rolling bearing failure analysis 2010[1]

8/11/2019 Rolling Bearing Failure Analysis 2010[1]

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ROLLING BEARING FAILURES ANALYSIS

1.0 INTRODUCTION

1.1 Types and Features of Rolling Bearings

Rolling bearings generally consist of two rings, rolling elements, an a cage, an t!ey are

classifie into raial bearings or t!r"st bearings e#ening on t!e irection of t!e main loa$

In aition, e#ening on t!e ty#e of rolling elements, t!ey are classifie into ball bearings or

roller bearings, an t!ey are f"rt!er segregate by ifferences in t!eir esign or s#ecific

#"r#ose$

Rolling bearings !a%e t!e following ma&or a%antages'

()* +!eir starting tor"e or friction is low an t!e ifference between t!e starting tor"e an

r"nning tor"e is small$

(-* .it! t!e a%ancement of worlwie stanari/ation, rolling bearings are internationally

a%ailable an interc!angeable$

(0* 1aintenance, re#lacement, an ins#ection are easy beca"se t!e str"ct"re s"rro"ning

rolling bearings is sim#le$

(2* 1any rolling bearings are ca#able of ta3ing bot! raial an a4ial loas sim"ltaneo"sly or

ine#enently$

(5* Rolling bearings can be "se "ner a wie range of tem#erat"res$

(6* Rolling bearings can be #reloae to #ro"ce a negati%e clearance an ac!ie%e greater

rigiity$

F"rt!ermore, ifferent ty#es of rolling bearings !a%e t!eir own ini%i"al a%antages$

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7ee# Groo%e Ball Bearing 8 +!ese bearings are t!e most common ty#e of rolling

bearings$ +!eir "se is %ery wies#rea$ +!e raceway groo%es on bot! t!e inner an o"ter

rings !a%e circ"lar arcs of slig!tly larger rai"s t!an t!at of t!e balls$ In aition to raial

loas, a4ial loas can be im#ose in eit!er irection$ Beca"se of t!eir low tor"e, t!ey are!ig!ly s"itable for a##lications w!ere !ig! s#ees an low #ower loss are re"ire$ In

aition to o#en ty#e bearings, t!ese bearings often !a%e steel s!iels or r"bber seals

installe on one or bot! sies an are #re9l"bricate wit! grease$ Also, sna# rings are

sometimes "se on t!e #eri#!ery$ As to cages, #resse steel ones are t!e most common$

Ang"lar :ontact Ball Bearing 8 Ini%i"al bearings of t!is ty#e are ca#able of ta3ing

raial loas an also a4ial loas in one irection$ Fo"r contact angles of )5;, -5;, 0owe%er, for !ig! #recision bearings wit! a contact angle less

t!an 0


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steel o"ter ring an t!e soli ty#e !as a mac!ine o"ter ring$ +!ere are also cage an roller

assemblies wit!o"t rings$ 1ost bearings !a%e #resse steel cages, b"t some are wit!o"t

cages$

+a#ere Roller Bearing 9 Bearings of t!is ty#e "se conical rollers g"ie by a bac39face

rib on t!e cone$ +!ese bearings are ca#able of ta3ing !ig! raial loas an also a4ial loas in

one irection$ +!e rollers are increase in bot! si/e an n"mber gi%ing it an e%en !ig!er loa

ca#acity$ +!ey are generally mo"nte in #airs in a manner similar to single9row ang"lar

contact ball bearings$ In t!is case, t!e #ro#er internal clearance can be obtaine by a&"sting

t!e a4ial istance between t!e cones or c"#s of t!e two o##ose bearings$ Since t!ey are

se#arable, t!e cone assemblies an c"#s can be mo"nte ine#enently$ 7e#ening "#on t!e

contact angle, ta#ere roller bearings are i%ie into t!ree ty#es calle t!e normal angle,

mei"m angle, an stee# angle$ 7o"ble9row an fo"r9row ta#ere roller bearings are also

a%ailable$ =resse steel cages are generally "se$

S#!erical Roller Bearing 9 +!ese bearings !a%e barrel9s!a#e rollers between t!e inner

ring, w!ic! !as two raceways, an t!e o"ter ring w!ic! !as one s#!erical raceway$ Since t!e

centre of c"r%at"re of t!e o"ter ring raceway s"rface coincies wit! t!e bearing a4is, t!ey are

self9aligning in a manner similar to t!at of self9aligning ball bearings$ +!erefore, if t!ere is

eflection of t!e s!aft or !o"sing or misalignment of t!eir a4es, it is a"tomatically correcte

so e4cessi%e force is not a##lie to t!e bearings$ S#!erical roller bearings can ta3e, not only

!ea%y raial loas, b"t also some a4ial loas in eit!er irection$ +!ey !a%e e4cellent raialloa9carrying ca#acity an are s"itable for "se w!ere t!ere are !ea%y or im#act loas$ Some

bearings !a%e ta#ere bores an may be mo"nte irectly on ta#ere s!afts or cylinrical

s!afts "sing aa#ters or wit!rawal slee%es$ =resse steel an mac!ine brass cages are "se$

+!r"st Ball Bearing 9 Single9irection t!r"st ball bearings are com#ose of was!er9li3e

bearing rings wit! raceway groo%es$ +!e ring attac!e to t!e s!aft is calle t!e s!aft was!er

(or inner ring* w!ile t!at attac!e to t!e !o"sing is calle t!e !o"sing was!er (or o"ter ring*$

In o"ble9irection t!r"st ball bearings, t!ere are t!ree rings wit! t!e mile one (centre ring*

being fi4e to t!e s!aft$ +!ere are also t!r"st ball bearings wit! an aligning seat was!erbeneat! t!e !o"sing was!er in orer to com#ensate for s!aft misalignment or mo"nting error$

=resse steel cages are "s"ally "se in t!e smaller bearings an mac!ine cages in t!e larger

ones$

1.2 Bearing aterials

+!e bearing rings an rolling elements of rolling bearings are s"b&ecte to re#etiti%e !ig!

