALARM LEVELS AT VARIOUS SPEEDS.........................................................................5-1

A. Spike Energy and Shock Pulse............................................................................................5-1

6. USE OF VIBRATION SIGNATURE ANALYSIS TO DIAGNOSE MACHINE PROBLEMS............... 6-1

6.0 Use of Vibration Signature Analysis.................................................................................6-1Overview of 5-Page "Illustrated Vibration Diagnostic Chart".....................................................6-4

6.01 Mass Unbalance........................................................................................................... 6-126.011 Force Unbalance............................................................................................... 6-156.012 Couple Unbalance............................................................................................. 6-156.013 Dynamic Unbalance...........................................................................................6-166.014 Overhung Rotor Unbalance................................................................................ 6-17

6.0141 Summary of Procedures for Balancing Overhung Rotors................................. 6-176.015 Allowable Residual Unbalance and ISO Balance Quality Grade............................. 6-21

6.02 Eccentric Rotors...........................................................................................................6-276.03 Bent Shaft.................................................................................................................... 6-306.04 Misalignment................................................................................................................ 6-32

6.041 Angular Misalignment.........................................................................................6-356.042 Parallel Misalignment......................................................................................... 6-366.043 Misaligned Bearing Cocked on the Shaft.............................................................6-376.044 Coupling Problems............................................................................................ 6-37

6.05 Natural Frequencies and Resonance...............................................................................6-396.051 Natural Frequency............................................................................................. 6-396.052 Resonance........................................................................................................6-42

6.06 Mechanical Looseness.................................................................................................. 6-436.061 Type A - Structural Frame/Base Looseness (1X RPM)........................................... 6-436.062 Type B - Looseness Due to Rocking Motion or Cracked

Structure/Bearing Pedestal (2X RPM)............................................................ 6-46 Type C - Loose Bearing in Housing or Improper Fit Between Component

Parts (Multiple Harmonics Due to Nonlinearity Often Induced ByImpulse Events)........................................................................................... 6-48

6.07 Tracking of Rolling Element Bearing Failure Stages Using Vibration and HighFrequency Enveloping and Demodulated Spectral Techniques......................................... 6-526.071 Optimum Vibration Parameter For Rolling Element Bearing Condition

Evaluation (Acceleration, Velocity or Displacement)..............................................6-556.072 Types of Vibration Spectra Caused by Defective Rolling Bearings......................... 6-57



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6.073 Typical Spectra for Tracking Failure Stages Through Which RollingElement Bearings Pass.......................................................................................6-70

6.08 Introduction to Gear Problem Detection..........................................................................6-806.081 Specification of Spectral Setup for Detecting Gear Wear....................................... 6-80

6.082 Indications of Gear Tooth Wear...........................................................................6-826.09 Introduction to Electrical Problem Detection................................................................... 6-866.091 Why Many Electrical Problems Occur at 2X Line Frequency................................. 6-866.092 Stator Problems.................................................................................................6-876.093 Eccentric Rotor and Variable Air Gap.................................................................. 6-916.094 Rotor Problems................................................................................................. 6-936.095 Important Closing Comments on Electrical Measurements.................................... 6-95

6.10 Belt Drive Problems...................................................................................................... 6-996.101 Worn, Loose or Mismatched Belts...................................................................... 6-1006.102 Belt/Sheave Misalignment....................................................................................6-1026.103 Eccentric Sheaves...............................................................................................6-1026.104 Belt Resonance..................................................................................................6-1036.105 Excessive Motor Vibration at Fan Speed Due to Motor Frame/Foundation

Resonances........................................................................................................6-1036.106 Loose Pulley or Fan Hub.....................................................................................6-103

7. PROVEN METHOD FOR SPECIFYING SPECTRAL BAND ALARM LEVELS ANDFREQUENCIES USING TODAY'S PREDICTIVE MAINTENANCE SOFTWARE SYSTEMS............ 7-1

7.0 Abstract....................................................................................................................... 7-17.1 Introduction to Specifying Spectral Alarm Bands & Frequency Ranges............................. 7-2

7.11 Two Types of Spectral Alarm Bands....................................................................7-37.12 Which Vibration Parameter to Use in Spectral Alarm Bands -

Displacement, Velocity or Acceleration?.............................................................. 7-47.13 Review of Problems Detectable by Vibration Analysis........................................... 7-57.14 Specification of Overall Vibration alarm Levels and Explanation of the

Origin of Table II "Overall Condition Rating" Chart................................................ 7-137.15 Specification of Spectral Alarm Levels and Frequency Bands Using Table III...........7-16

Case A - General Rolling Element Bearing Machine Without Rotating Vanes:(Motors, Spindles, Gearbox Lower Frequency Measurements, etc.).................7-17

Case B - General Sleeve Bearing Machine Without Rotating Vanes:(Sleeve Bearing Motors, Gearbox Lower Frequency Measurements, etc.).........7-18

Case C - Gearbox High Frequency Points with Known Number of Teeth................. 7-19Case D - Gearbox High Frequency Points with Unknown Number of Teeth............. 7-20Case E - Motor Electrical Rotor Bar Pass Frequency Point

(Single Point Usually Taken on Outboard Motor Bearing)............................... 7-21Case F - Motor Electrical 12,000 CPM Measurement Point

(Single Point Usually Taken on Inboard Motor Bearing).................................. 7-21Case G - Special Machine Types.........................................................................7-22

Type 1 - Centrifugal Machines with Known Number of Vanes (or Blades)and Rolling Element Bearings................................................................. 7-22

Type 2 - Centrifugal Machines with Unknown Number of Vanes (or Blades)and Rolling Element Bearings................................................................. 7-22

Type 3 - Centrifugal Machines with Known Number of Vanes (or Blades)and Sleeve Bearings.............................................................................. 7-23

Type 4 - Centrifugal Machines with Unknown Number of Vanes (or Blades)and Sleeve Bearings.............................................................................. 7-23

7.151 Examples - Specification of Spectral Alarm Bands for Sample Machines.....7-23



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7.16 Periodic Reevaluation of Spectral alarm Band Setups on Each Familyof Machines.......................................................................................................7-297.161 Procedure for Evaluating the Effectiveness of Specified Overall

Alarm Levels and Spectral Bands............................................................ 7-317.162 Example - "Statistical Analysis of Overall Vibration Velocity in 4

Client Power Plants Using the Procedure Recommended Above"...............7-327.17 Conclusions...................................................................................................... 7-34

8. COMMON PITFALLS IN EVERYDAY VIBRATION MEASUREMENTS..........................................8-1

8.0 Introduction....................................................................................................................... 8-18.1 General Consideration for Obtaining Consistent Quality Data.................................................8-1

8.11 Choosing Measurement Locations............................................................................8-18.12 Machine and Point Identification............................................................................... 8-38.13 Measurement Parameters.........................................................................................8-78.14 Instrument Selection, Setup, and Condition...............................................................8-128.15 Measurement Techniques.........................................................................................8-14

8.16 Transducer Mounting and Probes..............................................................................8-158.2 Effect of Transducer Mounting on Vibration Measurements.....................................................8-19

9. SETUP AND IMPLEMENTATION OF PREDICTIVE MAINTENANCE AND CONDITIONMONITORING PROGRAMS......................................................................................................9-1

• "Flow Chart of Recommended Predictive Maintenance Programs"..........................................9-7• "Sample PMP Machine Plant Layout"................................................................................... 9-8• "Sample Machinery Drawing and Configuration Chart"...........................................................9-9• "Criteria for Overall Condition Rating (Peak Overall Velocity, in/sec)"...................................... 9-10• "Sample Overall Condition Rating Report"............................................................................9-12• "Sample Overall Condition Rating Bar Graph".......................................................................9-14• "Sample Rank-Ordered Results and Recommendations Report".............................................9-16• "Sample PMP Machine Repair Log"..................................................................................... 9-17• "Latest Overall Measurement Report"................................................................................... 9-19• "Overall Alarm Exception Report"........................................................................................ 9-21• "Spectral Band Alarm Report"............................................................................................. 9-23• "Spectral Narrowband Envelope Alarm Report"..................................................................... 9-34• "Current Inspection Code Report"........................................................................................9-44• "Trends and Waterfall Plots".................................................................................................9-45• "Effect of Vibration Acceptance Testing"............................................................................... 9-49

10. REAL-WORLD CASE HISTORIES........................................................................................... 10-1

(See Individual Table of Contents At Beginning of This Section)



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RECOMMENDED PERIODICALS FOR THOSE INTERESTED INPREDICTIVE MAINTENANCE

1. Sound and Vibration Magazine P.O. Box 40416Bay Village, OH 44140Mr. J ack Mowry, Editor and PublisherPhone: 216-835-0101Fax : 216-835-9303

Terms: Normally free for bona fide qualified personnel concentrating in the Sound andVibration Analysis/Plant Engineering Technologies. Non-qualified personnel -$25/per year within the U.S.

