Vibration Field Tehcnician Guide

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PDM ENGINEERING

VIBRATION TROUBLE-SHOOTING FIELD GUIDE

Easy ReferenceGuide for the

Field Technician

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A L I G N M E N T & B A L A N C I N G B Y S A M P I C K E N S

VIBRATION ANAL YSIS TROUBLESHOOTER’S GUIDE


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Mr. Ray Dodd is held in esteem as the guy that fathered the modern day predictivemaintenance (PdM) programs. In the 1970’s we were basically carrying a portableinstrument and collecting vibration data via a velocity transducer and recordingthat data onto a chart with pen in hand. If overall level vibrations above a certain

magnitude were encountered, then we would pick out the predominate frequenciesand log them as well. A long process but one that proved its worth.

This writing is to serve as a help and guide to assist in diagnostics of vibration problems.First we’ll make sure that certain terms are defined to avoid confusion.

FUNDAMENTAL: This is running speed or 1X; referred to as the machine’sfundamental or fundamental component. Also component fundamental is sometimesused; i.e. the bearing’s ball pass frequency fundamental is expressed as componentfundamental to distinguish it from harmonics in a ball pass problem within the spectra.Always make sure it is defined if used for anything other than running speed.

FIRST HARMONIC: Some confuse this with the symbols 2X and 1X; the first harmonic(primarily an acoustical term) is also referred to as two times RPM (2X). So, be careful.One should visualize this as a bell. The bell is the center of concentric circles – circles orharmonics are numbered from the center. The bell is the center and the first circleencountered is the first harmonic. But in machinery terms, that would be the 2Xcomponent. [The bell, itself being the fundamental or 1X component].

BEARING COMPONENT: BPFI (Ball Pass Frequency Inner Race) BPFO (outer race frequency) BSF (Ball Spin Frequency)

FTF (Fundamental Train Frequency) cage problem

Here’s the so-called classic bearing equation= shaft speed in RPM = contact angle Pd = Pitch diameter Bd =

Ball diameter

BPOR = N/2 Bd /Pd (1 – Bd /Pd cos ) BPIR = N/2 (1 + Bd /Pd cos )


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BSF = Pd /2Bd (1 – ( Bd /Pd )2 cos2 ) FTF = /2(1 – Bd /Pd cos )And then there’s the easy way: The rule of thumb method.

BPOR rot ~ 0.4 n BPIR rot ~ 0.6 n FTF rot ~ 0.4

There’s no substitute for experience; again use acceleration, you can’t analyze what youcan’t see – you can’t trend what you can’t see or your instrumentation doesn’t see.Integration can often drive signals into the dirt or noise floor where they’ll be hidden orcamoflogged to death. The noise floor masks or covers them up, and a small accelerationsignal 0.05 g’s 0-P (the beginning of trending) at 600 Hz wouldn’t be noticed.

Always check the limitations of your system. Transducer selection for a paper machineshould be of very high resolution with as low a frequency response, as temperature willallow. There are a number of good transducers out there. Always make sure youracquisition system is within limits of your interest.

I’ve seen so-called consultants gather vibration data when the machine of interest wasoutside of the limitations of their instrumentation. Naturally, the sub sequential report thatfollows is a real gem. This writing is only a field book and to inform you of certain thingsthat should be of concern to you in your environment. Question your transducer supplier

but make certain they have the product you need or find one who does.

Bearing’s Natural Frequency: Lack of lubrication excites this frequency and creates a broad vibration pattern around this frequency region casing this phenomenon to displayin the vibration spectra.

Special Note: A broadened spike or spectral peak (from the peak on top, the shape of the peak will broaden toward the noise floor) will denote mechanical deterioration. If you seemechanical deterioration and the bearings natural frequency you may want to injectgrease lubricant very slowly as not to shock the bearing. If there’s only a small magnitudeof vibration then you may want to only note it in the report. If it’s severe and adetermination must be made the addition of lube may give insight as to time of failure orassist in analysis.

LINE FREQUENCY: 60 Hertz (Hz) USA Power Turbine RPM (3600)/60=60. In Europethe power is 50 Hertz so the generators are larger and operate at 3000 RPM.

