FAILURE OF ROLLING FAILURE OF ROLLING CONTACT BEARINGSCONTACT BEARINGS

Rolling Element BearingsRolling Element BearingsDEFINITION:DEFINITION:

The medium of contact in between the The medium of contact in between the rigid frame and movable parts in a machine is rigid frame and movable parts in a machine is called bearing.called bearing.

Bearings are common machine components.Bearings are common machine components.

Two types of bearingsTwo types of bearings

1.1. Plain or sleeve bearing.Plain or sleeve bearing.

2.2. Anti-friction or rolling element bearings.Anti-friction or rolling element bearings.

Types of roller bearings:Types of roller bearings:

1.1. Cylindrical roller bearing.Cylindrical roller bearing.

2.2. Spherical roller bearing.Spherical roller bearing.

3.3. Taper roller bearing.Taper roller bearing.

4.4. Needle roller bearing.Needle roller bearing.

5.5. Needle roller thrust bearing.Needle roller thrust bearing.

6.6. Spherical roller thrust bearing.Spherical roller thrust bearing.

Principle of roller bearing:Principle of roller bearing:it is designed on the principle of rolling contact it is designed on the principle of rolling contact

rather than sliding contact.rather than sliding contact. So the friction is very less in roller bearings So the friction is very less in roller bearings

compared to slider bearings.compared to slider bearings.

Types Of FailuresTypes Of Failures

Failure by wear:Failure by wear: wear failure is due to the insufficient lubrication in wear failure is due to the insufficient lubrication in

the roller bearing.the roller bearing.

If the lubrication is adequately and correctly applied If the lubrication is adequately and correctly applied there is no wear failure in the bearing.there is no wear failure in the bearing.

In practice wear is caused by the entry of dirt,hard In practice wear is caused by the entry of dirt,hard particles, or corrosive fluids causing increase in the particles, or corrosive fluids causing increase in the running clearances of the various parts of the running clearances of the various parts of the bearing,which may lead to noisy operation and early bearing,which may lead to noisy operation and early failure of machine.failure of machine.

Failure by dirt:Failure by dirt:Roller bearings are very sensitive to dirt or foreign Roller bearings are very sensitive to dirt or foreign

matter,because of the very high unit pressure matter,because of the very high unit pressure between the rollers and raceways.between the rollers and raceways.

Due to the rolling action of the balls dirt particles Due to the rolling action of the balls dirt particles will easily enter in to the bearing.will easily enter in to the bearing.

Foreign material may get in to the bearing during its Foreign material may get in to the bearing during its initial assembly,during repairs or by seepage from initial assembly,during repairs or by seepage from the atmosphere in to the bearing housing during the atmosphere in to the bearing housing during operation of the machine.due to this bearing operation of the machine.due to this bearing become loose and noisybecome loose and noisy

Failure by hard particles:Failure by hard particles:Usually non metallic, of any size that will scratch, Usually non metallic, of any size that will scratch,

cut or lap the surface of a bearing are called hard cut or lap the surface of a bearing are called hard particlesparticles

This failure is also known as third particle abrasive This failure is also known as third particle abrasive wear. This failure usually occurs on the load wear. This failure usually occurs on the load carrying surfaces.carrying surfaces.

Some metallic particles like iron oxide power will Some metallic particles like iron oxide power will ailso cause failure on the load surface. since iron ailso cause failure on the load surface. since iron oxide is a good lapping compound.oxide is a good lapping compound.

Failure by corrosive fluids:Failure by corrosive fluids:Water, acid, or other corrosive fluids, including Water, acid, or other corrosive fluids, including

corrosive formed by deterioration of the lubricant, corrosive formed by deterioration of the lubricant, produce a type of failure that is characterized by a produce a type of failure that is characterized by a reddish brown coating and very small itch points reddish brown coating and very small itch points over the entire exposed surfaces of the raceways.over the entire exposed surfaces of the raceways.

Usually corrosion will not occur on the ball path Usually corrosion will not occur on the ball path because of the rolling action the balls pushes the because of the rolling action the balls pushes the lubricantlubricant

Failure by electric pitting:Failure by electric pitting:It is produced by passage of electrical current It is produced by passage of electrical current

betweeen two surfaces.betweeen two surfaces.

Applications such as electric equipment,locomotives Applications such as electric equipment,locomotives and rail way cars there is a possibility of electric and rail way cars there is a possibility of electric current passing through the bearings.current passing through the bearings.

When current is broken at the contact surfaces When current is broken at the contact surfaces between raceways and rolling elements, arcing or between raceways and rolling elements, arcing or sparking occurs, producing high temperature and sparking occurs, producing high temperature and localised damage.localised damage.

The overall damage is proportional to the number and The overall damage is proportional to the number and size of localised points.size of localised points.

Failure by cumulative material transfer:Failure by cumulative material transfer:A type of failure some times called smearing or A type of failure some times called smearing or

galling results from considerable sliding between galling results from considerable sliding between bearing contact surface.it is characterized by the bearing contact surface.it is characterized by the formation of welded junctions between the contact formation of welded junctions between the contact surfaces.surfaces.

This welding usually occurs on a localized This welding usually occurs on a localized microscopic scale and results in removable of microscopic scale and results in removable of material from one or both bearing surfaces by material from one or both bearing surfaces by tearing action.tearing action.

Failure by rotational creep:Failure by rotational creep:Rotational creep occurs on inner ring of bearing.Rotational creep occurs on inner ring of bearing.

It may result from the ring being initially loose on It may result from the ring being initially loose on the shaft as a result of fitting errors or from plastic the shaft as a result of fitting errors or from plastic or elastic deformation due to abnormal loading or elastic deformation due to abnormal loading conditions or severe imbalance.conditions or severe imbalance.

