BEARING FAILURE
Load/Contact Patterns
As bearings rotate, the raceways of the inner ring and outer ring
make contact with the rolling elements. This results in a Wear path on
both the rolling elements and raceways. Running traces are useful,
because they indicate the load conditions. They should be carefully
observed when bearings are disassembled.
If the running traces are clearly defined, it is possible to determine
whether the bearing is carrying a radial load, axial load or moment load.
Additionally running traces can help determine the accuracy of bearing
roundness, confirm whether unexpected loads or large mounting errors
occurred, and shed light on probable causes of bearing damage. (R1)
Typical Running Traces of Deep Groove Ball Bearings
(a) shows the most common running trace generated when the inner
ring rotates under a radial load only. (e) through (h) show different
running traces that result in a shortened life due to their adverse effect on
bearings. (R2)
(a)
(b)
(c)
(d)
Inner ring rotation Outer ring rotation
Inner ring or
Inner ring rotation
Radial load
Radial load
outer ring rotation Radial and axial
Axial load in one
loads
direction
(e)
(f)
(g)
Inner ring rotation Inner ring rotation Inner ring rotation
Axial load and
Moment load
Housing bore is
misalignment
(Misalignment)
oval
(h)
Inner ring rotation
No radial internal
clearance
(Negative operating
clearance)
Typical Running Traces of Roller Bearings
(i) Shows the outer ring running trace when a radial load is properly
applied to a cylindrical roller bearing which has a load on a rotating inner
ring. (j) Shows the running trace in the case of shaft bending or relative
inclination between the inner and outer rings. This misalignment leads to
the generation of slightly shaded (dull) bands in the width direction.
Traces are diagonal at the beginning and end of the loading zone. For
double-row tapered roller bearings where a single load is applied to the
rotating inner ring, (k) shows the running trace on the outer ring under
radial load while (I) shows the running trace on the outer ring under axial
load. When misalignment exists between the inner and the outer rings,
then the application of a radial load causes running traces to appear on the
outer ring as shown in (m). (R3)
(i)
Inner ring
rotation
Radial load
(j)
(k)
Inner ring
Inner ring
rotation
rotation
Moment load Radial load
(Misalignment)
(l)
(m)
Inner ring
rotation
Axial load
Inner ring
rotation
Radial and
moment loads
(Misalignment)
Damage Types & Causes
In general, if rolling bearings are used correctly, they will survive to
their predicted fatigue life. Bearings, however, often fail prematurely due
to avoidable mistakes. The causes of, this premature failure include
improper mounting, mishandling, poor lubrication, entry of foreign
matter or abnormal heat generation.
For example, one cause of premature failure is rib Scoring which is
due to insufficient lubrication, use of improper lubricant, faulty
lubrication system, entry of foreign matter, bearing mounting error,
excessive deflection of the shaft or some combination of these. If all
conditions are known for the times both before and after the failure,
including the application, the operating conditions, and environment, then
a countermeasure can be determined by studying the nature of the failure
and its probable causes. A successful countermeasure will reduce similar
failures or prevent them from happening again.
Examples of bearing damage and countermeasures are presented in
the following sections. Please consult these sections when trying to
determine the cause of bearing damage. (R4)
Flaking
Peeling
Scoring
Smearing
Fracture
Cracks
Cage Damage
Denting
Pitting
Wear
Fretting
False Brinelling
Creep
Seizure
Electrical
Corrosion
Rust and
Corrosion
Mounting Flaws
Discoloration
1. Flaking (R5)
Damage Condition
Possible Causes
Countermeasures
Flaking occurs when small
Excessive load; Poor
 Reconfirm the bearing
pieces of bearing material are mounting (misalignment);
application and check the
split off from the smooth
Moment load; Entry of
load conditions
surface of the raceway or
foreign debris, water
 Improve the mounting
rolling elements due to
penetration; Poor lubrication,
method
rolling fatigue, thereby
improper lubricant;
 Improve the sealing
creating regions having rough Unsuitable bearing clearance; mechanism, prevent rusting
and coarse texture.