Comments: This is a monthly publication that normally will include approximately 4-6 issues per year devoted to Predictive Maintenance. Their Predictive Maintenance articles

are usually practical and in good depth; normally contain real “meat” for thePPM vibration analyst. Sound and Vibration has been published for over 25years.

2. Vibration s Magazine The Vibration Institute6262 South Kingery Hwy, Suite 212Willowbrook, IL 60514Institute Director - Dr. Ronald EshlemanPhone: 630-654-2254Fax : 630-654-2271

Terms: Vibration s Mag azine is sent to Vibration Institute members as part of their annualfee, (approx. $45 per year). It is available for subscription to non-members at$55/per year; $60/foreign.

This is a quarterly publication of the Vibration Institute. Always contains very practical and usefulPredictive Maintenance Articles and Case Histories. Well worth the small investment.

Comments: Yearly Vibration Institute fee includes reduced proceedings for that year if desiredfor the National Conference normally held in J une. They normally meet once peryear at a fee of about $675/per person, ($600/person for Institute members)including conference proceedings notes and mini-seminar papers. All of thepapers presented, as well as mini-courses, at the meeting are filled with “meat” forthe Predictive Maintenance Vibration Analyst. Vibration s Mag azine was firstpublished in 1985 although the Institute has been in existence since approximately1972, with their first annual meeting in 1977. The Vibration Institute has severalchapters located around the United States which normally meet on a quarterlybasis. The Carolinas' Vibration Institute Chapter normally meets in Greenville, SC;Charleston, SC; Columbia, SC; Charlotte, NC; Raleigh, NC; and in the WinstonSalem, NC areas. For Institute membership information, please contact: Dr. RonEshleman at 630-654-2254. When doing so, be sure to ask what regional chapteris located to your area. Membership fees for the “Annual Meeting Proceedings” are$30/per year (normal cost is approx. $60/per year for proceedings if annualmeeting is not attended). Please tell Ron that we recommended you joining theVibration Institute when you call or write to him.

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3. P/PM Techn olog y Magazine P.O. Box 1706Minden, NV 89423-1706 (Pacific Coast Time)Phone: 702-267-3970; 800-848-8324Fax : 702-267-3941Publisher- Mr. Ronald J ames; Assistant Publisher: Susan Estes

Terms: $42/per year for qualified USA subscribers, (individuals and establishments involved with

industrial plant and facilities maintenance; subscribers must be associated in engineering,maintenance, purchasing or management capacity). $60/year for unqualified subscribers.

Comments: This is a bi-monthly magazine with articles about all facets of PPM Technologies,including Vibration Analysis, Oil Analysis, Infrared Thermography, Ultrasonics, Steam

Trap Monitoring, Motor Current Signature Analysis, etc. These are normally goodpractical articles. Also includes some cost savings information, although does notnecessarily include how these cost savings were truly determined. P/PM Technology also hosts at least one major conference per year in various parts of the United States.Intensive training courses in a variety of condition monitoring technologies will also beoffered in vibration analysis, root cause failure analysis, oil analysis, thermographicanalysis, ultrasonic analysis, etc..)

4. Maintenance Techno logy Magazine 1209 Dundee Ave., Suite 8Elgin, IL 60120Phone: 800-554-7470Fax : 804-304-8603Publisher: Arthur L. Rice

Terms: $95/per year for non-qualified people This is a monthly magazine that usually has at leastone article relating to Predictive Maintenance using vibration analysis within each issue. Inaddition to vibration, it likewise always offers other articles covering the many other

technologies now within Predictive Maintenance.

5 . Re l ia b il it y M a g az in e PO Box 856Monteagle, TN 37356Phone: 423-592-4848Fax : 423-592-4849

Editor: Mr. J oseph L. Petersen

Terms: $49 per year in USA; $73 per year outside USA.

Comments: This bi-monthly magazine covers a wide variety of Condition Monitoring Technologies including Vibration Analysis, Training, Alignment, Infrared Thermography, Balancing, Lubrication Testing, CMMS and a unique category they entitle "Management Focus".

NOTE: In addition to these periodicals, many of the major predictive maintenance hardware andsoftware vendors put out periodic newsletters. Some of these in fact do include some “realmeat” in addition to their sales propaganda. We would recommend that you contact,particularly the vendor supplying your predictive maintenance system for their newsletter.

Their newsletter will likewise advise you of updates in their current products.

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© Copyright 2001 Technical Associates of Charlotte, P.C. 1-1

Technical Associates Level I

CHAPTER1

ANALYSIS I SEMINAR OVERVIEW

An effective Predictive Maintenance Program(PMP) is a total programof thefollowing:

1. DETECTION2. ANALYSIS3. CORRECTION4. VERIFICATION

This is a logical sequence of steps. The program firsthelps you detect theonset of a problem. Itthen provides means for analyzing theproblemin order to determine its cause. It puts you in aposition to correct the problem, effectivelyand efficiently, ata convenient time. And finally, itgives you a means to verify that any correction taken did in factcorrect the problem and that nootherproblems wereincluded.

PredictiveMaintenanceuses theprocess of tracking vibration levelson equipmentcomponentsto determine theconditionof themachinery. Theguiding purposeof this seminaris to provideinstructionon howvibrationsignatureanalysis canbeused to continually evaluatemachinecondition withinaPredictiveMaintenanceProgram.

PredictiveMaintenance Programs beginwithBaseline (or initial)surveys of machines. Later,followup surveys are conducted atperiodic intervals dependenton machinetype, criticality,operatingand maintenance cost, operatingspeed and design of components withinsuchasbearing type, gearing type, etc. Following either Baseline or Followup Periodic Surveymeasurements, ananalysis of collected data is madeand writtensurveyreports are compiledsummarizingdiagnostic results aswell asprovidingoverall recommendations suchas those whichfollow:

1) No ProblemFound.

2) MinorProblems Found - Trend Only atthis Time During Future PMP Surveys.

3) PotentiallySerious Problems Detected WhichMightTend to Deteriorate WithinWeeks -Continue toMonitor EquipmentatShorter Intervals.

4) Potentially Serious Problems Detected, butProblemSource NotYetConfirmed-PerformVibration Diagnostics toDetermineProblemSourceandSeverity.

5) SignificantProblem(s)DetectedWhichWarrantCorrectiveAction- ReplacePartsatNextScheduledShutdown.

6) VerySevere Problem(s)DetectedWhichMandateCorrective Action- ShutDownandReplace Immediately.


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Oneof theoverall purposes of this seminarwill be to teach thestudenthowto beginmakingthesediagnostic calls usingvibration signatureanalysis,howtoevaluateproblemseverities andwhatthoughtprocess should be used to either recommended further diagnostic tests, or howtogo ahead and recommend whatcorrective actions should be madein which chronologicalorder.

Importantly, theanalystwill thenbe instructed to verify all problems have been resolved (and nonewones have been introduced)aftercorrective actionshave been taken. Inother words, he

should repeathisanalysis to assess themachineconditionaftercompletionof suchrepairs.Documentingwhatactionsweretaken,alongwith thedate they werecompleted, is anessentialstep to any successful program. However, taking the extrastep to document the“Before”and“After”condition is whatis criticallyneeded to keep theprogramvisible and effective in the mindsof those associated, notonly with Maintenance and Plant Management, butalso to those in theProductionDepartment.

Following thecompletion of this course, theuser will have a good working knowledgeof theproperapplication instrumentation andsoftwarenecessaryforsetting up of andimplementing aneffectivepredictivemaintenanceprogramas well as receive instruction on basic conceptsinvolved in troubleshootingvibration problems. Thefollowingare brief introductions foreach of thechapters covered in this seminar text.

CHAPTER 2 - “WHAT IS VIBRATION AND HOW CAN IT BE USED TO EVALUATE `MACHINE CONDITION?”: describes thebasic theoryof vibrationwhich includes amplitude,frequencyand phase. Italso introduces the time waveformand howitis converted into avibration spectrum. The relationships betweenacceleration, velocity, anddisplacementareillustrated, as well as theirdirectrelation to machinewear, fatigue, and failure. The truemeaningof Overall Vibration is also givenhere, along with howit is measured. This sectionalso points outthe greatdifferenceinwhetherthis overall level is directlymeasured froma time waveform(Analogmethod), or if it is calculated fromonly the data within the spectrum itself between F MIN and F MAX(Digitalmethod). It shows howmuchof a difference this can make simplyby which method isemployed. Also included are examples of trending vibration levelson equipmentanddetermining vibration severity.