#ress"re wit! a small amo"nt of sliing$ +!e cages are s"b&ecte to tension an com#ression

an sliing contact wit! t!e rolling elements an eit!er or bot! of t!e bearing rings$

+!erefore, t!e materials "se for t!e rings, rolling elements, an cages c!aracteristics as !ig!

rolling contact, fatig"e strengt!, !ig! !arness, !ig! wear resistance, !ig! imensional

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stability an !ig! mec!anical strengt!$ Ot!er necessary c!aracteristics, s"c! as easy

#ro"ction, s!oc3 an !eat resistance, an corrosion resistance, are re"ire e#ening on

ini%i"al a##lications$

=rimarily, !ig! carbon c!romi"m bearing steel is "se for t!e bearing rings an rolling

elements$ 1ost bearings c!emical com#osition are mae of SU?- among t!e ?IS steel ty#es

a##ro4imately t!e same as AISI 5-)


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2.0 O!"R#TION#$ CONDITION

2.1 Bearings Operation

A bearing is a e%ice to allow constraine relati%e motion between two or more #arts,

ty#ically rotation or linear mo%ement$ Bearings may be classifie broaly accoring to t!emotions t!ey allow an accoring to t!eir #rinci#le of o#eration as well as by t!e irections

of a##lie loas t!ey can !anle$

A rolling9element bearing is a bearing w!ic! carries a loa by #lacing ro"n elements

between t!e two #ieces$ +!e relati%e motion of t!e #ieces ca"se t!e ro"n elements to roll

wit! %ery little rolling resistance an wit! little sliing$

A rolling9element rotary bearing "ses a s!aft in a m"c! larger !ole, an cyliners calle

rollers tig!tly fill t!e s#ace between t!e s!aft an !ole$ As t!e s!aft t"rns, eac! roller acts as

t!e logs$ >owe%er, since t!e bearing is ro"n, t!e rollers ne%er fall o"t from "ner t!e loa$

Bearings are "se in a wie range of a##lications an t!e esign life %aries wit! s#ecific

a##lications an o#erating conitions$ Below gi%es a %ario"s bearing a##lications'

)* Infre"ently "se or only for s!ort #erios 9 small motors for !ome a##liances

(%ac""m cleaners an was!ing mac!ines*, !an #ower tools an agric"lt"ral

e"i#ment$

-* Use only occasionally b"t reliability is im#ortant 9 motors for !ome (!eaters an air

conitioners*, constr"ction e"i#ment, con%eyors an ele%ator cable s!ea%es$

0* Use intermittently for relati%ely long #erios 9 rolling mill roll nec3s, small motors,

ec3 cranes, general cargo, cranes, #inion stans, #assenger cars, factory motors,

mac!ine tools, transmissions, %ibrating screens, cr"s!ers, crane s!ea%es, com#ressors

an s#eciali/e transmissions$

2* Use intermittently for more t!an eig!t !o"rs aily 8 escalators, centrif"gal

se#arators, air conitioning e"i#ment, blowers, woowor3ing mac!ines, large

motors, a4le bo4es on railway rolling stoc3, mine !oists, #ress flyw!eels, railway

traction motors, locomoti%e a4le bo4es an #a#er ma3ing mac!ines$

5* Use contin"o"sly an !ig! reliability is im#ortant 9 waterwor3s #"m#s, electric

#ower stations an mine raining #"m#s$

2.2 Bearing Failures

Bearings are among t!e most im#ortant com#onents in t!e %ast ma&ority of mac!ines an

e4acting emans are mae "#on t!eir carrying ca#acity an reliability$ +!erefore it is "ite

nat"ral t!at rolling bearings s!o"l !a%e come to #lay s"c! a #rominent #art an t!at o%er t!e

years t!ey !a%e been t!e s"b&ect of e4tensi%e researc!$ Inee rolling bearing tec!nology !as

e%elo#e into a #artic"lar branc! of science$

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Unfort"nately it sometimes !a##ens t!at a bearing oes not attain its calc"late rating life$

+!ere may be many reasons for t!is 8 !ea%ier loaing t!an !as been antici#ate, inae"ate

or "ns"itable l"brication, careless !anling, ineffecti%e sealing, or fits t!at are too tig!t, wit!

res"ltant ins"fficient internal bearing clearance$ Eac! of t!ese factors #ro"ces its own

#artic"lar ty#e of amage an lea%es its own s#ecial im#rint on t!e bearing$ :onse"ently, bye4amining a amage bearing, it is #ossible, in t!e ma&ority of cases, to form an o#inion on

t!e ca"se of t!e amage an to ta3e t!e re"isite action to #re%ent a rec"rrence$

2.% & Bearing Failures and T'eir Causes

.!en a rolling bearing rotates "ner loa t!e contacting s"rfaces of t!e rolling elements an

t!e raceways normally become somew!at "ll in a##earance$ +!is is no inication of wear in

t!e "s"al sense of t!e wor an is of no significance to t!e bearing life$ +!e "ll s"rface in an

inner or o"ter ring raceway forms a #attern calle t!e #at! #attern$ +!is #attern %aries in

a##earance accoring to t!e rotational an loaing conitions$ By e4amining t!e #at!

#atterns in a ismantle bearing t!at !as been in ser%ice, it is #ossible to gain a goo iea of

t!e conitions "ner w!ic! t!e bearing !as o#erate$ By learning to isting"is! between

normal an abnormal #at! #atterns t!ere is e%ery #ros#ect of being able to assess correctly

w!et!er t!e bearing !as r"n "ner t!e #ro#er conitions$

+!e following series of fig"res ill"strates normal #at! #atterns "ner ifferent rotational an

loaing conitions as well as ty#ical #atterns res"lting from abnormal wor3ing conitions$

In t!e ma&ority of cases t!e amage to t!e bearing originates wit!in t!e confines of t!e #at!

