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A b s

t r a c t

The role of vibration monitoring in predictive maintenancePart 1: Principles and Practice

Unexpected equipment failures can be

expensive and potentially catastrophic,

resulting in unplanned production downtime,

costly replacement of parts and safety and

environmental concerns. Predictive Maintenance

(PdM) is a process of monitoring equipment as

it operates in order to identify any deterioration,

enabling maintenance to be planned and

operational costs reduced. Rolling bearings are

critical too and used extensively in rottating equipment, and when they

fail unexpectedly can result in a catastrophic failure with associated

high repair and replacement costs. Vibration based condition

monitoring (CM) can be used to detect and diagnose machine faults and

form the basis of a PdM strategy.

In this, the first part of a comprehensive two-part review which builds

on an earlier contribution on this topic, the author discusses the role of

CM in maintenance, the importance of identifying asset criticality and

justifying the economics when implementing a CM programme, and

the basic principles and techniques of monitoring and analysing the

vibration of bearings in rotating machinery. In the second part, to be

published in the next issue, he will present examples of the successful

application of these techniques to a wide range of industrial plant.

INTRODUCTION

As greater demands are beingplaced on existing assets, eitherfor higher output or increased

efficiency, the need to understand when things are starting to go wrongis becoming more important. Also, asplant and equipment becomes morecomplex and automated the need tohave a properly structured and fundedmaintenance strategy is becoming

more important. There is also a needto understand the operation of yourown equipment so that improvementsin plant output and efficiency canbe realised. In today's increasingly

on investment (ROI) of a properlyfunded maintenance strategy can bedetermined.

The need to run a plant at ahigher efficiency, yet often with fewerpeople, puts increasing pressure on

all concerned when equipment failsprematurely. When equipment does failit is often at the most inconvenient time:either in the middle of a key process, ata weekend or in the middle of the night,

when obtaining replacement parts maybe difficult and labour costs high dueto overtime. While there is never agood time for equipment to fail, withthe technology available today thereis simply no excuse for not taking thenecessary steps to protect key assets.This can be achieved by minimising

the risk of early and unexpectedfailures through a properly structuredand funded maintenance strategy

which will ultimately reduce overalloperational costs.

The cost of not having a robustmaintenance strategy should not beunderestimated. It should not be lookedat simply as an up front cost, but

viewed as an investment to safeguardand protect key assets, reducing theneed for costly repairs and protectingthe output from key processes. Insome industries maintenance is now

the second highest or even the highestelement of operating costs. As a result,in the last two or three decades it hasmoved from almost nowhere to the topof the league as a cost control priority.

The need to contain costsand run plant for longer and morereliably means that there is a growingawareness of the need to preventunnecessary equipment failures.Central to this is having a maintenancestrategy which is based on monitoringkey assets to detect when things are

starting to go wrong, enabling plantoutage to be better planned, both interms of resource availability, sparecomponents, repairs etc. As a resultthe risk of missing important contract

Dr. S. J. Lacey,

Engineering ManagerSchaeffler UK(INA FAG)

competitive world all of these issuesare of key importance and can only beachieved through a properly structuredand financed maintenance strategy

which meets the business needs.Maintenance can often be a

casualty as businesses downsize to savecosts. How often have we heard the

words 'we have had no problems since

the equipment was installed so we don't

need condition monitoring ' This is oftenborn out of ignorance and the failureto undertake a proper risk assessmentto identify the criticality of existingassets so that the potential return

36 Jan / Feb 2010 | ME | maintenance & asset management vol 25 no 1



deadlines is reduced and customerconfidence is improved.

Until recently many industrieshave taken, and still take, the reactiveapproach to maintenance, because thishas no up front costs, but it can result inmany hours or days of plant downtimeor lost production. While this mayhave been acceptable in the past, theincreasing complexity and automationof equipment has meant this is now nota cost effective option.

Having a clear and robust

maintenance strategy that is fullysupported by senior management isbecoming more important, particularlyin industries where it not only hasa major impact on costs but also onthe health and safety of employees,particularly in situations wheresecondary damage and a catastrophicfailure may result.

MAINTENANCE APPROACH

Maintenance is traditionally

performed either at fixed time intervals– so called preventive maintenance, orby corrective maintenance when abreakdown or fault actually occurs.In the latter it is often necessary toperform the maintenance actionsimmediately, but in some cases itmay be deferred, depending on thecriticality of the equipment. Withpredictive maintenance an advanced

warning is given of an impendingproblem and repairs are only carriedout when necessary and can be plannedto avoid major disruption. These three

approaches are summarised in Figure 1 and briefly discussed in the next section.

Reactive maintenance

Often referred to as the 'runtill failure' approach, this involvesfixing problems only after they occur.Of course, this is the simplest andcheapest approach in terms of upfront maintenance costs, but oftenresults in costly secondary damagealong with high costs as a result ofunplanned downtime and increased

labour and parts costs. Because thereare no up front costs it is often seen asan easy solution to many maintenanceproblems– or as having no strategy at all.

In rotating equipment, rolling

element bearings are one of the mostcritical components, both as regardstheir initial selection and, just asimportantly, in terms of how they aremaintained. Bearing manufacturersgive detailed guidelines as to what

maintenance is required and when – which is often overlooked (and whichcan have disastrous consequences interms of poor quality output, reducedplant efficiency or equipment failure).Monitoring the condition of rollingbearings is therefore essential, and

vibration based monitoring has greatpotential for detecting the early onsetof a fault.