Improper precision for shaft
during non-running
or housing, unevenness in  Use a lubricant with a proper
housing rigidity, large shaft
viscosity, improve the
bending; Progression from
lubrication method
rust, corrosion pits,
 Check the precision of shaft
Smearing, dents (brinelling)
and housing
 Check the bearing internal
clearance
Photo 1-1
Photo 1-2
Photo 1-3
Part : Inner ring of an angular contact
ball bearing
Symptom : Flaking around half of the
circumference of the raceway surface
Cause : Poor lubrication due to entry of
cutting coolant into bearing
Part : Inner ring of an angular contact
ball bearing
Symptom : Flaking diagonally along
raceway
Cause : Poor alignment between shaft
and housing during mounting
Part : Inner ring of deep groove ball
bearing
Symptom : Flaking of raceway at ball
pitch
Cause : Dents due to shock load during
mounting
Photo 1-4
Part : Inner ring of an angular contact
ball bearing
Symptom : Flaking of raceway at ball
pitch
Cause : Dents due to shock load while
stationary
Part : Outer ring of Photo 1-4
Symptom : Flaking of raceway surface
at ball pitch
Cause : Dents due to shock load while
stationary
Photo 1-5
Part : Balls of Photo 1-4
Symptom : Flaking of ball surface
Cause : Dents due to shock load while
stationary
Photo 1-6
Part : Inner ring of a spherical roller
bearing
Symptom : Flaking of only one
raceway over its entire circumference
Cause : Excessive axial load
Photo 1-7
Part : Outer ring of Photo 1-7
Symptom : Flaking of only one
raceway over its entire circumference
Cause : Excessive axial load
Photo 1-8
Part : Inner ring of a spherical roller
bearing
Symptom : Flaking of only one row of
raceway
Cause : Poor lubrication
Photo 1-9
Photo 1-10
Part : Rollers of a cylindrical roller
bearing
Symptom : Premature Flaking occurs
axially on the rolling surfaces
Cause : Scratches caused during
improper mounting
2. Peeling (R6)
Damage Condition
Possible Causes
Countermeasures
Dull or cloudy spots appear
Unsuitable lubricant
Select a proper lubricant
on surface along with light
Entry of debris into
Improve the sealing
Wear. From such dull
lubrication
mechanism
spots, tiny Cracks are
Rough surface due to poor Improve the surface finish
generated downward to a
lubrication
of the rolling mating parts
depth of 5-10 µm. Small
Surface roughness of
particles fall off and minor
mating rolling parts
Flaking occurs widely.
Part : Inner ring of a spherical roller
bearing
Symptom : Rounded areas of Peeling
Cause : Poor lubrication
Photo 2-1
Part : Enlargement of Photo 2-1
Photo 2-2
Part : Convex rollers of Photo 2-1
Symptom : Rounded areas of Peeling
on the center of the rolling surfaces
Cause : Poor lubrication
Photo 2-3
Photo 2-4
Part : Outer ring of a spherical roller
bearing
Symptom : Peeling occurs near the
shoulder of the raceway over the entire
circumference
Cause : Poor lubrication
3. Scoring (R7)
Damage Condition
Possible Causes
Countermeasures
Scoring is surface damage Excessive load, excessive Check the size of the load
due to accumulated small
preload
Adjust the preload
seizures caused by sliding
Poor lubrication
Improve the lubricant and
under improper lubrication Particles are caught in the
the lubrication method
or severe operating
surface
Check the precision of the
conditions. Linear damage Inclination of inner and
shaft and housing
appears circumferentially
outer rings
on the raceway and roller
Shaft bending
surfaces. Cycloidal shaped Poor precision of the shaft
damage on the roller ends
and housing
and Scoring on the rib
surface contacting roller
ends also occur.
Part : Inner ring of a spherical roller
bearing
Symptom : Scoring on large rib face of
inner ring
Cause : Roller slippage due to sudden
Photo 3-1
acceleration and deceleration
Part : Inner ring of a tapered roller
thrust bearing
Symptom : Scoring on the face of inner
ring rib
Cause : Worn particles mixed with
lubricant, and breakdown of oil film
Photo 3-1
due to excessive load
Part : Inner ring of a spherical thrust
roller bearing
Symptom : Scoring on the rib face of
inner ring
Cause : Debris caught in surface, and
Photo 3-1
excessive axial load
Part : Cage of a deep groove ball
bearing
Symptom : Scoring on the pressed-steel
cage pockets
Cause : Entry of debris
Photo 3-1
Part : Convex rollers of Photo 3-1
Symptom : Scoring on roller end faces
Cause : Roller slippage due to sudden
acceleration and deceleration
Photo 3-2
Part : Rollers of a double-row
cylindrical roller bearing
Symptom : Scoring. on the roller end
faces
Cause : Poor lubrication and excessive
axial load
Photo 3-2
Part : Convex rollers of Photo 3-5
Symptom : Scoring. on the roller end
faces
Cause : Debris caught in surface, and
excessive axial load
Photo 3-2
4. Smearing (R8)
Damage Condition
Possible Causes
Smearing is surface
High speed and light load
damage which occurs from
Sudden
a collection of small
acceleration/deceleration
seizures between bearing
Improper lubricant
components caused by oil
Entry of water
film rupture and/or sliding.
Surface roughening occurs
along with melting.
