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A b s t

r a c t Dr S J Lacey

Engineering Manager Schaeffl er (UK) Ltd

An Overview ofBearing Vibration Analysis

surface stresses is not the limiting factorand probably accounts for less than 3 offailures in service

Unfortunately though many bearingsfail prematurely in service becauseof contamination poor lubrication

temperature extremes poor fittingfitsunbalance and misalignment All thesefactors lead to an increase in bearing

vibration and condition monitoring

has been used for many years todetect degrading bearings before theycatastrophically fail (with the associatedcosts of downtime or significant damage to

other parts of the machine)

Rolling element bearings are oftenused in noise sensitive applications eghousehold appliance electric motors whichoften use small to medium size bearings

Bearing vibration is therefore becomingincreasingly important from both anenvironmental consideration and because it

is synonymous with qualityIt is now generally accepted that quiet

running is synonymous with the form

and finish of the rolling contact surfacesAs a result bearing manufacturers havedeveloped vibration tests as an effective

method for measuring quality A commonapproach is to mount the bearing on a quietrunning spindle and measure the radial

velocity at a point on the bearingrsquos outerring and in three frequency bands viz 50-300 300-1800 and

1800-10000 Hz The bearing must meetRMS velocity limits in all three frequencybands

Vibration monitoring has now becomea well accepted part of many planned

maintenance regimes and relies on the wellknown characteristic vibration signatureswhich rolling bearings exhibit as the

rolling surfaces degrade However inmost situations bearing vibration cannotbe measured directly and so the bearing

vibration signature is modified by themachine structure this situation beingfurther complicated by vibration from other

equipment on the machine ie electricmotors gears belts hydraulics structuralresonances etc This often makes the

interpretation of vibration data difficultother than by a trained specialist and canin some situations lead to a mis-diagnosis

resulting in unnecessary machine downtime

and costsIn this paper the sources of bearing

vibration are discussed along with thecharacteristic vibration frequencies that arelikely to be generated

INTRODUCTION

Rolling contact bearings are usedin almost every type of rotating

machinery whose successful andreliable operation is very dependent onthe type of bearing selected as well as the

precision of all associated components ieshaft housing spacers nuts etc Bearingengineers generally use fatigue as the

normal failure mode on the assumption

that the bearings are properly installedoperated and maintained Today because ofimprovements in manufacturing technologyand materials it is generally the case that

bearing fatigue life which is related to sub-

Keywords education maintenance engineering reliability engineering

off-campus education distance education 1047298exible learning internet

Vibration produced by rolling bearings can

be complex and can result from geometrical

imperfections during the manufacturing

process defects on the rolling surfaces or

geometrical errors in associated components

Noise and vibration is becoming more critical

in all types of equipment since it is often perceived to be synonymous

with quality and often used for predictive maintenance In this article

the different sources of bearing vibration are considered along withsome of the characteristic defect frequencies that may be present Some

examples of how vibration analysis can be used to detect deterioration

in machine condition are also given

32 | Nov Dec 2008 ME | maintenance amp asset management vol 23 no 6



Improving the Reliability of a Coal Fired Power Plant using a SAP ndash REWOP Interface

SOURCES OF VIBRATION

Rolling contact bearings represents

a complex vibration system whosecomponents ndash ie rolling elements inner

raceway outer raceway and cage ndash interactto generate complex vibration signaturesAlthough rolling bearings are manufactured

using high precision machine tools andunder strict cleanliness and quality controlslike any other manufactured part they will

have degrees of imperfection and generatevibration as the surfaces interact through

a combination of rolling and slidingNowadays although the amplitudes ofsurface imperfections are in the order ofnanometres significant vibrations can still

be produced in the entire audible frequencyrange (20 Hz ndash 20 kHz)

The level of the vibration will depend

upon many factors including the energy ofthe impact the point at which the vibrationis measured and the construction of the

bearing

Variable compliance

Under radial and misaligning loadsbearing vibration is an inherent feature

of rolling bearings even if the bearing isgeometrically perfect and is not thereforeindicative of poor quality This type of

vibration is often referred to as variablecompliance and occurs because the externalload is supported by a discrete number

of rolling elements whose position withrespect to the line of action of the loadcontinually changes with time (see Figure 1)

As the bearing rotates individualball loads hence elastic deflections at therolling element raceway contacts change

class of the bearing For axially loaded ballbearings operating under moderate speedsthe form and surface finish of the critical

rolling surfaces are generally the largestsource of noise and vibration Controllingcomponent waviness and surface finish

during the manufacturing process istherefore critical since it may not only have

a significant effect on vibration but also mayaffect bearing life

It is convenient to consider geometrical

imperfections in terms of wavelengthcompared with the width of the rolling

element-raceway contacts Surface featuresof wavelength of the order of the contactwidth or less are termed roughness whereaslonger wavelength features are termed

waviness (seeFigure 2)

er r

to produce relative movement betweenthe inner and outer rings The movementtakes the form of a locus which under radial

load is two dimensional and contained ina radial plane whilst under misalignment

it is three-dimensional The movement isalso periodic with base frequency equal tothe rate at which the rolling elements pass

through the load zone Frequency analysisof the movement yields the base frequencyand a series of harmonics For a single row

radial ball bearing with an inner ring speedof 1800 revmin a typical ball pass rate is