#atterns an, once t!eir significance !as been learne, t!e a##earance an location of t!e

#atterns #ro%e to be "sef"l ais in iagnosing t!e ca"se of t!e amage$ 7ee# groo%e ball

bearings an t!r"st ball bearings !a%e been "se for ill"strati%e #"r#oses as t!ey is#lay s"c!

c!aracteristic #at! #atterns$ >owe%er, t!e fig"res are a##licable, wit! some moifications, to

ot!er ty#es of bearing as well$

Uni9irectional raial loa$ Rotating inner ring 8 fi4e o"ter ring$ Inner

ring' #at! #attern "niform in wit! #ositione in t!e centre an e4tene aro"n t!e entirecirc"mference of t!e raceway$ O"ter ring' #at! #attern wiest in t!e loa irection an

ta#ere off towars t!e ens$ .it! normal fits an normal internal clearance, t!e #attern

e4tens aro"n slig!tly less t!an !alf t!e circ"mference of t!e raceway$

Uni9irectional raial loa$ Fi4e inner ring 8 rotating o"ter ring$ Inner

ring' #at! #attern wiest in t!e loa irection an ta#ere off towars t!e ens$ .it! normal

fits an normal internal clearance, t!e #attern e4tens aro"n slig!tly less t!an !alf t!e

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circ"mference of t!e raceway$ O"ter ring' #at! #attern "niform in wit! #ositione in t!e

centre an e4tene aro"n t!e entire circ"mference of t!e raceway$

Raial loa rotating in #!ase wit! t!e inner ring$ Rotating inner ring 8

fi4e o"ter ring$ Inner ring' #at! #attern wiest in t!e loa irection an ta#ere off towars

t!e ens$ .it! normal fits an normal internal clearance, t!e #attern e4tens aro"n slig!tly

less t!an !alf t!e circ"mference of t!e raceway$ O"ter ring' #at! #attern "niform in wit!,

#ositione in t!e centre an e4tene aro"n t!e entire circ"mference of t!e raceway$

Raial loa rotating in #!ase wit! t!e o"ter ring$ Fi4e inner ring 8

rotating o"ter ring$ Inner ring' #at! #attern "niform in wit! #ositione in t!e centre an

e4tene aro"n t!e entire circ"mference of t!e raceway$ O"ter ring' #at! #attern wiest in

t!e loa irection an ta#ere off towars t!e ens$ .it! normal fits an normal internal

clearance, t!e #attern e4tens aro"n slig!tly less t!an !alf t!e circ"mference of t!e raceway$

Uni9irectional a4ial loa$ Rotating inner or o"ter ring$ Inner an o"ter

rings' #at! #attern "niform in wit! e4tene aro"n t!e entire circ"mference of t!e

raceways of bot! rings an laterally is#lace$

:ombination of "ni9irectional raial an a4ial loas$ Rotating inner

ring 8 fi4e o"ter ring$ Inner ring' #at! #attern "niform in wit! e4tene aro"n t!e entire

circ"mference of t!e raceway an laterally is#lace$ O"ter ring' #at! #attern e4tenearo"n t!e entire circ"mference of t!e raceway an laterally is#lace$ +!e #attern is wiest

in t!e irection of t!e raial loaing$

Uni9irectional a4ial loa$ Rotating s!aft was!er 8 fi4e !o"sing

was!er$ S!aft an !o"sing was!ers' #at! #attern "niform in wit!, e4tene aro"n t!e entire

circ"mference of t!e raceways of bot! was!ers$

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Uni9irectional raial loa C imbalance$ Rotating inner ring 8 cree#ing

o"ter ring$ Inner an o"ter rings' #at! #attern "niform in wit!, e4tene aro"n t!e entirecirc"mference of t!e raceways of bot! rings$

Fits too tig!t 8 #reloaing$ Uni9irectional raial loa$ Rotating inner

ring 8 fi4e o"ter ring$ Inner ring' #at! #attern "niform in wit! #ositione in t!e centre an

e4tene aro"n t!e entire circ"mference of t!e raceway$ O"ter ring' #at! #attern #ositione

in t!e centre an e4tene aro"n t!e entire circ"mference of t!e raceway$ +!e #attern is

wiest in t!e irection of t!e raial loaing$

O%al com#ression of o"ter ring$ Rotating inner ring 8 fi4e o"ter ring$

Inner ring' #at! #attern "niform in wit! #ositione in t!e centre an e4tene aro"n t!e

entire circ"mference of t!e raceway$ O"ter ring' #at! #attern #ositione in two iametrically

o##ose sections of t!e raceway$ +!e #attern is wiest w!ere t!e #inc!ing !as occ"rre$

O"ter ring misaligne$ Rotating inner ring 8 fi4e o"ter ring$ Inner

ring' #at! #attern "niform in wit! #ositione in t!e centre an e4tene aro"n t!e entire

circ"mference of t!e raceway$ O"ter ring' #at! #attern in two iametrically o##ose sections,

is#lace iagonally in relation to eac! ot!er$

Inner ring misaligne$ Rotating inner ring 8 fi4e o"ter ring$ Inner ring'

#at! #attern in two iametrically o##ose sections, is#lace iagonally in relation to eac!

ot!er$ O"ter ring' #at! #attern wiest in t!e loa irection an ta#ere off towar t!e ens$

+!e internal clearance is re"ce on acco"nt of t!e misalignment of t!e inner ringD t!e lengt!

of t!e #at! #attern e#ens "#on t!e magnit"e of t!e internal clearance re"ction$

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>o"sing was!er #ositione eccentrically relati%e to s!aft was!er$

Rotating s!aft was!er 8 fi4e !o"sing was!er$ S!aft was!er' #at! #attern "niform in wit!,e4tene aro"n t!e entire circ"mference of t!e raceway$ >o"sing was!er' #at! #attern

e4tene aro"n t!e entire circ"mference of t!e raceway an off9centre relati%e to raceway$