Photo 4-1
Photo 4-3
Photo 4-5
Countermeasures
Improve the preload
Improve the bearing
clearance
Use a lubricant with good
oil film formation ability
Improve the sealing
mechanism
Part : Inner ring of a cylindrical roller
bearing
Symptom : Smearing around
circumference of raceway surface
Cause : Roller slippage due to
excessive grease quantity
Part : Inner ring of a spherical roller
bearing
Symptom : Smearing around
circumference of raceway surface
Cause : Poor lubrication
Part : Inner ring of a spherical roller
bearing
Symptom : Partial Smearing around
circumference of raceway surface
Cause : Poor lubrication
Part : Convex rollers of Photo 4-5
Symptom : Smearing of rolling surfaces
Cause : Poor lubrication
Photo 4-7
Part : Outer ring of Photo 4-1
Symptom : Smearing around
circumference of raceway surface
Cause : Roller slippage due to
excessive grease quantity
Photo 4-2
Part : Outer ring of Photo 4-3
Symptom : Smearing around
circumference of raceway surface
Cause : Poor lubrication
Photo 4-4
Part : Outer ring of Photo 4-5
Symptom : Partial Smearing around
circumference of raceway surface
Cause : Poor lubrication
Photo 4-6
5. Fracture (R9)
Damage Condition
Fracture refers to small
pieces which were broken
off due to excessive load or
shock load acting locally on
a roller corner or rib of a
raceway ring.
Possible Causes
Impact during mounting
Excessive load
Poor handling such as
dropping
Countermeasures
Improve the mounting
method (shrink fit, use of
proper tools)
Reconsider the load
conditions
Provide enough back-up
and support for the bearing
rib
Part : Inner ring of a double-row
cylindrical roller bearing
Symptom : Chipping of the center rib
Cause : Excessive load during
mounting
Photo 5-1
Part : Inner ring of a spherical thrust
roller bearing
Symptom : Fracture of the large rib
Cause : Repeated shock load
Photo 5-3
Part : Inner ring of a tapered roller
bearing
Symptom : Fracture of the cone back
face rib
Cause : Large shock during mounting
Photo 5-2
Photo 5-4
Part : Outer ring of a solid type needle
roller bearing
Symptom : Fracture of the outer ring rib
Cause : Roller inclination due to
excessive loading (Needle rollers are
long compared to their diameter. Under
excessive or uneven loading, rollers
become inclined and push against the
ribs.)
6. Cracks (R10)
Damage Condition
Possible Causes
Countermeasures
Cracks in the raceway ring
Excessive interference
Correct the interference
and rolling elements.
Excessive load, shock load Check the load conditions
Continued use under this
Progression of Flaking
Improve the mounting
condition leads to larger
Heat generation and
method
Cracks or Fractures.
Fretting caused by contact Use an appropriate shaft
between mounting parts
shape
and raceway ring
Heat generation due to
Creep
Poor taper angle of tapered
shaft
Poor cylindricality of shaft
Interference with bearing
chamfer due to a large
shaft corner radius
Part : Inner ring of a spherical roller
bearing
Symptom : Rounded areas of Peeling
Cause : Poor lubrication
Photo 6-1
Photo 6-3
Part : Outer ring of a double-row
cylindrical roller bearing
Symptoms : Cracks propagated
outward in the axial and
circumferential directions from the
Flaking origin on the raceway surface
Cause : Flaking from a flaw due to
shock
Part : Raceway surface of outer ring
in Photo 6-4
Symptom : Outside surface crack
propagating to the raceway
Photo 6-5
Part : Cross section of cracked inner
ring in Photo 6-6
Symptom : Origin is directly beneath
the raceway surface
Photo 6-7
Photo 6-2
Photo 6-4
Photo 6-6
Part : Outer ring of a double-row
cylindrical roller bearing
Symptom : Thermal Cracks on the
outer ring side face
Cause : Abnormal heat generation
due to contact sliding between mating
part and face of outer ring
Part : Outer ring of a double-row
cylindrical roller bearing used for
outer ring rolling (Outer ring rotation)
Symptom : Cracks on outside surface
Cause : Flat Wear and heat generation
due to non-rotation of the outer ring
Part : Inner ring of a spherical roller
bearing
Symptom : Axial Cracks on raceway
surface
Cause : Large fitting stress due to
temperature difference between shaft
and inner ring
Part : Roller of a spherical roller
bearing
Symptom : Axial Cracks on rolling
surface
Photo 6-8
7. Cage Damage (R11)
Damage Condition
Cage damage includes:
Cage deformation,
Fracture and Wear
Fracture of cage pillars
Deformation of side face
Wear of pocket surface
Wear of guide surface
Possible Causes
Poor mounting (Bearing
misalignment)
Poor handling
Large moment load
Shock and large vibration
Excessive rotation speed,
sudden acceleration and
deceleration
Poor lubrication
Temperature rise
Countermeasures
Check the mounting
method
Check the temperature,
rotation and load
conditions
Reduce the vibration
Use an appropriate shaft
shape Select a different
cage type
Select a different
lubrication method and/or
lubricant
Part : Cage of a deep groove ball
bearing
Symptom : Fracture of pressed-steel
cage pocket
Photo 7-1
Part : Cage of an angular contact ball
bearing
Symptom : Fracture of machined hightension brass cage
Photo 7-3
Photo 7-5
Part : Cage of an angular contact ball
bearing
Symptom : Pressed-steel cage
deformation
Cause : Shock load due to poor
handling
Part : Cage of a cylindrical roller
bearing
Symptom : Deformation and Wear of
machined high-tension brass cage
Photo 7-7
Photo 7-2
Part : Cage of an angular contact ball
bearing
Symptom : Pocket pillar Fractures of a
cast iron machined cage
Cause : Abnormal load action on cage
due to misaligned mounting between
inner and outer rings
Part : Cage of a tapered roller bearing
Symptom : Pillar Fractures of pressedsteel cage
Photo 7-4
Photo 7-6
Part : Cage of a cylindrical roller
bearing
Symptom : Deformation of the side
face of machined high-tension brass
cage
Cause : Large shock during mounting
Part : Cage of an angular contact ball
bearing
Symptom : Stepped Wear on the
outside surface and pocket surface of
machined high-tension brass cage
Photo 7-8
8. Denting (R12)
Damage Condition
Possible Causes
Countermeasures
When debris such as small
Debris such as metallic
Clean the housing
metallic particles are
particles are caught in the
Improve the sealing
caught in the rolling
surface
mechanism
contact zone, Denting
Excessive load
Filter the lubrication oil
occurs on the raceway
Shock during transport or Improve the mounting and
surface or rolling element
mounting
handling methods
surface. Denting can occur
at the rolling element pitch
interval if there is a shock
during the mounting
(brinell dents).