LINE SYNC FREQ: 120 Hz (120 / # poles X 60 = RPM of a given machine ) Look forespecially in DC motors. In AC & DC this frequency phenomenon is generally found atupper harmonics of high frequency problems associated with electrical problems or the

beginning of potential problems.

The electrical frequency is also displayed in fluorescence light bulbs and is used tocalibrate a photo tach. I’ve also used it for a trigger.


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Field balancing using a tach for a trigger device to sync at once per rev.

MEASUREMENT PARAMETERS: Acceleration, Velocity, Displacement and Acoustics(dB).

There are contact type and non-contact type. Which is best? That may be impossible todetermine. Large turbo machinery – both. The contact type are cheaper and easier as arule but are affected by the machines dampening effect caused by bearing type and mass.Mass determines resonance so the sensor’s mass should be so that the frequencies ofinterest are below its natural frequency. The non-contact sensor (eddy current probe)

provides rotor dynamics and good down to DC. We are not getting into all systems, laser,all non-contacting as this is only a field reference book.

Bearing cap data are now taken almost exclusively in acceleration. I’ve used thatapproach since 1980 exclusively. A velocity transducer is sometimes useful for

balancing, apart from that I won’t use a velocity transducer and don’t integrate anacceleration spectrum into velocity or double integrate to a displacement spectrum. Thoseintegrations are only relative, not absolute. I will and do present digitized data in all three

parameters for some. I do not believe in producing a velocity spectrum because it is solimited as compared to an acceleration spectrum. Velocity (IPS) is a good language totranslate into words and is a common language as it is irrespective of frequency. So, youcan iterate an amplitude and one can understand its magnitude or severity.


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Overall or Overall level (OA): A term I use often, but not really sure if all are versed inits meaning. Normally an analyzer has 200, 400, 800 and 1600 lines of resolution.Imagine a spreadsheet of 400 columns, as that’s normal for most PdM programs. Theheight is normally 1 Volt with a resolution of 0.005 and automatic scaling. Time

waveform data are broken down into frequency spectra. So, each of the 400 windows hasamplitude: that amplitude for each window is squared. Overall level (OA) is the squareroot of the sum of the squares. It is the sixth band in my PdM software. Or in a 200 lineanalyzer it would be line 201.

A picture is worth a thousand words. You can’t analyze what you can’t see. Why wasteyour time with a velocity spectrum when it isn’t even a true integration from accelerationto velocity. Integration is relative, not absolute. Work in acceleration and get used to it;it’s the right thing to do now and the way of doing in the future. It took some transition

period to get from displacement to velocity. Machines fitted with anti-friction bearingsshould always be monitored in acceleration, and use acceleration spectra for diagnostics:

3600 RPM to 5 kHz. Likewise, if I’m using a velocity transducer or an eddy current probeI monitor IPS and mils respectively – never integrating the spectra only displayingdigitized data in all three parameters.

Both displacement and acceleration are respective of frequency, therefore velocity is thelanguage to speak in for clarity and ease of understanding. But in the analysis stage whengathering acceleration data use acceleration for analysis. Discipline yourself and you’llfind over time your PdM program will evolve into planned scheduled maintenance andyou’ll get maximum life from the machine and eliminate overtime. I have successfullyimplemented such programs. You’ll tear into a machine and realize that you have gottenmaximum life: no need to pull the magnifying glass out and look for scratches. I’ve seen

people do that – it stands out that it’s an excuse for inability or a poor program.

Relationships of sinusoidal velocity, acceleration, & displacementENGLISH METRICV=(Pi)fD V=(Pi)fDV=61.44 g/f V=1.56 g/fg=0.0511f 2D g=2.013 f 2Dg=0.016 2Vf g=0.64 VfD=0.3183 V/f D=0.3183 V/fD=18.57 g/f 2 D=0.4968 g/f 2

NOTE: D= inches pk to pk or meters pk to pkV= inches per second (IPS) meters per secondf= frequency in Hertz (Hz) or cycles per second (RPM/60)g= 386.1 IPS 2


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Constants for true sine waves only ------- _________________rms value = 0.707 X peak value avg rms rms value = 1.11 X avg value

peak value = 1.414 X rms value peak value = 1.57 X avg value

average value = 0.637 X pk value P-P pk to pk value = 2.0 X pk value

Microphones (acoustics) are not usually associated with machines except for OSHAsound level measurements.But, if you have a machine high and away (remote) and don’t have transducers mounted,then a highly directional microphone can assist for frequency analysis.

