The Why, What, How & Who Of Maintenance - AMMJ

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AMMJVibration Analysis and Condition Monitoring 34Enveloping TechniquesA variety of analysis techniques are available within commercial software packages to refine the detection of potentialfailures. Enveloping is such a technique whereby a shape is created around the spectrum plot that equates to alarmprofile values set for each monitored component of the machine. These individual alarms are triggered even thoughthe component signal may not be the highest amplitude signal within the spectrum (not of sufficient value to affectthe whole machine or overall value).Octave Band analysisDespite its name (which comes from the type of filters it uses) it is a vibrational technique which has to be set up(usually by an expert) to determined measurement parameters relating to the frequency bands of interest on themachine being monitored (based on rpm frequency relationships). Once set up it is fairly simple to use for overallmeasurements but has a limited diagnostic ability.Shock pulseIs a derivation of acoustic techniques. Shock pulses are generated within a machine by the impacting of surfaces andthe extent of this shock depends on the extent of damage, the rpm and the size of the components. The peak valueof the amplitude picked up by the transducer is directly propotional to the impact velocity and as deterioration occursshock pulse measurements increase significantly (up to 1000 times). It is a relatively quick and easy technique touse but needs information on bearing size and speeds and the trandsducer to be ‘tuned’.Energy spikingWorks on the principle that some faults excite the natural frequencies of components and structures within a machine.Repetitive impacts generate intense energy which can be sensed by a transducer (accelerometer) as periodicspikes of high frequency in a spectrum. Electronically processed and enhanced the fault frequency shows clearly.Diagnosis usually needs the services of an expert although latest software developments help.ChoicesDIY Or Specialist Help?With the equipment and softwareavailable the decision as to whether to goit alone or seek specialist advice/servicesis difficult. The decision depends largelyon the in house time, resources andbudget available.If you are using vibration or acousticanalysis for the first time you have toconsider:• Buying the equipment• Training• Learning how to do the job• Learning how to recognise problems(usually a recognised course of training)• Managing the whole thingThe author’s recommended route is touse simplified techniques such as WholeMachine vibration or acoustic emissionlevel monitoring for regular, routinemeasurements. Record and trend theresults and bring in the analytical powerof frequency analysis when problems aresuspected.SPECIALIST TECHNIQUESWe have examined the more common techniques available in vibration and acoustic analysis, but there are anumber of specialised techniques, for which you would normally call in a specialist, that are useful for diagnosisor in specific circumstances. A range of such techniques is listed in the following table by ‘application’ rather than‘analysis technique’, as this is the most likely trigger for their use.GLOSSARY OF TERMSAlternative What it involves Advantages DisadvantagesIn HouseCombinedServiceOutsourced to ThirdPartyYou collect and analyse the data relyingon vibration or acoustic equipmentmanufacturer for basic training (whichmay be a very minor requirement if yougo for acoustic emission or overallvibration monitoring).Possible use of a third party foroccasional specialist support.Where the system is set up jointly, youcollect the data and pass it on to a thirdparty for analysis and reporting.Where specialists set up system, collectdata, carry out analysis and send youperiodic reports on machine condition.Relatively cheapQuick response tosuspected andidentified problemsOwnership in houseExperience on tapSpecialist knowledgeavailable from theoutsetNo equipment costsLimited commitmentof time and resourcesExperience on tapSpecialist knowledgeavailable from theoutsetCommitment of timeand resourcesSteep learning curveInitial lack ofexperienceMore expensiveResponse time tosuspected andidentified problemsLack of ownershipThose collecting dataare still a commitmentof time and resourcesand training wouldnormally be required.Relatively expensiveResponse time toproblemsLack of ownershipAcoustic Emission – technique distinguishing thenatural frequencies of a machine’s components fromthose associated with deterioration in condition.Band - the collection of data within a specific rangeof a machines operation (usually targeted at anarrow range of values equating to specific machinecomponents {high speed bearings, gearwheels, etc.])Baseline Values – the range of monitored data valuesobtained at the adoption of condition monitoring thatidentify the subject machine’s normal operating range(variation) and allow alert and alarm values to bepredicted.Vol 25 No 1