Part : Inner ring of a double-row
tapered roller bearing
Symptom : Frosted raceway surface
Cause : Debris caught in the surface
Photo 8-1
Photo 8-3
Part : Inner ring of a tapered roller
bearing
Symptom : Small and large
indentations occur over entire raceway
surface
Cause : Debris caught in the surface
Part : Outer ring of a double-row
tapered roller bearing
Symptom : Indentations on raceway
surface
Cause : Debris caught in the surface
Photo 8-2
Part : Tapered rollers of Photo 8-3
Symptom : Small and large
indentations occur over the rolling
surface
Cause : Debris caught in the surface
Photo 8-4
9. Pitting (R13)
Damage Condition
Possible Causes
Pitting has a dull luster and Debris becomes caught in
appears on the rolling
the lubricant
element surface or raceway Exposure to moisture in
surface.
Poor lubrication
Countermeasures
Improve the sealing
mechanism
Filter the lubrication oil
thoroughly
Use a proper lubricant
Part : Outer ring of a slewing bearing
Symptom : Pitting on the raceway
surface
Cause : Rust
Photo 9-1
Part : Ball of Photo 9-1
Symptom : Pitting on the rolling
element surface
Photo 9-2
10. Wear (R14)
Damage Condition
Possible Causes
Wear is surface
Entry of debris
deterioration due to sliding Progression from rust and
friction at the surface of
electrical corrosion
the raceway, rolling
Poor lubrication
elements, roller end faces,
Sliding due to irregular
rib face, cage pockets, etc. motion of rolling elements
Photo 10-1
Photo 10-3
Photo 10-2
Photo 10-3
Countermeasures
Improve the sealing
mechanism
Clean the housing
Filter the lubrication oil
thoroughly
Check the lubricant and
lubrication method
Prevent misalignment
Part : Inner ring of a cylindrical roller
bearing
Symptom : Many pits occur due to
electrical corrosion, and wave-shaped
Wear on raceway surface
Cause : Electrical corrosion
Part : Inner ring of a double-row
tapered roller bearing
Symptom : Fretting Wear of raceway
and stepped Wear on the rib face
Cause : Fretting progression due to
excessive load while stationary
Part : Outer ring of a spherical roller
bearing
Symptom : Wear with a wavy or
concave-convex texture on loaded side
of raceway surface
Cause : Entry of debris under repeated
vibration while stationary
Part : Tapered rollers of Photo 10-3
Symptom : Stepped Wear on the roller
head end faces
Cause : Fretting progression due to
excessive load while stationary
11. Fretting (R15)
Damage Condition
Wear occurs due to
repeated sliding between
the two surfaces.
Fretting occurs at fitting
surface and also at contact
area between raceway ring
and rolling elements.
Fretting corrosion is
another term used to
describe the reddish brown
or black worn particles.
Possible Causes
Poor lubrication
Vibration with a small
amplitude
Insufficient interference
Countermeasures
Use a proper lubricant
Apply a preload
Check the interference fit
Apply a film of lubricant to
the fitting surface
Part : Inner ring of a deep groove ball
bearing
Symptom : Fretting on the bore surface
Cause : Vibration
Photo 11-1
Part : Outer ring of a double-row
cylindrical roller bearing
Symptom : Fretting on the raceway
surface at roller pitch intervals
Photo 11-3
Part : Inner ring of an angular contact
ball bearing
Symptom : Fretting over entire
circumference of bore surface
Cause : Insufficient interference fit
Photo 11-2
12. False Brinelling (R16)
Damage Condition
Possible Causes
Countermeasures
Among the different types Oscillation and vibration of
Secure the shaft and
of Fretting, false brinelling a stationary bearing during
housing during
is the occurrence of hollow such times as transporting
transporting
spots that resemble brinell Oscillating motion with a Transport with the inner
dents and are due to Wear
small amplitude
and outer rings packed
caused by vibration and
Poor lubrication
separately
swaying at the contact
Reduce the vibration by
points between the rolling
preloading
elements and raceway.
