Transcription

SKF Reliability SystemsVibrationDiagnosticGuideCM5003

Vibration Diagnostic GuideTable of ContentsPart 1Overview . 1How To Use This Guide 1Detection vs. Analysis . 1Vibration (Amplitude vs. Frequency) . 1“Overall” Vibration . 2Time Waveform Analysis . 5FFT Spectrum Analysis . 5Envelope Detection . 6SEE Technology . 7Phase Measurement . 7High Frequency Detection (HFD) . 7Other Sensor Resonant Technologies . 7Part 2Spectrum Analysis Techniques . 13Misalignment . 14Imbalance 16Looseness 18Bent Shaft 19Bearing Cocked on a Shaft . 19Bearing Defect 20Multi-Parameter Monitoring . 24Appendix AUnderstanding Phase . 25GlossaryGlossary . 27i

Overview / How To Use This Guide / Detection vs. Analysis / Vibration (Amplitude vs. Frequency)Vibration DiagnosticGuidePart 1OVERVIEWThis guide is designed to introduce machinery maintenanceworkers to condition monitoring analysis methods used fordetecting and analyzing machine component failures.This document was created by field experienced SKFapplication engineers using measurements obtained with SKFCondition Monitoring equipment. This guide is a “LivingDocument” and will continuously grow as application andexperience information becomes available.It is important to note that this guide is not intended to make thereader an analysis expert. It merely informs the reader about“typical” methods of analysis and how machinery problems“typically” show themselves when using these methods ofanalysis. It is intended to lay the foundation for understandingmachinery analysis concepts and to show the reader what isneeded to perform an actual analysis on specific machinery.Rule 1Know what you know and don’t pretend to know whatyou don’t know!Often, a situation arises where the answer is not obvious or notcontained within the analysis data. At this point “I don’t know”is the best answer. A wrong diagnosis can cost greatly and canrapidly diminish the credibility of the machinery maintenanceworker. Analysis of the problem by a vibration specialist isrequired.HOW TO USE THIS GUIDEThis guide is divided into two sections. The first section introduces concepts and methodsused to detect and analyze machinery problems. The second section examples “typical” ways in whichvarious machinery problems show themselves andhow these problems are “typically” analyzed.DETECTION VS. ANALYSISCAUSE AND EFFECTThere is a big difference between detecting a machinery problemand analyzing the cause of a machinery problem. Swapping outa bearing that is showing wear by vibrating heavily may or maynot solve your problem. Usually, some other machineryproblem is causing the bearing to wear prematurely. To solvethe bearing problem you must solve the cause of the bearingproblem (i.e. misalignment, looseness, imbalance). If not, youare not running a condition monitoring program, you’re runninga bearing exchange program.It is essential that machinery problems be detected early enoughto plan repair actions and to minimize machine downtime.Once detected, a cause and effect approach must be used to takefurther steps toward analyzing what caused the detectedproblem. Only then will you keep the problem from becoming arepeat problem.VIBRATION(AMPLITUDE VS. FREQUENCY)Vibration is the behavior of a machine’s mechanical componentsas they react to internal or external forces.Since most rotating machinery problems show themselves asexcessive vibration, we use vibration signals as an indication ofa machine’s mechanical condition. Also, each mechanicalproblem or defect generates vibration in its own unique way.We therefore analyze the “type” of vibration to identify its causeand take appropriate repair action.When analyzing vibration we look at two components of thevibration signal, its amplitude and its frequency. Frequency is the number of times an event occurs in agiven time period (the event being one vibrationcycle). The frequency at which the vibration occursindicates the type of fault. That is, certain types offaults “typically” occur at certain frequencies. Byestablishing the frequency at which the vibrationoccurs, we get a clearer picture of what could becausing it. Amplitude is the size of the vibration signal. Theamplitude of the vibration signal determines theseverity of the fault. The higher the amplitude, thehigher the vibration, the bigger the problem.Amplitude depends on the type of machine and isalways relative to the vibration level of a “good”;“new” machine!A glossary is provided at the end of this document. Referencethis glossary for any unfamiliar terms.Vibration Diagnostic Guide1