100 Hz and significant harmonics to morethan 500 Hz can be generated

Variable compliance vibration isheavily dependent on the number of rolling

elements supporting the externally appliedload the greater thenumber of loaded

rolling elements theless the vibrationFor radially loaded or

misaligned bearingslsquorunning clearancersquodetermines the extent

of the load regionand hence in generalvariable compliance

increases withclearance Runningclearance should

not be confusedwith radial internalclearance (RIC) the

former normally being lower than the RICdue to interference fit of the rings and

differential thermal expansion of the innerand outer rings during operation

Variable compliance vibration levels can

be higher than those producedby roughness and waviness ofthe rolling surfaces However

in applications where vibrationis critical it can be reduced toa negligible level by using ball

bearings with the correct level ofaxial pre-load

Geometrical imperfections

Because of the very nature

of the manufacturing processes

used to produce bearingcomponents geometricalimperfections will always bepresent to varying degrees

depending on the accuracy

Radial Load

Width of Contact

raceway

ball

h

Figure 1 Simple bearing model

Figure 2 Waviness and roughness of rolling surfaces

SURFACE ROUGHNESS

Surface roughness is a significantsource of vibration when its level is highcompared with the lubricant film thickness

generated between the rolling element-raceway contacts (seeFigure 2) Underthis condition surface asperities can break

through the lubricant film and interact withthe opposing surface resulting in metal-to-metal contact The resulting vibration

consists of a random sequence of smallimpulses which excite all the natural modesof the bearing and supporting structure

Surface roughness produces vibration

predominantly at frequencies above sixtytimes the rotational speed of the bearing Thus the high frequency part of thespectrum usually appears as a series of

resonances

vol 23 no 6 maintenance amp asset management | Nov Dec 2008 ME | 33



elements following the surface contours The relationship between surface geometry

and vibration level is complex beingdependent upon the bearing and contactgeometry as well as conditions of load and

speed Waviness can produce vibrationat frequencies up to approximately threehundred times rotational speed but is

usually predominant at frequencies belowsixty times rotational speed The upper limitis attributed to the finite area of the rolling

element raceway contacts which average outthe shorter wavelength features

In the direction of rolling elasticdeformation at the contact attenuates simpleharmonic waveforms over the contact width

(seeFigure 4) The level of attenuation increases as

wavelength decreases until in the limit for

a wavelength equal to the contact widthwaviness amplitude is theoretically zero The contact length also attenuates short

wavelength surface features Generallypoor correlation can exist between parallelsurface height profiles taken at different

points across the tracks and this averagesmeasured waviness amplitudes to a lowlevel For typical bearing surfaces poor

correlation of parallel surface heightsprofiles only exists at shorter wavelengths

Even with modern precision machiningtechnology waviness cannot be eliminated

completely and an element of wavinesswill always exist albeit at relatively lowlevels As well as the bearing itself thequality of the associated components

can also affect bearing vibration and anygeometrical errors on the outside diameterof the shaft or bore of the housing can be

reflected on the bearing raceways with theassociated increase in vibration Thereforecareful attention is required to the form

and precision of all associated bearingcomponents

Discrete defects

Whereas surface roughness and

waviness result directly from the bearingcomponent manufacturing processesdiscrete defects refer to damage of the rolling

surfaces due to assembly contaminationoperation mounting poor maintenanceetc These defects can be extremely small

and difficult to detect and yet can havea significant impact on vibration-criticalequipment or can result in reduced bearing

Λ

Region of lubricated

relatedsurfacedistress

Operatingregion for

mostindustrial

applcations

Region of increased life

Regionof

possiblesurfacedistress

withseveresliding P

e r c e n t F i l m

100

80

60

40

20

04 06 1 2 4 6 10

Attention due to ElasticDeformation

Raceway Waviness

Ball

Contact Width

A common parameter used to estimatethe degree of asperity interaction is the

lambda ratio (Λ) This is the ratio oflubricant film thickness to composite surfaceroughness and is given by the expression

Λ = h (σЪ2 + σr2)05

where Λ = degree of asperit y interacti on

h = the lubricant 1047297lm thickness

σЪ = RMS roughness of the ball

σr = RMS roughness of the raceway

If we assume that the surface finish ofthe raceway is twice that of rolling elementthen for a typical lubricant film thickness of

03microm surface finishes better than 006 micromare required to achieve a Λ value of threeand a low incidence of asperity interaction

For a lubricant film thickness of 01_msurface finishes better than 0025 _m arerequired to achieveΛ=3 The effect of Λ on

bearing life is shown in Figure 3If Λ is less than unity it is unlikely

that the bearing will attain its estimated

design life because of surface distresswhich can lead to a rapid fatigue failureof the rolling surfaces In general Λ ratios

greater than three indicate complete surfaceseparation A transition from full EHL(elastohydrodynamic lubrication) to mixed

lubrication (partial EHL film with someasperity contact) occurs in the Λ rangebetween 1 and 3

