4 couplings

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Couplings

Types of couplings and their functions

There are many types of couplings. Virtually all of themcan be put into two classes, rigid or exible. Flexiblecouplings can be divided into four categories-mechanicallyexible, elastomeric, metallic element, and miscellaneous-and three industrial disciplines miniature, general-purpose, and special-purpose.

Rigid couplings are used to connect euipment thatexperiences very small shaft excursions or with shafts

made long and slender enough that they can acceptforces and moments produced from exing anges andshafts.

Flexible couplings !oin two pieces of rotating euipmentwhile permitting some degree of misalignment or endmovement or both. The three basic functions of a exiblecoupling are to "Figure #.#$

%. Transmit power#. &ccommodate misalignment'. (ompensate for end movement

FIGURE 1 Rigid anged connection

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A-Transmit Poer

(ouplings are primarily used to transmit mechanicalpower from one machine to another. The power is in theform of mechanical torue at some speed or wor) per unitof time. *n general, the amount of power lost by a exiblecoupling is small, although some couplings are moree+cient than others.

!-Accommodate "isalignment

Flexible couplings must accommodate three types ofmisalignment "Figure '$1 # arallel oset.#. &ngular.

The axes of connected shafts are parallel but not in thesame straight line "Figure '&$.

The axes of connected shafts intersect at the center pointof the coupling, but not in the same straight line "Figure'/$.'. (ombined angular and oset. The axes of connectedshafts do not intersect at the center point of the couplingand are not parallel "Figure '($. *t is important torecogni0e that while the euipment may see these.

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$%er%ie of Couplings and &oints

FIGURE '# ' Functions of a exible coupling (A)Transmittorue1 "/$accommodate misalignment1 "($ compensatefor end movement..

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FIGURE '#*Types of misalignment."&$ arallel oset misalignment."/$ &ngular misalignment."($ (ombined angular and oset misalignment.

to prevent internal rubbing in the motor. 2ome pumps aredriven by motors with sleeve bearings that do not havethrust bearings. 3isalignment and end movement must beaccommodated without the introduction of abnormal loadsin the euipment. 4enerally, machines are set up atinstallation uite accurately. 3any things can forceeuipment to run out of alignment. The thermal eects ofhandling hot and cold uids cause some movement in thevertical and axial directions as do dierentials of

temperature in driver media such as gas and steam.Vertical motion could be a result of support structure

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expansion due to temperature dierences, distortion dueto solar heating, axial growth, or a combination of any ofthese. 5ori0ontal motion is usually due to piping forcescaused by poor installation practices and expansion or

contraction caused by changes in the temperature orpressure of the media in the system. *t is a fact of life thatmachinery appears to live and breathe, move, grow, andchange form and position1 this is one of the basic reasonsfor using exible couplings. 5owever, a exible coupling isnot the solution to all movement problems that can existin a sloppy system. *t is naive to use a exible coupling inthe hope that it will compensate for any and all motions.Flexible couplings have their limitations. The euipment or

system designer must ma)e calculations that will give areasonable estimate of the outer boundaries of theanticipated gyrations. 6nless those boundaries arede7ned, the euipment or system designer may !ust betransforming euipment failure into coupling failure "seeFigure #.8$.

FIGUR9 #.8 (oupling failure.

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:ne thing to remember is that when sub!ected to torueand misalignment, all couplings react on the connected

euipment components. 2ome produce greaterreactionary forces than others, and if these forces areoverloo)ed they can cause vibration, shaft failure, bearingfailure, and;or other operational and early failure of othercomponents of the drive train "see Figure #. with=euipment misalignment tolerance.> The capabilities of acoupling are usually substantially higher than the

euipment can accept "see Figure 8.? for couplingmisalignment limits and 2ection %%% of (hapter 8 forguidelines for euipment misalignment tolerances$. Rigidcouplings produce the greatest reactions on euipment.3echanically exible types such as gear, chain, and gridcouplings produce

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FIGURE '#+ 9uipment failures.

moderate to high moments and forces on euipment thatare a function of torue and misalignment. 9lastomericelement couplings produce moderate to low moments andforces that are slightly dependent on torue. 3etallicelement couplings produce relatively low moments andforces that are relatively independent of torue. The mostcommonly used exible couplings today are those thatexhibit the greatest exibility "misalignment and axialcapacity$ while producing the lowest external loads on thecoupled euipment.