This will sometimes produces good results and always is better than no approach or program at all.


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Sound isn’t our topic although sound is vibration and vibration is sound. We won’t begetting into media that has so many variables and dampening factors and backgroundnoise etc. But, here’s a chart nonetheless.


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In a PdM program we normally are concerned with bearing cap data and sometimes with permanently installed transducers. However, one must know that the transducer is incalibration. An outside consultant may choose to use known transducers such as his orher own accelerometers for bearing cap data. Normally, (for example) I will go into acontrol room and acquire data directly from the panel by plugging in to an existing

system such as a Bentley Nevada 3300; this gives me rotor dynamics. But, I also acquire bearing cap data.

NOTE: A normally good machinery analyses instrument system will cover 20 kHz to 0Hz or DC and have transducers covering 0.05 Hz to 20 kHz.

They may be phased matched having phase measuring capabilities down 30 RPM within2%. Phase gathering triggering devices either photo tachs, laser, eddy current probes orstrobes. Set transducers side by side to confirm amplitude/phase integrity as a field check.

High quality tachometer may be necessarywhen phase readings aren’t stable using conventional phase gathering instruments.


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OUT OF BALANCE CONDITION : Imbalance occurs at running speed and produces aclean sharp spike at 1X. Running speed vibration is very often misunderstood and givesmany technicians problems. I’ve seen technicians attempt to balance rotors with high 1Xand almost nothing else - no 2X or 3X and relatively smooth the rest of the base band.

Misalignment (gross) will totally manifest itself in 1X and you’ll play havoc attemptingto balance that problem. Bearing problems and faulty machine parts will manifestthemselves into running speed vibration at some percentage as the machine deteriorates.That percentage may be very small but a bearing flaw (for example) will produce dragand contribute some magnitude into running speed though its source may not bedetermined. Analyze the spectra, different data points, and different positions with phasedata. Correct faults then balance. All too often I see technicians balance and amplitude atrunning speed will become acceptable and it’ll be called good. However, that balance jobdidn’t magically cure the machine component that also has a fault.

Never just balance a machine to an acceptable vibration level and walk off with out some

analysis to determine machine condition. Imbalance in vertical machines is usuallyhighest transverse to the flow or in the most flexible direction. Bottom mountedmachines usually reveal higher 1X vibration in the horizontal direction.

Once imbalance has been determined to be the fault, balance the number of planesnecessary to achieve smooth machine operation.

When field balancing one may want to use eddy current probes or existing transducers.Most large turbo-machinery will have a permanent monitoring system installed that youcan plug into in a control room or local panel.

On paper machine dryer cans where you balance two planes and still have excessiveamplitude: does the phase match - yes, then whip is the problem and requires third plane

balance or further testing. The third plane is the center of the roll.

1X vibration problems . These are imbalance, resonance, bent shaft, external mechanicalrun-outs (coupling hub, pulley, etc., machine part, electrical, eccentrics) internal run-outs.

Phase measurements (see photos below) are very useful in determining misalignment and bent shaft but one of the easiest methods of detective work is to simply shut down themachine when you can and if practical. This enables you to monitor power off cascadingto a complete stop. Use a good analyzer with capturing capabilities or a tape recorderwhen you can.

Often the vibration disappears upon power off if electrical in nature. By monitoring coastdown one may record via instrument tape recorder, or internal capture in a real timeanalyzer, information that can identify resonance.