Preventive maintenance

With Preventive Maintenance

(PM), machinery is overhauled on aregular basis regardless of the conditionof the parts. This normally involvesthe scheduling of regular machine/ plant shutdowns, whether or not they

are required. The process may reducethe incidence of unwanted failures butit also leads to increased maintenancecosts because parts are replaced whenthis is not necessarily required. Thereis also the risk of infant mortality

due to human error during the timethe asset is taken out of service forrepair, adjustment, or installation ofreplacement parts. Other risks includeinstalling a defective part, incorrectlyinstalling or damaging a replacementpart, or incorrectly reassembling parts.

With some predictivemaintenance programmes a largeproportion of the plant will notexperience failure, which can leadto a significant cost savings fromunnecessary repair work. Often a direct

result of preventive maintenance is thatmuch of the maintenance is carried out when there is nothing wrong in the firstplace. With predictive maintenancethe only costs incurred are those that

Reactive Maintenance Preventive Maintenance Predictive Maintenance

DISADVANTAGES

High risk of catastrophic failureor secondary damage. Highrepair & replacement costs

High replacement costs - partsreplaced too early

High up front costs includingequipment & training

Loss of key assets due to highdowntime. Lost production &missed contract deadlines

Risk of early failure – infantmortality. Human error duringreplacement of repaired or newparts.

Additional skills or outsourcingrequired

Inventory - high cost of spareparts or replacement equipment

Parts may often have many yearsof serviceable life remaining

High labour cost – overtime, subcontract. High cost due to hire ofequipment

Increased Health & Safety risks

Environmental concernsADVANTAGES

No up front costs i.e.

equipment, training. Seen as

an easy option

Maintenance is planned &

helps prevent unplanned

breakdowns

Risk of unexpected

breakdowns are reduced

Fewer catastrophic failures

resulting in expensive

secondary damage

Equipment life is extended

Greater control over inventory Reduced inventory & labour

costs

Maintenance can be planned &

carried out when convenient

Risk of Health & Safety &

Environmental incidents are

reduced

Opportunity to understand

why equipment has failed &

improve efficiency

Figure 1 Comparison of different types of maintenance

The role of vibration monitoring in predictive maintenance

Part 1: Principles and Practice

vol 25 no 1 maintenance & asset management | ME | Jan / Feb 2010 | 37



or are not deemed critical a reactiveor preventive approach may well beappropriate. Even if an asset does

warrant condition monitoring then adecision must be made, not only on thetechnology but also whether the asset

warrants continuous (on-line system) or

non-continuous (patrol) monitoring. Tohelp with this decision, assets are oftencategorised as falling into one of threecategories depending on their criticality(see Figure 2)1.

become necessary as a result of anadvanced warning of a problem.

Predictive maintenance

Predictive maintenance (PdM) isthe process of monitoring the conditionof machinery as it operates in order topredict what might fail. In this waymaintenance can be planned and itgives an opportunity to change onlythose parts that are showing signs ofdeterioration or damage. The basicprinciple of predictive maintenance is

to take measurements that expeditethe prediction of when parts willbreakdown and what will go wrong.These can be measurements of suchthings as machine vibration or ofplant operating data such as flow,temperature, or pressure. Continuousmonitoring detects the onset ofcomponent problems in advance so thatmaintenance is performed only whenneeded. With this type of approachunplanned downtime is reducedor eliminated and the possibility of

catastrophic failure is minimised.It allows parts to be ordered moreeffectively (reducing parts inventory)and manpower can be scheduled(increasing efficiency and reducing thecosts of overtime).

The main benefits of PdM are – Improvement of machine reliability

through the effective prediction

of equipment failures.

Reduction of maintenance costs by

minimising downtime through the

scheduling of repairs. Increase of production through

greater machine availability.

Reduction of energy consumption.

Extension of bearing service life.

Improvement of product quality

Rolling bearings are often akey element in many different typesof plant and equipment, spanningall market sectors. On one hand theycan be of a standard design, readily

available and low cost commodity itemscosting only a few pounds (e.g. thoseon electric motors, fans, gearboxes)

while on the other hand they can beof bespoke design with long lead times

and costing hundreds of thousandsof pounds (e.g. those found on WindTurbines, Steel Plants etc. ) However,they have one thing in common: if theyfail unexpectedly they can result inplant and equipment outage resultingin lost production costing from a fewthousands to many millions of pounds.With a predictive maintenance strategysuch large costs can be avoided bygiving advanced warning of a potentialproblem enabling remedial action tobe planned and taken at a convenient

time. Replacing a bearing in a gearboxis preferable to replacing the wholegearbox, or replacing a motor bearingbetter than having to send the motor toa rewinder to make expensive repairsand replace parts.

At the heart of many predictivemaintenance strategies is conditionmonitoring, which detects potentialdefects in critical components (e.g.bearings, gears etc.) at an early stage,thereby enabling the maintenanceactivity to be planned, saving both time

and money and preventing secondarydamage to equipment, which can oftenbe catastrophic.