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C- $ther Functions of Fle,ile Couplings

/esides their basic functions, exible couplings aresometimes reuired to

%. @ampen vibration and reduce pea) or shoc) loads.#. rotect euipment from overload.'. 3easure the output torue of driven euipment.8. *nsulate the driver from the driven euipment.5. osition a rotor of a motor or generator.A. /e used to tune a system out of a torsion critical mode .

.escription of couplings

The basic construction of the most exible couplingsconsists of two anges or hubs, which attach to the shaftsbeing coupled, and a connecting element that may bemetallic "such as in disc couplings$, or a sleeve made fromelastomeric material such as 9@3 rubber, neoprene,5ytrel or urethane, or a mechanical connection "as in a u-

!oint or gear coupling$.

To be considered exible, a coupling must handle parallel

and angular misalignment. (ouplings with four-wayexibility accommodate both end oat and torsionalmovement.

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"isalignment prolems and their e/ects on theperformance of the e0uipment

The basic function of all couplings is to transmit power,accommodate misalignment and compensate for axialmovement "end movement of shafts$. 2ometimes, acoupling is as)ed to absorb shoc) or vibration. 2electingthe right coupling depends on four basic conditions ofshaft misalignment or movement.

Parallel misalignmentoccurs when the two shaftsdo not share the same rotation axis. Their end facesmay be parallel, but their center axes are laterallydisplaced with respect to each other.

Angular misalignment applies when shafts areneither coaxial nor parallel. The angle at which theshafts are misaligned may be symmetrical orasymmetrical.

End oatoccurs when either or both shafts display

axial movement, moving in and out. & sleeve-bearingmotor, for example, BoatsB as the rotor hunts for the

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magnetic center of the winding. Temperaturevariation can also cause thermal expansion andvariation in position of the shafts.

Torsional e,iilityis the torsional movement in

planes perpendicular to the shaft axis. 2hoc) orvibration typically causes this. & torsionally exiblecoupling absorbs and dampens these movements.

Figure 12 The four types of misalignment thatcouplings can accommodate

Gear couplings

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4ear couplings are torsionally rigid and are supplied totwo designs C completely exible and exible;rigid. &completely exible coupling comprises two hubs with anexternal gear and two outer sleeves with an internal gear.

*tDs a universal coupling for all sorts of applications andaccommodates all possible misalignments "angular, osetand combined$ as well as large axial moments. 3achines,bearings, seals, and shafts are therefore not sub!ected tothe additional forces, sometimes of considerablemagnitude, which arise from unavoidable misalignmentusually associated with rigid shaft couplings.& exible;rigid coupling comprises one exible geared halfand one rigid half. *t does not accommodate parallell

displacement of shafts but does accommodate angularmisalignment.This type of couplings are primarily used for =oatingshaft> applications.

Gear couplings

.isc couplings

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@isc couplings provide reliable transmission of mechanicalpower from driving to driven machine where lowmaintenance, nonlubricated couplings are reuired.@isc couplings are speci7cally designed to accommodate

general purpose drive system applications such ascentrifugal pumps, generators, cooling towers, machinetools, printing, and pulp and paper machines.

The coupling transmit torue and accommodate bothangular, oset and axial misalignment between shafts,with a coupling comprised of shaft mounted hubsconnected through exible disc pac)s with spacer orsleeve assemblies.

&ll covered disc couplings use stainless steel discs asexible members, providing high strength and goodcorrosion resistance. 9ach disc pac) accommodates up toE,


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.isc couplings

Elastic couplings

9lastic couplings of the elastomeric type incorporate a pre-compressed rubber component, which provides extrastrength and long life. These couplings cater for all types

of misalignment. The hubs are made of high strengthaluminium alloy, ma)ing the coupling light and corrosionresistant. The rubber component damps shoc)s andprovides smooth, uiet power transmission, thusprotecting both the driving and the driving machines.3ounting is very simple. &s the rubber component is asplit insert, it can be mounted after the shafts have beenaligned.

