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Bearing failure and its Causes and Countermeasures
BEARING FAILURE ITS CAUSES
Prepared By : Dutt Thakar
• How Bearing life is defined ?
• Why a Bearing fails ?
• Bearing Failure Chart
• Types of Bearing Damage and Failures
• Research work
• Some Other type of Common Failures.
• Bearings are the most important components in majority of
• Their demands are made upon their carrying capacity and
• The purpose of a ball bearing is to reduce rotational friction
and to support Radial and Axial loads.
How Bearing Life is defined ?
• Bearing life is defined as no. of revolutions a bearing can perform
before evidence of fatigue in any of its element.
• It is generally defined in terms of Millions of Revolutions or in
• It is generally based on 90 % Reliability.
Lh = (106 /60N) * (C/P)a
where, Lh = Life in terms of Hrs.
N = RPM
C = Dynamic load carrying capacity
P = Load or Equivalent Load
a = Constant.
Why a Bearing fails ?
• Match bearing life with the service life of the machine
• Unfortunately it sometimes happens that a bearing
does not attain its calculated rating life.
• There may be many reasons for this
1. Heavier loading
2. Inadequate or unsuitable lubrication
3. Careless handling
4. Ineffective sealing or fits that are too tight
5. Insufficient internal bearing clearance.
Types Of Bearing Damage and Failures
1. Primary Damage
• Surface distress
• Electric current
2. Secondary Damage
• Flaking (Spalling)
• Cage damage
• Wear may, however, occur as a result of the ingress of foreign
particles into the bearing or when, the lubrication is
unsatisfactory. Vibration in bearings which are not running
also gives rise to wear.
• Three types of Wear Damage
1. Wear caused by Abrasive particles
2. Wear caused by Inadequate lubrication
3. Wear caused by Vibration
Wear Damage by Abrasive Particles
Small indentations around the raceways
and rolling elements. Dull, worn surfaces.
Grease discolored green.
Lack of cleanliness before and during
mounting operation. Ineffective seals.
Lubricant contaminated by worn particles
from brass cage.
Keep workshop clean and use clean tools.
Check and possibly improve the sealing.
Always use fresh, clean lubricant. Filter
Wear caused by Vibration
Depressions in the raceways. These depressions
are rectangular in roller bearings and circular in
ball bearings. The bottom of these depressions
may be bright or dull and oxidized.
The bearing has been exposed to vibration
Secure the bearing during transport by radial
preloading. Provide a vibration-damping base.
Where possible, use ball bearings instead of
roller bearings. Employ oil bath lubrication,
• Raceways and rolling elements may become dented if the
mounting pressure is applied to the wrong ring, so that it
passes through the rolling elements, or if the bearing is
subjected to abnormal loading while not running. Foreign
particles in the bearing also cause indentations.
• Two types of Indentation Damage.
1. Indentations caused by faulty mounting or overloading
2. Indentations caused by foreign particles
Indentation due to faulty Mounting or
Indentations in the raceways of both rings
with spacing equal to the distance between
the rolling elements.
Mounting pressure applied to the wrong ring.
Excessively hard drive-up on tapered seating.
Overloading while not running.
Apply the mounting pressure to the ring with
the interference fit.
Avoid overloading or use bearings with higher
basic static load ratings.
• When two inadequately lubricated surfaces slide against
each other under load, material is transferred from one
surface to the other. This is known as smearing.
• Types of Smearing Damage
1. Smearing of roller ends and guide flanges
2. Smearing of rollers and raceways
3. Raceway smearing at intervals corresponding to the roller
4. Smearing of external surfaces
5. Smearing in thrust ball bearings
Hardness vs Time and Temperature
• As per American Bearing Manufacturer’s
Smearing of Roller end and Guide Flange
Scored and discoloured roller
ends and flange faces.
Sliding under heavy axial
loading and with inadequate
More suitable lubricant.
Transverse smear streaks - spaced at intervals equal to
the distance between the rollers - in the raceways of
cylindrical roller bearings.
Roller slipping due to excessive grease filling
Improve the preload
Improve the bearing clearance
Use a lubricant with good oil film formation ability
Improve the lubrication method
Improve the sealing mechanism
Smearing Of External Surfaces
Scored and discoloured ring bore or
outside surface or faces.
Ring rotation relative to shaft or
Select heavier interference fits.
Improve Lubrication method.
• If the lubricant film between
raceways and rolling elements
becomes too thin, the peaks of
the surface asperities will
momentarily come into contact
with each other.
• Small cracks then form in the
surfaces and this is known as
• The surface distress cracks are
microscopically small and
increase very gradually to such a
size that they interfere with the
smooth running of the bearing.
Initially the damage is not visible
to the naked eye. A more
advanced stage is marked by
small, shallow craters with
crystalline fracture surfaces.