>o"sing was!er misaligne$ Rotating s!aft was!er 8 fi4e !o"sing

was!er$ S!aft was!er' #at! #attern "niform in wit!, e4tene ro"n t!e entire circ"mference

of t!e raceway$ >o"sing was!er' #at! #attern in t!e centre of t!e raceway b"t wier aro"n

#art of its circ"mference$

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%.0 F#I$UR" #N#$()I)

%.1 Types of Bearing Da*age

Eac! of t!e ifferent ca"ses of bearing fail"re #ro"ces its own c!aracteristic amage$ S"c!

amage, 3nown as #rimary amage, gi%es rise to seconary, fail"re9in"cing amage 8fla3ing an crac3s$ E%en t!e #rimary amage may necessitate scra##ing t!e bearings on

acco"nt of e4cessi%e internal clearance, %ibration, noise, an so on$ A faile bearing

fre"ently is#lays a combination of #rimary an seconary amage$

+!e ty#es of amage may be classifie as follows'

=rimary amage 8 wear, inentations, smearing, s"rface istress, corrosion an electric

c"rrent amage$

Seconary amage 8 fla3ing an crac3s$

%.1.1 & +ear

In normal cases t!ere is no a##reciable wear in rolling bearings$ .ear may, !owe%er, occ"r as

a res"lt of t!e ingress of foreign #articles into t!e bearing or w!en t!e l"brication is

"nsatisfactory$ ibration in bearings w!ic! are not r"nning also gi%es rise to wear$

%.1.1.1 +ear Cause ,y #,rasi-e !artile

Small, abrasi%e #articles, s"c! as grit or swarf t!at !a%e entere t!e bearing by some means

or ot!er, ca"se wear of raceways, rolling elements an cage$ +!e s"rfaces become "ll to a

egree t!at %aries accoring to t!e coarseness an nat"re of t!e abrasi%e #articles$ Sometimes

worn #articles from brass cages become %erigrise an t!en gi%e lig!t9colo"re grease a

greenis! !"e$

+!e "antity of abrasi%e #articles gra"ally increases as material is worn away from t!e

r"nning s"rfaces an cage$ +!erefore t!e wear becomes an accelerating #rocess an in t!e

en t!e s"rfaces become worn to s"c! an e4tent as to rener t!e bearing "nser%iceable$

+!e abrasi%e #articles may !a%e entere t!e bearing beca"se t!e sealing arrangement was not

s"fficiently effecti%e for t!e o#erating conitions in%ol%e$ +!ey may also !a%e entere wit!

contaminate l"bricant or "ring t!e mo"nting o#eration$

O"ter ring of a s#!erical roller bearing wit! raceways t!at !a%e been worn by

abrasi%e #articles$ It is easy to feel w!ere t!e i%iing line goes between worn an "nworn

sections$

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%.1.1.2 +ear Cause ,y Inade/uate $u,riation

If t!ere is not s"fficient l"bricant, or if t!e l"bricant !as lost its l"bricating #ro#erties, it is not

#ossible for an oil film wit! s"fficient carrying ca#acity to form$ 1etal to metal contact

occ"rs between rolling elements an raceways$

In its initial #!ase, t!e res"ltant wear !as ro"g!ly t!e same effect as la##ing$ +!e #ea3s of t!e

microsco#ic as#erities, t!at remain after t!e #ro"ction #rocesses, are torn off an, at t!e

same time, a certain rolling9o"t effect is obtaine$ +!is gi%es t!e s"rfaces concerne a

%arying egree of mirror9li3e finis!$ At t!is stage s"rface istress can also arise$

If t!e l"bricant is com#letely "se "#, t!e tem#erat"re will rise ra#ily$

+!e !arene material t!en softens an t!e s"rfaces ta3e on bl"e to brown !"es$ +!e

tem#erat"re may e%en become as !ig! as to ca"se t!e bearing to sei/e$

:ylinrical roller wit! mirror9li3e s"rface on acco"nt of l"bricant star%ation$

O"ter ring of a s#!erical roller bearing t!at !as not been ae"ately l"bricate$

+!e raceways !a%e a mirror finis!$

%.1.1.% +ear Cause ,y i,ration

.!en a bearing is not r"nning, t!ere is no l"bricant film between t!e rolling elements an t!e

raceways$ +!e absence of l"bricant film gi%es metal to metal contact an t!e %ibrations

#ro"ce small relati%e mo%ements of rolling elements an rings$ As a res"lt of t!ese

mo%ements, small #articles brea3 away from t!e s"rfaces an t!is leas to t!e formation of

e#ressions in t!e raceways$ +!is amage is 3nown as false brinelling, sometimes also

referre to as was! boaring$ Balls #ro"ce s#!ere ca%ities w!ile rollers #ro"ce fl"ting$

In many cases, it is #ossible to iscern re r"st at t!e bottom of t!e e#ressions$ +!is is

ca"se by o4iation of t!e etac!e #articles, w!ic! !a%e a large area in relation to t!eir%ol"me, as a res"lt of t!eir e4#os"re to air$ +!ere is ne%er any %isible amage to t!e rolling

elements$

+!e greater t!e energy of %ibration, t!e more se%ere t!e amage$ +!e #erio of time an t!e

magnit"e of t!e bearing internal clearance also infl"ence e%elo#ments, b"t t!e fre"ency of

t!e %ibrations oes not a##ear to !a%e any significant effect$

Roller bearings !a%e #ro%e to be more s"sce#tible to t!is ty#e of amage t!an ball bearings$

+!is is consiere to be beca"se t!e balls can roll in e%ery irection$ Rollers, on t!e ot!er

!an, only roll in one irectionD mo%ement in t!e remaining irections ta3es t!e form ofsliing$ :ylinrical roller bearings are t!e most s"sce#tible$

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+!e fl"ting res"lting from %ibrations sometimes closely resembles t!e fl"ting #ro"ce by t!e