!earings

& bearing is a device to allow constrained relative motionbetween two or more parts, typically rotation or linearmovement. /earings may be classi7ed broadly accordingto the motions they allow and according to their principleof operation as well as by the directions of applied loadsthey can handle

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!earing types and their applications according tothe load and the speed

There are at least six common principles of operation

plain bearing, also )nown by the speci7c stylesbushings, !ournal bearings, sleeve bearings, riebearings

rolling-element bearings such as ball bearings androller bearings

!ewel bearings, in which the load is carried by rollingthe axle slightly o-center

uid bearings, in which the load is carried by a gas orliuid

magnetic bearings, in which the load is carried by amagnetic 7eld

exure bearings, in which the motion is supported bya load element which bends.

lain bearings use surfaces in rubbing contact, often witha lubricantsuch as oil or graphite. & plain bearing may ormay not be a discretedevice. *t may be nothing more thanthe bearing surfaceof a hole with a shaft passing through

it, or of a planar surface that bears another "in thesecases, not a discrete device$1 or it may be a layer ofbearing metaleither fused to the substrate "semi-discrete$or in the form of a separable sleeve "discrete$. Hithsuitable lubrication, plain bearings often give entirelyacceptable accuracy, life, and friction at minimal cost.

Therefore, they are very widely used.

5owever, there are many applications where a more

suitable bearing can improve e+ciency, accuracy, serviceintervals, reliability, speed of operation, si0e, weight, andcosts of purchasing and operating machinery.

Thus, there are many types of bearings, with varyingshape, material, lubrication, principal of operation, and soon. For example, rolling-element bearingsuse spheres ordrums rolling between the parts to reduce friction1reduced friction allows tighter tolerances and thus higher

precision than a plain bearing, and reduced wear extendsthe time over which the machine stays accurate. lain

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http://en.wikipedia.org/wiki/Plain_bearinghttp://en.wikipedia.org/wiki/Journal_bearinghttp://en.wikipedia.org/wiki/Rolling-element_bearinghttp://en.wikipedia.org/wiki/Jewel_bearinghttp://en.wikipedia.org/wiki/Fluid_bearinghttp://en.wikipedia.org/wiki/Magnetic_bearinghttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Flexure_bearinghttp://en.wikipedia.org/wiki/Lubricanthttp://en.wiktionary.org/wiki/discrete#Adjectivehttp://en.wikipedia.org/wiki/Bearing_surfacehttp://en.wiktionary.org/wiki/bear#Verbhttp://en.wikipedia.org/wiki/Babbitt_(metal)http://en.wikipedia.org/wiki/Roller_bearinghttp://en.wikipedia.org/wiki/Journal_bearinghttp://en.wikipedia.org/wiki/Rolling-element_bearinghttp://en.wikipedia.org/wiki/Jewel_bearinghttp://en.wikipedia.org/wiki/Fluid_bearinghttp://en.wikipedia.org/wiki/Magnetic_bearinghttp://en.wikipedia.org/wiki/Magnetic_fieldhttp://en.wikipedia.org/wiki/Flexure_bearinghttp://en.wikipedia.org/wiki/Lubricanthttp://en.wiktionary.org/wiki/discrete#Adjectivehttp://en.wikipedia.org/wiki/Bearing_surfacehttp://en.wiktionary.org/wiki/bear#Verbhttp://en.wikipedia.org/wiki/Babbitt_(metal)http://en.wikipedia.org/wiki/Roller_bearinghttp://en.wikipedia.org/wiki/Plain_bearing


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bearings are commonly made of varying types of metal orplastic depending on the load, how corrosive or dirty is theenvironment, and so on. *n addition, bearing friction andlife may be altered dramatically by the type and

application of lubricants. For example, a lubricant mayimprove bearing friction and life, but for food processing abearing may be lubricated by an inferior food-safelubricant to avoid food contamination1 in other situations abearing may be run without lubricant because continuouslubrication is not feasible, and lubricants attract dirt thatdamages the bearings

@ierent bearing types have dierent operating speed

limits. 2peed is typically speci7ed as maximum relativesurface speeds, often speci7ed ft;s or m;s. Rotationalbearings typically describe performance in terms of theproduct DNwhere Dis the diameter "often in mm$ of thebearing and N is the rotation rate in revolutions perminute.