Use a proper lubricant
Photo 12-1
Part : Inner ring of a deep groove ball
bearing
Symptom : False brinelling on the
raceway
Cause : Vibration from an external
source while stationary
Part : Outer ring of a thrust ball bearing
Symptom : False brinelling of raceway
surface at ball pitch
Cause : Repeated vibration with a small
oscillating angle
Photo 12-3
Part : Outer ring of Photo 12-1
Symptom : False brinelling on the
raceway
Cause : Vibration from an external
source while stationaryt
Photo 12-2
Photo 12-4
Part : Rollers of a cylindrical roller
bearing
Symptom : False brinelling on rolling
surface
Cause : Vibration from an external
source while stationary
13. Creep (R17)
Damage Condition
Possible Causes
Creep is the phenomenon Insufficient interference or
in bearings where relative
loose fit
slippage occurs between
Insufficient sleeve
fitting surfaces and thereby
tightening
creates a clearance between
them surface. Creep causes
a shiny appearance,
occasionally with Scoring
or Wear.
Countermeasures
Check the interference,
and prevent rotation
Correct the sleeve
tightening
Study the shaft and
housing precision
Preload in the axial
direction
Tighten the raceway ring
side face
Apply adhesive to the
fitting surface
Apply a film of lubricant to
the fitting surface
Part : Inner ring of a spherical roller
bearing
Symptom : Creep accompanied by
Scoring of bore surface
Cause : Insufficient interference
Photo 13-1
Photo 13-2
Part : Outer ring of a spherical roller
bearing
Symptom : Creep over entire
circumference of outside surface
Cause : Loose fit between outer ring
and housing
14. Seizure (R18)
Damage Condition
When sudden overheating
occurs during rotation, the
bearing becomes discolored.
Then, the raceway rings,
rolling elements, and cage
will soften, melt and deform
as damage accumulates.
Photo 14-1
Possible Causes
Poor lubrication
Excessive load (Excessive
preload)
Excessive rotational speed
Excessively small internal
clearance
Entry of water and debris
Poor precision of shaft and
housing Excessive shaft
bending
Countermeasures
Study the lubricant and
lubrication method
Reinvestigate the suitability
of the bearing type selected
Study the preload, bearing
clearance, and fitting
Improve the sealing
mechanism
Check the precision of the
shaft and housing
Improve the mounting
method
Part : Inner ring of a spherical roller
bearing
Symptom : Raceway is discolored and
melted. Worn particles from the cage
were rolled and attached to the
raceway.
Cause : Insufficient lubrication
Part : Inner ring of an angular contact
ball bearing
Symptom : Raceway Discoloration,
melting at ball pitch intervals
Cause : Excessive preload
Photo 14-3
Part : Balls and cage of Photo 14-3
Symptom : Cage is damaged by
melting, balls discolored and melted
Cause : Excessive preload
Photo 14-3
Part : Convex rollers of Photo 14-1
Symptom : Discoloration and melting
of roller rolling surface, adhesion of
worn particles from cage
Cause : Insufficient lubrication
Photo 14-2
Part : Outer ring in Photo 14-3
Symptom : Raceway Discoloration,
melting at ball pitch intervals
Cause : Excessive preload
Photo 14-4
15. Electrical Corrosion (R19)
Damage Condition
When electric current passes
through a bearing, arcing and
burning occur through the
thin oil film at points of
contact between the raceway
and rolling elements. The
points of contact are melted
locally to form "fluting" or
groove-like corrugations
which can be seen by the
naked eye.
Magnification of these
grooves reveals crater-like
depressions which indicate
melting by arcing.
Possible Causes
Electric current passing
through a bearing
Countermeasures
Design electric circuits
which prevent current flow
through the bearings
Insulate the bearing
Part : Inner ring of a tapered roller
bearing
Symptom : Striped pattern of corrosion
occurs on the raceway surface
Photo 15-1
Photo 15-3
Part : Inner ring of a cylindrical roller
bearing
Symptom : Belt pattern of electrical
corrosion accompanied by pits on the
raceway surface
Part : Tapered rollers in Photo 15-1
Symptom : Striped pattern of corrosion
occurs on the rolling surface
Photo 15-2
Part : Balls of a deep groove ball
bearing
Symptom : Dark color covering the
entire ball surfaces
Photo 15-4
16. Rust and Corrosion (R20)
Damage Condition
Possible Causes
Countermeasures
Bearing rust and corrosion
Entry of corrosive gas or
Improve the sealing
are pits on the surface of
water
mechanism
rings and rolling elements
Improper lubricant
Study the lubrication method
and may occur at the rolling Formation of water droplets Anti-rust treatment during
element pitch on the rings or
due to condensation of
periods of non-running
over the entire bearing
moisture
Improve the storage methods
surfaces.