#assage of electric c"rrent$ >owe%er, in t!e latter case t!e bottom of t!e e#ression is ar3 in

colo"r, not brig!t or corroe$

+!e amage ca"se by electric c"rrent is also isting"is!able by t!e fact t!at t!e rolling

elements are mar3e as well as t!e raceways$

Bearings wit! %ibration amage are "s"ally fo"n in mac!ines t!at are not in o#eration an

are sit"ate close to mac!inery #ro"cing %ibrations$ E4am#les t!at can be cite are

transformer fans, stan9by generators an s!i#s a"4iliary mac!inery$ Bearings in mac!ines

trans#orte by rail, roa or sea may be s"b&ect to %ibration amage too$

O"ter ring of ta#er roller bearing amage by %ibration "ring o#eration$

ibration amage to t!e ring of cyliner roller bearing$ +!e amage !as arisen w!ile

t!e bearing was not r"nning$ It is e%ient, from t!e fainter fl"ting iscernible between t!e

#rono"nce e#ressions wit! corrosion at t!e bottom, t!at t!e ring !as c!ange #osition for

s!ort #erios$

.!ere mac!ines s"b&ect to constant %ibration are concerne, it is essential t!at t!e ris3 ofamage to t!e bearings be ta3en into consieration at t!e esign stage$ :onse"ently, w!ere

#ossible, ball bearings s!o"l be selecte instea of roller bearings$ +!e ability of ball

bearings to wit!stan %ibrations wit!o"t being amage can also be consierably im#ro%e

by a##lying a4ial #reloaing wit! t!e ai of s#rings$ An oil bat!, in w!ic! all rolling elements

in t!e loa /one are immerse in t!e oil, !as also #ro%e to #ro%ie satisfactory #rotection$ A

%ibration9am#ing base !el#s to #re%ent amage too$

+!e bearings in mac!ines t!at are to be trans#orte can be #rotecte by loc3ing t!e s!aft, t!"s

#re%enting t!e small mo%ements t!at !a%e s"c! a amaging effect on t!e bearings$

Inner an o"ter ring of a cylinrical roller bearing e4#ose to %ibration$ +!e

inner ring !as c!ange #osition$

12


13/24

S#ring loaing a ee# groo%e ball bearing to #re%ent %ibration amage$

O"ter ring of a self9aligning ball bearing amage by %ibration$ +!e bearing

!as not rotate at all$

%.1.2 Indentations

Raceways an rolling elements may become ente if t!e mo"nting #ress"re is a##lie to t!e

wrong ring, so t!at it #asses t!ro"g! t!e rolling elements, or if t!e bearing is s"b&ecte to

abnormal loaing w!ile not r"nning$ Foreign #articles in t!e bearing also ca"se inentations$

%.1.2.1 Indentation Cause ,y Faulty ounting or O-erloading

+!e istance between t!e ents is t!e same as t!e rolling element s#acing$ Ball bearings are

#rone to inentations if t!e #ress"re is a##lie in s"c! a way t!at it #asses t!ro"g! t!e balls

"ring t!e mo"nting or ismo"nting o#erations$ Self9aligning ball bearings are #artic"larly

s"sce#tible to amage in s"c! circ"mstances$ In s#!erical roller bearings t!e amage

originates as smearing an s"bse"ently, if t!e #ress"re increases, e%elo#s into a ent$ +!e

same conitions a##ly in ta#er roller bearings t!at are "n"ly #reloae wit!o"t being

rotate$

Bearings t!at are mo"nte wit! e4cessi%ely !ea%y interference fits, an bearings wit! ta#ere

bore t!at are ri%en too far "# t!e s!aft seating or slee%e, also become ente$

.as!er of a t!r"st ball bearing s"b&ecte to o%erloaing w!ile not r"nning$ +!e

inentations are narrow an raially aligne, not s#!ere as in raial ball bearings$

A #eri#!ery camera %iew of t!e roller s!ows two iametrically o##ose

inentations$ +!e roller !as, in t"rn, ente t!e inner ring raceway$

13


14/24

%.1.2.2 Indentation Cause ,y Foreign !artiles

Foreign #articles, s"c! as swarf an b"rrs, w!ic! !a%e gaine entry into t!e bearing ca"se

inentations w!en rolle into t!e raceways by t!e rolling elements$ +!e #articles #ro"cing

t!e inentations nee not e%en be !ar$ +!in #ieces of #a#er an t!rea from cotton waste

an clot! "se for rying may be mentione as instances of t!is$ Inentations ca"se by t!ese

#articles are in most cases small an istrib"te all o%er t!e raceways$

Inentations ca"se by irt, in one of t!e raceways of a roller bearing 8 5<

magnifications$

%.1.% )*earing

.!en two inae"ately l"bricate s"rfaces slie against eac! ot!er "ner loa, material is

transferre from one s"rface to t!e ot!er$ +!is is 3nown as smearing an t!e s"rfaces

concerne become score, wit! a Htorn a##earance$ .!en smearing occ"rs, t!e material is

generally !eate to s"c! tem#erat"res t!at re9!arening ta3es #lace$ +!is #ro"ces localise

stress concentrations t!at may ca"se crac3ing or fla3ing$

In rolling bearings, sliing #rimarily occ"rs at t!e roller en9g"ie flange interfaces$

Smearing may also arise w!en t!e rollers are s"b&ecte to se%ere acceleration on t!eir entry

into t!e loa /one$ If t!e bearing rings rotate relati%e to t!e s!aft or !o"sing, t!is may also

ca"se smearing in t!e bore an on t!e o"tsie s"rface an ring faces$

In t!r"st ball bearings, smearing may occ"r if t!e loa is too lig!t in relation to t!e s#ee of

rotation$

%.1.%.1 & )*earing of Roller "nds and uide Flanges

In cylinrical an ta#er roller bearings, an in s#!erical roller bearings wit! g"ie flanges,

smearing may occ"r on t!e g"iing faces of t!e flanges an t!e ens of t!e rollers$ +!is

smearing is attrib"table to ins"fficient l"bricant between flanges an rollers$ It occ"rs w!en a