Vibration (Amplitude vs. Frequency) / “Overall” VibrationWhen measuring vibration we use certain standard measurementmethods: Overall Vibration Phase Acceleration Enveloping SEE Technology (Acoustic Emissions) High Frequency Detection (HFD) Other Sensor Resonant Technologies“OVERALL” VIBRATIONScale Factors on a Sinusoidal Vibration Waveform.Overall vibration is the total vibration energy measured within afrequency range. Measuring the “overall” vibration of amachine or component, a rotor in relation to a machine, or thestructure of a machine, and comparing the overall measurementto its normal value (norm) indicates the current health of themachine. A higher than normal overall vibration readingindicates that “something” is causing the machine or componentto vibrate more.Vibration is considered the best operating parameter to judgelow frequency dynamic conditions such as imbalance,misalignment, mechanical looseness, structural resonance, softfoundation, shaft bow, excessive bearing wear, or lost rotorvanes.FREQUENCY RANGEThe frequency range for which the overall vibration reading isperformed is determined by the monitoring equipment. Somedata collectors have their own predefined frequency range forperforming overall vibration measurements. Other datacollectors allow the user to select the overall measurement’sfrequency range. Unfortunately there is an ongoing debate onwhich frequency range best measures to measure overallvibration (even though the International Organization forStandardization (ISO) has set a standard definition). For thisreason, when comparing overall values, it is important that bothoverall values be obtained from the same frequency range.SCALE FACTORSWhen comparing overall values, the scale factors that determinehow the measurement is measured must be consistent. Scalefactors used in overall vibration measurements are Peak, Peakto-Peak, Average, and RMS. These scale factors have directrelationships to each other when working with sinusoidalwaveforms. The figure below shows the relationship of Averagevs. RMS vs. Peak vs. Peak-to-Peak for a sinusoidal waveform.2Peak 1.0RMS 0.707 PeakAverage 0.637 PeakPeak-to-Peak 2 PeakThe Peak value represents the distance to the top of thewaveform measured from a zero reference. For discussionpurposes we’ll assign a Peak value of 1.0.The Peak-to-Peak value is the amplitude measured from the topmost part of the waveform to the bottom most part of thewaveform.The Average value is the average amplitude value for thewaveform. The average of a pure sine waveform is zero (it is asmuch positive as it is negative). However, most waveforms arenot pure sinusoidal waveforms. Also, waveforms that are notcentered around zero volts produce nonzero average values.Visualizing how the RMS value is derived is a bit more difficult.Generally speaking, the RMS value is derived from amathematical conversion that relates DC energy to AC energy.Technically, on a time waveform, it’s the root mean squared(RMS). On a FFT spectrum, it’s the square root of the sum of aset of squared instantaneous values. If you measured a pure sinewave, the RMS value is 0.707 times the peak value.NOTE:Peak and Peak-to-Peak values can be either true orscaled. Scaled values are calculated from the RMSvalue.Don’t be concerned about the math, the condition monitoringinstrument calculates the value. What’s important to rememberis when comparing overall vibration signals, it is imperative thatboth signals be measured on the same frequency range and withthe same scale factors.Vibration Diagnostic Guide