IDENTIFYING ASSETCRITICALITY

Rolling bearings are usedextensively in almost every type ofrotating equipment whose successfuland reliable operation is very dependenton the type of bearing selected, thebearing fits, the installation andthe maintenance requirements (e.g.re-lubrication). The deterioration of

rolling bearings can result in expensiveequipment failures with high associatedcosts. Unplanned downtime, the costlyreplacement of equipment, healthand safety issues and environmentalconcerns are all potential consequencesof a maintenance strategy that failsto monitor and predict equipmentproblems before they escalate intosomething bigger. Assessing thecriticality of an asset to the overalloperation of the plant is thereforeessential in terms of determining the

type of condition monitoring requiredand whether it is necessary at all.In some cases where a plant has a

large number of low cost assets wherereplacements are readily available and/

Figure 2 Asset categorisation

Cat Description Economics

A Equipment assetsthat have a large

impact on plant

output; equipment

that represents

significant repair

costs; equipment

with significant H&S

impacts. Failures can

occur very suddenly

and not always give

advanced warning.

Failures very expensivedue to lost production,

H&S impact or

environmental impact.

Examples include

large horsepower,

large energy density

machines with very

large replacement and

maintenance costs.

Financial justification:

lost production

avoidance, reduced

maintenance costs,

protection of life andenvironment

B Equipment assets that

have lesser impact

on plant output;

equipment with

moderate repair costs;

equipment that can

have H&S related

implications if failure

occurs. Failures can

occur relatively quickly,

but usually with some

advance warning

Similar to economics

of critical equipment

assets, but of smaller

magnitude. Typical

examples include

medium horsepower

machines with

moderate replacement

and maintenance costs.

C Equipment assetsthat have little or no

direct impact on plant

output; equipment

that represents

limited repair costs;

equipment that

has minor safety

ramifications

Typically includesmaller assets with

small individual

replacement & repair

costs & little or no

costs related to lost

production. However,

collectively comprise

a large percentage of

annual maintenance

costs.

Small individual repair

& replacement costs.

Costs of failure do

not exceed costs tomonitor (or excessive

payback periods)




Assets falling in Category Aare deemed to be critical and theirfailure can result in one or more of thefollowing – Total or major interruption of the

process.

A significant safety risk, such as a fire,

toxic leak, or explosion.

Long lead times and/or significant

repair costs.

A good example of this would bethe main turbine-generator trains in

a large power plant. For such assets, itis the cost of failure that is of primaryconcern. Other examples would bethe main rotor bearing or gearboxbearings in a wind turbine. Because ofthe generally remote location, failureof these (which may lead to secondarydamage) makes costly the replacementof parts and the hire of equipment andlabour.

Category C, or non essential assets,are at the opposite end of the spectrumand the reasons for monitoring these, if

indeed they are monitored at all, mightbe to prevent failure by eliminatingroot cause and having more effectivemaintenance planning. Category B onthe other hand covers essential assetsand might, for instance, be pumps orcompressors where a standby unit isavailable in the event of a failure and

which then may become Category A.

RETURN ON INVESTMENT (ROI)

As already discussed, equipmentfailure can be expensive and potentially

catastrophic, resulting in unplanneddowntime, missed customer schedules,costly machine replacements or repairsas well as safety and environmentalconcerns. By initiating a predictivemaintenance strategy unforeseenfailures are minimised, which can yieldan impressive ROI.

Another major benefit ofintroducing CM as part of a predictivemaintenance programme is that itgenerates a greater understanding ofthe equipment critical to the process

and also allows more time to be spenton improving the overall condition ofthe assets and improving the efficiencyof key processes.

When justifying PdM the

following should be taken intoconsideration –(1) Direct costs

Labour

Normal and overtime labour for –

– planned repair activities

– unplanned repairs

Materials

Parts replaced

Machinery replaced

(2) Indirect costs

Lost production (£/hr)

Outside services

I nsurance

Parts inventory

Total annual potential cost

reduction (1 + 2)

(3) PdM programme costs

Site survey

Capital equipment

Any additional labour

Training

Initial set-up and baseline

Total annual PdM costs (3)

By contracting out your PdMthere will be no capital equipment andtraining costs and the benefits tendto be more immediate because of theuse of highly trained staff. However,it is often more beneficial to keep theactivities in house so that you becomemore familiar with your own plant,equipment and processes, whichgives benefits of not only preventing

unplanned downtime but enables moretime to be spent on mitigating potentialfailure modes and improving processefficiency.

The cement industry provides agood example of the implementationof CM reducing repair costs and lostproduction costs. The failure of a largegearbox could cause a three-weekshutdown and extensive repair costsmay be typically €50,000 to €100,000.To prevent such damages F'IS (FAGIndustrial Services, Schaeffler Group)

installed an eight-channel FAG DTECTX1 system and trained the customer'sstaff, who received three monthssupport – total cost €18,000. Detectingdeterioration of the gearbox early

resulted in a repair cost of €5000,saving the customer at least €27,000.More importantly, the company avoidedlost production amounting to around€6000/hour.

CONDITION MONITORING

Condition monitoring is a process where the condition of equipmentis monitored for early signs ofdeterioration so that the maintenanceactivity can be better planned,

reducing down time and costs. This isparticularly important for continuousprocess plants where failure anddowntime can be extremely costly.

