Table Of ContentCONDITION MONITORING SENSORSAND SYSTEMS
EskoVenalainen
Duetoincreasedspeedsofmachineryandsubstantialcostsofunexpectedshutdowns,general
interestinparametersinfluencingbearingperformancehasgreatlyincreased.Thisissuewas
extensively discussed at the Tappi Engineering conference in Boston, 1984. Further
development of data communication and computer hardware since 1984 has offered cost-
effective means to continuously monitor certain conditions of the machinery and its oil to
avoid unexpected bearing failures and to obtain more efficient and organized maintenance
workthroughscheduledshutdowns.
Sensortechnologytomeasurecertainparametershasbeenavailableforquitesometime,
but these applications have lacked the opportunity to communicate cost effectively into
centralizedcontrolrooms.Withoutpresentdatacommunication,everysensorhadtobehard-
wiredfromitslocationallthewaytothecontrolroomorannunciatorpanel.
OILFLOWSENSORS
Whilecommonintheindustry,conventional“sightglasses”cannotbeconsideredasoil
flowsensorsformonitoringpurposesbecausetheycannotprovideanyelectronicoutput.Quite
alargevarietyofoilflowmeteringorsensingdeviceshave,however,beenofferedoverrecent
years. Measurement of oil flow is rather specific and requires special attention to the
requirements of each application.This means that compromises may have to be made.An
idealoilflowmeteringdeviceorsensorshouldhavethefollowingfeatures:
• Noleaks.
• Easytomaintain,i.e.,takeapart.
• Relativelyaccurateandrepeatable.
• Easytoread,evenwithdark,dirtyoil.
• Built-invalveforflowadjustment.
• Influenceofoilviscosityshouldbeminimal.
• Reliableincontinuousoperationtoprovideflowindicationwithno“sticking”behavior.
Several types of oil flowmeter for continuous measurement of oil flows to the bearing are
available. These can be grouped basically as the two following types: (1) variable area
rotameter,and(2)meterswithacontinuouselectronicoutputsignal(on/offorcontinuous).
VariableAreaFlowMonitors
Differenttypesofvariableareameters,knownasrotametersorsimplyas“sightglasses”
are shown in Figure 1.Traditionally, they are made of different materials, including steel,
aluminum,brass,glass,andplastic.Theindicatingdeviceisafloatinsidetheflowtubewhich
canbeglass,plastic,ormetal.Connectingpartsarealwaysplasticormetal.Thewholemeter
can also be made of metal, in which case it is called an armored flowmeter, which is very
rarelyusedbecauseofitshighprice.Themajorbenefitisfiresafetyandfreedomfromleaks.
Useofplasticcomponentshasbeenincreasing.Onereasonistheircapabilitytoendure
highertemperatures;somearenowavailableforroutineuseupto150°C.Anotherreasonis
their elasticity: with proper construction plastic components can take some stress and are
relativelyeasytomakeleak-proof.
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FIGURE1.Variableareaoilflowsensors.
SensorswithContinuousElectronicOutput
Thefollowingtypesofoilflowsensorsgenerateanelectronicsignal,analogordigital,for
continuousmonitoringofoilflows.
Positive Displacement Meter.The most common of this type is the so-called oval gear
meter,developedduringWorldWarItoaccuratelydetectoilquantityinsubmarineperiscope
control.AsshowninFigure2,themeasuringelementsareovalshapedandtheflowisrotating
them.Thepistontypepositivedisplacementmeterhasalsooccasionallyappliedinlubrication.
A major drawback is the back and forth moving piston, which easily becomes a high
maintenanceitem.
Turbine Meter. Turbine meters are also applied for continuous oil flow measurement.
Theyarenotasaccurateaspositivedisplacementmeters.Whiletheyareviscositysensitive,
athigherviscositiesthesensitivityerrorisnotveryhigh,andthistypeofmeterhasbeenfound
tobegenerallyacceptable.
Gear Meter. While gear meters are also available, they have not often been used for
lubricationpurposes.Theyareveryaccurateandcanbemadetotakehighpressures.
PressureDifferentialOilFlowMeter.Inthesemeters,asillustratedinFigure2,thesharp-
edged orifice plate generates pressure differential which is proportional to the flow. It is
nonlinear,butviscosityimmune.Ifsupplypressureisconstant,thendownstreampressurecan
bemeasuredasanindicationofoilflow.