Waviness

For longer wavelength surface featurespeak curvatures are low compared withthat of the Hertzian contacts and rolling

motion is continuous with the rolling

Figure 3 Percent 1047297lm versus Λ (function of 1047297lm thickness and surface roughness)

Figure 4 Attenuation due to contact width




life This type of defect can take a variety offorms viz indentations scratches along and

across the rolling surfaces pits debris andparticles in the lubricant

Bearing manufacturers have adoptedsimple vibration measurements on thefinished product to detect such defects butthese tend to be limited by the type and

size of bearing An example of this type ofmeasurement is shown inFigures 5(a) and5(b) where compared to a good bearing

the discrete damage on a bearing outer ringraceway has produced a characteristicallyimpulsive vibration which has a high peak

RMS ratioWhere a large number of defects occurs

individual peaks are not so clearly defined

but the RMS vibration level is several timesgreater than that normally associated with abearing in good condition

Bearing characteristic frequencies

Although the fundamental frequenciesgenerated by rolling bearings are expressedby relatively simple formulas they cover a

wide frequency range and can interact togive very complex signals This is oftenfurther complicated by the presence on the

equipment of other sources of mechanical

structural or electro-mechanical vibrationFor a stationary outer ring and rotating

inner ring the fundamental frequenciesare derived from the bearing geometry as

follows ndash

f co = f r2 [1 ndash dD Cos α ]

f ci = f r2 [1 + dD Cos α ]

f bo = Z f co

f bi = Z f ci f b = D2d f r [1 ndash (dD Cos α)2]

where f r = inner ring rotational frequenc y

f co = fundamental train (cage)

frequency relative to outer ring

f ci = fundamental train frequenc y

relative to inner ring

f bo = ball pass frequenc y of outer ring f bi = ball pass frequenc y of inner ring

f b = rolling element spin frequenc y

D = Pitch circle diameter

d = Diameter of roller elements

Z = Number of rolling elements

α = Contact angle

The bearing equations assume that thereis no sliding and that the rolling elementsroll over the raceway surfaces However

in practice this is rarely the case and due

to a number of factors the rolling elementsundergo a combination of rolling and sliding

As a consequence the actual characteristicdefect frequencies may differ slightly from

those predicted but this is very dependenton the type of bearing operating conditionsand fits Generally the bearing characteristicfrequencies will not be integer multiples of

the inner ring rotational frequency whichhelps to distinguish them from other sourcesof vibration

Since most vibration frequencies areproportional to speed it is important whencomparing vibration signatures that data is

obtained at identical speeds Speed changeswill cause shifts in the frequency spectrumcausing inaccuracies in both the amplitude

and frequency measurement Sometimesin variable speed equipment spectral ordersmay be used where all the frequencies are

normalized relative to the fundamentalrotational speed This is generally calledlsquoorder n ormalisationrsquo where the fundamental

frequency of rotation is called the first order

The bearing speed ratio (ball passfrequency divided by the shaft rotational

frequency) is a function of the bearingloads and clearances and can therefore give

some indication of the bearing operatingperformance If the bearing speed ratiois below predicted values it may indicate

insufficient loading excessive lubricationor insufficient bearing radial internalclearance which could result in higher

operating temperatures and prematurefailure Likewise a higher than predictedbearing speed ratio may indicate excessive

loading excessive bearing radial internalclearance or insufficient lubrication

A good example of how the bearing

speed ratio can be used to identify apotential problem is shown in Figure6 which shows a vibration acceleration

spectrum measured axially on the end cap

of a 250 kW electric motorIn this case the Type 6217 radial ball

bearings were experiencing a high axialload as a result of the non-locating bearingfailing to slide in the housing (thermal

An

Figure 5(a) Signal from a good bearing

Figure 6 Axial vibration acceleration spectrum on end cap of a 250 kW electric motor

Figure 5(b) Signal from a damaged bearing




loading) For a nominal shaft speed of

3000 revmin the estimated outer ring ballpass frequency f bo was 2288 Hz giving

a bearing speed ratio of 4576 The actualouter ring ball pass frequency was 2335 Hzgiving a ball speed ratio of 467 an increase

of 2 A photograph of the inner ring isshown in Figure 7 showing the ball runningpath offset from the centre of the raceway

towards the shoulderEventually this motor failed

catastrophically and thermal loading (cross

location) of the bearings was confirmedA number of harmonics and sum anddifference frequencies are also evident in

the spectrumBall pass frequencies can be

generated as a result of elastic properties

of the raceway materials due to variablecompliance or as the rolling elementspass over a defect on the raceways The

frequency generated at the outer and innerring raceway can be estimated roughly as40 (04) and 60 (06) of the inner ring

speed times the number of rolling elementsrespectively

Unfortunately bearing vibration signals

are rarely straightforward and are further

complicated by the interaction of thevarious component parts but this can be

often used to advantage in order to detecta deterioration or damage to the rollingsurfaces

Imperfections on the surface ofraceways and rolling elements as a resultof the manufacturing process interact to

produce other discrete frequencies andsidebands (summarised inTable 1)

