Download - Failure Analysis of Bearings

Bearing Failure Analysis

OverviewThis section will give the attendee a broad overview of failure analysis. The best way to gain expertise in this subject is to examine as many damaged bearings as possible. Although each failure is unique, evidence will emerge that often allows the determination of root cause, which will lead to the application of the proper corrective action to reduce or eliminate future failures.

In designing the bearing mounting, the first step is to decide which type and size of bearings to use. This choice is usually based on a certain desired life for the bearing. The next step is to design the application, with allowance for prevailing service conditions. Unfortunately, too many of the ball and roller bearings installed never attain their calculated life expectancy because of something done, or left undone, in handling, installation, and maintenance.

The calculated life expectancy of any bearing is based on four assumptions:

1. Good lubrication in proper quantity will always be available to the bearing.

2. The bearing will be mounted without damage.

3. Dimensions of parts related to the bearing will be correct.

4. There are no defects inherent in the bearing.

However, even when properly applied and maintained, the bearing may be exposed to one further cause of failure; fatigue of the bearing material. Fatigue is the result of shear stresses cyclically applied immediately below the load carrying surfaces, and is observed as spalling away of surface metal. Although spalling can be readily observed, it is necessary to discern between spalling produced at the normal end of a bearing’s useful life and that which is triggered by causes found in the three major classifications of premature spalling: lubrication, mechanical damage, and material defects.

Most bearing failures are attributed to one or more of the following causes:

Defective bearing seats on shafts and in housings

Misalignment

Faulty mounting practice

Incorrect shaft and housing fits

Inadequate lubrication

Ineffective sealing

Vibration while the bearing is not rotating

Passage of electric current through the bearing

Transportation, storage, and handling

Bearing Life

The life of a rolling bearing is defined as the number of revolutions (or the number of operating hours at a given constant speed) which the bearing is capable of enduring before the first sign of fatigue occurs on one of its rings or rolling elements (flaking, spalling).

It is, however, evident from both laboratory tests and practical experience that seemingly identical bearings operating under identical conditions have different lives. A clearer definition of the term “life” is therefore essential for the calculation of bearing size. All information presented by SKF on dynamic load ratings is based on the life that 90 percent of a sufficiently large group of apparently identical bearings can be expected to attain or exceed. This is called “basic rating life,” and agrees with the ISO definition. The median life is approximately five times the calculated basic rating life.

There are several other bearing “lives.” One of these is the “service life,” which is the actual life achieved by a specific bearing before it fails. Failure is not generally by fatigue in the first instance, but by wear, corrosion, seal failure, mishandling, etc. Another is “specification life.”

SKF Reliability Systems - Bearing Maintenance and Service 1


This is the life specified by an authority, based on hypothetical load and speed data supplied by the same authority. It is generally a requisite L10 (basic rating life), and is assumed that the authority has related the specification to experience gained with similar machinery, to obtain adequate service life.

Load-Path Patterns and their Meanings

There are many ways bearings can be damaged before and during mounting, and in service. The pattern or load zone produced by the action of the applied load and the rolling elements on the internal surfaces of the bearing is a clue to the cause of failure.

To benefit from a study of load zones, you must be able to differentiate between normal and abnormal patterns. The figure illustrates how an applied load of constant direction is distributed among the rolling elements of a bearing. The large arrow indicates the applied load. The series of small arrows show the share of this load supported by each ball or roller in the bearing.

The rotating ring will have a continuous 360 degrees zone, while the stationary ring will show a pattern of approximately 150 degrees. The figure illustrates the load zone found inside

a ball bearing when the inner ring rotates and the load has a constant direction.

The figure illustrates the load zone resulting if the outer ring rotates relative to a load of constant direction, or where the inner ring rotates and the load also rotates in phase with the shaft.

Combined thrust and radial load will produce a pattern shown in the figure above. With combined load, the loaded area of the inner ring is slightly off-center, and the length in the outer ring is greater than that produced by radial load, but not necessarily 360 degrees. In a double-row bearing, a combined load will produce load zones of unequal length. The thrust-carrying row will have a longer stationary load zone. If the thrust is of sufficient magnitude, one row of rolling elements can be completely unloaded.

