Presentation on theme: "Vibration Analysis (cont)"— Presentation transcript:
1 Vibration Analysis (cont) Adam AdgarSchool of Computing and Technology
2 The FFT ProcessBased upon principle that any periodic signal (e.g. measured vibration signal) can be broken down into a series of simple sinusoidsCombining these sinusoids will generate the periodic signal we have just analyzedFFT process can generate a spectrum from a time domain signalBy plotting amplitude versus frequency (instead of time), it becomes far easier to analyze.Plot displays a certain number of amplitude values (400, 800, 1600, etc.) over a range of frequencies.
4 FFT Terminology Commonly used terms include: Fundamental Frequency 1x rpmBut remember that a belt drive, for instance, has three fundamental frequenciesDominant FrequencyFrequency at which the highest amplitude occursSynchronous VibrationVibration harmonically related to a fundamental frequencyNon-synchronous VibrationVibration not harmonically related to a fundamental frequencySub-synchronous VibrationVibration occurring at a frequency below the fundamental frequency
5 What Does Frequency Tell Us ? Synchronous VibrationMachines generate mechanical vibration at multiples (harmonics) of running speeds.For example:An unbalanced rotor causes a force that moves the bearing (causes vibration) in any direction (plane) at exactly once per revolution (1x RPM)A pump with 5 vanes on the impeller can generate hydraulic pulses (which can be measured as mechanical vibration) at exactly 5 times per rev (5x rpm)Different mechanical problems tend to generate their own characteristic vibration patterns.Non-Synchronous VibrationOther vibration generators may not be tied specifically to the machine's rotational speed. For example:Bearing problems and electrical problems tend to generate vibrations at specific frequencies other than exact multiples (harmonics) of running speed.Source of the problem can be identified by correctly linking the frequency to the various possible sources,
6 Trend Plot Y-Axis Units: Amplitude X-Axis Units: Time (typically days or months)Plot of a number of overall amplitude values - snapshots of the total vibration (vibration at all frequencies) - over a period of timeInterval between readings is time elapsed between those readings. Could be anything from months to milliseconds depending on the specifics of the vibration program and system(s) involvedTrend plots offer limited analysis tools (no identification of specific frequencies) but can be an important indicator of developing problems
7 Spectrum Plot Y-Axis Units: X-Axis Units: AmplitudeX-Axis Units:FrequencyPlot of many amplitude measurements over a range of frequenciesUses “Fast Fourier Transform”, or “FFT”.Most commonly used analysis toolAllows for preliminary identification of the source of the vibration by enabling frequency identification
8 Envelope Spectra Y-Axis Units: X-Axis Units: Amplitude (acceleration)X-Axis Units:FrequencyEnveloping spectra is sensitive to impact related events, not sinusoidal motion as processed by FFTAllows quantification of both the frequency of impacts and their intensityImpacts are destructive forces - normally indicate developing problem.Most typically, this plot is used to detect bearing defects.Signal processing focuses on the transient, impact type events (spikes on the time domain signal) that the FFT process missesLooks for a consistent period between impacts (i.e. the impacts are occurring at a regular interval)Similar in appearance (amplitude vs. frequency) to a conventional spectrum but displays different informationFilters used to help process the signal and focus on any impacts that may be occurring.
9 Interpretation of Spectra Examples of Fault TypesUnbalanceMisalignmentRolling Element BearingsGears
10 Unbalance Very common and simplest problem to diagnose Unbalance caused by centrifugal forcePure sinusoid signal when single fault existsHence generates peak at 1× rpmSingle-Plane Unbalance Symptoms:Radial vibration at 1× rpm.Little or no phase shift across bearingsTwo-Plane Unbalance Symptoms:Axial vibration at 1× rpm.Significant phase shift (> 60°) across bearingOne of the only problems that does not distort the signal shape in some manner
11 Misalignment Most common vibration problem Unlike unbalance, no single vibration symptom. As a result, it should always be considered as a possibilityPerfect alignment - Shaft centrelines are parallel and intersectTypes of misalignment (always a combination):Angular - Shaft centerlines intersect but are not parallelOffset - Shaft centerlines are parallel but do not intersectGeneral SymptomsHigh axial vibration at 1× rpm, possible harmonics at 2× and 3× .2× rpm axial vibration may be as high or even higher than at 1×.Combination of angular/offset gives wide variation of symptoms
12 Rolling Element Bearings One of most important areas of CbMVibration symptoms can vary greatly but are fairly predictableRequiresPreferably acceleration spectra covering frequency range between 30,000 and 120,000 cpmEnveloped spectraTime domain show the impacts better than spectrum - especially on slow speed equipment
13 Early Stage RE Bearing Defects Generates frictional or impact-related high frequency vibrationShows up earliest on the enveloping spectraTime signal being collected will contain the impact spikes showing up at an interval equal to the defect frequencyAcceleration spectrum shows high frequency peaks far more clearly than velocity spectrumThe defect frequency harmonics can be very low amplitude (not even noticeable) in the early stagesThere will typically be no peak at 1× defect frequency on the velocity or acceleration FFTs in the defect's early stages
14 Gears and Gear TrainsGears generate vibration under normal circumstancesThe most common frequency generated is the number of teeth x RPMThis is known as 'gear mesh frequency' (GMF)Consider 2 mating gears where one is eccentric. At one point during that gear's rotation, it will bottom out with the mating gear and the vibration at GMF will be very highThis causes modulation of the amplitude at gear mesh frequency.Also it goes from its minimum amplitude to its maximum and back again to its minimum at a rate of once per shaft revolution - 1x rpm.From this signal, the FFT generates a peak at GMF with sidebands at 1x rpm.A limited amount of amplitude modulation on a gear train is normalThe number of and size of the sidebands should be closely monitored.Even more significant can be the development of an amplitude peak at the natural frequency of the gear or gears. Wear or impacting due to problems such as backlash can cause the excitation of the natural frequency of a gear.
15 Further Work Read the Application Note Look at the web resources Detecting Faulty Rolling Element BearingsLook at the web resources