Table 1 Frequencies related to surface

imperfections

Analysis of bearing vibration signalsis usually complex and the frequencies

generated will add and subtract and arealmost always present in bearing vibrationspectra This is particularly true where

multiple defects are present Howeverdepending upon the dynamic range ofthe equipment background noise levels

and other sources of vibration bearingfrequencies can be difficult to detect inthe early stages of a defect However

over the years a number of diagnosticalgorithms have been developed to detectbearing faults by measuring the vibration

signatures on the bearing housing Usuallythese methods take advantage of both thecharacteristic frequencies and the lsquoringing

frequenciesrsquo (ie natural frequencies) of thebearing (see later)

Raceway defect

A discrete defect on the inner racewaywill generate a series of high energy pulses

at a rate equal to the ball pass frequencyrelative to the inner raceway Because theinner ring is rotating the defect will enter

and leave the load zone causing a variation

in the rolling element-raceway contactforce hence deflections While in the loadzone the amplitudes of the pulses willbe highest but then reduce as the defect

leaves the load zone resulting in a signal

which is amplitude modulated at innerring rotational frequency In the frequencydomain this not only gives rise to a discrete

peak at the carrier frequency (ball passfrequency) but also a pair of sidebands

spaced either side of the carrierfrequency by an amount equal to themodulating frequency (inner ring

rotational frequency) (seeFigure 8 )Generally as the level of

amplitude modulation increases so

will the sidebands As the defectincreases in size more sidebands

are generated and at some point theball pass frequency may no longerbe generated but instead a series ofpeaks will be generated spaced at the

inner ring rotational frequencyA discrete fault on the outer

raceway will generate a series of

high energy pulses at a rate equalto the ball pass frequency relative

to the outer ring Because the outer ring is

stationary the amplitude of the pulse willremain theoretically the same and hencewill appear as a single discrete peak within

the frequency domainAn unbalanced rotor will producea rotating load so as with an inner ring

defect the resulting vibration signal canbe amplitude modulated at inner ringrotational frequency

Likewise the ball pass frequency canalso be modulated at the fundamentaltrain frequency If a rolling element has

a defect it will enter and leave the loadzone at the fundamental train frequency

causing amplitude modulation and result insidebands around the ball pass frequencyAmplitude modulation at the fundamental

train frequency can also occur if the cage islocated radially on the inner or outer ring

Although defects on the inner and

outer raceways tend to behave in a similarmanner for a given size defect the amplitudeof the spectrum of an inner raceway defect

is generally much less The reasons for thismight be that a defect on the inner ringraceway only comes into the load zone once

per revolution and the signal must travelthrough more structural interfaces beforereaching the transducer location ie rolling

element across an oil film through the outer

ring and through the bearing housing tothe transducer position The more difficult

transmission path for an inner raceway faultprobably explains why a fault on the outerraceway tends to be easier to detect

Figure 7 Photograph of Type 6217 inner ring

showing running path offset from centre of

raceway

Surface DefectFrequency

Component Imperfection

Inner Raceway Eccentricity

Waviness

Discrete Defect

f r

nZf ci

plusmnf r

nZf ci

plusmnfr

OuterRaceway

Waviness

Discrete Defect

nZf co

nZf c

oplusmnf r

nZf co

plusmnf co

RollingElement

DiameterVariation

Waviness

Discrete Defect

Zf co

2nfb plusmnf co

2nfb plusmnf co




Rolling element defect

Defects on the rolling elements cangenerate a frequency at twice ball spinfrequency and also harmonics and the

fundamental train frequency Twice therolling element spin frequency can begenerated when the defect strikes both

raceways but sometimes the frequencymay not be this high because the ball is notalways in the load zone when the defect

strikes and energy is lost as the signal

passes through other structural interfacesas it strikes the inner raceway Also whena defect on a ball is orientated in the axialdirection it will not always contact the inner

and outer raceway and therefore may be

difficult to detect When more than one

rolling element is defective sums of the ballspin frequency can be generated If thesedefects are large enough then vibration

at fundamental train frequency can begenerated

Cage defect

As already shown the cage tends torotate at typically 04 times inner ring

speed generally has a low mass and

therefore unless there is defect from themanufacturing process is generally notvisible

Unlike raceway defects cage failures do

not usually excite specific ringing frequencies

and this limits the effectiveness of theenvelope spectrum (see later) In the caseof cage failure the signature is likely to have

random bursts of vibration as the balls slideand the cage starts to wear or deform and awide band of frequencies is likely to occur

As a cage starts to deteriorate for examplefrom inadequate lubrication wear can start to

occur on the sliding surfaces ie in the cagepocket or in the case of a ring guided cage onthe cage guiding surface This may gives rise

to a less stable rotation of the cage or a greaterexcursion of the rolling elements resulting in

increased sideband activity around the otherbearing fundamental frequencies eg the ballspin frequency

Excessive clearance can cause vibration

at the fundamental train frequency (FTF) asthe rolling elements accelerate and deceleratethrough the load zone which can result

in large impact forces between the rollingelements and cage pockets Also outer racedefects and roller defects can be modulated

with the FTF fundamental frequency

Other sources of vibration

Contamination is a very common sourceof bearing deterioration and premature

failure and is due to the ingress of foreignparticles either as a result of poor handlingor during operation By its very nature

the magnitude of the vibration caused bycontamination will vary and in the earlystages may be difficult to detect but this

depends very much on the type and natureof the contaminants Contamination cancause wear and damage to the rolling

contact surfaces and generate vibrationacross a broad frequency range In the earlystages the crest factor of the time signal will

increase but it is unlikely that this will bedetected in the presence of other sources ofvibration