OILFLOWSENSORS PROVIDING ON/OFFANDALARM SIGNALS
Thesearetypicallyvariableareaflowmeterswithasignalfromafloatusedtoindicatethe
lack of adequate oil flow. Of on-off sensors in this category, Figure 3 illustrates the free-
floating type and shows three types of detectors used, and Figure 4 illustrates the technical
characteristicsofinductiveproximitysensors.Themaindifferencebetweenfree-floatingand
spring-loadedunits(seeFigure5)isthemounting.Thefree-floatingonehastobemounted
vertically,butthespring-loadedflowmetercanbemountedinotherpositionsaswell.
Anotherbasicdifferencebetweenthesetwotypesofsensorsistheirviscositysensitivity.
Free-floatingoilflowmetersaremoreviscositysensitivethanspring-loadedones.Thefloat
onafree-floatingunitisoftenmadewithsharpedgestoreducetheviscositysensitivity.Ifwe
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FIGURE2.Ovalandpressuredifferentialtypesofflowmeteringdevices.
FIGURE3.Differenttypesofdetectorstoprovidelowflowalarmwithfree-floatingdevices.
trytomaximizetheflow,however,thenasharpedgemayhavetoberemovedfromthefloat
andthemeteristhenmoreviscositysensitive.
Afree-floatingdevicealsocannotbecomestuck,whileaspring-loadedfloatneedssupport
from tight side clearances which introduces the possibility of getting stuck. This sticking
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FIGURE4.Electricalfunctionandcharacteristicsofaninductiveproximitysensor. d
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FIGURE5.Spring-loadedvariableareaflowmeterattop,swingingvanetypeatbottom.
behavior may cause a false indication of adequate oil flow, even when the bearing may be
starving.
An alarm arrangement with a spring-loaded float is often controlled by micro-switch.
Tightclearanceconstructionofthespring-loadedswitchcallsforcleanoiltomakethismeter
reliable.
Figure3illustratesthefollowingthreedifferentmeanstodetectthepositionofthefloat:
• Magneticreedrelay
• Infrared
• Inductiveproximitysensors
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FIGURE6.Standardovalgearmeterscoverflowrangesupto100l/min(25gpm).
Themagneticreedrelayislowincost,butitsreliabilityislessthanthatofinductiveproximity
sensors.Inductiveproximitysensorsdonotattractsteelparticleslikemagneticones.Infrared
israrelyusedbecauseitissensitivetooilcolorandcontamination.
TheinductiveproximitysensorshowninFigure4hasproventobethemostreliableand
accuratedevicetodetectthepositionoftheindicatingfloatinavariableareaflowmeter.At
the tip of the sensor, there is an electromagnetic field. Presence of the metallic float will
“overload”thiselectromagneticfieldandinthatwayprovideon/offindication.
Anotherbasicproximitysensoristhecapacitivetype.Itsfunctionsarereversed,butthe
basicoperationprincipleisthesameasthatoftheinductivesensor.
Thebasicfunctioninallthesetypesofflowalarmsistodetectthepositionofthefloator
pointerintheindicatingmechanism.Wheneverthefloatenterstheproximityofasensor,then
analarmwillbeprovided,asillustratedinFigure3.Also,aninductiveproximitysensoris
often applied as a low flow alarm in the bottom of the meter. Basic features of inductive
proximitysensorsaregiveninFigure4.
OILFLOWSENSORS PROVIDING CONTINUOUS DIGITALOR
ANALOG SIGNALS
Thesetypesofsensorswillprovideremoteindication.Alarmingwiththesesensorsisnormally
arrangedinreceivingelectronics,abasicdifferencefromthesensorsmentionedabove.For
continuousoilflowmeasurement,fourbasictypesareavailable:
• Ovalgearmetersorpistonmeters
• Gearmeters
• Turbinemeters
• Orificeplate/pressuredifferentialmeter
OvalGearMeters
AtypicalexampleofanovalgearmeterisillustratedinFigure6.Thistechnologywas
developedseveraldecadesagoforsubmarineperiscopecontrol.Itwasappliedforthefirsttime
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in lubrication applications almost 10 years ago in Finland, and today there are tens of
thousands of these meters employed successfully in oil flow measurement. The meter
generates a pulse increment proportional to flow and in practical terms provides reliable
viscosity-immune measurements. Lubrication oval gear meters typically use inductive
proximity sensors to detect rotation: in every rotation, the meter gives two pulses.Table 1
givesatabulationofdifferentsizesofovalgearmeterswiththeirpulserateperU.S.gallon
andperlitersperminute.Theovalgearmeternormallyhasacapabilityof1:10flowratio;for
instance,asize2meteriscapableoffrom200cm3/minupto2l/minatitsnormalspeedlimit
of900rpm.