!ea%y a4ial loa acts in one irection o%er a long #erio, for instance w!en ta#er rollerbearings are s"b&ect to e4cessi%e #reloaing$ In cases w!ere t!e a4ial loa c!anges irection,

smearing is m"c! less common as t!e o##ort"nity is #ro%ie for t!e ingress of l"bricant

w!en t!e roller en is tem#orarily relie%e of loa$ S"c! smearing can be a%oie to a

consierable e4tent by selecting a s"itable l"bricant$

Smearing on t!e s"rface of a roller from a s#!erical roller bearing 8 )


15/24

A cylinrical roller wit! en smearing ca"se by !ea%y a4ial loaing an im#ro#er

l"brication$

G"ie flange smearing attrib"table$

%.1.%.2 & )*earing of Rollers and Raeays

In certain circ"mstances, smearing may occ"r on t!e s"rface of rollers an in raceways of

s#!erical an cylinrical roller bearings$ +!is is ca"se by roller rotation being retare in t!e

"nloae /one, w!ere t!e rollers are not ri%en by t!e rings$ :onse"ently t!eir s#ee of

rotation is lower t!an w!en t!ey are in t!e loae /one$ +!e rollers are t!erefore s"b&ecte to

ra#i acceleration an t!e res"ltant sliing is so se%ere t!at in may #ro"ce smearing$

S3i smearing in bot! raceways of a s#!erical roller bearing o"ter ring$

%.1.%.% & Raeay )*earing at Inter-als Corresponding to t'e Roller )paing

Far too often, w!en cylinrical roller bearings are being mo"nte, t!e ring wit! t!e roller an

cage assembly is entere as3ew, wit!o"t being rotate$ +!e rollers t!en scratc! t!e raceway

of t!e ot!er ring, ca"sing smearing in t!e form of long, trans%erse strea3s$ +!e rollers may be

smeare too$ +!is ty#e of amage can be a%oie if t!e bearing is well l"bricate an one of

t!e rings is rotate$ .!en large n"mbers of bearings are to be mo"nte it is e4#eient to

em#loy a mo"nting ring$ Similar amage may arise if t!e bearing rings are mo"nte wit! fits

t!at are too tig!t in relation to t!e internal clearance, so t!at #reloaing occ"rs$

Smear strea3s may also be fo"n in t!e raceways of s#!erical an ta#er roller bearings$ +!ese

strea3s are t!e res"lt of careless !anling or incorrect mo"nting #ractice$ Blows or !ea%y

#ress"re a##lie to t!e wrong ring, wit!o"t rotating t!e bearing, ca"se t!e rollers to #ro"ce

narrow, trans%erse strea3s of smearing in t!e raceways$

A cylinrical roller bearing wit! smear strea3s in t!e inner ring raceway an

on t!e rollers$ +!e smearing !as been ca"se by t!e roller assembly being entere as3ew

wit!o"t being rotate$

15


16/24

One of t!e smear strea3s 5


17/24

formation of s"b9s"rface fatig"e crac3s an t!"s s!orten t!e life of t!e bearing$ If t!e

l"brication remains satisfactory t!ro"g!o"t, i$e$ t!e l"bricant film oes not become too t!in

beca"se of l"bricant star%ation or %iscosity c!anges in"ce by t!e rising tem#erat"re or on

acco"nt of e4cessi%e loaing, t!ere is no ris3 of s"rface istress$

S"rface istress in t!e form of a ban encircling a roller from a s#!erical roller bearing$

%.1.5 & Corrosion

R"st will form if water or corrosi%e agents reac! t!e insie of t!e bearing in s"c! "antities

t!at t!e l"bricant cannot #ro%ie #rotection for t!e steel s"rfaces$ +!is #rocess will soon lea

to ee# seate r"st$ Anot!er ty#e of corrosion is fretting corrosion$

%.1.5.1 & Deep )eated Rust

A t!in #rotecti%e o4ie film forms on clean steel s"rfaces e4#ose to air$ >owe%er, t!is film

is not im#enetrable an if water or corrosi%e elements ma3e contact wit! t!e steel s"rfaces,

#atc!es of etc!ing will form$ +!is e%elo#ment soon leas to ee# seate r"st$ 7ee# seate

r"st is a great anger to bearings since it can initiate fla3ing an crac3s$ Aci li"is corroe

t!e steel "ic3ly, w!ile al3aline sol"tions are less angero"s$ +!e salts t!at are #resent in

fres! water constit"te, toget!er wit! t!e water, an electrolyte w!ic! ca"ses gal%anic

corrosion, 3nown as water etc!ing$ Salt water, s"c! as sea water, is t!erefore !ig!ly

angero"s to bearings$

7ee# seate r"st in t!e o"ter ring of a cylinrical roller bearing$ E4tensi%e water

etc!ing on t!e inner ring of a s#!erical roller bearing$

%.1.5.2 & Fretting Corrosion

If t!e t!in o4ie film is #enetrate, o4iation will #rocee ee#er into t!e material$ An

instance of t!is is t!e corrosion t!at occ"rs w!en t!ere is relati%e mo%ement between bearing

ring an s!aft or !o"sing, on acco"nt of t!e fit being too loose$ +!is ty#e of amage is callefretting corrosion an may be relati%ely ee# in #laces$ +!e relati%e mo%ement may also

ca"se small #articles of material to become etac!e from t!e s"rface$ +!ese #ar9ticles

o4iise "ic3ly w!en e4#ose to t!e o4ygen in t!e atmos#!ere$

As a res"lt of t!e fretting corrosion, t!e bearing rings may not be e%enly s"##orte an t!is

!as a etrimental effect on t!e loa istrib"tion in t!e bearings$ R"ste areas also act as

fract"re notc!es$

17


18/24


19/24

+!e ca"ses of #remat"re fla3ing may be !ea%ier e4ternal loaing t!an !a been antici#ate,

#reloaing on acco"nt of incorrect fits or e4cessi%e ri%e9"# on a ta#ere seating, o%al

istortion owing to s!aft or !o"sing seating o"t9of9ro"nness, a4ial com#ression, for instance

as a res"lt of t!ermal e4#ansion$ Fla3ing may also be ca"se by ot!er ty#es of amage, s"c!