CHAPTER 3 - “OVERVIEWOF THE STRENGTHS AND WEAKNESSES OF TYPICALVIBRATION INSTRUMENTS”: introduces theuser to the types of vibrationmeasurementequipmentcommonlyfound ina Predictive Maintenance Programas well as thevibrationinstruments and softwarerequired to analyzeand diagnosevibrationproblems. A briefhistoryof thedevelopmentand enhancementof these tools is provided, along with updated information onsomeof the more state-of-the-artPMP systems available today.

CHAPTER 4 - “OVERVIEWOF VARIOUS VIBRATIONTRANSDUCERS ANDTHEIROPTIMUM APPLICATIONS”: describes thevarioustransducers (vibration pickups)thatmeasurethe vibration oscillation and transmit this information back to the measuring instrument. Transducers arecoveredwhichcan directlymeasureanyof thethreeprimaryvibrationparameters including acceleration, velocity, or displacement. Information on which particulartransducershould beemployedbased on machinespeed, machinecomponents and overall

frequencyresponseis also covered.

CHAPTER 5 - “ROLE OF SPIKE ENERGY, HIGH FREQUENCY DETECTION (HFD) ANDSHOCK PULSE (SPM), ALONG WITH PROPER SPECIFICATION OF THEIR ALARM LEVELSAT VARIOUS SPEEDS”: describes thetheorybehindeach of theseparameters and illustratesthe types of machineryproblems thatare and are notresponsive to these high frequency signals.Acceptable spike energy,HFD and shockpulse levels are provided fromseverity charts whichhavebeendevelopedfortheseparameters.


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CHAPTER 6 - “INTRODUCTION TO VIBRATION SIGNATURE ANALYSIS ANDHOW IT ISUSEDTO DIAGNOSE MACHINE OPERATING CONDITION”: introduces theuser to TechnicalAssociates’ renown5-Page“Illustrated VibrationDiagnosticsChart”which showstheusera typicalspectrumforeachof 48problemconditions (unbalance, misalignment, rolling elementbearingproblems, etc.),demonstrates phase responseforthoseproblems whichhave aneffecton phase,andprovides supporting remarkssummarizing themore importantsymptoms eachproblemwillnormally display. Followingpresentation of thediagnostic chartwill be discussions separatelycoveringmany of theproblemconditions listed in thechart. However, sincethis seminar isintended to be an introductory course, only the more common problems will be covered (allremainingchartitems are covered in thenext seminar module). Included in many of the theorydiscussions arevibration spectra takenfrommachines having theparticularproblembeingcovered. Finally, real-world casehistories representingeach of theproblems covered inChapter6 are included in the last chapterof the seminartext.

CHAPTER 7 - “PROVEN METHOD FOR SPECIFYING SPECTRAL BAND ALARM LEVELSANDFREQUENCIES USINGTODAY'S PREDICTIVEMAINTENANCE SOFTWARE SYSTEMS”:illustrates howto properly specify notonly overall vibrationalarms for each pointon eachmachine in a program, but also howand whyvarious portions of each spectrumshould be setapart, allowingmuchmoreforsome of these so-called“spectral alarmbands”than forothers.

For example, ifoneband is setaround operating speed (1X RPM), whereas another band isspecifiedaroundbearingdefectfrequencies forthemachine, muchmorevibration is mostalwaysallowed for 1XRPM as compared to that for bearing defect frequencies (which are covered inChapter6). Note in this chapterthatdifferentmachinetypes will have a differentspectral bandsetup (for example, a pump havinga 6-vaned impellerand rolling elementbearings versus amotor outfitted with sleeve bearings). Thesespectralbands are available in several PredictiveMaintenanceSoftware systems. Thevibration amplitudeswithinthesebandsare summed toproduce a so-called “power” level ineach band which is compared with anallowable level for thatband specified bytheuser. Information is givenon howthis so-called “RSS Power Level” iscalculated foreach band and why this is importantwhenattempting to specifymeaningful alarmlevels foreach spectral band. Details of thestartingand ending frequencies foreach band arediscussed, along with theband alarm as per the type of equipment being monitored. Alsoprovided is an importantparameter - wherethe maximumfrequency (F MAX) shouldbeset foreachtypeof machine, bearing typeand operating speed.

CHAPTER 8 - “COMMON PITFALLS IN EVERYDAY VIBRATIONMEASUREMENTS AND THEIR EFFECT ON PROPERLY DIAGNOSING MACHINE PROBLEMS”: informs theuserontheproperchoiceand useofvibration transducers based on transducermounting, frequencyrange, and outputparameter(acceleration,velocity, or displacement) desired. Thepros andcons of thevibration measurement location (bearinghousing, shaft, etc...)are also evaluated. Italso clearlydemonstrates thedetrimental effecton accuracy of diagnostic calls if thewrongtransducer(ortransducermount) is used; ifthewrongfrequencyspanis measured; if measurements are madefromthewrong locationon a machine; etc.

CHAPTER 9 - “SETUP ANDIMPLEMENTATIONOF EFFECTIVE PREDICTIVEMAINTENANCE ANDCONDITION MONITORING PROGRAMS”: pulls togetherthe informationon all thePMP hardware and softwaretools, signature analysis and overall/spectral band alarmsettings covered in theprevious chapters and shows theuser howto usethis knowledgeto puttogethera completeandcost effective Predictive Maintenance Program(PMP). Implementationcostsvs. paybacksavings are presented. A flow chartdescribing the steps necessaryto createaneffectivePredictiveMaintenanceProgramis presented,aswell as thenecessarydata sheetsthatarerequired fordeterminingproperalarmsettings, frequencyranges,measurementparameters (i.e., acceleration,velocity, displacement, etc.), as well as howto determineoptimumdata collection routes. Examples of machinecondition reports are provided which includefrequencyspectra fromdamagedmachines; overall condition foreachmachineevaluated;


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severity rank-ordered reports forproblemmachines;alarmexception reports;andmachinemeasurementreports whichshowtheoverall levelmeasuredand recorded foreach pointon eachmachineevaluated.

CHAPTER 10 - “ACTUAL CASE HISTORIES OF VIBRATION DIAGNOSTIC ON VARIOUSMACHINE TYPES”: offers anarray of real-world problems whichhave been detected by

Predictive Maintenance using the tools taughtin this seminar. They showhowproblems weredeterminedbyvibration diagnostics, andsubsequentlycorrected withoutcatastrophic failure.Impressive“before”and“after”frequencyspectraare displayed toshowtheeffectof completingthe recommended corrective actions on themachine, and thereby prolonging the life of theequipmentby reducingvibration amplitudes.

SEMINAR GOALS

Attheconclusionof this seminar, itis hoped theuser of this manual will be able to effectivelycommunicate on a technical levelwith others in the field of vibrationanalysis. In addition, heshould also gain a fundamentalunderstanding of howto effectively startup and fullyimplementaneffective Predictive Maintenance Program. Furthermore, an introduction into the use of vibration analysis forpurposesof troubleshooting equipmentproblems will have beenpresentedby this manual. Yourquestions and comments pertaining to this course’s educational contentare welcomeand will providethebasis forenhancements to bettertrainfuture seminar students.


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2-1© Copyright 2001 Technical Associates Of Charlotte, P.C.

WHAT IS VIBRATION AND HOW CAN IT BE

USED TO EVALUATE MACHINERYCONDITION?

2.0 INTRODUCTION

Vibration is defined by Webster’s New World Dictionary as “to swing back and forth; to oscillate”.For those more closelyassociated with thevibration industry, vibration is a pulsating motionof amachineor a machinepartfromits original place of restand can be represented by this formula:

VIBRATIONAMPLITUDERESPONSE ∝ DYNAMIC FORCE Equation1DYNAMIC RESISTANCE

CHAPTER2

FIGURE 1MASS IN NEUTRAL POSITION WITH NO APPLIED FORCE

A good way to illustrate vibration of a machine is shown in Figure 1 below. Equation 1 showsthat vibration amplitudevaries with the quotientof Dynamic Force divided by Dynamic Re-sistance (that is, a machinewith acceptable vibration mayhave a marked higherlevel ifgiveninsufficientsupportframeand foundation).

Vibration is theresponse of a systemto some internal or external stimulus or force applied to thesystem. Vibration has three importantparameterswhich canbe measured - Amplitude(howmuch); Frequency(howmanytimesperminute orpersecond); and Phase(whichdescribes howitis vibrating).Eachof these importantparameters will be discussed in sections of Chapter2whichfollow.

