journal berings

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JOURNAL BEARINGS

Introduction

Bearings are used for reducing friction between the two surfacesand increase eciency.

The properties of Engine bearing materialsdetermine how abearing functions under conditions of alternating load,intermittent metal-to-metal contact with the journal and in thepresence of impurities transported in the oil.

The structure and the characteristics of the materials used in ahigh performance bearing are particularly important because of

the severe conditions under which it must operate, including highloads and high rotation speeds.

Here are the main properties of materials for engine bearings

Load capacity (fatigue strength)is the ma!imum value

of cycling stress a bearing can withstand without

developing "atiguecrac#s after an in$nite number of cycles.

ear resistance is the ability of the bearing material to

maintain its dimensional stability %oil clearance& under

conditions of mi!ed lubrication regime and in the presence of

foreign particles carried by the lubricant.

!o"pati#i$ity (sei%ure resistance)is the ability of the

bearing material to resist physical joining with the cran#shaft

journal when it contacts the bearing surface.

!onfor"a#i$ityis the ability of the bearing material to

accommodate geometry imperfections of the journal, housing

or bearing itself.

E"#eda#i$ityis the ability of the bearing material to absorb

small foreign particles transported in the lubricating oil.

!orrosion resistanceis the ability of the bearing material

to resist chemical attac# from the lubricant or substances that

may enter and contaminate the lubricant.
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!a&itation resistanceis the ability of the bearing material

to withstand impact stresses caused by collapsing cavitation

bubbles, which form as a result of sharp and locali'ed drops of

pressure in the circulating lubricant.

Thus, in order to achieve durability and reliability from an enginebearing, its materials should parado!ically combine contradictoryproperties high strength %load capacity, wear resistance,cavitation resistance& with softness %compatibility, conformability,embedability.&

'hic She$$ Bearings

This type of bearing has a steel bac# with the re(uired sti)ness

*. To ensure against the distortion of the sliding surfacegeometry.

+. To support the cast-on white metal in regions where the shelllac#s support.

These are lined with tin-base white metal upto about *+mm th#.arious formulations and speci$cations e!ist bearing a closerelationship to babbits original composition which had ./0 tin,.0 1ntimony and *.0 2u. 1ddition of * to *.30 2admiumraises the tensile strength.

The bearing life depends to a large e!tent on the strength of bondbetween white metal and shell. 1 2u layer is deposited inbetween to ensure good bonding.

'hin She$$ Bearings

Thin shell bearings have a wall thic#ness between +0 to +.30 ofthe journal diameter. The steel bac# does not have sucientthic#ness to support the cast-on white metal alone. The bearing

must therefore be supported rigidly over its full length. This typeof bearing is manufactured with a circumferential overlength%nip4crush& which, when the shells are mounted and tightened up,will produce the re(uired radial pressure between the shell andthe bearing housing.
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5uring recent years there has been a swing from traditional thic#white metal bearings in order to improve their relatively poorcompressive strength at high operating temperatures. 6n modernhigh power engines bearing loads are very high, tending to cause

fatigue failure of bearing metal.

'hin She$$ Bering !onstruction

'ri*"eta$ Bearing Structure

The contradictory combination of strength and softness may beachieved if the bearing material has a composite structure.

Engine bearings are composed of a steelbac#, onto which isapplied a relatively strong base %copperor aluminum& combinedwith a solid lubricant in the form of either a thin overlay or smallparticles distributed throughout the base material.

Bearing material having a thin overlay is called tri-metal, in

contrast to materials without any overlay 7 bi-metal.

Typical tri-metal and bi-metal engine bearing construction isshown in "ig. *
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8icrostructure of a typical tri-metal bearing is shown in "ig. +


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2onventional tri-metal bearings are composed of the followinglayers Stee$ #acsupporting the bearing structure and providing its

rigidity and press $t under severe conditions of increasedtemperature and cycling loads.

Bearing +eta$Bearing metal is coated on the steel bac#. 6t is

applied onto the steel bac# surface by either

castingor sintering.Three di)erent types of materials are used as bearing material,

namely Tin based white metal, 2opper-lead and Tin 1luminium

%1l9n:;&.

The bearing metal thic#ness is ;.3 to /mm. Inter"ediate $ayer, 1n intermediate layer may be used

between the overlay and main bearing metal to avoid di)usion.This is particularly found where bearing loads are very such asin the lower half of the cross head bearing. The layer is

galvanically applied.


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in the overlay, causing a deterioration of corrosion resistance of

the lead-based overlay alloy. >ic#el di)usion barrier thic#ness is

about ;.;;;;:?-;.;;;;?.

O&er$aywhich is commonly made of a leaded alloycontaining *;0 tin and +-/0 copper. The overlayer is a soft

and ductile coating, its main objective is to ensure good

embedability and conformity between the bearing sliding

surface and pin surface geometry. The addition of tin increases

the corrosion resistance of the alloy. 2opper improves its

fatigue strength %load capacity&.


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The intermediate layer of conventional tri-metal bearings iscomposed of copper %the basic element&, ++-+30 lead and +-/0 tin. ead in the form of separated particles distributedthroughout the copper matri! serves as a solid lubricantand

imparts anti-friction properties to the alloy. Tin forms tinyintermetallic inclusions with copper atoms which strengthenthe material. 6t has higher load carrying capacity at higheroperating temperatures.

