An Introduction
To
Failure Modes
of
Rolling Element
Bearings
Overview
SKF Center for Learning
1
An introduction to failure modes of rolling element bearings
This 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
damage.
will
be
mounted
without
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
2
•
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.”
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.
SKF Center for Learning
An introduction to failure modes of rolling element bearings
Load-Path
Meanings
Patterns
and
their
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 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 doublerow 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 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 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.
SKF Reliability Systems - Bearing Maintenance and Service
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An introduction to failure modes of rolling element bearings
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.
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.
Misalignment
Roll 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.
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
4
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-ofround 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.
SKF Center for Learning
An introduction to failure modes of rolling element bearings
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.
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 Classification
1. Causes of Failures
Characteristics
Have
Identifiable
2. Failure Mechanisms
Failure Modes
Have
Identifiable
3. Observed Damage Can Identify Failure
Causes
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-ofround where the applied load is superimposed
most on the internal preload.
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
Characteristics
Have
Identifiable
Although there are many causes for failures,
each one can be uniquely identified.
•
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.
Failure Mechanisms
Failure Modes
Have
Identifiable
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.
In unbalanced applications, the load does not
rotate in phase with either ring, producing a
SKF Reliability Systems - Bearing Maintenance and Service
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An introduction to failure modes of rolling element bearings
“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.
Fatigue
Wear
Corrosion
Electrical erosion
Plastic deformation
Fracture
6
Subsurface fatigue
Surface initiated fatigue
– Surface distress
– Reduced
lubrication regime
– Sliding motion
– Burnishing,
glazing
– Asperity
microcracks
– Asperity
microspalls
40 µm
SKF Center for Learning
An introduction to failure modes of rolling element bearings
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
SKF Reliability Systems - Bearing Maintenance and Service
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An introduction to failure modes of rolling element bearings
other. As metal is pulled from the surface, pits
appear and frosting advances to pulling.
Fatigue
Wear
Abrasive wear
Adhesive wear
Corrosion
Electrical erosion
Plastic deformation
Fracture
–
–
–
–
Progressive Removal of Material
Accelerating Process
Inadequate Lubrication
Ingress of Dirt Particles
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:
8
•
Sufficient elastohydrodynamic film prevents
surface distress (glazing, frosting).
•
Proper lubrication guards against smearing
and sliding surface wear.
SKF Center for Learning
An introduction to failure modes of rolling element bearings
•
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)
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.
– Higher temperatures may cause loss of Hardness
– Loss of 2-4 points of Rockwell Hardness reduces life 50%
Fatigue
Wear
Corrosion
Temperature Discoloration
Straw Color:
Moisture corrosion
Frictional corrosion
Electrical erosion
~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
Plastic deformation
Fracture
Fretting corrosion
False brinelling
– Micro
movement of
mating parts
– Oxidation of
asperities
– Powdery rust
– Loss of material
– Occurs in fit
interfaces
Above 500°C (~1000°F)
Fatigue
Wear
Corrosion
Electrical erosion
Plastic deformation
Fracture
Moisture corrosion
Frictional corrosion
Fretting corrosion
False brinelling
– Oxidation / rust
– Chemical
reaction
– Corrosion pits /
flaking
– Etching (water /
oil mixture)
SKF Reliability Systems - Bearing Maintenance and Service
When an interference fit is required, it must be
sufficient to prevent fretting corrosion. Fretting
corrosion is the mechanical wearing of material
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An introduction to failure modes of rolling element bearings
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.
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.
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.
Fatigue
Wear
Corrosion
Moisture corrosion
Frictional corrosion
Electrical erosion
Plastic deformation
Fracture
Fretting corrosion
False brinelling
–
–
–
–
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
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.
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.
Since false brinelling is a true wear condition,
such damage can be observed even though the
10
SKF Center for Learning
An introduction to failure modes of rolling element bearings
False Brinelling Caused by Static
Vibration
False brinelling vs. True Brinelling
True brinell (denting) will still show machine
marks in the dented area.
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.
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.
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.
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.
SKF Reliability Systems - Bearing Maintenance and Service
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An introduction to failure modes of rolling element bearings
Fatigue
Wear
Corrosion
Electrical erosion
Plastic deformation
Fracture
Overload
Indents from debris
Indentation
Indents by handling
– Static or shock loads
– Plastic deformations
– Depressions at rolling element
spacing
– Handling damage
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.
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
Fatigue
Wear
Corrosion
Electrical erosion
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12
Plastic deformation
Fracture
Overload
Indentation
Indents from debris
Indents by handling
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.
SKF Center for Learning
An introduction to failure modes of rolling element bearings
– Localized
overloading
– Nicks caused by
hard / sharp
objects
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.
Fatigue
Wear
Corrosion
Electrical erosion
Plastic deformation
Fracture
Overload
Indentation
Indents from debris
Indents by handling
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.
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.
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.
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.
SKF Reliability Systems - Bearing Maintenance and Service
13
An introduction to failure modes of rolling element bearings
Fatigue
Wear
Corrosion
Electrical erosion
Plastic deformation
Forced fracture
Fracture
Fatigue fracture
Thermal cracking
Fatigue
Wear
Corrosion
Electrical erosion
– Exceeding
fatigue strength
under bending
– Crack initiation
/ propagation
– Finally forced
fracture
– Rings and
Cages
– High sliding and
/ or insufficient
lubrication
– High friction heat
– Cracks at right
angle to sliding
direction
Plastic deformation
Forced fracture
Fracture
Fatigue fracture
Thermal cracking
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.
Fatigue
Wear
Corrosion
Electrical erosion
Plastic deformation
Subsurface fatigue
Surface initiated fatigue
Abrasive wear
Adhesive wear
Moisture corrosion
Frictional corrosion
Excessive voltage
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
Fretting corrosion
False brinelling
Current leakage
Overload
Indentation
Forced fracture
Fracture
guesswork. Speedier root cause identification
for corrective action is the result.
Indents from debris
Indents by handling
Fatigue fracture
Thermal cracking
Using the failure mode chart to assist in
analyzing
failed
bearings
will
reduce
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SKF Center for Learning
An introduction to failure modes of rolling element bearings
SKF Reliability Systems - Bearing Maintenance and Service
15