Rotational creep may also occur in thrust bearings if Rotational creep may also occur in thrust bearings if the shaft against which the ring about is not the shaft against which the ring about is not perpendicular to the axis.perpendicular to the axis.

Failure by damage:Failure by damage: bulk damage failures are the result of damage to bulk damage failures are the result of damage to

bearing components outside the contact zone.bearing components outside the contact zone.

Deterioration of the bearing results from one or Deterioration of the bearing results from one or more cause, inducing overheating, more cause, inducing overheating, overloading, bulk fatigue fretting, and overloading, bulk fatigue fretting, and permanent dimensional changes.permanent dimensional changes.

Failure MechanismFailure Mechanism Wear mechanism:Wear mechanism:There is a limited number of modes or mechanisms There is a limited number of modes or mechanisms

by which small amounts of material can be by which small amounts of material can be removed from a larger body. these are ductile removed from a larger body. these are ductile fracture, brittle fracture, high cycle fatigue, low fracture, brittle fracture, high cycle fatigue, low cycle fatigue, and melting, chemical dissolution. cycle fatigue, and melting, chemical dissolution. hydrogen ebrittlement is an example of a hybrid hydrogen ebrittlement is an example of a hybrid mechanism in which ordinary ductile material mechanism in which ordinary ductile material may fracture in a brittle manner. may fracture in a brittle manner.

Fatigue:Fatigue:Segments of a long lived bearing may fail by fatigue. Segments of a long lived bearing may fail by fatigue.

A bearing may have many cycles of varying load A bearing may have many cycles of varying load on it, as in engines, or may have an unbalanced on it, as in engines, or may have an unbalanced load, as in other machinery. Fatigue failure will be load, as in other machinery. Fatigue failure will be hastened by high stress and by corrosive hastened by high stress and by corrosive environment.environment.

Electrical erosion:Electrical erosion:some bearings, particularly those in steam turbines, some bearings, particularly those in steam turbines,

may have an electrical potential applied across may have an electrical potential applied across them, usually anadvertently.them, usually anadvertently.

When the gap spacing in the bearing is small and the When the gap spacing in the bearing is small and the applied potential is high enough,applied potential is high enough,

sparks will pass through the oil film and causesparks will pass through the oil film and cause

Erosion. The bearing surfaces will not be reflective, Erosion. The bearing surfaces will not be reflective, but will appear frosted.but will appear frosted.

Fretting:Fretting:

When oscillatory sliding occurs with an amplitude When oscillatory sliding occurs with an amplitude of the order of 0.05 m, fretting occurs. This may of the order of 0.05 m, fretting occurs. This may happen on the outside of the bearing backing, happen on the outside of the bearing backing, against the housing of the machine against the housing of the machine

Corrosion:Corrosion:Several products of degradationSeveral products of degradation of oil (alcohols, of oil (alcohols,

aldehydes, ketones, and acids) contaminants (such aldehydes, ketones, and acids) contaminants (such as water), and some anti-wear additives in oil as water), and some anti-wear additives in oil cause corrosion. A corrosive may selectively cause corrosion. A corrosive may selectively attack an alloy, removing only one element.attack an alloy, removing only one element.

Cavitation:Cavitation:In certain areas of fluid flow along a solid boundary, In certain areas of fluid flow along a solid boundary,

high pressures and low pressures can develop. If high pressures and low pressures can develop. If the negative pressure is high, cavities can form in the negative pressure is high, cavities can form in the fluid. the fluid.

Foreign particles:Foreign particles:Dirt, chips, and abrasive grit cause wear by abrasion. Dirt, chips, and abrasive grit cause wear by abrasion.

This is a problem in all applications of sliding This is a problem in all applications of sliding bearings. One insidious effect of foreign particles bearings. One insidious effect of foreign particles is that they may scratch or score the shaft surface is that they may scratch or score the shaft surface severely, which adds move foreign particles to the severely, which adds move foreign particles to the system.system.

Misalignment:Misalignment:Some bearings are improperly assembled and Some bearings are improperly assembled and

fracture very early in use. Some bearing-holder fracture very early in use. Some bearing-holder systems deflect excessively. The result of systems deflect excessively. The result of misalignment is a distorted wear pattern.misalignment is a distorted wear pattern.

PRVENTIVE METHODS:PRVENTIVE METHODS: For dirt failure:For dirt failure:i.i. Proper maintenance.Proper maintenance.

ii.ii. Grinding and polishing the journal surfaces if Grinding and polishing the journal surfaces if necessary.necessary.

iii.iii. Changing the oil and filters at regular interval.Changing the oil and filters at regular interval. For fatigue failures:For fatigue failures:i.i. Proper lubricationProper lubrication

ii.ii. Maintenance of oil-film thickness at desired Maintenance of oil-film thickness at desired level.level.

For hot shot phenomenon:For hot shot phenomenon:i.i. Temperature at contact surface should not be Temperature at contact surface should not be

allowed to rise more than the specified level.allowed to rise more than the specified level.

ii.ii. Flow of oil, free from dirt should be ensured.Flow of oil, free from dirt should be ensured. For fretting failure:For fretting failure:i.i. Mating faces of the bearing assembly should be Mating faces of the bearing assembly should be

cleared and made burr free while installation.cleared and made burr free while installation.

ii.ii. Attention should be given regarding load Attention should be given regarding load carrying capacity.carrying capacity.

For excessive interference problems:For excessive interference problems:i.i. Care should be taken during mounting, Care should be taken during mounting,

specially regarding fit selection.specially regarding fit selection. For corrosion problems:For corrosion problems:i.i. oil suppliers may be consulted.oil suppliers may be consulted.

ii.ii. Investigation may be made for possible coolant Investigation may be made for possible coolant leakage into oil.leakage into oil.

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