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VIBRATION GUIDELINE CHART

Frequency Probable Cause Other Possible Causes

Sixth 6X Machine Part 1. Vane passing

2. Rotor slippage on shaft(usually found with 4X)

Sub-rotative Oil Whirl 1. Lubrication43% typical 2. Oil Whipcan occur @ 3. FTF Fundamental Train Frequency39%-48%

Half Running Rub 1. In a sleeve bearing - rub (0.01 IPS)Speed 1/2X 2. Resonance Drive Belts

3. Drive Belts

4. Defective machine partsLine Freq. Electrical * 1. Rotor Bar Problem(60 Hz) 2. Eccentric rotor

3. Unequal air gap4. Unbalanced phases5. Insulation Breakdown

Line Sync Rotor Problem 1. Rotor resonanceFrequency 2. Torque pulses(120 Hz)

Upper Harmonic Gears 1. Gear wear, look for side bands ofdefective shaftFrequencies Aerodynamic speed

Hydraulic 2. Fan speed x # blades3. Cavitation, impeller x # vanes4. Machine part

Upper freq. Bearing Defects 1. Inner race - BPIR not harmonic Lubrication 2. Outer race - BPOR

3. Ball Spin - BSF 4. Cage - FTF

Synchronous 2 pole motors operate @ 60 Hz (3600 RPM): A good real time analyzermay have a resolution of 0.0039 Hz or better with a dynamic range of 120 dB - good foranalysis when a fault is detected: A must for third party consulting. Traceable to the NIST is a requirement.


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OK, how do we check and solve for these problems?

Let’s go to misalignment; it is usually associated with a 2X component along with 3X inthe axial direction. However, gross radial misalignment will manifest itself in 1X and noteven show 2X or 3X. Angular misalignment is usually found at 2X in the axial direction.

Using phase as a tool measure both ends and across the coupling. When searching for anunknown source, don’t form an opinion before you’ve gathered sufficient data. I’ve seen

people try fruitlessly to balance a misaligned machine. It happens too often. Of course agood maintenance program will have alignment records. And those alignment recordsshould be truthful and exact so they can be used for an analysis tool. If physicalmeasurements must be taken one may do so if the situation permits.

Use good field proven techniques and procedures to gather alignment data and/or correctalignment.

Bentley Nevada Fixture Laser

alignment instrument. Dual Beam


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Of course this one with the big picture is my personal kit (shims are extra): PdMK-2.Visit http://www.pdm-engineering.com

The procedures below are for all types of alignment. These are field proven and work.

They will enable you to align in the shortest period of time. The types of alignment are:Indicator Reverse, Long Spacer Coupling and Rim & Face. The methods for these typesare: Laser (single and dual beam), dial indicators, parallels and micrometers.

LIGNMENT PROCEDURES

SAFETY; lockout and tag. Rack out fuses and chain valves when necessary

Ensure driven machine has no stress from piping or soft foot and that the anchor bolts are tightInspect the feet and base of the machine to be aligned for burrs, roughness, dirtEvaluate if a temporary spacer is need or useful for this alignment process; noteInspect coupling and check for run outsCoupling halves must if possible rotate together for dynamic alignmentEvaluate method of alignment to be usedWhich is best suited for this job


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Is axial thrusting a factorCollect SAG data and recordInstall brackets on machines and record machine dimensionsCheck for soft foot; correct as necessaryRough align within 1/16 inch

Record alignment readingsAre they valid?Mathematically correct and repeat?s thrust a consideration?Calculate shim and lateral shift changeVia computer? Print and file.Calculator? Record, plot and file.Make changes and verify & PLOTIf spacer was used, remove it at this timePLOT THE FINAL ALIGNMENT DATAUNLOCK AND REMOVE TAGS; NOTIFY OPERATIONSPLACE RECORD AND PLOTS IN MAINTENANCE FILE

2X running speed - mechanical looseness is a prime factor when this type of vibrationoccurs. It is radial in nature and usually predominate in the vertical direction. Itsometimes pays to acquire data at the foot or split of a bearing cap. Reciprocating forces;are you analyzing a compressor?

If you are fortunate enough to have a good vibration lab, you can duplicate most vibrationsituations or create experiments for further study. Bentley Nevada markets a good lab kitcomplete with eddy current probes and oil whirl attachment.

Measurement techniques are important as well as applying common sense. Approach themachine like a crime scene. Notice everything; does the foundation appear correct and

properly isolated? Are there cracks on the Floor; stand on each side of the crack - is therea difference - measure? Can you feel or hear a beat? Use a stopwatch, count the beatsover a five minute period. I’ve uncover the source of vibration using this technique. Peckon a sole plate, is there thud? Are there splits in the case or bearing housings - measure

both sides. Eddy current probes give rotor dynamics, but also take cap readings.