Note that a vibration transducer is mounted on (oras nearas possibleto) thebearing housing. This transducer will sense thevibration and pass thesignal through a connecting cable to ananalyzer. Figure 1 shows that themachine’s bearing housing can bemodeled bya masssuspended bya coil spring. Until a force (stimulus) is applied to this mass, itwill remain sus-pended in a neutral or unstimulated position. When a force is applied to themass (i.e., in anupward directionin this case)as shown inFigure2, themassmoves upward and thespringcompresses (stimulated)bytheforce.


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FIGURE 2MASS BEING STIMULATED BY AN APPLIED FORCE

FIGURE 4CONTINUED RESPONSE TO APPLIED FORCE

FIGURE 3MASS RESPONDING TO THE RELEASE OF THE APPLIED FORCE

Once anupper limitof motion is reached theforce is removed and themass begins to drop. Themass will drop through theneutral positionand continue to travel to its lowerlimitas shown inFigure 3.

Oncethe lowerlimitis reached, themasswill stop its downwards motionand reverse directionagain passing through theneutral position to theupper limit; then stop and return to the lowerlimitrepetitively as long as anexternal force is applied as shown in Figure4.


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If a pen were attached to the mass as it repetitively responds to theapplied force and a stripchartrecorder placednearby, thevibrationresponse could bedocumentedas shown inFigure5.

2.01 WHAT IS VIBRATION FREQUENCY AND HOW DOES IT RELATE TO A TIMEWAVEFORM?

When examined, the trace drawn on thestrip chartrecorder of Figure5 would show a uniformsinewave havinganamplitude(peak-to-peak displacementin this case). Frequencycanbecalculated fromitby measuring the time period (T)of onecycleand inverting to determine thefrequency. SeeFigure6.Frequencyis expressed inunits of either Cycles per Minute (CPM) or inCycles perSecond (CPS),which is nowcalledHertz (where 1Hertz = 60CPM).The commonlyused abbreviationforHertz is “Hz”.

FIGURE 5PEN ADDED TO MASS TO TRACE ITS OSCILLATING MOTION ON A

CONSTANT SPEED STRIP CHART RECORDER

2.02 WHAT IS VIBRATIONDISPLACEMENT?

As its name implies, displacement is a measure of the total travelof themass - that is, itshowshowfar themass travels back and forth when itvibrates. Displacementof themasscan beexpressedeither inunits ofmils (where1 mil= .001 inch)or inmicrons (where1 micron = .001millimeter). Furtherextrapolation of the same displacementsinewaveformwouldyield velocityand accelerationvalues as seeninFigures 7 and 8.

FIGURE 6DISPLACEMENT AND FREQUENCY FROM THE TIME WAVEFORM


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2.03 WHAT IS VIBRATIONVELOCITY?

Vibrationvelocity is a measure of thespeed atwhich themass is moving or vibrating during itsoscillations. The speed of themass is zero atthe upper and lowerlimits since itmust come to astop atthese points before it reverses direction and begins to move in theopposite direction.Velocity reaches its maximum(orpeak) attheneutral positionwhere themass has fullyac-celerated and nowbegins to decelerate as shown inFigure 7.Velocity is expressed inunits of

inchespersecond (in/sec)ormillimeters persecond (mm/sec).

2.04 WHATIS VIBRATIONACCELERATION?

Acceleration is defined as therate of changeof thevelocity and is measured ing’s of accelerationrelative to theacceleration of gravity. Atsea level, 1.0 g equals 32.2 ft/sec 2 which equals 386.087in/sec/sec or 9806.65mm/sec/sec - theaccepted values fortheacceleration of gravity in theEnglishand Metric systems (where in/sec/sec isnormallyexpressedas in/sec 2). Accelerationasnoted inFigure8 is greatestwherevelocity is ata minimum. This is wherethemass has

decelerated to a stop and is aboutto beginaccelerating again (that is, moving faster).

Acceleration is probablythemostdifficult measure of vibration amplitudeto grasp. To bring itinto clearerfocus, let us return to themid-1950’s and to a groupof sevenmenhaving the“rightstuff”and follow themon their journey towards thecenterof theearth fromwithina speciallydesigned compartmentin a jetplane. When the plane began its descentatapproximately400miles per hour and started increasing speed attherate of about26miles per hour each second(or386 in/sec/sec), these menbegan to experience that temporarilywonderful feeling of weightlessness as the planeovercame theforce ofgravity.Thatis, whentheplaneacceleratedatthis rate (400miles per hour in thebeginning; then 426 MPH onesecond later; 452 MPH thenextsecond and so on), a “force”of 1.0g was applied to each manwhether heweighed 150 lbs, or250 lbs, and each manbegan to floataround within thecabin. This was all well and good until...itwas time for theplane to pull outof its downward plunge(ofcourse, thealternative was muchless fun). When theplanepulled outof thedive, each manwas then subjected to accelerationlevels of 6g or more (that is, theskeletal structure of a 200 lb mansuddenly had to withstand aforce of roughly1200lbs!).

In thesame way,whenamachinehousing vibrates, itexperiences the force of accelerationsinceitcontinuallychanges speed as itmoves back and forth. Thegreater the rate of changeof thisspeed (orvelocity), thehigher will be theforces on this machine dueto thehigheracceleration. Therefore, thegreater this amountof acceleration, thehigherwillbe theforces (andthus,stresses) applied tothevibrating machinemember.

FIGURE 7VELOCITY FROM THE DISPLACEMENT WAVEFORM


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FIGURE 8ACCELERATION FROM THE DISPLACEMENT WAVEFORM

2.05 WHAT IS VIBRATIONPHASE?

Vibration phase is the final descriptive characteristic of vibration.Phase is the relative shiftof avibrating partto a fixed referencepointon another vibrating part. That is, phaseis a measure of the vibration motion atone location relative to the vibration motion atanother location. Or, inother words, it is the “timing”of a vibration in relation to a stationary or moving parton the

machine. This is similarto the“timing”referenceused to tune a reciprocating engine.Phase is apowerful tooluseful intheanalysisofmachinefaults whichwill bediscussedlater. Since phase isa measure of relative motion,examples canbeshown with twomass weights and springsattached to thesame referencepoint. Figure 9 shows the two systems in-phase with each otheror vibratingatthesame rate with 0°phase differenceand theresulting time waveform.

FIGURE 9 TWO MASSES WITH ZERO PHASE DIFFERENCE

Figure 10shows twomasses vibratingwith 90°phasedifference. Thatis,Mass #2is one-fourthof a cycle(or90°) ahead of Mass #1. In general vibration literature, this is what is meantwhensomeone states “Mass #1has a 90°phaselag relative to themotionof Mass #2".

FIGURE 10 TWO MASSES WITH 90°PHASE DIFFERENCE


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In thesame way,Figure 11shows thesame twomasses vibratingwith a 180°phase difference. That is, at any instantof time,Mass #1moves downwards at thesame time as Mass #2movesupwards,andviceversa.

Vibrationphaseis measured inangulardegrees byusing a strobe lightor electronic photocell.

Figure 12shows howphase relates to machine vibration. Theleftsketch shows a 0°phasedifferencebetweenbearingPositions 1and 2 (in-phase motion); whiletherightsketch pictures a180°out-of-phasedifferencebetweenthesepositions(out-of-phasemotion).

FIGURE 11 TWO MASSES WITH 180° PHASE DIFFERENCE

FIGURE 12

PHASE RELATIONSHIP AS USED WITH MACHINERY VIBRATION


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2.1 WHAT IS A VIBRATION SPECTRUM (ALSO CALLED AN “FFT” OR“SIGNATURE”)?

A vibrationspectrumplots orgraphs amplitude (mils, in/sec org’s) versusfrequency(CPM orHz)

as shown inFigure 16c. Gettingbackto theso-called vibrationmeasurementsystemshown inFigure 5, note that this directrecording method has many limitations. To overcome theseproblems, transducers are used to convertvibration into anelectrical signal.This electrical signalis then translated throughelectronics into a vibration displaywithinananalyzer to which thetransducerisconnected.

There are manyshortcomings to using theaforementioned directrecordingmethod to measurevibrationdata. A muchmore effective method is to useelectronics which is used to convert thevibrationsensedbythetransducer itself into anelectronic signal.Onesuchsetup is shown inFigure 13inwhich thetransducersignal travels throughanamplifier to a servo motorwhichdrivesa pen to graph the vibration motion on a chartrecorder.