The load path shows uniform wear on both the inner and outer ring. Pure thrust (axial) load is rare. If axial load is present, it is usually accompanied by radial load.

2 SKF Reliability Systems - Bearing Maintenance and Service


Certain types of bearings can tolerate only very limited amounts of misalignment. A deep groove ball bearing, when misaligned, will produce load zones not parallel to the ball groove on one or both rings, depending on which ring is misaligned. The figure illustrates the load zone when the outer ring is misaligned relative to the shaft.

Here, the inner ring is misaligned with respect to the outer ring. Cylindrical roller bearings and angular contact ball bearings are also sensitive to misalignment, but it is more difficult to detect this condition from the load zones.

Misalignment is a common source of premature spalling, occurring when a shoulder is not square with the journal, or where a housing shoulder is out-of-square with the housing bore. Misalignment arises when two housings are not on the same centerline. A bearing ring can be misaligned even though it is mounted on a tight fit, yet not pressed against its shoulder causing it to be left cocked on its seat. Bearing outer rings in slip-fitted housings that are cocked across their opposite corners can also result in misalignment.

Using self-aligning bearings does not cure some of the foregoing misalignment faults. When the inner ring of a self-aligning bearing is not square with its shaft seat, the inner ring is required to wobble as it rotates. This results in

smearing and early fatigue. Where an outer ring is cocked in its housing across corners, a normally floating outer ring can become axially held and can be radially pinched in its housing.

MisalignmentRoll the bearing rings on a flat surface and note the position of the wear patterns. Misaligned patterns will slalom back and forth across the raceway surface. Thrust loads will simply move the wear path to one side.

Distorted or out-of-round housing bores can radially pinch an outer ring. The figure above illustrates the load zone found in a bearing where the housing bore was initially out-of-round or became out-of-round by bolting the housing to a concave or convex surface. In this case, the outer ring will show two or more load zones depending on the type of distortion.



The figure is a picture of a bearing that had been mounted in an out-of-round housing that pinched the stationary outer ring. This is a mirror view and shows both sides of the outer ring raceway.

If the fit is too tight, the bearing can be internally preloaded by compressing the rolling elements between the two rings. In this case, the load zones observed in the bearing indicate that this is not a normal life failure. Both rings are loaded through 360 degrees, but the pattern will usually be wider in the stationary ring out-of-round where the applied load is superimposed most on the internal preload.

Some applications, such as shaker screens, polishing machines, and other vibratory sorters, employ a weight attached to the shaft to produce eccentric motion in the machine. Since the load rotates in phase with inner ring raceway, a stationary load zone results.

In unbalanced applications, the load does not rotate in phase with either ring, producing a

load zone on both rings. This condition often produces creep if the outer ring is loosely fit. Fan applications are a common source of this load pattern.

Failure Mode Classification1. Causes of Failures Have Identifiable

Characteristics

2. Failure Mechanisms Have Identifiable Failure Modes

3. Observed Damage Can Identify Failure Causes The primary cause of failure analysis is to identify the true cause of failure. Corrective actions and verification of success are impossible without this first step. This classification system is in the development stage, and may change significantly prior to ISO submission. The proposed system arose from a desire to standardize terminology and methodology for analyzing bearing failures. Three underlying principles were adopted in developing the system: Causes of Failures Have Identifiable Characteristics

Although there are many causes for failures, each one can be uniquely identified.

Failure Mechanisms Have Identifiable Failure Modes

Failure mechanisms can be organized into logical groups. These groupings can be used to more quickly identify the root cause of failure.

Observed Damage Can Identify Failure Causes

Careful observation of the failed parts and associated components will eliminate other causes and lead to true root cause.



“Bearings only fail in two ways, either from the inside or the outside.” This adage is a basic way to begin to eliminate failure causes and begin the search for root cause.