With grease lubricated bearingsvibration may be initially high as thebearing lsquoworksrsquo and distributes the grease

The vibration will generally be irregularbut will disappear with running time andgenerally for most applications doesnrsquot

present a problem For noise-criticalapplications special low-noise-producinggreases are often used

VIBRATION MEASUREMENT

Vibration measurement can be generallycharacterised as falling into one of three

categories viz detection diagnosis and

An

Amplitude

Ac 2

Am 4

frequency

f c - f

m

f c

f c + f

m

Figure 8 Amplit ude mod ulation (AM) (a) Ampl itude m odulate d time s ignal

Figure 8 Amplit ude mod ulation (AM) (b) Spec trum of amplitu de modu lated si gnal




frequencies and sidebands followed by adrop in these amplitudes and an increasein the broadband noise with considerable

vibration at shaft rotational frequency Wheremachine speeds are very low the bearings

generate low energy signals which againmay be difficult to detect Also bearingslocated within a gearbox can be difficult to

monitor because of the high energy at thegear meshing frequencies which can maskthe bearing defect frequencies

Overall vibration level

This is the simplest way of measuringvibration and usually consists of measuringthe Root Mean Square (RMS) vibration ofthe bearing housing or of some other point

on the machine with the transducer locatedas close to the bearing as possible Thistechnique involves measuring the vibration

over a wide frequency range eg 10-1000Hz or 10-10000 Hz The measurements canbe trended over time and compared with

known levels of vibration or pre-alarm andalarm levels can be set to indicate a changein the machine condition Alternatively

measurements can be compared withgeneral standards Although this methodrepresents a quick and low cost method of

vibration monitoring it is less sensitive toincipient defects ie it detects defects in

the advanced condition and has a limiteddiagnostic capability Also it is easilyinfluenced by other sources of vibration

eg unbalance misalignment loosenesselectromagnetic vibration etc

In some situations the Crest Factor

(the ratio PeakRMS) of the vibration iscapable of giving an earlier warning ofbearing defects The development of a

local fault produces short bursts of highenergy which increase the peak level of thevibration signal but have little influence

on the overall RMS level As the faultprogresses more peaks will be generateduntil finally the Crest Factor will reduce but

the RMS vibration will increase The maindisadvantage of this method is that in theearly stages of a bearing defect the vibration

is normally low compared with othersources of vibration present and is thereforeeasily influenced so any changes in bearing

condition may be difficult to detect

Frequency spectrum

Frequency analysis plays an importantpart in the detection and diagnosis of

machine faults In the time domain the

individual contributions eg unbalance tothe overall machine vibration are difficultto identify In the frequency domain they

become much easier to identify and cantherefore be much more easily related toindividual sources of vibration As we have

already discussed a fault developing in abearing will show up as increasing vibration

at frequencies related to the bearingcharacteristic frequencies making detectionpossible at a much earlier stage than with

overall vibration

Envelope spectrumWhen a bearing starts to deteriorate

the resulting time signal often exhibitscharacteristic features which can be used to

detect a fault Also bearing condition canrapidly progress from a very small defectto complete failure in a relatively short

period of time so early detection requiressensitivity to very small changes in thevibration signature As we have already

discussed the vibration signal from the earlystage of a defective bearing may be maskedby machine noise making it difficult to

detect the fault by spectrum analysis alone The main advantage of envelopeanalysis is its ability to extract the periodic

impacts from the modulated random noiseof a deteriorating rolling bearing This is

even possible when the signal from therolling bearing is relatively low in energyand lsquoburiedrsquo within other vibration from the

machineLike any other structure with mass and

stiffness the bearing inner and outer rings

have their own natural frequencies whichare often in the kilohertz range Howeverit is more likely that the natural frequency

of the outer ring will be detected due tothe small interference or clearance fit in thehousing

If we consider a fault on the outerring as the rolling element hits the faultthe natural frequency of the ring will be

excited and will result in a high frequencyburst of energy which decays and then isexcited again as the next rolling element

hits the defect In other words the resultingtime signal will contain a high frequencycomponent amplitude-modulated at the ball

pass frequency of the outer ring In practice

this vibration will be very small and almostimpossible to detect in a raw spectrum so a

method to enhance the signal is requiredBy removing the low frequency

components through a suitable high pass

prognosis Detection generally uses themost basic form of vibration measurementwhere the overall vibration level is measured

on a broadband basis in a range say of10 to 1000 Hz or 10 to 10000 Hz In

machines where there is little vibrationother than from the bearings the spikinessof the vibration signal indicated by the Crest