The monitoring of vibration,temperature, voltage or current, andoil analysis are probably the mostcommonly used techniques in thisarea. Vibration is the most widely usedand not only has the ability to detectand diagnose problems but has thepotential to provide a prognosis, i.e.an indication of the remaining useful

life and possible failure mode of themachine. However, prognosis is muchmore difficult and often relies on thecontinued monitoring of the fault todetermine a suitable time when theequipment can be taken out of service(or relies on known experience withsimilar problems).

Vibration monitoring

Probably the most widelyused PdM technique, and with fewexceptions can be applied to a wide

variety of rotating equipment. Because

the mass of the rolling elements isgenerally small compared to themachine, the velocities generated areusually small and result in even smallermovements of the bearing housing,difficult for the vibration sensor todetect.

Machine vibration comesfrom many sources, e.g. bearings,gears, imbalance etc., and even smallamplitudes can have a severe effecton the overall machine vibration,depending on the transfer function,

damping and resonances (see Figure 3).Each source of vibration will have itsown characteristic frequencies whichcan be discrete frequencies or sum and/ or difference frequencies.






At low speeds it is still possible

to use vibration, but a greater degreeof care and experience is required andother techniques, such as measuringshaft displacement or Acoustic Emission(AE), may yield more meaningfulresults – although the former is notalways easy to apply. Also, while AEmay detect a change in condition it haslimited diagnostic capability.

Vibration is used successfullyon wind turbines, where the mainrotor speed is typically between 5and 30 RPM. In a wind turbine thereare two main groups of vibration

frequencies generated – gear meshand bearing defect frequencies. Thiscan result in complex vibration signals

which can make frequency analysis aformidable task. However, techniquessuch as enveloping (see later), whichhas a high sensitivity to faults thatcause impacting, can help reduce thecomplexity of the analysis. Bearingdefects can excite higher frequencies

which can be used for the basis ofdetecting incipient damage.

Vibration measurement can be

generally characterised as falling intoone of three categories, viz. detection,diagnosis and prognosis. Detectiongenerally uses the most basic formof vibration measurement, where the

overall vibration level is measured on a

broadband basis in a range, for example,of 10 to 1,000 Hz or 10 to 10,000Hz. In machines where there is little vibration other than from the bearingsthe spikiness of the vibration signal,indicated by the Crest Factor (peak/ RMS), may imply incipient defects, whereas the high energy level givenby the RMS level may indicate severedefects.

Generally speaking, other thanto the experienced operator thistype of measurement gives limitedinformation, but can be useful when

used for trending, where an increasing vibration level is an indicator of adeteriorating machine condition. Trendanalysis involves plotting the vibrationlevel as a function of time and usingthis to predict when the machine mustbe taken out of service for repair, or atleast a more in-depth survey must beperformed. Another way of using themeasurement is to compare the levels

with published vibration criteria fordifferent types of equipment.

Although broadband vibration

measurements may provide a goodstarting point for fault detection ithas limited diagnostic capability, andalthough a fault may be identifiedit may not give a reliable indication

of where the fault is, i.e. bearingdeterioration or damage, unbalance,misalignment etc. Where an improveddiagnostic capability is requiredfrequency analysis is normallyemployed, which usually gives a muchearlier indication of the development ofa fault and, secondly, the source of thefault.

Having detected and diagnoseda fault the prognosis – what is theremaining useful life and possiblefailure mode of the machine or

equipment? – is much more difficultand often relies on the continuedmonitoring of the fault to determinea suitable time when the equipmentcan be taken out of service, or relieson known experience with similarproblems.

Generally, rolling bearingsproduce very little vibration whenthey are fault free and have distinctivecharacteristic frequencies when faultsdevelop. A fault that begins as a singledefect, e.g. a spall on the raceway, is

normally dominated by impulsiveevents at the raceway pass frequency,resulting in a narrow band frequencyspectrum. As the damage worsensthere is likely to be an increase in thecharacteristic defect frequencies andsidebands, followed by a drop in theseamplitudes and an increase in thebroadband noise with considerable

vibration at shaft rotational frequency.Where machine speeds are very low thebearings generate low energy signals

which again may be difficult to detect.Also, bearings located within a gearbox

can be difficult to monitor because ofthe high energy at the gear meshingfrequencies, which can mask thebearing defect frequencies.

Overall vibration level

This is the simplest way ofmeasuring vibration and usuallyconsists of measuring the Root MeanSquare (RMS) vibration of the bearinghousing or some other point on themachine with the transducer locatedas close to the bearing as possible. This

technique involves measuring the vibration over a wide frequency range,e.g. 10 to 1,000 Hz or 10 to 10,000 Hz.The measurements can be trendedover time and compared with known

Bearings

Imbalance

Misalignment

Looseness

Gears

Belts

Rubs

Vanes or Blades

m1

m2

K 2

x(t)

F(t)

k 1

Figure 3 Simple machine model




levels of vibration, or pre-alarm andalarm levels can be set to indicatea change in the machine condition.Alternatively measurements can becompared with general standards.Although this method represents aquick and low-cost method of vibrationmonitoring, it is less sensitive toincipient defects, i.e. defects in theadvanced condition, and has a limiteddiagnostic capability. Also, it is easilyinfluenced by other sources of vibration,e.g. unbalance, misalignment, looseness,

electromagnetic vibration etc.In some situations, the Crest

Factor (Peak-to-RMS ratio) of the vibration is capable of giving an earlier warning of bearing defects. As a localfault develops this produces shortbursts of high energy which increasethe peak level of the vibration signal,but have little influence on the overallRMS level. As the fault progresses, morepeaks will be generated, until finallythe Crest Factor will reduce but theRMS vibration will increase. The main

disadvantage of this method is that inthe early stages of a bearing defect the vibration is normally low compared with other sources of vibration presentand is therefore easily influenced, soany changes in bearing condition canbe difficult to detect.