High temperature and high
Improve the handling
humidity while stationary
method
Poor rust-preventive
treatment during transporting
Improper storage conditions
Improper handling
Part : Outer ring of a cylindrical roller
bearing
Symptom : Rust on the rib face and
raceway surface
Cause : Water entry
Photo 16-1
Part : Inner ring of a spherical roller
bearing
Symptom : Rust on raceway surface at
roller pitch intervals
Cause : Entry of water into lubricant
Photo 16-3
Photo 16-2
Photo 16-4
Part : Outer ring of a slewing ring
Symptom : Rust on raceway surface at
ball pitch intervals
Cause : Moisture condensation during
stationary periods
Part : Rollers of a spherical roller
bearing
Symptom : Pit-shaped rust on rolling
contact surface. Corroded portions.
Cause : Moisture condensation during
storage
17. Mounting Flaws (R21)
Damage Condition
Straight line scratches on
surface of raceways or
rolling elements caused
during mounting or
dismounting of bearing.
Photo 17-1
Possible Causes
Countermeasures
Inclination of inner and outer Use appropriate jig and tools
rings during mounting or Avoid shock loads by using a
dismounting
press machine
Shock load during mounting Center the relative mating
or dismounting
parts during mounting
Part : Inner ring of a cylindrical roller
bearing
Symptom : Axial scratches on raceway
surface
Cause : Inclination of inner and outer
rings during mounting
Part : Rollers of a cylindrical roller
bearing
Symptom : Axial scratches on rolling
surface
Cause : Inclination of inner and outer
rings during mounting
Photo 17-3
Part : Outer ring of a double-row
cylindrical roller bearing
Symptom : Axial scratches at roller
pitch intervals on raceway surface
Cause : Inclination of inner and outer
rings during mounting
Photo 17-2
18. Discoloration (R22)
Damage Condition
Discoloration of cages,
rolling elements and raceway
rings occurs due to their
reacting with lubricant at
high temperature
Photo 18-1
Photo 18-2
Possible Causes
Poor lubrication
Oil stain due to a reaction
with lubricant
High temperature
Countermeasures
Improve the lubrication
method
Part: Inner ring of an angular contact
ball bearing
Symptom: Bluish or purplish
Discoloration on raceway surface
Cause: Heat generation due to poor
lubrication
Part : Inner ring of a 4-point contact
ball bearing
Symptom : Bluish or purplish
Discoloration on raceway surface
Cause : Heat generation due to poor
lubrication
Bearing Fits (26)
1. Interference
For rolling bearings the bearing rings are fixed on the shaft or in the
housing so that slip or movement does not occur between the mated
surface during operation or under load. This relative movement
(sometimes called creep) between the fitted surfaces of the bearing and
the shaft or housing can occur in a radial direction, or in an axial
direction, or in the direction of rotation. This creeping movement under
load causes damage to the bearing rings, shaft or housing in the form of
abrasive wear, fretting corrosion or friction crack. This, in turn, can also
lead to abrasive particles getting into the bearing, which can cause
vibration, excessive heat, and lowered rotational efficiency. To ensure
that slip does not occur between the fitted surfaces of the bearing rings
and the shaft or housing, the bearing is usually installed with an
interference fit. The most effective interference fit is called a tight fit (or
shrink fit). The advantage of this “tight fit” for thin walled bearings is that
it provides uniform load support over the entire ring circumference
without any loss in load carrying capacity. However, with a tight
interference fit, ease of mounting and dismounting the bearings is lost;
and when using a non-separable bearing as a non-fixing bearing, axial
displacement is impossible.
2. Calculation of interference
1) Load and interference
The minimum required amount of interference for inner rings
mounted on solid shafts when acted on by radial loads, is found by
formula (7.1) and (7.2).
2) Temperature rise and interference
To prevent loosening of the inner ring on steel shafts due to
temperature increases (difference between bearing temperature and
ambient temperature) caused by bearing rotation, an interference fit must
be given. The required amount of interference can be found by formula
(7.3).
3) Effective interference and apparent interference
The effective interference (the actual interference after fitting) is
different from the apparent interference derived from the dimensions
measured value. This difference is due to the roughness or slight
variations of the mating surfaces, and this slight flattening of the uneven
surfaces at the time of fitting is taken into consideration. The relation
between the effective and apparent interference, which varies according
to the finish given to the mating surfaces, is expressed by formula (7.4).
4) Maximum interference
When bearing rings are installed with interference fit on shafts or
housings, the tension or compression stress may occur. If the interference
is too large, it may cause damage to the bearing rings and reduce the
fatigue life of the bearing. For these reasons, the maximum amount of
interference should be less than 1/1000 of the shaft diameter, or outside
diameter.
3. Fit selection
Selection of the proper fit is generally based on the following
factors: 1) the direction and nature of the bearing load, 2) whether the
inner ring or outer ring rotates, 3) whether the load on the inner or outer
ring rotates or not, 4) whether there is static load or direction
indeterminate load or not. For bearings under rotating loads or direction
indeterminate loads, a tight fit is recommended; but for static loads, a
transition fit or loose fit should be sufficient (see Table 2).