Factor (the ratio PeakRMS) may implyincipient defects whereas the high energylevel given by the RMS level may indicate

severe defectsGenerally other than to the experienced

operator this type of measurementgives limited information but can beuseful when used for trending where anincreasing vibration level is an indicator of

a deteriorating machine condition Trendanalysis involves plotting the vibrationlevel as a function of time and using this to

predict when the machine must be takenout of service for repair Another way ofusing the measurement is to compare the

levels with published vibration criteria fordifferent types of equipment

Although broadband vibration

measurements may provide a good startingpoint for fault detection it has limiteddiagnostic capability and although a fault

may be identified it may not give a reliableindication of where the fault is ie bearingdeterioration or damage unbalance

misalignment etc Where an improveddiagnostic capability is required frequencyanalysis is normally employed which

usually gives a much earlier indication ofthe development of a fault and secondly

the source of the faultHaving detected and diagnosed a fault

the prognosis ndash ie what the remaining

useful life and possible failure mode ofthe machine or equipment are likely tobe ndash is much more difficult and often relies

on the continued monitoring of the faultto determine a suitable time when theequipment can be taken out of service or

relies on known experience with similarproblems

Generally rolling bearings produce very

little vibration when they are fault free andhave distinctive characteristic frequencieswhen faults develop A fault that begins as

a single defect eg a spall on the raceway

is normally dominated by impulsive eventsat the raceway pass frequency resulting

in a narrow band frequency spectrumAs the damage worsens there is likely tobe an increase in the characteristic defect






cage outer diameter showed clear signs ofdamage with some fragments of plasticmaterial which had broken away but was

still attached to the outer diameter As aresult the spectrum had sum and difference

frequencies related to the shaft (fr) and cage(fc) eg 1740 Hz (5fr+fc)

As already discussed the

deterioration of rolling element bearingswill not necessarily show at the bearingcharacteristic frequencies but the vibration

signals are complex and produce sum anddifference frequencies which are almost

always present in the spectra

Raceway damage

High axial load

An example of a vibration spectrum

measured axially on the drive side end capof a 250 kW electric motor is shown inFigure 12

The rotational speed was approximately3000 revmin (50 Hz) and the rotor wassupported by two type-6217- C4 (85 mm

bore) radial ball bearings grease lubricated The vibration spectrum shows dominantpeaks between 1 kHz and 15 kHz which

can be related to the outer raceway ball passfrequency The calculated outer raceway

ball pass frequency f bo is 229 Hz and thefrequency of 1142 Hz relates to 5f bo with

a number of sidebands at

rotational frequency frWhen the bearings

were removed from the

motor and examined theball running path wasoffset from the centre

of the raceways towardsthe shoulders of theboth the inner and outer

rings indicative of highaxial loads The cause ofthe failure was thermal

pre-loading as a result ofthe non-locating bearingnot sliding in the housing

to compensate for axialthermal expansion of theshaft this is often referred

to as lsquocross locationrsquo The

non-drive end bearinghad severe damage to the

raceways and the rollingelements which was

Figure 11 Vibration acceleration measured on the spindle housing of an internal grinding machine

Figure 12 Vibration acceleration measured axially on the DE of a 250 kW electric motor

Figure 13 Vibration acceleration measured radially on the housing of a Type 23036 spherical roller bearing




critical operations is grinding of the bearingraceways which have to meet very tighttolerances of roundness and surface finish

and any increase in machine vibration canresult in a severe deterioration in workpiece

qualityFigure 17 which shows the vibration

acceleration spectrum 0-500 Hz measured

on the spindle housing of an external shoecentreless grinding machine during thegrinding of an inner ring raceway where

the typical values for out-of-roundness andsurface roughness were gt4 microm and 03

micromRa respectively The most distinctivefeature on the finished raceway was thepresence of 21 lobes which when multipliedby the workpiece rotational speed (370 rev

min or 62 Hz) corresponded to a frequencyof 1295 Hz This was very close to the 126Hz component in the spectrum which was

associated with the ball pass frequencyrelative to outer raceway of a ball bearingin the drive head motor Also present are

harmonics at 256 and 380 Hz The discretepeaks at 38 116 and 190 Hz correspondto the spindle rotational speed and its

harmonicsFigure 18 shows that after replacingthe motor bearings the vibration at 126

Hz reduced from 0012g to 000032g andthe associated harmonics were no longerdominant This resulted in a dramatic

improvement in workpiece out-of-roundnessof lt04 microm and the surface finish improvedto 019 micromRa This demonstrates that with

some critical equipment such as machinetools it is possible to assess directly the

condition of the machine by measuring theresultant workpiece quality [2 3]

Figure 14 Type 23036 spherical roller bearing

outer ring raceway showing black corrosion stains

Figure 15 Envelope spectrum of the Type 23036 spherical roller bearing

Figure 16 Accelerat ion time signal o f the Type 23036 sphe rical ro ller b earing

consistent with the highly modulated signal

and high amplitude of vibration at 5f bo The overall RMS vibration level of the motorincreased from typically 022g to 164g

Another example of a vibrationacceleration spectrum obtained from thehousing of a Type 23036 (180 mm bore)

spherical roller bearing located on the maindrive shaft of an impact crusher is showninFigure 13 The spectrum shows a number

of harmonics of the outer raceway ball passfrequency 101 Hz with a dominant peakat 404 Hz (4f bo) with sidebands at shaft

rotational frequency 9 Hz

When the bearing was removed fromthe machine and examined one part of the

outer raceway had black corrosion stain (seeFigure 14)