The amplitude of therecorded waveformcan be adjusted to be thesame as thatof theactualvibrating pieceby adjusting thegain of theamplifier.Figure13is a very simplified illustrationof vibration recording.However,most vibrations are complicated combinations of variouswave-forms thatrequiremoresophisticated indirectrecording devices. Figure 14shows howwave-forms combine to make morecomplicated waveforms. Note howthe total waveformis actuallymadeup ofa series of smallerwaveforms, each of whichcorrespond to anindividual frequency(1X RPM, 2XRPM, 3XRPM, etc.). Each of these individualwaveforms then algebraicallyadd tooneanother to generate the totalwaveform which canbe displayed by oneof today’soscilloscopes oranalyzers.

Anoscilloscope is useful inviewingthese combinedwaveforms. Itfunctionsbypassing thesignalfroma transducer into two electronic plates that are able to displace anelectronic beaminto theshapeof thewaveform. Figure 15illustrates thisprocess.

FIGURE 13INDIRECT MEANS OF RECORDING VIBRATION


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Until nowin thispaper,all the illustrated measurements have been shown in the time domainwhere theX-axis is time (secor min.) whiletheY-axis is a vibrationamplitudemeasure(displacement, velocity or acceleration). Displayingand using the time domain is a very precisemethod in which to display the actual (total)machinemotion and to analyze thevarious vibrationparameters.However,analyzing the timewaveformitselfcanbeverycumbersomeandlaborintensive whenfrequency needs to be determined. Here, the time duration fromonepeak of interestto the next similar peak mustbe determined to calculate the vibration period (sec/cycle)as seeninFigures 6 and 16. This cycletime or period (T) must then be inverted to obtainfrequency(F) and then converted to theproper frequencyunits (CPM, CPS, Hertz).

To simplify this process, vibration instruments are able to develop what is known as a Fast

FourierTransform(FFT).AnFFT is thecomputer(microprocessor)transformationfromtimedomaindata (amplitudeversus time) into frequencydomaindata (amplitudeversusfrequency). This FFT calculation technique was developed byBaron J ean Baptiste Fourier over 100 yearsago.Fourierstated that any real-world sinewaveformcancombine to make another morecomplicated waveformas was seen inFigure 14; and viceversa, any complicated real-worldwaveformcan beseparated into its simplesinewaveformcomponents. This is illustrated inFigure16where: (a) a total time waveformis captured in the time domain; wherein (b) this time domainwaveformis separated into its separate sinewaveforms anddisplayed in threedimensionalcoordinates of amplitude, time and frequency.As thesinewaves are separated fromthecombinedwaveform, thefrequencyofeach sinewave is determined and thesinewaves are

FIGURE 14

COMPARISON OF TIME & FREQUENCY DOMAINS (Ref. 14)

Where: t MAX = Total Sampling Period setting how often amplitude is measured and stored (sec).Sample Size = Number of Analog to Digital Conversions to be used to Construct the Time Waveform

(Samples- most often 1024Samples to provide a 400 line FFT).FMAX = Maximum Spectral Frequency or Frequency Span (CPM).


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FIGURE 16FREQUENCY DOMAIN DEVELOPED FROM THE TIME DOMAIN

WAVEFORMS

placed in therespective positions along the frequency axis. In Figure16C, a view looking alongeach individualwaveformis shown allowingtheanalyst to seethefrequencydomain of amplitudeversusfrequency.

The frequencydomain view(Figure16C) ofthetimewaveform(Figure16A)visuallyshows eachsimplesine wave as a vertical line thathas amplitude(asdetermined by its height) and frequency(asdetermined by its positionalong thefrequencyaxis).This frequencydomain representationof a time waveformis called a spectrum(with spectrabeing its plural form). A spectrumissometimes referred to asa “signature”oran“FFT”.Spectra(frequencydomain displays) are veryuseful tools forthevibrationanalystwho would otherwisehave the laborious taskof distinguishingandseparating timewaveforms into discrete frequencycomponents foranalysis.

FIGURE 15VIBRATION SIGNAL DISPLAYED ON AN OSCILLOSCOPE

a) Capturedtimedomainwaveform

b) Three-dimensional viewof thecaptured timedomainwaveformafterseparation into itsseparate simplesinewavesand determinationof the frequencyofeach sinewave.Thethreeaxes are amplitude, time andfrequency.

c) Frequencydomainviewdevelopedfromthecaptured timewaveform(s).


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FIGURE 17STEPS IN THE CONVERSION OF A VIBRATION INTO AN FFT SPECTRUM

Figure 17illustrates the full transformationof vibrationas equated to a real-world transducermountedon a bearing housing (a) as representedbythespring mass system(b) capturing a timewaveformof thevibration(c)which is then processed into anFFT spectruminthefrequencydomain (d).


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2.2 DIFFERENCE BETWEEN RMS, PEAK AND PEAK-TO- PEAK VIBRATION AMPLITUDE?

Since electronics is already employed to create the various vibration displays in the time andfrequency domains, additional electronics and softwareare used to convert thedisplayedamplitude into measurementunits of displacement, velocityandacceleration. Theelectronicsalsoperforms all thenecessaryconversions forpeak to peak, peak, RMS (root-mean-square)oraverageamplitude. Table I gives theformulas forthevariousmeasurementunitconversions.Itisinterestingto note herethatEuropeans normallyuseRMS velocityamplitudes whileAmericanshave adopted peak velocityvalues eventhough the instruments themselves collectRMS data andthen multiply themby the conversion factor (1.414) to obtain so-called peakvelocity.This is mostlikelydue to the factthat mostall the severity charts for various equipment types have beendeveloped using this so-called peak velocity in America.

Figure18compares thevariouscommon Englishand Metric units ofvibrationmeasurement.

TABLE I

CONVERSION FORMULAS FOR VARIOUS AMPLITUDE UNITS (Ref. 1)(See Figure 22also which graphically shows these and gives further formulas)

FIGURE 18COMPARISON OF ENGLISH AND METRIC VIBRATION UNITS


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Note fromFigure19that for a sinusoidal waveformlike that shown (which is likelycaused byalmostpureunbalance):

A. Peak-to-PeakVibration = 2.000 X Peak VibrationB. Peak-to-Peak Vibration = 2.828 X RMS VibrationC. Peak Vibration = 0.500 X Peak-to-Peak VibrationD. Peak Vibration = 1.414 X RMS VibrationE. RMS Vibration = 0.354 X Peak-to-Peak VibrationF. RMS Vibration = 0.707X Peak Vibration

FIGURE 19COMPARISON OF PEAK, PEAK-TO-PEAK, RMS, AND AVERAGE

Figure 19 shows howonecan convert fromoneunitof amplitude(i.e., RMS, Average,Peak andPeak-to-Peak) to another (for apure sinusoidal sinewave whichcanbecausedbyunbalance forexample). Thatis, ifonefound hehad anamplitude unitwhich heneeded to convert forcomparison to another unitof amplitude, hewould find themultiplier inFigure19. For example, if onemeasuredanamplitudeof1.0 in/sec RMS, this would equal 1.414 in/sec peak; whereas 1.0mil RMS would equal 2.828mils Pk-Pk.


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2.3 WHEN TO USE DISPLACEMENT, VELOCITY OR ACCELERATION?

Whenattempting to analyzea machine’s vibration, itis essential thatas much informationaspossible is available (bearing types and modelnumbers, accurate measures of thespeed of each

shaft, geartooth counts, quantity of vanes or blades, etc.). Notknowing this informationcanseriously affect theaccuracy of thediagnosis.Each oneof these elements is very important indiagnosing vibration.Forces both inside and outside the machineresultin some vibration atthemachine. Sometimes the vibration is acceptable; atother times, the vibration is notacceptableandwill eventuallyhave anadverse affecton the machine health. Evaluating the vibrationamplitude, frequencyand phase canidentify thecondition themachine is in, both atthe time of originalbaseline signatures aswell as in followup ReliabilitySurveys.

Amplitudeis oneof themost frequentlyused parameters invibrationanalysis.Vibrationamplitudeis proportional to the severity of potential machine problems and is oneof the prime indicators of machinecondition.Amplitudeunits of either displacement, velocity or acceleration canbe used.Each of these units ofmeasurementcanbeused. However, generallyspeaking, velocity ispreferred.