The actual beginning of spalling (or flaking) is invisible because its origin is usually below the surface. The subsurface crack grows under continued cyclic stresses and eventually breaks the surface, where the damage can be detected by condition monitoring equipment. By the time spalling reaches larger proportions, the condition should make itself known by noise. If the surrounding noise level is great enough, a bearing’s condition can be evaluated with a monitoring device. The time between incipient and advanced spalling varies with speed and load. Spalling is generally not a sudden condition that causes destructive failure within a matter of hours. Complete bearing failure and consequent damage to machine parts is usually avoided due to the noise the bearing produces, and the erratic performance of the shaft supported by the bearing.

Cylindrical and tapered roller bearings can accommodate only very small misalignments, even if crowned. If misalignment is

appreciable, edge loading, a source of premature fatigue, results. Edge loading from misalignment was responsible for the spalling in the bearing ring shown in the above figure.

The above figure shows fatigue spalling caused by improper handling (impact damage). The damage can occur from blows to the bearing during mounting, or from damage while mounting external components, or other heavy shock loads, such as transportation damage. If the cage pocket spacing matches the dent spacing, it is corroborative of impact damage, also known as true brinelling.

– Surface distress– Reduced

lubrication regime– Sliding motion– Burnishing,

glazing– Asperity

microcracks– Asperity

microspalls

40 µm

Surface initiated fatigue

Electrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Subsurface fatigue

All bearings need lubricants for reliable operation. The curvature of the contact areas between rolling element and raceway in normal operation results in minute amounts of sliding motion, in addition to the rolling. Also, the cage must be carried on either the rolling elements or some surface of the bearing rings, or a combination of these. In most types of roller bearings, there are roller end faces that slide against a flange or a cage. These reasons give even more importance to adequate lubrication at all times.

The term “lubrication failure” is too often taken to imply that there was no oil or grease in the



bearing. While this does happen occasionally, failure analysis is usually not that simple. Many cases require a thorough examination of the lubricant’s properties, the amount of lubricant applied to the bearing, and the operating conditions. If any one of these factors does not meet requirements, the bearing can be said to have failed from inadequate lubrication.

Viscosity of the oil, either as oil itself or as the oil in grease, is the primary characteristic of adequate lubrication. The nature of a grease’s soap base, and its consistency, along with the viscosity of the oil, are the main quality points when considering a grease. The quantity of lubricant required in a bearing at any one time is usually rather small, but the supply must be constant and consistent.

If the lubricant is oil, and is being used for heat removal as well as for lubrication, then a larger quantity is required. An insufficient quantity of grease at medium to high speeds generates a temperature rise and, usually, a whistling sound. An excessive amount of grease results in churning, which produces a temperature rise in all, but exceptionally slow, speed bearings. A lubricant that is adequate under normal conditions can be made inadequate when operational conditions produce abnormally high temperatures.

Inadequate lubrication causes surface damage. This damage progresses rapidly to failures that are often difficult to differentiate from a failure due to material fatigue or spalling. Spalling will occur and often destroy the evidence of inadequate lubrication. However, if caught early, indications that pinpoint the real cause of the short bearing life can be found.

One form of surface damage is shown in stages in the following figure. The first visible indication of trouble is usually a fine roughening or waviness on the surface. Later, fine cracks develop, followed by spalling. If there is insufficient heat removal, the temperature may rise high enough to cause discoloration and softening of the hardened bearing steel.

In some cases, inadequate lubrication initially appears as a highly glazed or glossy surface, which, as damage progresses, takes on a frosty appearance and eventually spalls.

In the frosty stage, it is sometimes possible to feel the “nap” of fine slivers of metal pulled from the bearing raceway by the rolling element. The frosted area will feel smooth in one direction, but have distinct roughness in the other. As metal is pulled from the surface, pits appear and frosting advances to pulling.

– Progressive Removal of Material– Accelerating Process– Inadequate Lubrication – Ingress of Dirt Particles

Adhesive wear

Electrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Abrasive wear



Although foreign matter can enter a bearing during mounting, its most direct and sustained area of entry can be the housing seals. The result of gross change in bearing internal geometry has been detailed. Bearing manufacturers realize the damaging effect of dirt and take extreme precautions to deliver clean bearings. Not only assembled bearings, but also parts in process are washed and cleaned. Freedom from abrasive matter is so important that some bearings are assembled in air-conditioned white rooms.