“Overall” VibrationNOTE:NOTE:As discussed in future sections, for comparison purposes,measurement types and locations must also be identical.These descriptions are given as guidelines for “typical”machinery only. Equipment that is vertically mounted,overhung, or in someway not typical may show differentresponses.MEASUREMENT SENSOR POSITIONSelect the best measurement point on the machine. Avoidpainted surfaces, unloaded bearing zones, housing splits, andstructural gaps.When measuring vibration with a hand-held sensor, it isimperative that you perform consistent readings, paying closeattention to the sensor’s position on the machinery, the sensor’sangle to the machinery, and the contact pressure with which thesensor is held on the machinery.Position - When possible, vibration should be measured in threedirections: the axial direction (A) the horizontal direction (H), and the vertical direction (V).Since we generally know how various machinery problemscreate vibration in each plane, vibration readings taken in thesethree positions can provide insight as to what may be causingany excessive vibration. Note that measurements should betaken as close to the bearing as possible. If possible, avoidtaking readings on the case as the case could be vibrating due toresonance or looseness.NOTE:Enveloping and SEE measurements should be taken asclose to the bearing load zone as possible.If possible, choose a flat surface to press the sensor tip against.Measurements should be taken at the same precise location forcomparison (moving the probe only a few inches can producedrastically different vibration readings). To ensuremeasurements are taken at the exact same spot, mark themeasurement point with permanent ink or machine a shallowconical hole with a drill point.Magnetic mounts are even better for consistency andpermanently mounted sensors are the best for consistency. Angle – Always perpendicular to the surface(90 10 ). Pressure – Even, consistent hand pressure must beused (firm, but not so firm as to dampen the vibrationsignal).OPTIMUM MEASUREMENT CONDITIONS Horizontal measurements typically show the mostvibration due to the machine being more flexible inthe horizontal plane. Also, imbalance is one of themost common machinery problems and imbalanceproduces a radial vibration, that is, part vertical andpart horizontal. Because the machine is usually moreflexible in the horizontal plane, excessive horizontalvibration is a good indicator of imbalance. Vertical measurements typically show less vibrationthan horizontal because of stiffness due to mountingand gravity. Under ideal conditions, axial measurements shouldshow very little vibration as most forces are generatedperpendicular to the shaft. However, misalignmentand bent shaft problems do create vibration in theaxial plane.Vibration Diagnostic GuidePerform measurements with the machine operating undernormal conditions. For example, when the rotor, housing, andmain bearings have reached their normal steady operatingtemperatures and with the machine running under its normalrated condition (for example, at rated voltage, flow, pressure andload). On machines with varying speeds or loads, performmeasurements at all extreme rating conditions in addition toselected conditions within these limits.TRENDING OVERALL READINGSProbably the most efficient and reliable method of evaluatingvibration severity is to compare the most recent overall readingagainst previous readings for the same measurement, allowingyou to see how the measurement’s vibration values arechanging, “trending” over time. This trend comparison betweenpresent and past readings is easier to analyze when the valuesare plotted in a “trend plot”.A trend plot is a line graph that displays current and past overallvalues plotted over time. Past values should include a base-line(known good) reading. The base-line value may be acquiredafter an overhaul or when other indicators show that the machine3

“Overall” VibrationWhen the mass is set in motion it oscillates on the spring.Viewing the oscillation as position over time produces a sinewave. The starting point (when the mass is at rest) is the zeropoint. One complete cycle of the mass displays a positive and anegative displacement of the mass in relation to its reference(zero). Displacement is the change in distance or position of anobject relative to a reference. The magnitude of thedisplacement is measured as amplitude.There are two measurable derivatives of displacement: velocityand acceleration. Velocity is the change in displacement as a function oftime, it is speed at which the distance is traveled, forexample 0.2 in/sec.is running well. Subsequent measurements are compared to thebase-line to determine machinery changes.Comparing a machine to itself over time is the much preferredmethod for detection of machinery problems as each machine isunique in its operation. For example, some components have acertain amount of vibration that would be considered a problemfor most machines, but is normal for them. The current readingby itself might lead an analyst to believe that a problem exists,whereas the trend plot and base-line reading would clearly showthat a certain amount of vibration is normal for this machine.ISO Standards are good for a start (until you develop a machinehistory). However, ISO charts define “good” or “not good”conditions for various wide-ranged machinery classifications. Acceleration is the rate of change of velocity. Forexample, if it takes 1 second for the velocity toincrease from 0 to 1 in/sec, then the acceleration is 1in/sec2.Thus, vibration has three measurable characteristics:displacement, velocity, and acceleration. Although these threecharacteristics are related mathematically, they are threedifferent characteristics, not three names for the same quantity.It is necessary to select a vibration measurement and sensor typethat measures the vibration most likely to reveal the expectedfailure characteristics.DISPLACEMENTEvery machine is: Manufactured differently Installed differently (foundation) Operated under different conditions (load, speed,materials, environment) Maintained differentlyIt is unrealistic to judge a machine’s condition by comparing itscurrent measurement value against a wide classification ISOStandard or other general rule or levels. By comparing currentvalues to historical values, you are able to easily see how aspecific machine’s condition is changing over time. You’recomparing apples to apples.OVERALL VIBRATION MEASUREMENTS METHODSMeasuring vibration is the measurement of periodic motion.Vibration is exampled using a spring-mass setup.Measured in mils or micrometers, displacement is the change indistance or position of an object relative to a reference.Displacement is typically measured with a sensor commonlyknown as a displacement probe or eddy probe. A displacementprobe is a non-contact device that measures the relative distancebetween two surfaces. Displacement probes most often monitorshaft vibration and are commonly used on machines with fluidfilm bearings.Displacement probes measure only the motion of the shaft orrotor relative to the casing of the machine. If the machine and4Vibration Diagnostic Guide