Peakto peak displacementmeasurements are often thefavored unitof measurementin theEnglish systemsincedisplacementmeasurements relatedirectlyto dialindicatormeasurements.Displacementis usuallythoughtto bemostuseful infrequency ranges less than approximately600 CPM (10 Hz).Frequencymustbe used along with displacement to evaluate vibrationseverity. That is, simply stating that thevibration at 1X RPM is 2 mils (25.4microns Pk) is notenough information to relate if this 2 mil level is good or bad. For instance, 2 mils Pk-Pk of vibrationat3600CPM is muchmore destructive than is thesame 2 mils at300CPM (see Figure20). Thus, displacementaloneis unableto evaluate vibrationseverity throughouta full spectrumfrequency range. Thiswill becoveredagainbelowand recommendedvibrationseverity charts forvariousmachinetypes will begiven.

Acceleration hassimilardisadvantagestodisplacementexceptitfavors thehigher frequencyranges. Acceleration is also frequency dependentin terms of severity or damagecriteria. Forexample, 2g’s at18,000 CPM is muchmore severe than thesame 2g’s ata frequencyof 180,000CPM (3000Hz) (seeFigure21). Acceleration is typicallyrecommendedforusewhensourceswithinamachinegeneratefrequenciesoverapproximately300,000CPM (5000Hz).Thesesourcesmayincludegearmeshfrequencies (# teeth X RPM), rotorbarpassing frequencies (#rotorbarsX RPM), blade passing frequencies (# blades X RPM), etc. Don’tforgetthat inmany cases, thesesourcesgenerateharmonics (ormultiples)ofthesefrequencies.

Velocity on theother hand is notnearly so frequencydependent in the range fromabout600 to120,000 CPM (10 to 2000 Hz)and is generally theunitof choice when vibrationsources within amachinegenerate frequencies ranging fromapproximately300to 300,000CPM (5to 5000Hz).Velocity amplitudes relate directly to machinecondition, no matter the frequency in the range

fromapproximately 600 to about100,000CPM (10 to 1670Hz). That is, it is generally feltamachineexperiencing .30in/sec atforexample1800RPM is subjected to thesame severity asanother machine subjected to .30 in/sec atabout 10,000RPM operating speed. This is shown bythe“Contours ofEqualSeverity”inFigure 22.


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2.4 HOW MUCH IS TOO MUCH VIBRATION?

Figure 20is a typicaldisplacement/velocity severity chartdeveloped years ago for“generalrotatingmachines”with vibrationseveritynotedasbeing “GOOD”, “FAIR”, “ROUGH”, etc. Thischartclearly illustrates the frequencydependenceof displacement. For instance, a peak-to-peakdisplacementof2mils (25.4microns, PK) ranges from“SMOOTH”to “VERY ROUGH”in severity.For this sample, frequency mustalso be identified to accuratelydetermineseverity (i.e., 2 mils of vibration @ 400 CPM is “GOOD”while2 mils at3600 CPM is rated“ROUGH”). Velocity on theother hand needs only its amplitude to determine severity on this chart(i.e., .157to .314in/sec of vibration is “SLIGHTLY ROUGH”). Figure 21is a similarvibrationseverity chartforacceleration.Accelerationseverity is againnoted as being frequency dependentand velocity as independentof frequency. For example2g’sat18,000CPM is rated “ROUGH”, while2g’s at180,000 CPM israted justwithinthe“GOOD”range.

Figure 22shows theconsistency which velocity has overa wide flatfrequency rangeascompared to displacement and accelerationwhich tend to favor the lowand high ends of thefrequencyscale respectively.NotethatFigure22graphs “CONTOURS OF EQUAL SEVERITY”for each of the3 amplitude measurements. ExampleA shows howto useequations giveninFigure 22 to convert velocity to displacement, velocity to acceleration, displacement toacceleration, etc.

Figure 23shows 3 spectrain (a) displacement, (b)velocity and (c) accelerationof thesamewaveformlooking fora possible bearing defectproblem.

Note thepresence of thepeak labeled 300 CPM ineach of the3 spectra. This is thefrequency atoperating speed (commonly called 1XRPM). However, note that as onemoves fromdisplacement to velocity into theacceleration spectrum, 1XRPM appears to be less and less acontributor. That is, looking atFigure23A, note that 1XRPM clearlydominates thedisplacementspectrum. However, it is only slightlyhigher than 2 or 3 other peaks in thevelocity spectrumof Figure 23B. Then, the1XRPM peakalmostvanishes in theaccelerationspectrumofFigure23C

where its amplitudewas justenoughtoreachtheuser-defined threshold so that its amplitudeandfrequencycould be printed outon the plot.

Possiblyof greaterimportance whendetectingproblems and evaluatingmachinecondition iswhathappens to whatare known as the bearing defectfrequencies in thissamefigure (as thename implies, bearingdefectfrequencies are generatedwhenwearbegins to occuron either thebearing races, rolling elements or cage). Whether or nottheanalystwill see these importantbearing frequencies inhisspectramaydepend on hischoiceofamplitudemeasure asdemonstrated byFigure23. With respect to thebearing frequencies at4860 CPM and 9720CPM,note that these frequencies are clearly visible on both thevelocity and acceleration spectra(Figures23B and 23C). Whatare known assideband frequencies spacedatequaldistances totheleftandrightof4860CPM, sideband frequencies can indicate a more seriousbearing wearproblemso it is important that the spectrum show theirpresence if they are there. Looking at

Figure 23, note thatwhilethe4860CPM bearing frequency was clearly present in thevelocityspectrum, itwas justhigh enoughin thedisplacementspectrumtoreachthe user-definedthreshold. Ofgreaterconcernwas thefact that thedisplacementspectrummissed thesidebandfrequencies surrounding 4860CPM almostaltogether,whileitin factdid entirely miss thesecondbearing frequencyat9720CPM. Of course, thereason for this is thatdisplacement tends to“amplify”oremphasizelowfrequencieswhereasitsuppresseshigherfrequencypeaks (asillustrated inFigure22). On theotherhandwhileacceleration emphasizes higher frequencies, ittends to suppress lowfrequencies (seeFigure22).


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FIGURE 20VIBRATION DISPLACEMENT & VELOCITY CHART FOR

GENERAL HORIZONTAL ROTATING MACHINERY (Source: Entek IRD International, Milford, OH)


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Technic al Assoc iates Level I

FIGURE 21VIBRATION ACCELERATION & VELOCITY SEVERITY CHART FOR

GENERAL HORIZONTAL ROTATING MACHINERY (Source: Entek IRD International, Milford, OH)


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FIGURE 22COMPARISON OF VIBRATION

DISPLACEMENT, VELOCITY & ACCELERATION


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FIGURE 23COMPARISON OF DISPLACEMENT, VELOCITY AND ACCELERATION

SPECTRA ON A 300 RPM FAN WITH BEARING PROBLEMS


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Therefore, itis importantto note that a velocity spectrumhas a widerusable frequency range foralmost all rotatingmachinerythan do either displacementor acceleration.Combining thischaracteristicwith velocity’s direct relationshipwithvibration severityalmost always makesvelocitythe bestvibration measurementunitto use (particularlywhenfrequencies arebelow120,000 CPM, or2000 Hz).

Through theyears, thegeneral vibration severity chartofFigure 20 has probably been themostcommonlyused vibration severity chartforrotatingmachinery. Using this chartand velocityamplitudes,onecan beginto assess howbad thevibrationmightbeon his machine. However,this chartwas nevermeantto apply to all machinetypes and configurations whenchoosingvibration limits thatwill give adequate warning of anexistingor impending problem. To help meetthisneed,Technical Associateshas also developedamorecomprehensivevibrationseveritychartshowninFigure24whichisentitled “CRITERIAFOR OVERALL CONDITION RATING”. Thischartapplies to a widevariety of machines overa widerangeof operating speeds from 600 to60,000RPM. It is important to pointoutthese levels are peak overall velocity levels (in/sec). Notethat two columns entitled“GOOD”and “FAIR”are included with thechart. These columns areused to give a machinean“Overall Condition Rating”based on thehighestoverallvibration levelfound on anyof themachinemeasurementpoints.

However, these ratings are onlyapplied if spectral band alarms specified by theuser forthesemachines are notexceeded(ineffect, these spectral alarms arebands whichare placedaroundindividual frequencypeaks orgroups of peaks and allowtheuser to specify a muchhigher alarmfor1XRPM than hewould forother frequencies such asbearing defectfrequencies).Theinformation givenin thecondition ratingchartof Figure 24was acquired throughmany years of actual vibration data acquisition on a diverse array of machinetypes. Notice that analarmvaluefor a cooling towerfan drive outfitted with a long, hollowdrive shaft(.600in/sec) or areciprocatingcompressor (.500in/sec) is muchhigher than thatallowedfora hermetic centrifugalchiller(.225 in/sec) or a machine tool motor (.175in/sec). Also note that two alarmlevels aregivenin the table (ALARM 1 and ALARM 2). In general, machines allowed to operate above Alarm1 will likely fail prematurelyifproblems are not identifiedand corrected; while those allowed tooperateabove Alarm2maysuffercatastrophicfailure if leftunaddressed.