Frequency spectrum

Frequency analysis plays animportant part in the detection anddiagnosis of machine faults. In the timedomain the individual contributions, e.g.unbalance, bearing faults, gear faults,

etc., to the overall machine vibrationare difficult to identify. In the frequencydomain they become much easier toidentify and can therefore be muchmore easily related to individual sourcesof vibration.

It is not always possible to relyon the amplitude of bearing discretefrequencies to provide information ondefect severity because each machine

will have different mass, stiffness anddamping properties. Even identicalmachines could have different

system properties and this affects theamplitudes of similar sized bearingdefects. Often of real significanceis the pattern of the bearing defectfrequencies in determining the defect

severity. Study of the number ofbearing-related harmonic frequencies,frequency sidebands and characteristicfeatures within the time waveform datacan be a much more reliable methodof determining when action needs tobe taken than relying on monitoringamplitude alone.

As already discussed, a faultdeveloping in a bearing will show upas increasing vibration at frequenciesrelated to the bearing characteristicfrequencies, making detection possible

at a much earlier stage than with overall vibration.

Envelope spectrum

When a bearing starts todeteriorate the resulting time signaloften exhibits characteristic features

which can be used to detect a fault. Also,bearing condition can rapidly progressfrom a very small defect to completefailure in a relatively short periodof time, so early detection requiressensitivity to very small changes in

the vibration signature. As alreadydiscussed, the vibration signal from theearly stage of a defective bearing maybe masked by machine noise making itdifficult to detect the fault by spectrumanalysis alone.

The main advantage of envelopeanalysis is its ability to extract theperiodic impacts from the modulatedrandom noise of a deteriorating rollingbearing. This is even possible whenthe signal from the rolling bearing isrelatively low in energy and 'buried'

within other vibration from the

machine.Like any other structures with

mass and stiffness the bearing innerand outer rings have their own naturalfrequencies which are often in thekilohertz range. However, it is morelikely that the natural frequency of theouter ring will be detected due to thesmall interference or clearance fit in thehousing. If we consider a fault on theouter ring, as the rolling element hitsthe fault the natural frequency of thering will be excited and will result in a

high frequency burst of energy whichdecays and then is excited again as thenext rolling element hits the defect.In other words the resulting timesignal will contain a high frequency

component amplitude modulated at theball pass frequency of the outer ring.In practice, this vibration will be verysmall and almost impossible to detectin a raw spectrum, so a method toenhance the signal is required.

By removing the low frequencycomponents through a suitable highpass filter, rectifying and then usinga low pass filter, the envelope of thesignal is left, the frequency of whichcorresponds to the repetition rate of thedefect. This technique is often used to

detect early damage in rolling elementbearings and is also often referredto as the High Frequency ResonanceTechnique (HFRT) or EnvelopeSpectrum.

Cepstrum analysis

Vibration spectra from rotatingmachines are often very complex,containing several sets of harmonicsand also sidebands as a result of

various modulations. When trying toidentify and diagnose possible machine

faults a number of characteristics ofthe vibration signal are considered,including harmonic relationships andthe presence of sidebands. Cepstrumanalysis can simplify this, becausesingle discrete peaks in the cepstrumrepresent spacing of harmonics andsidebands in the spectrum, i.e. thecepstrum identifies periodicity withinthe spectrum. Cepstrum analysisconverts the spectrum back into thetime domain i.e. plots amplitude versustime ('quefrency') and harmonics('rhamonics').

Vibration monitoring can also beused to gain valuable information aboutthe condition of machining processes.In the manufacture of rolling bearingsgrinding of the raceways is a criticalprocess in the achievement of a highsurface finish and roundness, essentialto achieving the required service life.

Figure 4 shows cepstra ofshoe force obtained during the shoecentreless grinding of bearing outerring raceways2.

In this case, as the severity

of the dressing process increases,i.e. increasing diamond infeed, theamplitude of the first peak, 2.38 ms,increases along with the number ofrhamonics. The quefrency of 2.38 ms






corresponds with the wheel rotationalfrequency, 420 Hz., because as theseverity of the dressing operationincreases it has a significant effect on

wheel form, and hence work piecequality, and the vibration signalbecomes more highly modulated at

wheel rotational speed.

ROLLING ELEMENT BEARINGS

Rolling contact bearings areused in almost every type of rotatingmachinery, whose successful andreliable operation is very dependant

on the type of bearingselected as well as theprecision of all associatedcomponents, i.e. shaft,housing, spacers, nutsetc. Bearing engineersgenerally use fatigue asthe normal failure mode,on the assumption thatthe bearings are properlyinstalled, operated andmaintained. Today,because of improvements

in manufacturingtechnology and materials,bearing fatigue life (whichis related to sub-surfacestresses) is generally notthe limiting factor andprobably accounts for lessthan 3% of failures inservice.