as inentations, ee# seate r"st, electric c"rrent amage or smearing$

Fla3e cone an rollers of ta#er roller bearing$ >ea%y loaing an inae"ate

l"brication are t!e ca"ses of t!is amage$

O"ter ring of a self9aligning ball bearing t!at !as been ri%en too far "# its

ta#ere seating$ +!e bearing !a only #erforme a few re%ol"tions w!en t!e amage was

etecte$ Fla3ing wo"l soon !a%e occ"rre if t!e bearing !a been #"t into ser%ice$

Fla3ing in t!e o"ter ring o"ter ring of s#!erical roller bearing t!at !as been

mo"nte in a !o"sing wit! o%al bore$

Fla3e inner ring of a s#!erical roller bearing$ +!e e4tent of t!e fla3ing aro"n one

entire raceway inicates t!at t!e a4ial loa !as been %ery !ea%y in relation to t!e raial loa$

:ylinrical roller bearing inner ring wit! fla3ing at one sie of t!e raceway, as a res"lt

of o%erloaing "e to misalignment$

Fla3ing (t!e ar3 area* initiate by t!e a&acent inentations 8 )


20/24

Inner ring of a cylinrical roller bearing wit! e4tensi%e fla3ing ca"se by smearing of

t!e 3in e#icte$

Fla3ing originating from ee# seate r"st on t!e roller of a s#!erical roller bearing$

%.1.; & Cra8s

:rac3s may form in bearing rings for %ario"s reasons$ +!e most common ca"se is ro"g!

treatment w!en t!e bearings are being mo"nte or ismo"nte$ >ammer blows, a##lie irect

against t!e ring or %ia a !arene c!isel, may ca"se fine crac3s to form, wit! t!e res"lt t!at

#ieces of t!e ring brea3 off w!en t!e bearing is #"t into ser%ice$ E4cessi%e ri%e "# on ata#ere seating or slee%e is anot!er ca"se of ring crac3ing$ +!e tensile stresses, arising in t!e

rings as a res"lt of t!e e4cessi%e ri%e9"#, #ro"ce crac3s w!en t!e bearing is #"t into

o#eration$ +!e same res"lt may be obtaine w!en bearings are !eate an t!en mo"nte on

s!afts man"fact"re to t!e wrong tolerances$ +!e smearing escribe in an earlier section

may also #ro"ce crac3s at rig!t angles to t!e irection of slie$

:rac3s of t!is 3in #ro"ce fract"res rig!t across t!e rings$ Fla3ing, t!at !as occ"rre for

some reason or ot!er, acts as a fract"re notc! an may lea to crac3ing of t!e bearing ring$

+!e same a##lies to fretting corrosion$

Fract"re o"ter ring of a self9aligning ball bearing$ +!e inentations %isible at

t!e bottom ege of t!e ring were ca"se by ro"g! treatment an t!e crac3 originate at one of

t!ese inentations$

Inner ring of a s#!erical roller bearing wit! o"ter flange fract"re #ro"ce by irect

!ammering$

S#!erical roller bearing inner ring t!at !as crac3e rig!t across following smearing of

one face$ +!e ring !as been mo"nte to ab"t a s#acer t!at !as not !a a s"fficiently tig!t fit

on t!e s!aft$ :onse"ently t!e s#acer !as rotate relati%e to t!e s!aft an t!e bearing ring$

20


21/24

S#!erical roller bearing inner ring wit! fretting corrosion an a trans%erse crac3 rig!t

t!ro"g! t!e ring$ +!e fretting corrosion !as ca"se t!e crac3ing$

%.1.< & Cage Da*age

If, on e4amination of a faile bearing, t!e cage is fo"n to be amage, it may in many cases

#ro%e iffic"lt to ascertain t!e ca"se$ Us"ally ot!er com#onents of t!e bearing are amage

too an t!is ma3es it e%en more iffic"lt to isco%er t!e reason for t!e tro"ble$ >owe%er,

t!ere are certain main ca"ses of cage fail"re, %i/$ %ibration, e4cessi%e s#ee, wear an

bloc3age$

.!en a bearing is e4#ose to %ibration, t!e forces of inertia may be so great as to ca"se

fatig"e crac3s to form in t!e cage material after a time$ Sooner or later t!ese crac3s lea to

cage fract"re$

If t!e bearing is r"n at s#ees in e4cess of t!at for w!ic! t!e cage is esigne, t!e cage is

s"b&ecte to !ea%y forces of inertia t!at may lea to fract"res$ Fre"ently, w!ere %ery !ig!

s#ees are in%ol%e, it is #ossible to select bearings wit! cages of s#ecial esign$

:age wear may be ca"se by inae"ate l"brication or by abrasi%e #articles$ +!e iea wit!

rolling bearings is of co"rse to a%oi sliing friction$ >owe%er, w!ere t!e cage is concerne,

sliing cannot be eliminate in t!e contacts wit! t!e ot!er com#onents of t!e bearing$ +!is

e4#lains w!y t!e cage is t!e first com#onent to be affecte w!en t!e l"brication becomesinae"ate$ +!e cage is always mae of softer material t!an t!e ot!er com#onents of t!e

bearing an conse"ently it wears com#arati%ely "ic3ly$ As t!e cage #oc3ets increase in

si/e, "e to wear, t!e rolling element g"iance eteriorates an t!is also a##lies to t!e cage in

cases w!ere t!e cage is centre on t!e rolling elements$ +!e res"ltant forces may lea to cage

fail"re wit!in a s!ort s#ace of time$

Fragments of fla3e material or ot!er !ar #articles may become wege between t!e cage

an a rolling element, #re%enting t!e latter from rotating ro"n its own a4is$ +!is leas to

cage fail"re$

If t!e rings of a ee# groo%e ball bearing are fitte o"t of alignment wit! eac! ot!er, t!e #at!