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Cap readings make up machine and rotor movements. Is there a difference from machineto base? What about phase end to end, radically. Phase axially: measure every ninetydegrees.

In low frequency problems, resonance is usually a consideration. Can you alter speed orother factors? A coast down provides good information. While the unit is down, performring testing - this will give you natural frequency. One problem can excite another.

Another consideration in taking low frequency measurements; low pass filters. They’re

available in a variety of ranges. Low pass filters can let you blow up low frequencysignals. This works well especially with an analyzer having a 90 dB range or better. Alsoreal time zooming is a good tool. This can give a resolution of 0.0156 Hz or better; verygood to separate frequency components. Sixty Hertz from running speed; side bandsaround 60 Hz such as around 1.2 Hz delta freq and so on.

While we’re in low frequency stuff, let’s consider half running speed. A rub in a sleeve bearing will show up at ½ and may be very severe at 0.03 IPS (inches per second).What is a machine’s mass, dampening factor? A rub at 0.05 IPS is trouble. Also look forsub-harmonics. In reality; you never want to see a rub.

Sleeve bearings: rubs, excessive clearance, misalignment, lubrication and looseness aremain concerns. Since these bearings ride on a film of oil and the hydraulic pressure

provides certain lift, clearance is an important function. Excessive clearance will causeharmonic content. Oil whirl is - a riding up and slipping back on the journal; when it

becomes violent it is called whip. Oil viscosity is an important factor and one mustconsider the type of oil system.


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We have oil bath, oil circulation, ring oiled, mist lubed; etc. Lubrication usually isn’t a big problem anymore. I once encountered an oil whirl in a machine with five tilting pad bearings. If a sleeve bearing is loose in it’s housing it can produce a 4X component; makesure the 4X isn’t being produced from another source. Again, always consider a phasestudy.

The fifth and sixth harmonic (5X & 6X); machine parts and looseness are maincontributors but, the 6X can be generated by a loose rotor and is usually accompanied by4X. You may have never seen this in print before, but this comes from practicalexperience. Just like the misalignment condition manifesting itself exclusive in 1X.Physically setup that condition in a machine and let it prove itself.

I made this statement in 1983 about acceleration; within ten years everyone willwork in acceleration. Velocity will only be used as the universal language and rightly so.

However, you can’t argue with success; if you’re using velocity successfully and almostonly worked in velocity: give acceleration a try. Back in 1980 velocity was the new kid on

the block for most. I think you’ll be glad you switched. I’d rather fight than switch back.There was also a transition period between displacement and velocity. Who works in milstoday for a PdM program? Remember, acceleration is the right thing to do: integratingto velocity is not absolute only relative. There is no pure integration and you can’tanalyze what you can’t see.

Anti-friction bearings: use both time waveform and frequency spectra. These bearingstransmit forces more readily than sleeve bearings and the stiffest direction usually revealsa higher magnitude of vibration. Banding analysis and overall level diagnostics workwell.

On 3600 RPM machines use a base band of 5 kHz. For purposes of resolution I normallyview 2 kHz as my primary base band. But, never limit anything when troubleshooting.Defective anti-friction bearings are a product of mechanical deterioration. When thisoccurs, a sharp spike will broaden: study spectral shape and use shape as a diagnostictool. Overall analysis is also useful but always with the frequency spectra. Windowing asin different base bands is also a tool.

There’s more than just the Hanning window too. Explore! Technical Associates ofCharlotte have a good section on A-F bearings; R-0792-3, page 2. A good analyzer is avery valuable too. Just any ole black box won’t do. There’s more than shown.