Wear of the bearing as a whole also results from inadequate lubrication. The areas subject to sliding friction such as locating flanges and the ends of rollers in a roller bearing are the first parts affected. The figure shows a large bore tapered roller bearing failure due to an insufficient amount of lubricant resulting from too low a flow rate in a circulating oil system. The area between the guide flange and the large end of the roller is subject to sliding motion.

A peculiar type of smearing occurs when rolling elements slide, as they pass from the unloaded to the loaded zone. The top right figure illustrates the patches of skid smearing, one in each row. Insufficient load, a lubricant that is too stiff, excessive clearance, and insufficient lubrication in load zone can all contribute to smearing.

To avoid lubrication-related surface failures, be aware of the following:

Sufficient elastohydrodynamic film prevents surface distress (glazing, frosting).

Proper lubrication guards against smearing and sliding surface wear.

Clean lubricants prevent significant wear of rolling surfaces.

As long as the rolling element and raceway surfaces in rolling contact can be separated by an elastohydrodynamic oil film, surface distress is avoided. The continuous presence of the film depends on contact area, the load it carries, the speed, operating temperature, the surface finish, and the oil viscosity.

In unusual applications, when viscosity selection must be governed by the sliding areas, experience has proven that the viscosity chosen is capable of maintaining the necessary elastohydrodynamic film in the rolling contacts.



» 150° - 177° C (300° - 350° F)

» 177° - 205° C (350° - 400° F)

» 205° - 260° C (400° - 500° F)

» + 260° C (+ 500° F)

» + 540° C (+ 1000° F)

– SKF Bearings can be used at temperatures up to 125° C (~ 250° F)

– Higher temperatures may cause loss of Hardness

– Loss of 2-4 points of Rockwell Hardness reduces life 50%

Temperature Discoloration

Straw Color: ~150-175°C (~300 - 350°F)

Darker Brown: ~175-200°C (~350 - 400°F)

Blue: ~200-250°C (~400 - 500°F)

Black: Above 260°C (~ 500°F)

Black, Gray, Loose Scale

Above 500°C (~1000°F)

– Oxidation / rust – Chemical

reaction– Corrosion pits /

flaking– Etching (water /

oil mixture)

Frictional corrosionElectrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Moisture corrosion

False brinelling

Fretting corrosion

In addition to abrasive matter, corrosive agents should be excluded from bearings. Water, acid, and those agents that deteriorate lubricants resistance to corrosion must all be excluded. The figures above illustrate how moisture in the

lubricant can rust rollers and raceways. The etching in the bearing on the right occurred when the bearing was not rotating. Acids forming in lubricant with water present etch the surface. Even small amounts of water are dangerous: 0.1 percent water in the lubricant can reduce the effective viscosity by 50 percent.

– Micro movement of mating parts

– Oxidation of asperities

– Powdery rust

– Loss of material

– Occurs in fit interfaces


Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Moisture corrosion

False brinelling

Fretting corrosion

When an interference fit is required, it must be sufficient to prevent fretting corrosion. Fretting corrosion is the mechanical wearing of material from movement between two surfaces resulting in oxidation or rust colored appearance. The corrosion is usually found on the inner ring bore or outer ring OD, and corresponds to load zone areas.



Bearing damage is also caused by bearing seats that are concave, convex, tapered or excessively worn. On such a seat, a bearing ring cannot make contact throughout its width. The ring therefore deflects under the loads and fatigue cracks commonly appear axially along the raceway.

– Rolling element / raceway– Micro movements / elastic deformations– Vibrations– Corrosion / wear: shiny or reddish

depressions– Stationary: Damage at rolling element

spacing – Rotating: Damage exhibits parallel flutes


Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Moisture corrosion

False brinelling

Fretting corrosion

Rolling bearings exposed to vibration while the shafts are not rotating are subject to damage called false brinelling. The evidence can be either bright polished depressions or the characteristic red-brown stain of fretting. The oxidation rate at the point of contact determines the appearance. Variation in the vibration load causes minute sliding in the area of contact between rolling elements and raceways. Small particles of material are set free from the contact surfaces and may, or may not be, immediately oxidized. The debris formed acts as a lapping agent, and accelerates the wear.