Unfortunately,however, many bearingsfail prematurely in servicebecause of contamination,

poor lubrication, mis-alignment, temperatureextremes, poor fitting/ fits or unbalance. Allthese factors lead toan increase in bearing

vibration, and conditionmonitoring has been usedfor many years to detectdegrading bearings beforethey catastrophically failand incur the associatedcosts of downtime or ofsignificant damage to other

parts of the machine.Rolling element

bearings are often usedin noise sensitive applications, e.g.household appliances or electric motors,

which often use small to medium sizebearings. The elimination of bearing

vibration is therefore becomingincreasingly important from bothan environmental consideration andbecause it is synonymous with theachievement of quality.

Vibration monitoring has

now become a well accepted part ofmany PdM regimes and relies on the well known characteristic vibrationsignatures which rolling bearingsexhibit as the rolling surfaces degrade.

However, in most situations bearing vibration cannot be measured directly,and so the bearing vibration signatureis modified by the machine structure,and this situation is further complicatedby vibration from other equipment onthe machine, e.g. electric motors, gears,belts, hydraulics, structural resonancesetc. (see Figure 3). This often makes theinterpretation of vibration data difficultother than by a trained specialistand can in some situations lead to amis-diagnosis resulting in unnecessary

machine downtime and costs.

Bearing characteristic frequencies

Although the fundamentalfrequencies generated by rollingbearings can be expressed in relativelysimple formulae they cover a widerange and can interact to give verycomplex signals. This is often furthercomplicated by the presence of othersources of mechanical, structural orelectromechanical vibration on theequipment.

For a stationary outer ring androtating inner ring the fundamentalfrequencies are derived from thebearing geometry as follows: f c/o = f r /2 [1 – d/D Cos ] f c/i = f r /2 [1 + d/D Cos ] f b/o = Z f c/o

f b/i = Z f c/i

f b = D/2d f r [1 – (d/D Cos )2 ] where f r = Inner ring rotationalfrequency,

f c/o = Fundamental train (cage)frequency relative to outer ring,

f c/i = Fundamental train frequency

relative to inner ring, f b/o = Ball pass frequency of outer ring

(BPFO), f b/i = Ball pass frequency of inner ring

(BPFI), f b = Rolling element spin frequency, D = Pitch circle diameter, d = Diameter of roller elements, Z = Number of rolling elements, = Contact angle.

The bearing equations assumethat there is no sliding and that therolling elements roll over the raceway

surfaces. However, in practice this israrely the case and due to a number offactors the rolling elements undergo acombination of rolling and sliding. Inaddition, the operating contact angle

Figure 4 Cepstra of top shoe force during the internal

grinding of bearing outer r ing raceways.

(a) Diamond infeed 0.01mm/rev

(b) Diamond infeed 0.0 64mm/rev

(c) Diamond infeed 0.125mm/rev




á may be different from the nominal value. As a consequence, the actualcharacteristic defect frequencies maydiffer slightly from those predicted, butthis is very dependent on the type ofbearing, operating conditions and fits.Generally the bearing characteristicfrequencies will not be integer multiplesof the inner ring rotational frequency,

which helps to distinguish them fromother sources of vibration.

Since most vibration frequenciesare proportional to speed, it is

important when comparing vibrationsignatures that data is obtained atidentical speeds. Speed changes willcause shifts in the frequency spectrum,causing inaccuracies in both theamplitude and frequency measurement.In variable speed equipment spectralorders may sometimes be used, whereall the frequencies are normalisedrelative to the fundamental rotationalspeed. This is generally called 'ordernormalisation', where the fundamentalfrequency of rotation is called the first

order frequency.Because of the elastic propertiesof the raceway materials ball passfrequencies can be generated due to

variable compliance or as the rollingelements pass over a defect on theraceways. The frequency generated atthe outer and inner ring raceway canbe roughly estimated as 40 % (0.4)and 60% (0.6) of the inner ring speedtimes the number of rolling elementsrespectively.

Unfortunately, bearing vibrationsignals are rarely straight forward and

are further complicated by the interactionof the various component parts, but thiscan be often used to our advantage inorder to detect a deterioration or damageto the rolling surfaces.

Analysis of bearing vibrationsignals is usually complex, and thefrequencies generated will add andsubtract and are a lmost always presentin bearing vibration spectra. This isparticularly true where multiple defectsare present. However, depending uponthe dynamic range of the equipment,

background noise levels and othersources of vibration bearing frequenciescan be difficult to detect in the earlystages of a defect. Nevertheless, over the

years a number of diagnostic algorithms

have been developed to detect bearingfaults by measuring the vibrationsignatures on the bearing housing.Usually these methods take advantageof both the characteristic frequenciesand the 'ringing' frequencies (i.e. thenatural frequencies) of the bearing.

By measuring the frequenciesgenerated by a bearing it is oftenpossible not only to identify a problembut also to identify the cause. While itmay be only be necessary to identifythat a bearing is starting to deteriorate,

and to then plan the timing of itschange, a more detailed analysis ofthe vibration can often give some vitalclues as to what caused the problemin the first place. This can be furtherenhanced by inspecting the bearingafter removal from the equipment,especially when the fault has beenidentified early.