GearMeter
The gear meter gives a more accurate measurement than the oval gear meter, because
gearmetersnormallygiveapulseoneachtoothofthegear.Gearmetersarealsoavailablefor
highpressurehydraulicapplicationsrangingupto9000psi.Inprinciple,thereisnodifference
between these two, except that today the oval gear meter dominates the market because of
price.Theelectronicpickupisaspecialcrystalwhichwillveryreliablypickupmagnetization
ineachtoothofthegear.Thisprovidesoutstandingaccuracyascomparedtoanyothermeter.
TurbineMeter
The turbine meter is not a positive displacement meter, although it does provide
continuous indication of the oil flow. The basic limit for a turbine meter in lubrication
applications is its sensitivity to viscosity variations above 30 cSt. There have been
developmentsandtrials,butthismeterhasnotachievedpopularityinlubricationmonitoring.
While piston-type metering devices have found some use, they are declining in popularity
becausetheyaredifficulttomaintain.
Orifice-TypeOilFlowmeter
Anothertypeofcontinuousflowmeasurementisthewell-knownandreliableorificeplate.
AsindicatedinFigure2,thesharp-edgedorificeplatewillprovidelowpressuredifferential
proportionaltotheflow.Becauseofthesharp-edgedorificeplate,itisconsideredpractically
viscosityimmune.
Sinceconstantsupplypressurecanbearranged,itispossibleinoilflowapplicationsto
useapressuregaugeonthedownstreamsideoftheorificeplateasasecondarymeasurement.
Obviously,thereisapotentialerrorifsupplypressurevaries.Oftenthisapproachisusedonly
foralarmingbymountingthepressuregaugewithanalarmcontactonthedownstreamside.
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10 CRCHandbookofLubricantsandTribology
FIGURE7.Thermowellandsensorsusedformonitoringbearingtemperatures.
OILAND BEARINGTEMPERATURESENSORS
Asmentionedbefore,excessivebearingtemperatureshavebeenacknowledgedasamain
cause or indication of bearing damage.At certain temperatures, thermal stresses occur and
bearingswillbedamagedandtheirlifeshortened.Also,athightemperatures,theviscosityand
lubricatingeffectivenessoftheoilaresignificantlyreduced.
Extraheatcancomefromseveralsources.Ifsteamispresent,thenobviouslyinsulation
ofthebearingandtheoilisveryimportant.Often,inadequatedesuperheatingcanintroduce
overheatedsteamadjacenttothebearing.
Bearings can also be overheated because of excessive friction as from overloading.
Obviously, lack of sufficient oil flow also will lead to increased friction and reduced heat
transferfromthebearing,resultinginoverheating.
To sense temperatures, several technologies are available. Heat-sensitive paints which
changecolorsatcertaintemperaturehavebeenused.Forcontinuousmonitoring,thefollowing
arepossibilities:
• Resistancetemperaturedetectors(RTDs)
• Thermocouples
• Semiconductors
• Infraredtemperaturesensors
The electrical resistance of all metals will vary as the temperature varies. For this kind of
temperaturesensor,typicallyplatinumandnickelareused.Thesesensorsaretheninsertedinto
thethermowellshowninFigure7.WhenapplyinganRTDfortemperaturemeasurement,a
varying resistance signal has to be converted into an electronic signal acceptable for the
monitoring systems. The required converter often increases the price of the system and
thereforeisnotused.
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FIGURE8.Anexampleofinfraredtemperaturedetectionsystemwithafocusedlensusedtoviewheated
surfacefrompredetermineddistanceawayfromR.F.field.