Also a number of the rollers had blackcorrosion stains which was consistent withthe vibration at cage rotational frequency

fc=4 Hz in the envelope spectrum (see

Figure 15) The modulation of the time signal at

cage rotational frequency can be clearly seenin the time signal Figure 16

Effect of bearing vibration oncomponent quality

Even low levels of vibration can havea significant impact on critical equipment

such as machine tools that are required toproduce components whose surface finishand form are critical A good example ofthis is during the manufacture of bearing

inner and outer rings One of the most

er r




Figure 17 Vibration spectrum and roundness before replacing

wheel head drive motor bearings (a) Vibration spectrum on

spindle housing (b) Roundness of raceway

CONCLUSIONS

The various sources of bearing vibrationhave been discussed also how each such

source can generate characteristic vibrationfrequencies which can combine to givecomplex vibration spectra which at times

may be difficult to interpret other than by

the experienced vibration analyst Howeverwith rolling bearings characteristicvibration signatures are often generatedusually in the form of modulation of the

fundamental bearing frequencies This

can be used to advantage and vibration

conditioning monitoring software is oftendesigned to identify these characteristicfeatures and provide early detection of an

impending problem This usually takesthe form of signal de-modulation andestablishment of the envelope spectrum

where the early indications of sidebandactivity and hence bearing deteriorationcan be more easily detected

As long as there are natural frequenciesof the bearing and its nearby structuresndash which occur in the case of a localized

defect on the outer raceway or on theinner raceway or on a rolling element

ndash the envelope spectrum works wellHowever cage failures do not usually excitespecific natural frequencies The focus of

demodulation is on the lsquoringingrsquo frequency(the carrier frequency) and the rate it isbeing excited (the modulating frequency)

Simple broad band vibrationmeasurements also have their place butoffer a very limited diagnostic capability

and will generally not give an early warningof incipient damage or deterioration

REFERENCES1 Harris T A Rollin g Bear ing A na lysis (4 th Ed)

Wiley New York 2001

2 Lacey S J Vibration m onitoring of theinternal centreless grinding process Part 1mathematical models Proc Instn MechEngrs Vol 24 1990

3 Lacey S J Vibration m onitoring of the

internal centreless grinding process Part2 experiment al results Proc Instn MechEngrs Vol 24 1990

4 Wardle F P and Lacey S J Vibration Research in RH P Acoustics Bulletin

stevelaceyschaeffl ercom

Figure 18 Vibration spectrum and roundness

after replacing wheel head drive motor

bearings (a) Vibration spectrum measured on


42 | NovDec 2008 ME | maintenance amp asset management vol 23 no 6



Improving the Reliability of a Coal Fired Power Plant using a SAP ndash REWOP Interface

SOURCES OF VIBRATION

Rolling contact bearings represents

a complex vibration system whosecomponents ndash ie rolling elements inner

raceway outer raceway and cage ndash interactto generate complex vibration signaturesAlthough rolling bearings are manufactured

using high precision machine tools andunder strict cleanliness and quality controlslike any other manufactured part they will

have degrees of imperfection and generatevibration as the surfaces interact through

a combination of rolling and slidingNowadays although the amplitudes ofsurface imperfections are in the order ofnanometres significant vibrations can still

be produced in the entire audible frequencyrange (20 Hz ndash 20 kHz)

The level of the vibration will depend

upon many factors including the energy ofthe impact the point at which the vibrationis measured and the construction of the

bearing

Variable compliance

Under radial and misaligning loadsbearing vibration is an inherent feature

of rolling bearings even if the bearing isgeometrically perfect and is not thereforeindicative of poor quality This type of

vibration is often referred to as variablecompliance and occurs because the externalload is supported by a discrete number

of rolling elements whose position withrespect to the line of action of the loadcontinually changes with time (see Figure 1)

As the bearing rotates individualball loads hence elastic deflections at therolling element raceway contacts change

class of the bearing For axially loaded ballbearings operating under moderate speedsthe form and surface finish of the critical

rolling surfaces are generally the largestsource of noise and vibration Controllingcomponent waviness and surface finish

during the manufacturing process istherefore critical since it may not only have

a significant effect on vibration but also mayaffect bearing life

It is convenient to consider geometrical

imperfections in terms of wavelengthcompared with the width of the rolling

element-raceway contacts Surface featuresof wavelength of the order of the contactwidth or less are termed roughness whereaslonger wavelength features are termed

waviness (seeFigure 2)

er r

to produce relative movement betweenthe inner and outer rings The movementtakes the form of a locus which under radial

load is two dimensional and contained ina radial plane whilst under misalignment

it is three-dimensional The movement isalso periodic with base frequency equal tothe rate at which the rolling elements pass

through the load zone Frequency analysisof the movement yields the base frequencyand a series of harmonics For a single row

radial ball bearing with an inner ring speedof 1800 revmin a typical ball pass rate is