Since false brinelling is a true wear condition, such damage can be observed even though the forces applied during vibration are much smaller than those corresponding to the static carrying capacity of the bearing. However, the damage is more extensive as the contact load on the rolling elements increases.

False brinelling occurs most frequently during transportation of assembled machines. Vibration fed through a foundation can generate false brinelling of a shaft that is not rotating. False brinelling during transportation can always be minimized and usually eliminated by temporary structures that prevent any rotation or axial movement of the shaft.

Another identification of damage of this type is the spacing of the marks on the raceway. The spacing of false brinelling will be equal to the distance between the rolling elements, just as it is in some types of true brinelling. If the bearing has rotated slightly between periods of vibration, more than one pattern of false brinelling damage may be seen.

False Brinelling Caused by Static Vibration

False brinelling vs. True Brinelling

True brinell (denting) will still show machine marks in the dented area.



A combination of vibration and abrasion in a rotating bearing is seen in the wavy pattern. When these waves are more closely spaced, the pattern is called fluting and appears similar to electric erosion. False Brinelling may be distinguished from Electrical Erosion by the presence of small pits in the raceway surfaces, visible under magnification. Another indicator is color. Brinelling damage is typically light grayish in color, while electrical erosion is often dark gray, or nearly black. Metallurgical examination may be necessary to distinguish between fluting caused solely by abrasive and vibration or by vibration and passage of electric current.

Individual electric marks, pits, and fluting have been produced in test bearings. Both alternating and direct current can cause the damage. Amperage rather than voltage governs the amount of damage. When a bearing is under radial load, greater internal looseness in the bearing appears to result in greater electrical damage for the same current. In a double-row bearing loaded in thrust, little, if any damage results in the thrust-carrying row, although the opposite row may be damaged.

In certain electrical machinery applications, there is the possibility that electric current will pass through a bearing. Current that seeks ground through the bearing can be generated from stray magnetic fields in the machinery. It can also be caused by welding on some part of the machine with the ground attached, requiring the circuit to pass through the bearing.

An electric current can be generated by static electricity, emanating from charged belts or from manufacturing processes involving leather, paper, cloth, or rubber. This current can pass through the shaft to the bearing and then to ground. When the current is broken at the contact surfaces between rolling elements and raceways, arcing results. This produces very localized high temperature and consequent damage. The overall damage to the bearing is in proportion to the number and size of individual damage points.

Another type of electrical damage occurs when current passes during prolonged periods and the number of individual pits accumulate drastically. The result is fluting. This condition can occur in ball or roller bearings. Flutes can develop considerable depth, producing noise and vibration during operation and eventual



fatigue from local overstressing. The formation of flutes rather than a homogeneous dispersion of pits cannot be clearly explained. It is possible that it is related to initial synchronization of shocks or vibrations and the breaking of the current. Once the fluting has started, it is probably a self-perpetuating phenomenon.

SKF Electric Arcing Solutions

Silicon Nitride Balls are natural electrical insulators, providing a simple solution instead of shaft grounding or housing insulation. Costs have declined significantly since ceramic balls were introduced in the 1990s. Insocoat, a proprietary coating process, is available from stock on popular sizes, and made-to-order on request. Check SKF for availability.

– Static or shock loads

– Plastic deformations

– Depressions at rolling element spacing

– Handling damage

Indentation

Electrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformationOverload

Indents by handling

Indents from debris

Plastic deformation implies that the material’s elastic deformation limit has been exceeded and has flowed permanently into a new shape.

In this case, hammer blows applied directly to the bearing have caused the plastic deformation observed. In addition to the damage caused to the bearing, personal safety can be compromised if the bearing fractures.