“Overall” Vibration / Time Waveform Analysis / FFT Spectrum Analysisrotor are moving together, displacement is measured as zero,while in fact the machine could be vibrating heavily.Displacement probes are also used to measure a shaft’s phase.The shaft’s phase is the angular distance between a known markon the shaft and the vibration signal. This relationship is usedfor balancing and shaft orbital analysis (reference the PhaseSection).By mounting accelerometers at strategic points on bearings, wecan measure the acceleration and derive the velocity. Thesemeasurements are recorded, analyzed, and displayed as tablesand plots by condition monitoring equipment. A plot ofamplitude vs. time is called a time waveform.VELOCITYMeasured in in/sec or mm/sec, velocity measures the vibrationsignal’s rate of change in displacement. It is the most commonmachine vibration measurement. Historically the velocitysensor was one of the first electrical sensors used for machinecondition monitoring. This because for an equal amount ofdynamic motion being generated, velocity remains constantregardless of frequency. However, at very low frequencies(under 10 Hz) velocity sensors lose their effectiveness.Likewise at higher frequencies (above 2 kHz).The original velocity transducer employed a coil vibrating in amagnetic field to produce a voltage proportional to themachine’s surface velocity. Today, with the arrival of low costand versatile accelerometers, most velocity values are obtainedby integrating an acceleration reading into the velocity domain.ACCELERATIONAcceleration is the rate of change in velocity. Vibration in termsof acceleration is measured with accelerometers. Anaccelerometer usually contains one or more piezoelectric crystalelements and a mass.Time waveforms display a short time sample of the rawvibration. Though typically not as useful as other analysisformats, time waveform analysis can provide clues to machinecondition that are not always evident in the frequency spectrumand, when available, should be used as part of your analysisprogram.MassPiezo ElementBaseThe above time waveform plot illustrates how the signal from anaccelerometer or velocity probe appears when graphed asamplitude over time. This type of vibration plot is also called atime domain plot or graph.FFT SPECTRUM ANALYSISSpringHousingTIME WAVEFORM ANALYSISConnectorA method of viewing the vibration signal in a way that is moreuseful for analysis is to apply a Fast Fourier Transformation(FFT). In non-mathematical terms, this means that the signal isbroken down into specific amplitudes at various componentfrequencies.When the piezoelectric crystal is stressed it produces anelectrical output proportional to acceleration. The crystal isstressed by the mass when the mass is vibrated by thecomponent to which they are attached.Accelerometers are rugged devices that operate in a very widefrequency range from almost zero to well above 400 kHz. Thisability to examine a wide frequency range is the accelerometer’smajor strength. However, since velocity is the most commonmeasurement for monitoring vibration, accelerationmeasurements are usually integrated (either in the accelerometeritself or by the data collector) to get velocity. Acceleration unitsare G’s, in/sec2, or m/sec2.Vibration Diagnostic GuideFrequency scale showingcomponent vibration signalsat various frequencies.5

FFT Spectrum Analysis / Envelope DetectionFor example, we measure the signal’s amplitude at 10 Hz, thenagain at 20 Hz, etc., until we have a list of values for eachfrequency contained in the signal. These values or amplitudesare then plotted over the frequency scale. The number ofcomponent frequencies the waveform is divided into is referredto as the number of lines of resolution. The resulting plot iscalled an FFT spectrum.causes a small, repetitive vibration signal at the bearing’s defectsfrequencies. However, this vibration signal is of such lowenergy that, with overall vibration monitoring, it is lost in themachine’s other rotational and structural vibration noise.SpallAn FFT spectrum is an incredibly useful tool. If a machineryproblem exists, FFT spectra provide information to helpdetermine the location of the problem, the cause of the problema