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This has been the common analytical approach since the late 70’s. A word of warning;spike energy is not a good evaluation parameter, never rely on it for a tool. And, it isn’t

good at all on low frequency machines; i.e.. paper machine dryer cans. On high RPM machines note the harmonics of bearing fundamentals. Another factor of detrimental highfrequency vibrations related to A-F bearings - shaft currents. This can be measured (shaftcurrents) with a good DMM . You can also have a buildup of static current that dischargesthrough the bearing. Again, use 5 kHz base band on 3600 & 1800 RPM machines inconjunction with your normal observance frequencies. If magnitudes of vibration areobserved near the end of the spectra (high frequency region), then you may want to lookat the next higher base band on your analyzer.

With the absence of machine defects, bearing defects having a magnitude of 4-7 g’s 0-Pover 5 kHz are nearing failure. This is a broad statement, mass & dampening must be

taken into account. Example: 5 g’s 0-P over 5 kHz @ 3600 RPM usually will fail within amonth; maybe a little less, maybe a little more.

But, the same machine situation @ 1800 RPM may take two months to fail plus or minusa little. Please, this is an illustration, so don’t hold me to exact numbers although thescenario is close. In PdM program mode use trending, if the program is old enough.Continuous monitoring systems provide good trending information much better than amonthly or quarterly route program. Monthly or quarterly monitoring programs require agood chief analysis. Trending is valuable on continuous monitoring programs but hasshort comings on anything less.

Lastly, lubrication or lack of it. Check schedules, quantity per time interval, type, etc. A bearings natural frequency will be excited in the absence of lubrication. Lubricationmakes the wheels go around. Alignment makes them run longer, PdM makes them costefficient.Got a lube program? Your grease guns should be labeled – one shot equals so manygrams! Each grease lubed bearing should be given and exact quantity over a given timeinterval (i.e. 28 grams per month; 14 grams per two weeks). Classify machines as tospeed and environment to determine frequency of lubrication.


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Lubrication is your single most important program; not alignment or balancing or PdM. Ihave papers I’ve written on grease lubed bearings and quantities per given time interval. Ilike to calculate that time/time interval as accurately as possible. If I have a good PdM

program it will fine tune those numbers. Therefore, when a good PdM program is in place one can know when the lube is lacking. With this in mind; calculations and

estimations can be ‘backed off’ slightly and fined tuned over time through frequencyanalysis.

Gearboxes: These can be very complicated; I don’t consider any to be categorizes assimple. Correct gearbox set up is essential. It sometimes becomes necessary to stress agearbox to achieve internal alignment. I was call on a very high H.P. unit operating at12.0 g’s 0-P @ gear mesh. Upon shut down I inspected and measured clearances. Therewas originally internal misalignment. But, it had already worn a pattern. I recommendedto continue operation as distorting the case to correct internal misalignment would onlycause an increase in vibration level. They opted to distort the case. Vibration increased. Iagain recommended no action as it would now wear a new pattern and reduce in

magnitude. It was left there and monitored regularly. At 18.0 g’ it was inspected and then put back in service to 20.0 g’s. This is unusual but not odd. Always use time waveform asan analytical tool. The defective gear will be modulated or side-banded by the defectivegear’s shaft speed. Also, look for a small `side spike’ on the side of defect peak in timewaveform if a gear is cracked or chipped.

A voith coupling (high H.P.) may give you a bit of diagnostic problem as they can havean amplitude/frequency display in an axial position that appears excessive. But, It’s acharacteristic of that box.

Some of these things are only obtained from experience or being around someone withexperience. In troubleshooting always draw on all natural resources available to you.I believe that’s a good philosophy in most all aspects of everything you do. Gear meshfrequency in most cases is in trouble at or around 10.0 g’s (high frequency). Also, lookfor half gear mesh. How is the fundamental affected? Use phase.

Sometimes you are put in a position of doing diagnostics above your head. If you gotta,you gotta: get your feet wet. But, a critical, costly machine?Maybe it’s time to call the outside consultant, who? Again, these are guidelines to helpyou troubleshoot and solve your problem. Know when it’s time to call in help. I havedeveloped these standards with and through others:


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The chief analyst must have at least ten years experience as an analyst with five yearsspent as an outside consultant. It shouldn’t insult him if you ask for a resume’ and samplereport as well as references. Is it warranty work; then the work must be traceable to the

NIST, formerly The National Bureau of Standards, now, National Institute of Standardsand Technology.

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Vibration Field Tehcnician Guide