The interference should be tighter for heavy bearing loads or
vibration and shock load conditions. Also, a tighter than normal fit should
be given when the bearing is installed on hollow shafts or in housings
with thin walls, or housings made of light allows or plastic.
In applications where high rotational accuracy must be maintained,
high precision bearings and high tolerance shafts and housings should be
employed instead of a tighter interference fit to ensure bearing stability.
High interference fits should be avoided if possible as they cause shaft or
housing deformities to be induced into the bearing rings, and thus reduce
bearing rotational accuracy.
Because mounting and dismounting become very difficult when
both the inner ring and outer ring of a non-separable bearing (for example
a deep groove ball bearing) are given tight interference fits, one or the
other rings should be given a loose fit.
Table (2) Radial load and bearing fit
Lubrication (R25)
Lubrication of rolling bearings
The purpose of bearing lubrication is to prevent direct metallic
contact between the various rolling and sliding elements. This is
accomplished through the formation of a thin oil (or grease) film on the
contact surfaces. However, for rolling bearings, lubrication has the
following advantages.
(1) Friction and wear reduction
(2) Friction heat dissipation
(3) Prolonged bearing life
(4) Prevention of rust
(5) Protection against harmful elements
In order to achieve the above effects, the most effective lubrication
method for the operating conditions must be selected. Also, a good
quality, reliable lubricant must be selected. In addition, an effectively
designed sealing system prevents the intrusion of damaging elements
(dust, water, etc.) into the bearing interior, removes dust and other
impurities from the lubricant, and prevents the lubricant from leaking
from the bearing.
Almost all rolling bearings use either grease or oil lubrication
methods, but in some special applications, a solid lubricant such as
molybdenum disulfide or graphite may be used.
Grease lubrication
Grease type lubricants are relatively easy to handle and require only
the simplest sealing devices—for these reasons, grease is the most widely
used lubricant for rolling bearings.
Type and characteristics of grease
Lubricating grease is composed of either a mineral oil base or a
synthetic oil base. To this base a thickener and other additives are added.
The properties of all greases are mainly determined by the kind of base
oil used by the combination of thickening agent and various additives.
Standard greases and their characteristics are listed in (Table 3). As
performance characteristics of even the same type of grease will vary
widely from brand to brand, it is best to check the manufacturers’ data
when selecting grease.
Replenishment
As the lubricating efficiency of grease declines with the passage of
time, fresh grease must be re-supplied at proper intervals.
The replenishment time interval depends on the type of bearing,
dimensions, bearing’s rotating speed, bearing temperature, and type of
grease. An easy reference chart for calculating grease replenishment
intervals is shown in Fig. 1
This chart indicates the replenishment interval for standard rolling
bearing grease when used under normal operating conditions. As
operating temperatures increase, the grease re-supply interval should be
shortened accordingly. Generally, for every 10°C increase in bearing
temperature above 80°C, the relubrication period is reduced by exponent
“1/1.5”.
(Example)
Find the grease relubrication time limit for deep groove ball bearing
6206, with a radial load of 2.0 kN operating at 3,600 r/min.
Cr/Pr=19.5/2.0 kN=9.8, from Fig. 2 the adjusted load, fL, is
0.96.
From the bearing tables, the allowable speed for bearing 6206 is
11,000 r/min and the numbers of revolutions permissible at a radial load
of 2.0 KN are
therefore,
Using the chart in Fig. 1, find the point corresponding to bore
diameter d=30 (from bearing table) on the vertical line for radial ball
bearings. Draw a straight horizontal line to vertical line I. Then, draw a
straight line from that point (A in example) to the point on line II which
corresponds to the no/n value (2.93 in example). The point, C, where this
line intersects vertical line III indicates the relubrication interval h. In this
case the life of the grease is approximately 5,500 hours.
Fig. 1 Diagram for relubrication interval of greasing
Fig. 2 Value of adjustment factor fL depends on bearing load (R27)
Oil lubrication
Generally, oil lubrication is better suited for high speed and high
temperature applications than grease lubrication. Oil lubrication is
especially effective for those application requiring the bearing generated
heat (or heat applied to the bearing from other sources) to be carried away
from the bearing and dissipated to the outside.
Lubricating oil
Under normal operating conditions, spindle oil, machine oil, turbine
oil and other minerals are widely used for the lubrication of rolling
bearings. However, for temperatures above 150°C or below –30°C,
synthetic oils such as diester, silicone and fluorosilicone are used.
For lubricating oils, viscosity of the oil is one of the most important
properties and determines the oil’s lubricating efficiency. If the viscosity
is too low, the oil film will not be sufficiently formed, and it will damage
the load carrying surface of the bearing. On the other hand, if the
viscosity is too high, the viscosity resistance will also be high and cause
temperature increases and friction loss. In general, for higher speed,
lower viscosity oil should be used, and for heavy loads, higher viscosity
oil should be used.