– Localized overloading

– Over-rolling of particles = dents

– Caused by soft / hardened steel / hard mineral particles

Indentation

Electrical erosion

Wear

Corrosion

Fracture

Fatigue


Indents by handling

Indents from debris

Denting eventually leads to spalling. Close examination of spalls will reveal their true origin (subsurface or surface initiated). Recent studies by SKF indicate that denting of as little as ten percent of the rolling surfaces of the bearing causes a 90 percent reduction in predicted life.

– Localized overloading

– Nicks caused by hard / sharp objects

Indentation

Electrical erosion

Wear

Corrosion

Fracture

Fatigue


Indents by handling

Indents from debris

Dents from debris or handling damage leave a depression in the bearing surface. Under load, the front and back edges of the dent act as a stress riser. When over-rolled under elastohydrodynamic pressures, higher than normal local stresses result. This leads to



localized spalling in the dented area. The spalled material creates additional dents, further accelerating the bearing’s premature failure.

This type of damage is commonly seen when a puller is used to remove the bearing via the outer ring. Identification of root cause is aided by noting that the indentations are at intervals equal to the roller spacing. It may be possible to disassemble Spherical or CARB bearings to allow inner ring removal without damage.

Cylindrical roller bearings are subject to damage if care is not taken to support the rollers during mounting. One technique is to insert a plastic or cardboard sleeve inside the roller set to prevent roller drop. As the roller set is mounted, the sleeve is pushed out by the inner ring.

In addition to plastic deformation, high impact loads or local overstress may fracture bearing components. Common causes include hammer blows and improper distribution of forces from bearing pullers. This is one reason three-arm jaw-type pullers are generally preferred over two-jaw types.

Excessive press fit or burrs trapped under bearing rings can also lead to fractures. With modern inspection techniques, material failure is rare. In this case, cut out the cracked section of the ring to examine the crack propagation marks. The crack pattern may reveal the cause of the fracture.

– Exceeding fatigue strength under bending

– Crack initiation / propagation

– Finally forced fracture

– Rings and Cages

Forced fracture

Fatigue fracture

Thermal cracking

Electrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformation



– High sliding and / or insufficient lubrication

– High friction heat– Cracks at right

angle to sliding direction

Forced fracture

Fatigue fracture

Thermal cracking

Electrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Often accompanied with fretting, rings unsupported by proper fit may fracture catastrophically. Components subjected to large or cyclic moment (bending) loads may also fracture.

Also called “heat checking,” thermal cracking results from relatively high speeds generating extreme temperatures between sliding surfaces. Surfaces may also be discolored from the heat. Lacks of proper fit and improper repair practices, allowing ring creep, are two common causes of thermal cracking.

Electrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Forced fracture

Fatigue fracture

Thermal cracking

Overload

IndentationIndents from debris

Indents by handling

Moisture corrosion

Frictional corrosionFretting corrosion

False brinellingExcessive voltage

Current leakage

Adhesive wear

Abrasive wear


Subsurface fatigue

Electrical erosion

Wear

Corrosion

Fracture

Fatigue

Plastic deformation

Forced fracture

Fatigue fracture

Thermal cracking

Overload

IndentationIndents from debris

Indents by handling

Moisture corrosion

Frictional corrosionFretting corrosion

False brinellingExcessive voltage

Current leakage

Adhesive wear

Abrasive wear


Subsurface fatigue

Using the failure mode chart to assist in analyzing failed bearings will reduce guesswork. Speedier root cause identification for corrective action is the result.

Securing Evidence Collect Operating Data, Monitoring Data

Collect Lubricant Samples

Check Bearing Environment(s)

Assess Bearing(s) in Mounted Condition

Mark Mounting Position(s)

Remove, Mark, and Bag Bearing(s) and Parts

Check Bearing Seats

Standardized, written failure analysis procedures are recommended to achieve consistent, reliable root cause identification. External failure analysis services may be justified for high-cost or critical machinery. A failure analysis form for data collection is available from SKF Applications Engineering.

Conducting the Analysis Examine bearing(s) and parts

Record visual observations

Use the failure modes to eliminate possible improbable causes and determine the original cause of the failure

Contact external resources for assistance, if needed

Initiate corrective action, if desired