The Technical Associates rating chartbynomeans covers all types ofmachines. Formachinesnotincluded in thechart of Figure 24, onecould use theFigure20 severity chartor a statisticalmethod to develop other alarmlevels.Thestatistical method is especiallyeffective whenseveralidentical machines arepresent, severalsurveys on themachines havebeen conductedand themachines’ vibration levels canbe compared to establishanacceptable vibration level.Usingstatistics tomake this comparison lends a powerful mathematical approach. A statisticalcomparisoncanbe conducted if themachines are similar inconstruction,drive configuration(directcoupled, belt driven, etc.), operatingspeeds, loading and internal components (bearings,gears and gear tooth count, etc.). If a statistical method is used, it is wise to revise thealarms asoriginallyhigher vibration levels are reduced througha vibration program.

Shock Pulse,HFD and SpikeEnergyare stillanothervibrationmeasurementof special interest. Theirpurpose is to measure“bursts”of energythatoccur intheultrasonicfrequency ranges andgive veryearly indicationsof things suchasminute surface flaws inbearings and gears.Until veryrecentlythesemeasurements wereonlyviewedasanabstractnumeric value.However, with newinnovations in instrumentcircuitry theseultrasonic measurements arenowabletobe“demodulated”producingameaningfulspectrum.Theseultrasonicmeasurementswillbediscussed ingreaterdetail inanother chapter.


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FIGURE 24CRITERIA FOR OVERALL CONDITION RATING

(PEAK OVERALL VELOCITY, IN/SEC)


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2.5 UNDERSTANDING A VIBRATION SPECTRUM

A spectrumis a graphic displayof vibration in the frequency domain with vibration amplitude(displacement, velocity oracceleration) on theY-axis and frequency on theX-axis (CPM orHz).Figure 25 is anexample of a spectrum. However, note there is no way to tell how much or at whatfrequency thevibration is. Figure 26is thesame spectrumwith units ofmeasurementand otheridentifying information added. Note that theX-axis is delineated in CPM with a maximumfrequency (F MAX) of 12,000CPM and theY- axis is delineated in in/sec of vibrationwith .050in/secpeak/division (with .050 in/sec per division X 10divisions, this gives a FullScale Amplitude of .500in/sec). In this spectrum, both thehorizontaland vertical scales have tendivisions with eachfrequency divisionbeing one-tenth of 12,000 CPM. Note theother identifying informationpresentedon the spectrumthat includes therotational speed, machinename(TEST 12K400L),themeasurementposition(4H), thedate and time thedata was takenand theunits of measurement(in/sec).

The largepeak about1.5divisions over in thespectrumis thedominantvibration. Itwould bepossibleto scale theamplitude and frequencyvalues byusing a measuring devicesuch asa rulerbut that would onlybe a close estimate. However, theaddition of a cursoras shown inFigure 27makes determining frequency and amplitudeof anindividualpeak easy. In Figure 27, thecursoris atthedominant peakand indicates this vibration is .2474in/sec at1770 RPM (the rotatingspeed of themachine). Additionof anamplitudethreshold levelof 1%places animaginary lineacross thespectrumat1%of thefull amplitude scaleof .500in/sec as shown inFigure27. Whatresults fromthe threshold is a listing of all thepeaks that exceed the1%level (.005 in/sec in thiscase), theirorder (order= multipleof running speed) and theiramplitude(in/sec in this case).Pleasenote thateach vibrationsoftwarepackagewill present theinformation inFigure 27usingits ownformat.

2.51 EFFECT ON FREQUENCY ACCURACY OF #FFTLINES USED

It is very important thatboth amplitudeand frequency values are known as precisely as possible

for all peaks in thespectrumwhen doing ananalysis.Amplitudeof course is important as itgivesonean idea of the severity of theproblem. Frequencyon theother hand is used to determine thesource of thevibration.For instance thedominantvibration, (vibration withthe largestpeak) inthespectrumof Figure 27 is at1770 CPM. The rotational speedof themachine (1X RPM) is also1770RPM. Therefore, thecomponent’s rotating speedat1770 RPM is thesource of thedominantvibration. Knowing that theproblemsource is at 1X RPM helps one know a listof possibleproblemsources, and atthe same time, eliminates thepossibility of other sources being theproblemsuchas rolling elementbearing, blade pass, or most all electrical problems.

Probably thegreatestrequirementindetermining potentialmachineproblemsource(s) is knowingtheactual value of thefrequency as accuratelyas possible.Determining anaccurate vibrationfrequencycanbecome clouded orenhanced depending on howmany lines of spectral resolutionare used in collecting and displaying thedata. Mostmoderncomputermonitors are able to

display 400 lines of spectral resolution (and mostdata collectors today are usually setup tocapture 400 linespectraon theirPMP routes). Thatis, a spectral display is made of 400 individualvertical lines (or“bins”) located adjacent to oneanother along thefrequencyaxis.Eachoneof these bins has amplitude information stored in it relative to the amountof vibration atthat specificfrequency or bin location.Notallbins contain information sincethevibration atsome frequencybinlocations is zero. Thus inFigure27, thebinthat contains the1770CPM frequencyis the59thof 400 bins as counted fromthe left (or zero) end of thefrequency scale. Looking atFigure 27,the400th bincontaining 12,000CPM (12K CPM) has zero amplitude indicating no vibrationispresentatthatfrequency. In thecase of Figure27, each bin(line of resolution) covers a rangeof


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frequencies.Thefrequencyrangeperbin(knownasfrequencyresolution)is calculatedbydividing F MAX(12,000 CPM) bythenumber of lines of resolution(400 lines of resolution in thiscase)or12,000CPM/400 lines of resolution= 30CPM/lineof resolution(bin). Thereforeeachbinhas a 30CPM range. Thesize of this resolutionhas a definite effecton thefrequency informationgained froma spectrum. For instance, the1770 CPM binof Figure27actually contains vibrationinformation in the range of 1755 to 1785CPM. So whether theretrulyare oneor more peaks

withinthis rangeis accumulativelyrepresentedbythe1770CPM frequencyand .2474 in/secamplitude(i.e., if a vibration existed at1760CPM in addition to the 1770 CPM vibration, theamplitudeof the1760CPM frequency would contribute to the total amplitudeshown forthe1770CPM bin.

The higherthe lines of resolution specified fora spectrum, themore precise will be thefrequencyread by theanalyzerand displayed in the spectrum. Note importantly that the precisionof anydisplayed frequency will be thefrequency reading plusor minus one-half theresolution.That is, if a 12,000CPM spectrumwith 30CPM frequencyresolutionwere capturedand showeda peakof 1800 CPM, theaccuracywould be1800 CPM (±15 CPM), or from1785 to 1815 CPM. On theotherhand, ifa 120,000 CPM spectrumusing 400 lines were taken(300CPM resolution) atthesame location and showed thesame 1800 CPM peak, theaccuracy would be ± 150 CPM or from1650to 1950CPM. Figures 27through 31are spectrawith 100, 200,400 and 3200 lines of resolution.Note in each spectrumhowthevibrationpeaks are displayed.The100 line resolutionspectrumof Figure 28has a very “blocked”appearance with thedominantvibrationshown asbeing 1800 CPM. With 100 lines of resolutionavailable, each binhas a frequencyrange of 120CPM/lineof resolutionincluded. Therefore,considerable inaccuracyis seen indetermining theactual vibration frequency.Figure 29has 200 lines of resolution.Therefore, its resolution isslightlybetterthan thatof Figure28with 60CPM resolution. Herewe seethedominant vibrationis said to be at1740 CPM. Figure 30 is a 400 linespectrumwith 30 CPM/line of resolutionaccuracy and identifies thepeak at1770CPM, a muchmoreaccurate vibration representationthan the100 or200 linespectra. Further accuracycan beachievedbyusing highernumbersof lines of resolution. Figure31is a portionof a 3200 lineresolutionspectrumknown as a “zoomspectrum”. Calculatingtheresolutionof the3200line spectrumyields 3.75 CPM/line of resolutionaccuracy for the12,000CPM frequencyspan spectrum. Thus, looking atthe 3200 linespectra of Figure31, thedominantvibration is shown as 1758.7 CPM (and is trulytherefore1758.7 ± 1.87CPM). Thisdegreeof accuracy is notalways needed, butis extremelyhelpful whenvibrationfrequencies fromdifferentmachinesourcesareveryclose to each other.For example, bearingdefectfrequencies are commonlyclose to exactmultiples of operatingspeed. Consider a pumpimpeller with 6 vanes and bearings which have a defectfrequencyof say 6.03X RPM. In this case,itmayrequirea high resolution spectrumto separate thebearing frequency fromwhatis knownas theblade (orvane)passing frequencywhich equals thenumber ofvanesX RPM (6X RPMhere).