In regard to operating temperature and bearing lubrication, Table 4
lists the minimum required viscosity for various bearings. Fig. 3 is a
lubricating oil viscosity-temperature comparison chart is used in the
selection of lubricating oil.
It shows which oil would have the appropriate viscosity at a given
temperature. For lubricating oil viscosity selection standards relating to
bearing operating conditions, see Table 5.
Table 4 Minimum viscosity of lubricating oil for bearings
Fig. 3 Relation between viscosity and temperature
Table 5 Selection standards for lubricating oils
Oil quality
In forced oil lubrication systems, the heat radiated away by housing
and surrounding parts plus the heat carried away by the lubricating oil is
approximately equal to the amount of heat generated by the bearing and
other sources.
For standard housing applications, the quantity of oil required can be
found by formula (11.1).
where,
Q : Quantity of oil for one bearing cm3/min
K : Allowable oil temperature rise factor (Table 6)
q : Minimum oil quantity cm3/min (From chart)
Because the amount of heat radiated will vary according to the shape
of the housing, for actual operation it is advisable that the quantity of oil
calculated by formula (11.1) be multiplied by a factor of 1.5 to 2.0. Then,
the amount of oil can be adjusted to correspond to the actual machine
operating conditions. If it is assumed for calculation purposes that no heat
is radiated by the housing and that all bearing heat is carried away by the
oil, then the value for shaft diameter, d, (second vertical line from right in
Fig. 4) becomes zero, regardless of the actual shaft diameter.
Table 6 Factor K
(Example)
For tapered roller bearing 30220U mounted on a flywheel shaft with
a radial load of 9.5 kN, operating at 1,800 rpm; what is the amount of
lubricating oil required to keep the bearing temperature rise below 15°C?
d=100 mm, dn=100´1,800=18´104 mm r/min from Fig. (4), q=180
cm3/min.
Assume the bearing temperature is approximately equal to the outlet
oil temperature, from Table (6), since K=1, Q=1´180=180 cm3/min.
Fig. 4 Guidance for oil quantity
Sealing Devices (R24)
Bearing seals have two main functions: 1) to prevent lubricant from
leaking out and 2) to prevent dust, water and other contaminants from
entering the bearing. When selecting a seal the following factors need to
be taken into consideration: the type of lubricant (oil or grease), seal
sliding speed, shaft fitting errors, space limitations, seal friction and
resultant heat, and cost.
Sealing devices for rolling bearings fall into two main
classifications: contact and non-contact types.
Non-contact seals
Non-contact seals utilize a small clearance between the seal and the
sealing surface; therefore, there is no wear, and friction is negligible.
Consequently, very little frictional heat is generated making noncontact seals very suitable for high speed applications. As shown in
(Fig. 5), non-contact seals can have the simplest of designs. With its
small radial clearance, these types of seal are best suited for grease
lubrication, and for use in dry, relatively dust free environments.
When several concentric oil grooves (Fig. 6) are provided on the
shaft or housing, the sealing effect can be greatly improved. If grease is
filled in the grooves, the intrusion of dust, etc. can be prevented.
For oil lubrication, if helical concentric oil grooves are provided in
the direction opposite to the shaft rotation (horizontal shafts only),
lubricating oil that flows out along the shaft can be returned to the inside
of the housing (see Fig. 7). The same sealing effect can be achieved by
providing helical grooves on the circumference of the shaft.
Labyrinth seals employ a multistage labyrinth design which
elongates the passage, thus improving the sealing effectiveness. Labyrinth
seals are used mainly for grease lubrication, and if grease is filled in the
labyrinth, protection efficiency (or capacity) against the entrance of dust
and water into the bearing can be enhanced.
The axial labyrinth passage seal shown in (Fig. 8) is used on onepiece housings and the radial seal shown in (Fig. 9) is for use with split
housings.
In applications where the shaft is set inclined, the labyrinth passage
is slanted so as to prevent contact between the shaft and housing
projections of the seal (Fig. 10).
Fig. 5 Clearance seal
Fig. 7 Helical oil groove seal
Fig. 9 Radial labyrinth seal
Fig. 6 Oil groove seal
Fig. 8 Axial labyrinth seal
Fig. 10 Aligning labyrinth seal
Contact seals
Contact seals accomplish their sealing action through the constant
pressure of a resilient part of the seal on the sealing surface. Contact seals
are generally far superior to non-contact seals in sealing efficiency,
although their friction torque and temperature rise coefficients are
somewhat higher.
The simplest of all contact seals are felt seals. Used primarily for
grease lubrication (Fig. 11), felt seals work very well for keeping out fine
dust, but are subject to oil permeation and leakage to some extent.
Therefore, the Z type rubber seal shown in (Fig. 12) and GS type shown
in (Fig. 13), have been used more widely.
Fig. 11 Felt seal
Fig. 12 Felt seal
Fig. 13 Felt seal