4enerally there is considerable speed range overlapbetween bearing types. lain bearings typically handleonly lower speeds, rolling element bearings are faster,

followed by uid bearings and 7nally magnetic bearingswhich are limited ultimately by centripetal forceovercoming material strength

/earings vary greatly over the si0e and directions of forcesthat they can support. Forces can be predominately radial,axial "thrust bearings$ or /ending momentsperpendicularto the main axis.

/all /earings

/all bearings are extremely common because they canhandle both radial and thrust loads, but can only handle asmall amount of weight. They are found in a wide array ofapplications, such as roller blades and even hard drives,but are prone to deforming if they are overloaded.

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http://en.wikipedia.org/wiki/Thrust_bearinghttp://en.wikipedia.org/wiki/Bending_momenthttp://www.thomasnet.com/products/ball-bearings-3740800-1.htmlhttp://en.wikipedia.org/wiki/Thrust_bearinghttp://en.wikipedia.org/wiki/Bending_momenthttp://www.thomasnet.com/products/ball-bearings-3740800-1.html


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Roller !earings

Roller bearings are designed to carry heavy loadsItheprimary roller is a cylinder, which means the load isdistributed over a larger area, enabling the bearing tohandle larger amounts of weight. This structure, however,means the bearing can handle primarily radial loads, but isnot suited to thrust loads. For applications where space isan issue, a needle bearing can be used. Jeedle bearingswor) with small diameter cylinders, so they are easier to

7t in smaller applications.

!all Thrust !earings

These )inds of bearings are designed to handle almostexclusively thrust loads in low-speed low-weightapplications. /ar stools, for example, ma)e use of ballthrust bearings to support the seat.

Roller Thrust !earings

Roller thrust bearings, much li)e ball thrust bearings,handle thrust loads. The dierence, however, lies in theamount of weight the bearing can handle roller thrust

bearings can support signi7cantly larger amounts of thrustload, and are therefore found in car transmissions, where

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they are used to support helical gears. 4ear support ingeneral is a common application for roller thrust bearings.

Cylindrical Roller !earing Types

Tapered Roller !earings

This style of bearing is designed to handle large radial andthrust loadsIas a result of their load versatility, they arefound in car hubs due to the extreme amount of both radialand thrust loads that car wheels are expected to carry.

single ro tapered roller earings ( 3g

Angular contact all earings

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&ngular contact ball bearings have raceways in the innerand outer rings that are displaced with respect to eachother in the direction of the bearing axis. This means thatthey are designed to accommodate combined loads, i.e.

simultaneously acting radial and axial loads.The axial load carrying capacity of angular contact ballbearings increases with increasing contact angle. Thecontact angle K is defined as the angle between the line

!oining the points of contact of the ball and the racewaysin the radial plane, along which the load is transmittedfrom one raceway to another, and a line perpendicular to

the bearing axis " 7g$.

2LF angular contact ball bearings are produced in a widevariety of designs and si0es. Those commonly used and

included in this catalogue are

C single row angular contact ball bearings "

7g$

C double row angular contact ball bearings "

7g$

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the bearing "deprives the bearing of its hardness$. :ncethe bearing losses its hardness, its load carrying capabilityalso reduces because of the softness of the balls, innercage and outer cage. The common causes for this type of

failure are insu+cient lubrication, presence of debris in thelubricating oil and insu+cient cooling to be bearing. &good remedy for this would be continuous monitoring ofthe temperature of the bearing at least in critical areas.

He have now seen the two most important modes offailures in bearings. Jow let us loo) at an important modeof failure, /rinelling. /rinelling is uite important inbearings and exists in two types as explained below.

/rinelling is indicated by the presence of elliptical;straightmar)s in the inner and outer races of the bearing. Themar)s appear signi7cantly at eual distances which iseual to the pitch of the rollers in the bearings. /rinellingis a clear indication of the plastic deformation of thematerial.

1-False !rinelling2

&s explained above, brinelling mar)s appear as brightspots on the inner and outer races of the bearing withsharp mar)s in the axial direction of the bearing. :ftenthis leads the observer to thin) that a subtle amount ofdeformation has occurred in the raceways of the bearing.

These mar)s are very feeble and can be seen by na)edeye, yet on ma!ority times any deformation cannot be feltby feeling it.