Thermocouples
Inathermocouple,thejunctionoftwodifferentmetalswillprovidealowmillivoltsignal
whichisafunctionofitstemperature.Thissignalagaincanbeconvertedintoacurrentsignal
andthenappliedtothemonitoringsystems.ThissensorhasthesamerequirementasRTD,i.e.,
itrequiresaconvertertogenerateamilliamperesignalformonitoringpurposes.
SemiconductorSensor
The semiconductor sensor utilizes a tiny transistor having an accurately known
temperaturecoefficient.Thistypeofsensorprovidesalinearsignalfrom0to150°C(32to
302°F).Agoodfeatureofsemiconductorsensorsisthatreceivingelectronics,normallydata
acquisition,willacceptthislowmilliampsignalasaninputwithoutaconverter.Inpractice,
thismeansthateliminationofconverterswillmakethistypemostcosteffective.
InfraredSensors
Infrared temperature measurement as illustrated in Figure 8 detects radiation from the
object for which temperature is being measured. The infrared thermometer can measure
temperaturefromadistancewithoutbeingincontactwiththeobject.Thesignalprovidedcan
betransmittedelectronicallyorthroughfiberoptics.ThemainadvantagesofanIRsystemare
fastdetectionoftemperaturevariation,thefactthatthesensordoesnotneedtobephysically
incontact,andhighreliabilityinahostileenvironment.Itisalsoreliableineliminatingso-
callednoiseatthepick-uppoint.Goodexamplesarebearingtemperaturemeasurementson
inductionmotorsoronheat-generatingdevicesthatcaninterferewithothertypesofsensing
devices.
OILCONTAMINATIONANALYZERS
Oilcontaminationhasaproveneffectonbearinglife.Oilcanbecontaminatedbyparticles
orbyotherliquids,typicallywater.Particlescanbeofdifferenttypes,andiftheparticlesize
is larger than the thickness of the oil film, its effect on bearing life through wear and extra
stress can be significant. Harder metal particles are known to be more damaging than
nonmetallicparticles.Sensorshavebeendevelopedforbothparticlesandwatercontamination.
WaterContamination
Watercontaminationcanbeverydamagingtoballandrollerbearings,gears,andother
machinecomponents,althoughthereisafairamountofdebateastowhatisactuallyaffected
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FIGURE9.Effectofwatercontaminationonballbearinglife.(DatafromR.E.Conlley,ASLETrans.,20,244–
248,1977.)
bywaterintheoil.Figure9givesanindicationofhowwateraffectsbearinglifeincertain
conditions.Verylowconcentrations,downto0.01%(100ppm)canreducebearinglifetoone
half.Ontheotherhand,watercontaminationhasverylittleeffectinreasonableamountson
tinbabbittbearingsinpowerplantequipment(turbines,etc.).
Whilemanytraditionalmeasurementsdonothavesufficientsensitivity,waterabsorption
of infrared light through the oil sample will provide measuring sensitivity below 100 ppm.
These units have an oil sample flow through a measuring cell between two glass surfaces,
after which the sample flow is directed back to the system. Two different wavelengths of
infrared are commonly applied, one for measuring and the other as a reference.While the
measuringwavelengthisbeingabsorbedbywatermolecules,thereferenceoneisunaffected
by water.These two signals are then amplified and processed by the monitoring system to
indicatewatercontaminationlevel.Theseunitsnormallyofferthefollowingfeatures:
• Measuringrangefrom100to5000ppm
• Accuracy±50ppm
• Outputs:analog4to20ordigitalRS422
• Alarmoutput:dryrelaycontactnormallyopenornormallyclosed
A water evaporation technique is also used. Its drawback is insensitivity for low
concentrations.Itisreliableforhighconcentrationsonly.
MetallicParticleSensors
Quite recently, sensors to detect metallic particle distribution in lubrication oil have
becomecommerciallyavailable.Theyarebasedoninductivebehaviortodetectthequantity
of contamination. They are relatively low cost and easily applicable for lubricating oil
applications.
VIBRATION SENSORS
Vibration measurement and vibration analyses can become very complex and are
discussedinalaterchapterofthishandbook.Whileveryeasytoincludetheminacontinuous
monitoringsystem,detailedcontinuousanalysison-lineinvolvesacostlysystemandisoften
difficulttojustify.Continuousmeasurementfromseveralhundredbearingsuptoafrequency
of30to60kHzrequiresverysophisticatedelectronics.
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