AMMJVibration Analysis and Condition Monitoring 35Broadband– the collection of datathroughout the normalrange of a machinesoperating parametersBump Test – theinducing of an acousticsignal into a subjectmachine, exciting itsnatural or resonantfrequencies & throughprocessing of the signalsemitted determiningthe condition of themachine.Condition BasedMaintenance (CBM)maintenance carried outaccording to the needindicated by conditionmonitoringConditionMonitoring (CM)– the continuous orperiodic measurementof data (duringoperation) to indicate thecondition of an item todetermine the need formaintenanceCM routes – conditionmonitoring machinemeasurement tasksarranged into a logicaldata collection sequenceCM Routines – acollection of scheduledCM routes raised as ascheduled job within aplanned maintenanceprogrammeAPPLICATIONRotatingmachineryshaftsgearboxesetc.As above +roller & journalbearings,electric motorspumpsturbines+ diagnosticapplicationsGearteethdamagepumpsroller bearingsetc.Gearboxesgearteeth,roller bearings,shaftsRollersbanks of fansCriticality analysis– a quantitative analysis of events or faults and theranking of these in order of the seriousness of theirconsequencesANALYSISTECHNIQUEReal TimeAnalysisReal TimeConstantBandwidthTimeWaveformAnalysisTimeSynchronousAveragingAnalysisFast Fourier Transform – a mathematicaltransformation technique applied to vibration signal datathat allows the display of amplitude against frequency.Monitoring – activity performed either manually orautomatically intended to observe the state of an item.p-f interval - the period between which a defectbecomes detectable and the point where failure occursPlanned Maintenance – downtime due to theprogrammed or scheduled taking out of an item fromservicePredictive Maintenance – tasks carried out to gainevidence of the condition of an item and whether it isdeteriorating towards failurePreventative Maintenance – the scheduled(regardless of condition) restoration or discard of itemswith proven age related failure characteristics or dominantmodes of failureADVANTAGESAnalyses of allfrequencybandssimultaneouslyinstant graphicdisplay,constantlyupdated.Fixed orPortableSimple to useonce set up,good range,good detail athighfrequencies,PortableGood fortransientloads, slowpulses etc.Often used toanalyserandom noise.PortableGood forindividualgears analysisin gearbox orany machinewithcomponentsrotating atsimilar speedsTechniques By ApplicationDISADVANTAGESTime consumingLong analysistime high level ofmachineknowledgerequired tointerpret resultsMultiple signalscan be confusingand it is difficult toisolate sourceRoller elementbearings needcare due tobearing tonesWHAT ITDETECTSAcoustic &vibrationsignals +shock andtransient loadsAs above +identification ofmultipleharmonics andsidebandsGear teethdamage,misalignment,pumpcavitation, etcWear, fatigue,stress waves,micro weldingPro-active Maintenance – a generic term for Predictive andPreventative MaintenanceOpportunity Maintenance – the taking of an item out ofservice for maintenance when time and resources allow, i.e.during other scheduled or unscheduled production downtimeResonance – a condition in which an object or system issubjected to an oscillating force with a frequency close to itsown natural frequencyRun to Failure (breakdown) – the deliberate decision notto carry out any form of maintenance other than replacement orrefurbishment upon failureShutdown maintenance – maintenance that can be carriedout only when the item is out of service (or the planned shuttingdown of an operation solely to perform maintenance)Swept Filter Frequency Analysis - analogue systemwhere a fixed frequency range of the subject machine is sweptVibration - the act or an instance of oscillationHOW IT WORKSSignal recorded andplayed back throughreal time analyser –transformed intofrequency – operatesthrough 0-10Hz and0-20kHz. Highresolution and slowmotion capabilityVibration detected byaccelerometer, signalamplified, filtered andanalysed.Bandwidths &frequency can bechanged to suitdiagnostic needs(Function option withmost FFT analysers)Oscilloscope viavibration meter or realtime analyser,measures peak topeak amplitudeagainst time – needsband filters to dealwith complex signals(FFT)Tachometer triggeredpulse cleans signal soyou see mainlyrunning speed relatedcomponentsSignals not related tothe RPM areaveraged out leavingonly those related toa single rotatingspeedVol 25 No 1
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