Causes2

The ma!or cause for this is excessive external vibration.For example, components transported on a very bad areprone to more vibrations. roper insulation of thecomponent to insulate it from the external vibration needsto be provided to prevent false brinelling. The outcomedue to false brinelling can be uite dangerous. &nybearing in standstill condition would not be able to createits own lubrication 7lm, thus a false brinelling mar)

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created tends to get oxidi0ed easily leading to furtherwear and failure.Pre%ention2

9nsuring a good lubricant or grease to be present at alltimes is a good means of preventing false brinelling.9nsuring proper insulation during transport can alsoprevent this.

#. True !rinelling2

&s explained above, brinelling is caused due to plasticdeformation.

Causes2

This normally happens when the bearing is heavily loadedabove its static load rating. The excessive loading tends toincrease the pressure on the raceways of the bearings.

This ultimately leads to the raceways bearing the brunt ofthe load. 2ome good examples of instantaneous staticloads are sudden hitting of the bearings for installation by

hammers, suddenly dropped loads on the shaft on whichthe bearing is mounted, etc.

Pre%ention2

True brinelling can be prevented only by utmost careduring installation, mounting and transportation of thecomponent in which the bearing is mounted.

Fatigue Failure (5palling)2

This is also referred to as 2palling. This is caused due tothe rupture of the contact surfaces.

Causes2

The rolling elements or the balls running on the racewaystend to create high contact stress. The rolling contactcontinuously stresses the complete raceways again and

again and at a point the contact point in either theraceway surface or the ball surface tends to rupture

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unable to bear the stresses. This rupture does not remainas it is and tends to rupture further. This is caused due tothe increased stress in the surrounding areas of ruptureand this increases continuously until failure of the bearing.

Further this normally happens after the desired life of thebearing " M%EMife$ for which it is intended to be used for. &good example for this is applicable in the aerospaceapplications where a bearing is compulsorily replacedonce its reaches the M Mife

Remedy2

&ny increase in the life of the bearing can be done so byselecting a bigger bearing which has a fatigue life higherthan the currently used bearing.

He will now loo) into another important cause for thefailure of the bearing. &ny system properly safeguardedfrom extremes tends to survive easily. The same applies tobearings also. reventing it from (ontamination issomething li)e that.

Contamination Failure2

:f all the causes of failure in bearings, (ontaminationcontributes to nearly ?


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unauthori0ed or spurious lubricants can also lead to thistype of failure.

Remedy2

6sing clean and original lubricants. &ssembling thebearings in a controlled atmosphere where dirt, etc., canbe maintained to the minimum level possible.

Pust li)e all living beings, which are designed by nature tomove in the forward direction, but they tend to fall downor slow down drastically when they are made to movebac)wards. The same principle applies to the bearings too.(ertain bearings are speci7cally designed to carry loads in

only one direction. For example a cylindrical roller bearing"J6 Type$ can only carry radial loads and no axial load canbe carried by the bearing. &ny axial load occurring in theshaft which is held by a cylindrical bearing "J6 type$ will

!ust be passed over to the member which stands in theway of the force.

Re%erse 4oading Failure

Reverse loading failure is one such failure which occursdue to the bearing getting loaded in the opposite directionin which it is intended to bear the load. This type of failureis uite common in angular contact and thrust bearings.

The failure is generally characteri0ed by wear band in therolling elements "balls or rollers$ and a subseuentdiscoloration in the path of disturbed contact of the rollingelements.

Causes

/y construction angular, thrust and taper roller bearingscontain certain barriers which enable them to carry thethrust or axial loads only in one direction. /ut thesebarriers are normally present in one side and not on theother. Hhen the load is applied opposite direction, thebearing is made to move away from its path of running onthe raceways. This leads to increase in temperature,vibration and ultimately to premature failure. This isnormally caused due to improper mounting of the bearings

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without prior )nowledge to determine the direction offorce that will occur on the bearing.

Remedy

*nstallation of the bearing in the proper position anddirection is the only remedy. These types of bearings areclearly mar)ed stating the side it can carry the axial load.(ertain cases where loading tends to come from theopposite direction, it is best to choose a suitable bearingcapable of bearing loads from both directions and use it.