Bearing defects

A rolling contact bearing is acomplex vibration system in which

the components – i.e. rolling elements,inner raceway, outer raceway andcage – interact to generate complex

vibration signatures3. Although rollingbearings are manufactured using highprecision machine tools and understrict cleanliness and quality controls,like any other manufactured part they

will have degrees of imperfection and will generate vibration as the surfacesinteract through a combination ofrolling and sliding. Nowadays, althoughthe amplitudes of surface imperfectionsare in the order of nanometres,

vibrations can still be produced in theentire audible frequency range (20 Hzto 20 kHz).

Whereas surface roughnessand waviness result directly from thebearing component manufacturingprocesses, discrete defects refer todamage of the rolling surfaces due toassembly, contamination, operation,mounting, poor maintenance etc. Thesedefects can be extremely small anddifficult to detect and yet can have asignificant impact on vibration critical

equipment, or can result in reducedbearing life. This type of defect cantake a variety of forms: indentations,scratches along and across the rollingsurfaces, pits, debris and particles in the

lubricant. During the early developmentof the fault the vibration tends to beimpulsive but this changes as the defectprogresses and becomes larger.

The type of vibration signalgenerated depends on many factors,including the loads, internal clearance,lubrication, installation and type ofbearing. Because defects on the innerring raceway have to travel across anumber of interfaces – e.g. lubricantfilm between both inner ring racewayand rolling elements, and rolling

elements and outer ring raceway andouter ring-housing interface – they tendto be more attenuated than outer ringdefects and therefore can sometimes bemore difficult to detect.

When a defect first starts asingle spectral line can be generatedat the ball pass frequency and then, asthe defect becomes larger, it allowsmovement of the rotating shaft and theball pass frequency becomes modulatedat shaft rotational speed. Thismodulation generates a sideband at

shaft speed. As the defect increases insize more sidebands may be generated,until at some point the ball passfrequency may no longer be generated,but a series of spectral lines spaced atshaft rotational speed.

A defective rolling element maygenerate vibration at twice rotationalspeed as the defect strikes the innerand outer raceways. The vibrationproduced by a defective ball may not be

very high, or may not be generated atall, as it is not always in the load zone

when the defect strikes the raceway.

Also, as the defect contacts the cage itcan often modulate other frequencies(i.e. ball defect frequency, ball passfrequency or shaft rotational frequency)and show up as a sideband. The cagerotational frequency can be generatedin a badly worn or damaged cage. In aball bearing the rolling elements maynever generate ball rotational frequency,or twice ball rotational frequency,because of the combination of rollingand sliding and the constant changingof the ball rotational axis. In cylindrical

roller bearings the damage often occursall the way around the majority of therolling element surface, so the rollingelement rotational frequency may neverbe generated.






Variable compliance

This occurs under radial ormisaligning loads; it is an inherentfeature of rolling bearings and it iscompletely independent of quality.Radial or misaligned loads aresupported by a few rolling elementsconfined to a narrow region and theradial position of the inner ring withrespect to the outer ring depends onthe elastic deflections at the rollingelement-raceway contacts (see Figure 5).

The outer ring of the bearing is usuallysupported by a flexible housing whichgenerally has asymmetric stiffnessproperties described by the linearsprings with different stiffnesses.

rings due to raceway elastic deflections.Variable compliance vibration

is heavily dependent on the numberof rolling elements supporting theexternally applied loads; the greater thenumber of loaded rolling elements, theless the vibration. For radially loaded ormisaligned bearings 'running clearance'determines the extent of the loadregion, and hence, in general, variablecompliance increases with radialinternal clearance. A distinction is madebetween running clearance and radial

internal clearance (RIC). When fittedto a machine the former is normallyless than the RIC owing to differentialthermal expansion and interference fitof the rings. In high speed applicationsthe effect of centrifugal force shouldalso be considered.

Variable compliance vibrationlevels can exceed those produced byroughness and waviness of the rollingsurfaces. However, in applications

where vibration is critical it can bereduced to a negligible level by using

ball bearings with the correct level ofaxial pre-load.

Bearing speed ratio

The bearing speed ratio (ballpass frequency divided by the shaftrotational frequency) is a function ofthe bearing loads and clearances andcan therefore give some indication ofthe bearing operating performance.When abnormal or unsatisfactorylubrication conditions are encountered,or when skidding occurs, the bearingspeed ratio will deviate from the normal

or predicted values. If the bearing speedratio is below predicted values it mayindicate insufficient loading, excessivelubrication or insufficient bearing radialinternal clearance, which could resultin higher operating temperatures andpremature failure. Likewise, a higherthan predicted bearing speed ratio mayindicate excessive loading, excessivebearing radial internal clearance orinsufficient lubrication.

An experienced analyst cannotonly use vibration monitoring to detect

deterioration in bearing condition butalso to make an assessment of whetherthe equipment is operating satisfactorilyat initial start-up.