100 Hz and significant harmonics to morethan 500 Hz can be generated

Variable compliance vibration isheavily dependent on the number of rolling

elements supporting the externally appliedload the greater thenumber of loaded

rolling elements theless the vibrationFor radially loaded or

misaligned bearingslsquorunning clearancersquodetermines the extent

of the load regionand hence in generalvariable compliance

increases withclearance Runningclearance should

not be confusedwith radial internalclearance (RIC) the

former normally being lower than the RICdue to interference fit of the rings and

differential thermal expansion of the innerand outer rings during operation

Variable compliance vibration levels can

be higher than those producedby roughness and waviness ofthe rolling surfaces However

in applications where vibrationis critical it can be reduced toa negligible level by using ball

bearings with the correct level ofaxial pre-load

Geometrical imperfections

Because of the very nature

of the manufacturing processes

used to produce bearingcomponents geometricalimperfections will always bepresent to varying degrees

depending on the accuracy

Radial Load

Width of Contact

raceway

ball

h

Figure 1 Simple bearing model

Figure 2 Waviness and roughness of rolling surfaces

SURFACE ROUGHNESS

Surface roughness is a significantsource of vibration when its level is highcompared with the lubricant film thickness

generated between the rolling element-raceway contacts (seeFigure 2) Underthis condition surface asperities can break

through the lubricant film and interact withthe opposing surface resulting in metal-to-metal contact The resulting vibration

consists of a random sequence of smallimpulses which excite all the natural modesof the bearing and supporting structure

Surface roughness produces vibration

predominantly at frequencies above sixtytimes the rotational speed of the bearing Thus the high frequency part of thespectrum usually appears as a series of

resonances





















Discrete defects





Λ



Operatingregion for

mostindustrial

applcations


Regionof


withseveresliding P

e r c e n t F i l m

100

80

60

40

20

04 06 1 2 4 6 10


Raceway Waviness

Ball

Contact Width



Λ = h (σЪ2 + σr2)05













Waviness






















follows ndash



f bo = Z f co












α = Contact angle























An


























imperfections








Raceway defect

































raceway




Waviness

Discrete Defect

f r

nZf ci

plusmnf r

nZf ci

plusmnfr

OuterRaceway

Waviness

Discrete Defect

nZf co

nZf c

oplusmnf r

nZf co

plusmnf co

RollingElement

DiameterVariation

Waviness

Discrete Defect

Zf co

2nfb plusmnf co

2nfb plusmnf co














Cage defect





























An

Amplitude

Ac 2

Am 4

frequency

f c - f

m

f c

f c + f

m


























Frequency spectrum







overall vibration
























































Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001






























Discrete defects





Λ



Operatingregion for

mostindustrial

applcations


Regionof


withseveresliding P

e r c e n t F i l m

100

80

60

40

20

04 06 1 2 4 6 10


Raceway Waviness

Ball

Contact Width



Λ = h (σЪ2 + σr2)05













Waviness






















follows ndash



f bo = Z f co












α = Contact angle























An


























imperfections








Raceway defect

































raceway




Waviness

Discrete Defect

f r

nZf ci

plusmnf r

nZf ci

plusmnfr

OuterRaceway

Waviness

Discrete Defect

nZf co

nZf c

oplusmnf r

nZf co

plusmnf co

RollingElement

DiameterVariation

Waviness

Discrete Defect

Zf co

2nfb plusmnf co

2nfb plusmnf co














Cage defect





























An

Amplitude

Ac 2

Am 4

frequency

f c - f

m

f c

f c + f

m


























Frequency spectrum







overall vibration
























































Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001



























follows ndash



f bo = Z f co












α = Contact angle























An


























imperfections








Raceway defect

































raceway




Waviness

Discrete Defect

f r

nZf ci

plusmnf r

nZf ci

plusmnfr

OuterRaceway

Waviness

Discrete Defect

nZf co

nZf c

oplusmnf r

nZf co

plusmnf co

RollingElement

DiameterVariation

Waviness

Discrete Defect

Zf co

2nfb plusmnf co

2nfb plusmnf co














Cage defect





























An

Amplitude

Ac 2

Am 4

frequency

f c - f

m

f c

f c + f

m


























Frequency spectrum







overall vibration
























































Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001
































imperfections








Raceway defect

































raceway




Waviness

Discrete Defect

f r

nZf ci

plusmnf r

nZf ci

plusmnfr

OuterRaceway

Waviness

Discrete Defect

nZf co

nZf c

oplusmnf r

nZf co

plusmnf co

RollingElement

DiameterVariation

Waviness

Discrete Defect

Zf co

2nfb plusmnf co

2nfb plusmnf co














Cage defect





























An

Amplitude

Ac 2

Am 4

frequency

f c - f

m

f c

f c + f

m


























Frequency spectrum







overall vibration
























































Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001























Cage defect





























An

Amplitude

Ac 2

Am 4

frequency

f c - f

m

f c

f c + f

m


























Frequency spectrum







overall vibration
























































Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001

































Frequency spectrum







overall vibration
























































Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001






















Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001




















Raceway damage

High axial load



























































er r







CONCLUSIONS

















Wiley New York 2001
















































er r







CONCLUSIONS

















Wiley New York 2001
















CONCLUSIONS

















Wiley New York 2001











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