"isalignment

3isalignment is a common terminology in the engineering7eld each engineer is signi7cantly aware of. He have seen3isalignment destroy shafts, couplings and various othermachine elements, but they also tend to have a severeeect on bearings 3isalignment tends to causeunpredictable reactions in any machine element, startingright from vibrations to increase in temperature and incertain cases instant failure. Jow let us see the eects of3isalignment on bearings.

Causes

2ome of the most prevalent causes for misalignment inbearings are

/ent shafts.

/urrs or dirt on bearing mounting surfaces and bearingabutment surfaces.

2haft shoulders which are not perpendicular to the bearingabutment surface

Moc)ing nut faces which are not perpendicular to thebearing seating face.

Jon- (oncentric bores on which the bearings are mountedleading to an angular axis of rotation of the shaft and thus

misaligning the rotating axis of the bearings.

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microns, thus preventing metal to metal contact betweenthe rollers and the raceways of the bearing. Pust providinga lubricant doesnDt su+ce the lubrication of the bearing.(are needs to be ta)en in the following aspects

&mple supply of lubricant to the bearing

3aintaining an adeuate uantity of lubricant to thebearing

3aintaining the correct viscosity of the lubricant

9nsuring the correct uality of the lubricant is used

Jow let us loo) how the above points can aect the life ofbearing when not adhered to by the designer and the userof the bearing. /earings which are deprived of the aboveaspects can be identi7ed by the following phenomena.

%. @iscolored raceways and balls "/lue blac) or silver gold$are the best )nown indications for a bearing deprived oflubrication. This happens due to the metal to metalcontact which leads to increase in temperature and

overheating.

#. 9xcessive wear in the raceway, balls and cages are alsoindicative for depriving the bearing of lubrication.

This can be prevented by

a$ 9nsuring a trouble free consistent ow of lubricant tothe bearing. *f even reuired ensure a forced lubricationsystem to the bearing, where the lubricant is supplied

continuously by means of a pump to the bearing.

b$ *n certain cases the bearings tend to wor) in anelevated temperature, causing the lubricant to loose itsproperty. This also leads to lubrication failure because theviscosity reduces with increase in temperature and doesnot form a uid 7lm boundary between contactingsurfaces to prevent metal to metal contact. *n such casesproper cooling of the bearings or lubricant must be

ensured.

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9ach bearing must be ensured a proper 7t for the bearingon its inner race and the outer race. roper 7t is stressedbecause it carries a lot of importance for the properwor)ing of the bearing. There are two important types of

failures that occur due to improper mounting i.e. withoutthe proper 7t. The 7rst one in that is Failure of /earingsdue to Moose Fits.

/y loose 7ts, we mean to say that the bearing is eitherloosely 7tted on the inner race i.e. on the shaft or on theouter race i.e. onto the housing. This can occur under thefollowing conditions.

%. The si0e of the shaft is lesser that the bearing innerrace bore diameter. This dierence between the diameterof the shaft and the diameter of the bearing bore can beas less as E.E' mm.

#. The si0e of the bore of the housing is larger than theouter race diameter of the bearing. The dierencebetween the outer race diameter and the bore diameter ofthe housing can be as less as E.E# mm.

2o what happens if the above two point happenQ Thefailure of the bearing due to loose 7ts occurs. This has acharacteristic indication. The bearing can be easilyremoved "!ust by hand$ without any external aid. Thevisual indications could be as follows

@iscoloration and scoring on the surface which has beenfree from any 7t. The discoloration is mainly due to thetemperature raise cause due to the running in of the

bearing as a whole. To put in simple words, in thiscondition the bearing does not wor) li)e a bearing andinstead wor)s as a scraper scraping the housing bores orthe shaft diameter !ournals.

This also leads to Fretting. Fretting is a process ofgeneration of 7ne metal particles which oxidi0e and leavea distinct brownish mar) on the surface. 3oreover the 7neparticles are abrasive in nature and can cause furtherdamage to the bearing and the areas on which the bearingis mounted upon.

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The ultimate result of this will cause the bearing to run onthe loose surface causing further wear and tear. &characteristic point to be noted during running of abearing with looseness is that is tends to produce more

noise and runs with a run out.

The best remedy or precaution for this would be to ensurea proper 7t is available before assembly of the bearing onthe corresponding location whether it is a shaft or a bore.

4 couplings

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