In electrical machines two

deep-groove radial ball bearingsare commonly used to support theshaft, one bearing fully located whilethe other is non-locating and free toslide in the housing to compensatefor axial thermal expansion of theshaft. It is not unusual for bearingsto fail catastrophically due to thermalpre-loading or cross-location as a resultof insufficient clearance betweenthe bearing outer ring and housing,resulting in the non-locating or 'floating'bearing failing to slide in the housing,

i.e. the bearings become axially loaded.The effect of this axial load

is to increase the operating contactangle, which in turn increases theball pass frequency of the outer ring.For a ball bearing the contact anglecan be estimated from the followingexpression – = Cos-1 [1 – RIC / [(2 (ro+ri-D)] ], where = Contact angle, RIC = Radial internal clearance, ro = Raceway groove radius of outer

ring,

ri = Raceway groove radius of innerring, D = Ball diameter.

Because a radial ball bearingis designed to have a radial internalclearance in the no-load condition, itcan also experience axial play. Underan axial load this results in the ball-raceway contact having an angle otherthan zero. As the bearing radial internalclearance, hence axial play, increases sodoes the contact angle. For a correctlyassembled motor under pure radial load

the contact angle will be zero and theBPFO will be given by the expression –f b/o = Zf r /2 [1 – d/D ]

On the other hand, if crosslocation occurs (outer ring not freeto move in housing) then the bearingradial internal clearance will be lost bythe relative axial movement betweenthe inner and outer rings, the bearingsbecome axially loaded and the BPFO

will increase due to the increase incontact angle. The amplitude of BPFOis likely to be small until the bearing

becomes distressed and it may notalways be possible to detect the BPFO,particularly if using a linear amplitudescale. A log or dB amplitude scale maybe better, but again care should be

Figure 5 Simple model of a bearing under

radial load

As the bearing rotates individualball loads, and hence elastic deflections,change to produce a relative movementbetween the inner and outer rings.The movement takes the form of alocus which under radial load is twodimensional and contained in a radialplane, while under mis-alignment itis three dimensional. The movementis also periodic with base frequencyequal to the rate at which the rollingelements pass through the load zone.Frequency analysis of the movement

yields the base frequency and a seriesof harmonics. So even a geometricallyperfect bearing will produce vibrationbecause of the relative periodicmovement between the inner and outer




exercised because there may be otherfrequencies which may be close to theBPFO.

A good example of how thebearing speed ratio can be used toidentify a potential problem is shownin Figure 6, which shows a vibrationacceleration spectrum measuredaxially at the drive end (DE) on the endcap of a 250 kW electric motor. Themeasurements were obtained during

that it could not movefreely in the housing asthe shaft of the motorexpanded and contracted.

Soon afterinstallation the motorfailed catastrophically.Examination of the innerring revealed that theball running path wasoffset from the centre of

the racewaytowards the

shoulder.After athoroughinvestigationof all thebearingfits, it wasconfirmedthat there wasinsufficientclearancebetween theouter ring and

housing ofthe non- locating bearing,resulting in cross location(thermal loading) which was consistent with the vibration measurementstaken prior to installation.

A number ofharmonics and sum anddifference frequencies –relating to the ball passfrequency of the outerring (233.5 Hz), the cagerotational frequency (21

Hz) and the inner ringrotational frequency –are also evident in thespectrum of Figure 6.

When the motor was rebuilt with newbearings and the correct bearing fits,the 'run-up' test was repeated prior toinstallation (see Figure 7 ).

The raw spectrum shows nocharacteristic bearing frequencies, but

when both the amplitude and frequencyscales are expanded a discrete peak at229 Hz becomes evident

[see Figure 7(b)], which matches very closely with the predicted ballpass frequency, f b/o, of the outer ring, of228.8 Hz. This motor went on to operatesuccessfully.

REFERENCES

1. Bently Nevada, Application Note, AssetCategorisation.

2. Lacey S J, Vibration monitoring of theinternal centreless grindingprocess, Part 2: experimental results,Proc. Inst. Mech. Eng., Vol 24, 1990

3. Lacey S J, An overview of bearing

vibration analysis, Schaeffler (UK)Technical Publication.

[email protected]

www.schaeffler.co.uk

IN THE NEXT ISSUE – PART II: APPLICATIONS

This second and concluding partof the paper will present examples ofthe use of vibration monitoring to detectand diagnose problems on rotatingequipment ranging from electric motorsto large crushing machines used for

mining and processing. Also included will be examples taken from the FAGWiPro Condition Monitoring Systemused for monitoring the condition of

wind turbine drive trains.



(a) Base spectrum

(a) Base spectrum

(b) Base spectrum with zoomed amplitude

(b) Base spectrum with zoomed amplitude and frequency scale

Figure 6 Axia l vibra tion acceleration spectrum at the DE on en d cap

of a 250 kW electric moto r and frequency scale

Figure 7 Axial vibration acceleration

spectrum at the DE on end cap of a 250 kW

electric motor after new bearings fitted

a 'run-up' test prior to installing in theplant.

For a nominal shaft speed of 3000RPM the calculated BPFO was 228.8 Hz,giving a bearing speed ratio of 4.576.The measured BPFO was 233.5 Hz (seeFigure 6), giving a bearing speed ratioof 4.67, an increase of 2%. The BPFO of233.5 Hz corresponds to a contact angle

of 25°, which strongly suggested thatthe Type 6217 bearing was experiencinga high axial load. The most probablecause was that the bearing had beeninstalled too tightly in the housing so

vol 25 no 1 maintenance & asset management | ME | Jan/Feb 2010 | 45

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