of t!e balls !as an o%al config"ration$ If t!e cage is centre on t!e balls, it !as to c!ange

s!a#e for e%ery re%ol"tion it #erforms$ Fatig"e crac3s t!en form in t!e material an sooner or

later t!ey lea to fract"res$ +!ere is a similar case w!en a t!r"st ball bearing is fitte toget!er

wit! raial #lain bearings$ If clearance arises in t!e #lain bearings, t!e was!ers of t!e t!r"st

bearing become is#lace in relation to eac! ot!er$ +!en t!e balls o not follow t!eir normal

#at! an !ea%y stresses may arise in t!e cage$ :ages in bearings s"b&ect to se%ere

acceleration an retaration, in con&"nction wit! fl"ct"ations in s#ee, are affecte by forces

of inertia$ +!ese gi%e rise to consierable #ress"re between t!e contacting s"rfaces, wit!

conse"ent !ea%y wear$

21


22/24

:age of a s#!erical roller bearing$ Fatig"e crac3s !a%e forme in t!e fillets$

%.2 Bearing $ife I*pro-e*ents

%.2.1 Rated $ife of Bearings

+!e life of rolling bearing is efine as t!e n"mber of re%ol"tions, w!ic! t!e bearing is

ca#able of en"ring before t!e first sign of fatig"e occ"rs on one of its rings or rolling

elements$ Ser%ice life of bearing is t!e act"al life ac!ie%e by a s#ecific bearing before it

fails$ S#ecification life is life s#ecifie by an a"t!ority an base on !y#ot!etical loa an

s#ee ata s"##lie by t!e same a"t!ority$ It can be also 3nown as a basic rating life$ +!e

sim#lest met!o for calc"lating basic rating life is to "se ISO e"ation$

L)< J (:K=*#

.!ere

L)< JBasic rating life in millions of re%ol"tions

:J Basic ynamic loa rating in N

=J E"i%alent ynamic bearing loa in N

# J 0 for ball bearing an 0$0000 for roller bearing

Basic rating life in r"nning !o"rs J (:K=*# M)


23/24

bearing can be re#lace$ +o get ma4im"m ser%ice life it is re"ire #reloaing an re

l"bricating t!e bearings$

%.2.2 !reloading of Bearings

7e#ening "#on t!e ty#e of bearing t!e #reloa may be eit!er raial or a4ial$ +!e a4ialis#lacement in t!e bearing arrangement "ner #reloa is smaller t!an bearing agreement

wit!o"t #reloas$ +!e main effects of #reloa are gi%en below$

9 Re"ces noise$

9 En!ance stiffness$

9 Gi%e long ser%ice life

.!en t!e #reloa e4cees a gi%en o#tim"m %al"e, increases friction an conse"ently

increase !eat generation an res"lt in s!ar# ro# of bearing life$ +!e Fig"re No - s!owsrelations!i# between bearing life an #reloa Kclearance$

%.2.% & Bearing $u,riation

+!e rolling element bearings are to be ae"ately l"bricate to o#erate reliably$ +!is l"bricant

!as aitional tas3 t!an of l"brication, s"c! as sealing from "st, remo%al of !eat etc$ So t!e

larger "antity of l"bricant is re"ire$ +!e grease fille in bearing arrangement loses its

l"bricant #ro#erties "ring o#eration as res"lt of mec!anical wor3, ageing an b"il9"# of

contamination$ It is t!ere fore necessary for grease to be re#lenis!e or renewe from time to

time$ +!e re9l"brication inter%al is a f"nction of bearing s#ee an bore iameter of a certain

bearing ty#e$

+!ese inter%als are base "#on t!e wor3ing conition$ B"t as t!e more life #ast t!ese

inter%als are re"ire to c!ange$ +!e most reason for re9l"brication is entry of fine "st,

w!ic! is 3nown as res#irable "st in bearings$ Ot!erwise it will re"ire increasing t!e

fre"ency of re9l"brication$

%.2.3 & i,ration onitoring of Bearings

23


24/24

In rolling element bearings, %ibration res"lts from t!e im#act generate by a ball rolling o%er

a efect$ ibration e4cite by s!oc3 #"lses can be "se for etecting bearing efects$ It is

#ossible to etect se%eral ty#es of rolling s"rface wear efects wit! s!oc3 #"lse e4citation at

t!e beginning stage of t!eir e%elo#ment$ 1eas"ring t!e le%el of !ig! fre"ency %ibration,

e4cite by s!oc3 #"lses, an t!e magnit"e of t!e crest (#ea3* factor, are "ite eno"g! w!enetecting c!anges in rolling element9bearing conition wit! minimal e4#ense$ >owe%er, it

m"st be recogni/e t!at o%erall meas"rements may not be s"fficient to e%al"ate efect e#t!

(se%erity* beca"se t!e magnit"e of !ig! fre"ency %ibration is m"c! more e#enent on t!e

rise time of t!e leaing ege of t!e s!oc3 #"lse t!an t!e am#lit"e of t!e #"lse$ +!is

e#enency becomes m"c! stronger wit! increasing fre"ency of t!e meas"re %ibration$

Fa"lts in any of t!e fo"r bearing com#onents will generate s#ecific fre"encies e#enent

"#on t!e bearing geometry an rotating s#ee$ (B=FO JBall =ass Fre"ency, O"ter Race,

B=FI JBall =ass Fre"ency, Inner Race, BSF 9 Ball S#in Fre"ency, F+F 9 F"namental

+rain Fre"ency or cage rotating fre"ency*

%.2.5 & Te*perature onitoring Of Bearings

>ig!er t!an normal o#erating tem#erat"res, w!et!er ca"se by ambient conitions or

generate wit!in t!e bearing itself, !a%e t!e #otential to !arm rolling bearings$ Normal

o#erating tem#erat"res iffer, e#ening on t!e a##lication$ Normal bearing o#erating

tem#erat"res range from 55< to Q-

rolling bearing failure analysis 2010[1]

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