So why shouldn’tonealways take 3200line resolutionspectraand avoid thehassleof inaccuratedata? Collecting 3200 line data is costly in time as well as in data collector and computerstorage. First, a 3200 line spectrum takes 8 times longer to collect than does a 400 line spectrumas seen by Equation 2 and can really slowdown thedata collection process. For example, for a

frequencyspan of12,000CPM (200Hz) and a frequencyresolutionof400 lines, therequireddataacquisition time (t MAX) would be 60 X 400/12,000 = 2 sec. If eight (8) averages were desired, atotalof 16seconds would be required just to capture thedata - notincluding either analyzersettling time nor the time to performthemathematical calculation of the FFT (assumingnooverlap processingis used in theanalyzer).


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Second, some data collectors and PMP software currentlyonly allow3200lines of resolution inaspecial analysis mode, and notduring regularroute data collection. For mostgeneral datacollection400 lines of resolutionproduces adequate accuracyunless onehappens tohavesources producing frequencies located close to oneanother suchas theabove examplepump.Somerecentsoftwarepackages are nowpermitting3200line spectraforroute data collection,butjudgement mustbe used since thedata collector spectral storage capacity will be

significantlyreduced forsuch highresolution spectra.

Where:DataSamplingTime = Total Sampling Time Required to Capture Data (sec.);

(AssumingnoOverlapProcessing is involved).# FFT Lines = Numberof# FFT Lines orBins in Spectrum.# Avg. = NumberofAverages.Freq. Span = FrequencySpan(CPM).

2.52 EFFECT ONFREQUENCYACCURANCY OF FREQUENCY SPAN USED

Anotherfactoraffecting resolution is the frequencyrange(F MAX) setting. Thelarger thefrequencyrange, the less accurate will be thefrequencyreading and thewider will be each line(bin)of resolution.Figure 32, is a 400 linespectrumwith a 24,000 CPM (24K CPM) F MAX. The resolutionof this spectrumis 24,000CPM/400 lines of resolution= 60CPM/line of resolution. Thus, itsfrequencydefinition is only one-half thatof the12,000CPM (12K CPM) F MAX400 line spectrumof Figure 30. Further, Figure 33 is a 120,000 CPM F MAXspectrumwith 400 lines of resolution.Here,Figure33 has 300 CPM per lineof resolutionmeaning that theaccuracyof any displayedfrequencywill be± 150 CPM (orplus orminus one-half theresolution). In some cases, thisamountof accuracyis all that is needed. Again, trade-offs mustbe made to retain accuracy.Theanalyst needs to have a large enough F MAX to include all thevibrationdata heneeds, butdoesn’twant to loseaccuracy in theprocess. Technical Associates has a provenmethod for choosingFMAX values for data collection. The method is discussed indetail in another paper.

Equation 2


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2.6 WHAT IS OVERALL VIBRATION (DIGITAL AND ANALOG OVERALLLEVEL)?

The vibration seenina spectrumis thetotal sumofall thevibration measuredbythetransducerwithinthefrequencyspanchosen.Thetransducermayevensensevibrationfromanadjacentmachineand include itwithinthevibrationspectrum. Whateveris seen bythe transducer, nomatterwhat its source or from where itmight originate, becomes partof both the spectrumandthe overall vibration. Overall vibration is differentfromvibration atspecific frequencies asdiscussed thus far in that it is a total summationof all thevibration, no matterwhat the frequency.Figure 34shows the overall vibration levelof a spectrum(this is sometimes called theDigitalOverall. Simplysaid, ifyou take theamplitude of each frequencybin(Ai) and square it(A i

2); addall thesquared amplitudes together; take the square rootof that sum; and dividethis sumby thenoise factor for theFFT Windowchosen(theHanning windowis mostalways used for PMP routemeasurements and has a 1.5noisefactor), theend resultis theoverall vibrationof thespectrum.Of course, this is a very lengthyprocess for a 400 linespectrum(and evenmore so fora 3200 lineFFT). However, thisspectral overall is automaticallycalculatedeitherwithintheanalyzerand/orbackwithin thehostsoftwareof thecomputer.

Figures 35and 36show anapproximate formula for calculating theoverall vibrationand on ac-companyingsamplespectrumshowinghowthis approximation is applied. Here, theamplitudesfrom5 separate frequency peaks werecombined to compute the approximate overall level of thespectrum. Figure 36is a sampleestimatedoverall vibrationcalculationusing anactual spectrum.Note theestimated overallvibration calculated is .161 in/sec. Theactual overallas measured bythedata collector was .182 in/sec. Theactual overall should always be higherthan theestimatedmethod as it includes all the individual amplitudes withineach bin, not justthedetectable peaksover theuser-defined threshold. Multiplying theestimated valueby a 1.1 correction factor willusually make the estimated vibration fairlyclose to the actual (i.e., 1.1 X .161 = .177in/sec, veryclose to the actual .182in/sec overall vibration). Fortunately, the data collectors and computersoftwareperformall these calculationsand oneonlyneeds to do thiscalculation underunusualcircumstances.

Oneof theproblems with looking atonly theSpectrumOverall is that significantvibrationcanoccur outside the frequencyrange (0 - F MAX) specified by theanalyst. For example, if a high levelof .60 in/sec were occurring out at a frequency of 100,000 CPM and an F MAX of only60,000 CPMwerechosen, thishigh amplitude100,000 CPM frequency would notbe included withintheSpectral Overall. In this case, theSpectral Overall might calculate up to only .20 in/sec, far belowthat ifa frequencyrange outto 120,000CPM had beenchosen.

To overcome this problem, some PMP systems determine theoverall by looking directlyatthetime waveformwhichitselfhas a verywide frequency range. Then, theoverall measuredbytheanalyzer will be totally independentof any frequency spanspectrumchosen by theuser.This iswhat is known as theAnalog Overall.For example, onesystemusesa time waveformhaving afixed frequencyspan from300 to 3,900,000CPM (5to 65,000 Hz). Then, in theexamplegivenabove with the .60 in/sec at100,000 CPM, theAnalog Overall would be dramaticallyhigher thantheSpectrum(orDigital) Overall ifanF MAX ofonly60,000 CPM were chosen.For this reason if yourPMP systemallows you theoptionof finding either theSpectrum (orDigital) Overall orAnalog Overall, choose theAnalog Overalloption.


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FIGURE 34EXACT EQUATION FOR CALCULATING DIGITAL OVERALL LEVEL

OF A SPECTRUM


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FIGURE 35APPROXIMATE FORMULA FOR CALCULATING DIGITAL OVERALL LEVEL

OF A SPECTRUM


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2.7 UNDERSTANDING PHASE AND ITS APPLICATIONS

2.71 DEFINITION OF PHASE:

Phase analysis is a powerful tool onecan employto diagnose dominant problemsources.For

example, thereare many problems which cancause high vibration atoperatingspeed (i.e.,unbalance, misalignment, eccentricity, bentshaft, soft foot,cracked/brokengeartooth,resonance, loosehold-down bolts, etc.).Likewise, thereare severalproblems whichcangeneratehigh vibration ateither 2Xor 3XRPM. With all these possibleproblems potentially capableof generatinghighvibrationatthese frequencies, ananalystcanbehardpressed to determinewithconviction thedominant source(s).However, when high levels do occur at1X,2X and/or3XRPM,phase measurements ateachbearing housing inquestioncango far indoing just this - if theanalystclearlyunderstandsphase.

Phase is therelationship vibrationhas with respectto anothervibratingpartor fixed referencepoint (it can also be thought of as the vibration motion atone location relative to the vibrationmotion of another location). It canalso be thought of as the timing relationship between twosignals occurring atthesame frequency. Phase is easiest to visualize ifone is familiarwith usinga timing lightto setthe timing of an automobile engine. Thetiming light is actuated by the sparkgoing to a certain sparkplug (usually the“Number 1”plug). Here, theobject is to have the sparksynchronized with thepositionof the“Number 1”piston for proper firing. This is done byadjustingthespark timingso thetimingmark on thecrankshaftaligns (synchronizes)with a fixedreferencemark on theengineblock.This assures thesparkfiring and piston positions aresynchronized.

Technical Associate I Metric Textbook

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