These consist of relatively thin steel shell, having a lining ofcopper-lead ;.3mm to /.;mm thic# with a white metal overlayfrom about ;.3mm to /.; mm.

The greater the thic#ness of overlay the better theconformability and embedability, but this is achieved at thee!pense of load carrying capacity.

0)'in A$u"iniu" (A$Sn12)

9n 1l is a composition of aluminium and tin where the tin istrapped in /-dimensional mesh of aluminium.

The sliding properties of this composition are similar to tinbased white metal but the loading capacity of this material ishigher than tin based white metals for same wor#ing

temperature %almost same as copper-lead bearing&.

Tin gives the good embedability and sliding properties and 1lmesh functions as an e)ective load absorber.

'34ES O- 5E-E!'S

A6 ear

The relatively softer surface of the bearings rubs against theharder surface to which they are providing the bearing surface.Hence when two metals of di)erent hardness rub together, theone with the lesser hardness with wear more.
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Cnder normal service conditions bearing wear is negligible. Thewear is accelerated by the fact that impurities and abrasiveparticles can enter between these surfaces thus speeding up thewear.

Bearing ear rate The reduction of shell thic#ness in theloaded area of the bearing in a given time interval represents thewear rate of the bearing. 1verage bearing wear rate based onservice e!perience is ;.;*mm 4 *;;;; hrs.

B6 iping 5a"age

/) iping of O&er$ayer

6t manifests itself by parts of the overlayer being smearedout. Diping of overlayer can ta#e place when running-in anew bearing, e!cessive load at start-up or run down, lowspeed turning or barring.

7) iping of hite +eta$

This is due to metal contact between sliding surfaces which

causes increased fricitional heat, resulting in plasticdeformation %wiping&.

!auses of iping

*. 6ncreased journal4pin surface roughness.

+. 6nade(uate lub oil supply

/.


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!6!orrosion

Bearings are lubricated with lub oilfrom the engine systems andthis lub oil can turn acidic in due course of time with e)ect

of combustion products, temperature and so on. This acidic lub oilcan cause corrosion of the bearings especially the white metalparts of the bearings.

56 Bearing Scratched #y dirt in the Oi$

Embeddability enabled a bearing to protect itself by allowingparticles to embed in the bearing. Then they will not gouge outbearing material or scratch the rotating journal. The metal ispushed up around the particle, reducing oil clearance in the area.Csually the metal can @ow outward enough to restore ade(uateoil clearance. However, if the dirt particles are too large they donot embed completely. They are carried with the rotating journal,gouging out scratches in the bearing. 1lso, if the oil is very dirty,the bearing becomes overloaded with particles. 6n either case,bearing failure soon occurs.

E6 -atigue fai$ure of #earing

=epeated application of loads on a bearing, fatigue the bearingmetal. 6t starts to crac# and @a#e out. 2raters, or poc#ets, form inthe bearing. 1s more and more of the metal is lost, the remaindercarries a greater load and fatigues at a faster rate. Then completebearing failure occurs.

-6 E$ectrica$ 4itting

Electrical discharge through the oil $lm between the shaft and

bearing in electrical machinery or in fans and turbines may occur

due to faulty insulation or grounding, or due to the build-up of

static electricity. This electric discharge can occur at very low
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voltages and may cause severe pitting of the bearing or shaft

surfaces, or both.

G6 O&er$oading8O&erheating

1 bearing can get damaged due to local concentration of stress ortemperature and certain regions which in turn can be due toseveral factors. This can lead to various types of defects such asgalling, scung, scoring and so forth.

iven below are a few pictures which show di)erent types ofdefects which have occurred due to wear, loss of lubrication etc.

This should give you an idea about the manner in which adefective bearing appears from outside without using anymeasuring tool.

Graphics of Bearing 5efects

Shell Bearings


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Thin shell bearings are used for most bearing applications in the mainengine. They consist of a steel backing strip coated with a layer of white

metal. Bearings may be bi-metal or tri-metal. typical materials are steel-

babbitt, steel-brone or steel-tin!aluminium " tin-aluminium has slightlygreater load bearing capacity than white metal and maintains its fatigue

strength o#er a greater range of temperatures. The bearing metal thicknessis 0.$ to %mm. &n Overlayof 20 -40 micrometersmay be applied to impro#e

conformity . This is generally a ductile coating of lead and tin. 'n additionnew bearings may ha#e a flashlayerof a few microns of tin to pre#ento(idation

&n intermediate layer may be used between the o#erlay and main bearingmetal to a#oid diffusion. This is particularly found where bearing loads are

#ery high such as in the lower half of the cross head bearing. The layer isgal#anically applied.

ompared with the traditional cast bearing they ha#e a number of

ad#antages.

*hells are prefinished thus allowing for +uicker and easier

replacement. The bearings are made under strict controlled conditions gi#ing

consistent high +uality products

'n many cases the top and bottom hal#es are interchangeable in an

emergency.

Thin layer of white metal cools +uickly gi#ing fine grain structure which

has high strength and fatigue resistance.


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*hell bearings sometimes ha#e a layer of copper or brone between the steeland the white metal in order to impro#e adhesion of the white metal. This

layer will also pro#ide safe guard in the e#ent that the white metal beingworm away.

journal berings

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