Inadequate or improper
Damage due to Corrosion
• Rust will form if water or
corrosive agents reach the inside
of the bearing in such quantities
that the lubricant cannot provide
protection for the steel surfaces.
This process will soon lead to
deep seated rust.
Grey-black streaks across the raceways, mostly
coinciding with the rolling element spacing. At a
later stage, pitting of raceways and other surfaces
of the bearing.
Entry of corrosive gas or water Improper lubricant
Formation of water droplets due to condensation
High temperature and high humidity while
Poor rust preventive treatment during
Improper storage conditions
Improve the sealing mechanism
Study the lubrication method
Anti-rust treatment for periods of non-running
Improve the storage methods
Improve the handling metheod
Damage due to Electric Current
• When an electric current passes
through a bearing, i.e. proceeds from
one ring to the other via the rolling
elements, damage will occur. At the
contact surfaces the process is similar
to electric arc welding.
Dark brown or grey-black fluting
(corrugation) or craters in raceways and
Balls have dark discoloration only.
Sometimes zigzag burns in ball bearings
Localized burns in raceways and on
Passage of electric current through
Passage of electric current through non-
Design electric circuits which prevent
current flow through the bearings
Insulation of the bearing
Four stages of Flaking.Fatigue is the result of shear
stresses cyclically appearing
immediately below the load
After a time these stresses
cause cracks which gradually
extend up to the surface. As the
rolling elements pass over the
cracks fragments of material
break away and this is known
as flaking or spawling.
Heavily marked path pattern in raceways of both rings. Flaking usually in the most
heavily loaded zone.
Rough and Coarse texture.
Poor mounting (misalignment),Entry of foreign debris, Poor lubrication, Improper
lubricant, Unsuitable bearing clearance
Improper precision for shaft or housing
Progression from rust, corrosion pits, smearing, dents.
Improve the mounting method, Improve the sealing mechanism. Use a lubricant with a
Improve the lubrication method , Check the precision of shaft and housing
Check the bearing internal clearance
• Cracks may appear in Bearing due to so many reasons.
• The most common cause is rough treatment when the
bearings are being mounted or dismounted.
• Hammer blows, applied direct against the ring which
results in breaking of the parts of Bearing.
• Cracks may appear due to
1. Rough treatment
4. Excessive Drive-up.
Cracks or pieces broken off, generally at one face of the bearing.
Excessive load, shock load
Progression of flaking
Heat generation and fretting caused by contact between mounting parts and
Heat generation due to creep
Poor taper angle of tapered shaft
Poor cylindricality of shaft
Correct the interference
Check the load conditions
Improve the mounting method
Use an appropriate shaft shape
• There are certain main causes of cage failure
2. Excessive speed
Cracks, cage deformation, Fracture.
Poor mounting (Bearing misalignment).
Shock and large vibration.
Excessive rotation speed, sudden acceleration and deceleration.
Check the mounting method.
Check the temperature, rotation, and load conditions.
Reduce the vibration and use vibration damping base.
Select a cage type properly.
Select a lubrication method and lubricant.
Failure analysis of bearing in wind turbine generator gearbox.
Presented By:- Sanskar S., Nataraj M. and Prabhu Raja V.
• This research paper describes the failure analysis of bearing in Wind Turbine Generator
(WTG) gearbox. The two-stage filter element and the gearbox were examined at turbine
tower top called nacelle to find out the reason for filter choke alarm in the turbine
controller. Drive train alignment was ensured between the asynchronous generator
shaft and the gearbox shaft to conclude the mode of bearing failure.
The visual observation and the microstructure examination confirmed that an
excessive damage of the roller as well as the surface of the race is due to the
debris collected at the filter unit resulted from contact wear (Scoring) followed by
surface and sub-surface wear (Spalling) on rollers and races of an intermediate
non-drive end bearing.
It is obvious from failure analysis that the failure of bearing is due to high cyclic
fatigue fracture. It is clear that the failure of the bearing has happened due to
continuous peak power generation of the WTG during high wind season and the
presence of bauxite element such as aluminum oxide, calcium oxides and silicon
oxides in the gearbox oil.
Some Other types of Common Failures
• Failure due to Creep
• Failure due to Pitting
• Failure due to False Brinneling.
• Failure due to Seizure or Excessive Heating.
• Failure Due to Reverse Loading.
• “Failure Analysis Of Bearing In Wind Turbine Generator Gearbox” by
Sanskar S., Nataraj M. and Prabhu Raja V. in Journal of Information
Systems and Communication Vol-III, 2012.
• “Tech Talk Roller bearing life at High Temperatures” Published by NSK
Americas. NSK Bearings, NSK America Corp.
• “ New Bearing Doctor” Published by NSK Europe. NSK Bearings. NSK
• “Bearing Failure Causes and Curses” Published by Wilcoxon Research.
• An Article on “Bearing Life” and catalogue Published by SNR bearings.
• “Bearing Failures and its Causes” published by SKF bearings Sweden.