Bonus Module Understanding Bearing Failures Failures.pdf · 2020. 5. 30. · Lubricant Failure :...
Transcript of Bonus Module Understanding Bearing Failures Failures.pdf · 2020. 5. 30. · Lubricant Failure :...
Rolly Angeles
ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
Bonus Module
Understanding Bearing Failures
2004 All Rights Reserve
Rolly Angeles
ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
DEFENITION OF TERMS
FRACTOGRAPHY • The science and art of evaluating broken
things. The study of fractography can
provide many clues to the person who
wishes to understand why the component
failed or broke down. More advance frac-
tography can also be used to shed light on
many types of wear mechanisms. Fracto-
graphy is getting information about how &
why the part cracked by examining the fracture
assembly of the failed component or part
BRITTLE FRACTURE • Brittle fracture is a fracture that involves
little or no permanent deformation. Many
non-metals lack ductility and are subject
to brittle fracture. Brittle fracture occurs
when a part is overloaded and breaks with
no visible distortion. In a brittle overload
failure, separation of the 2 halves is instan-
taneous but proceeds at a tremendous rate
where the crack begins at the point of maximum stress
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ROOTCAUSE FAILURE ANALYSIS
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DEFENITION OF TERMS
DUCTILE FRACTURE
A failure is termed ductile if it occurs by plastic deformation, if the yield strength of
the material is exceeded. This causes a permanent change in shape of the part, w/c
becomes apparent if we attempt to reassemble broken components. There is a great
BRITTLE DUCTILE
deal of distortion on the failed part. There are some
instances when a brittle fractures appear in a normally
ductile materials which indicates that the load was app-
lied rapidly and instances that when you heat up a
a material it becomes ductile and deforms
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
UNDERSTANDING BEARING FAILURE
Bearing life is determined by the number of hours it
will take for the metal to fatigue and that is a function
of the load on the bearing, the number of rotations and
the amount of lubricant a bearing receives
When a bearing fails what do we do ?
• When a bearing fails we simply replace it
like any other part, we change the bearings
every time we disassemble the equipment
• Most often a bearing does not attain its use-
ful or calculated life for a variety of reasons
such as heavier loading that had been anti-
cipated, inadequate lubrication, careless
handling, ineffective sealing or fits that are
too tight or too loose. A failed bearing will
leave some sort of clues for the maintenance
to figure out what it is.
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ROOTCAUSE FAILURE ANALYSIS
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Causes of Bearing Failures
Loose and
Tight Fits
Corrosion
Fatigue (Excessive Loads)
Lubricant Failure
Misalignment (Poor Installation)
Contamination
Poor Lubrication
False Brinelling
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
Most common type of bearing failures :
Poor Lubrication (36%)
• Using wrong lubricant type
• Mixing lubricants
• Improper lubrication amounts
• Lack of lubrication
• Loss or improper additives
Fatigue (34%)
• Bearings are overloaded
• Due to misalignment unbalance
• Premature fatigue cause by
lubrication problems
Poor Installation (16%)
• Improper installation can lead
to failures such as load imbalance,
misalignment, tight or loose fits
Contamination (14%)
• Typical failures due to contamination
may include excessive wear, abnormal
surface stress, corrosion such as
water and liquid contamination
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
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Lubricant Failure :
Oil Starvation" can be attributed to a number of causes.
Wiping, Blue Discoloration and "Adhesion" wear to shaft
or pin are ]common indicators
• In many cases, a bearing subjected to abnor-
mal operating conditions exhibits signs of
lubrication failure.
• Abnormal operating condition often produces
excessive heat and this lowers the fluid film
thickness which allows metal to metal contact
between the raceways and the balls.
• Discolored (blue/brown) raceway and balls are symptoms of lubricant
failure. Excessive wear of balls, rings and cages will follow which
will result in overheating and premature catastrophic failures
• Poor lubrication can lead to other problems such as overheating,
contamination, fatigue and corrosion.
• Bearings depend on the continuous presence of a very thin film
between the ball and the races. Once lubrication is inadequate or
restricted it will give rise to excessive temperature and will degrade
or destroy the lubricant oil properties
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
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Restricted lubricant
flow or excessive
temperatures that
degrade the lubricant's
properties typically
cause failures.
Lubricant failure will
lead to excessive wear,
overheating and subse-
quent bearing failure.
Temperatures in excess of 400
deg Fahrenheit can anneal the
ring and ball materials causing
them to be ductile and deform.
The resulting loss in hardness
reduces the bearing’s capacity
causing premature failures.
Lubricant Failure :
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
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Lubricant Failure :
Extreme cases result in
deformation of balls,
rollers and rings.
Extensive fretting corrosion on the outer
race of a deep groove ball bearing
Rust will form if water or corrosive agents reach the
inside of the bearing in such quantities that the lubri-
cant cannot provide protection for the steel surfaces.
This process will soon lead to deep seated rust.
Another type of corrosion is fretting corrosion.
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
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Lubricant Failure :
• Heat is another cause of premature bearing failure.
Heat will cause the lubricant to decrease in viscosity,
causing more heat as it looses its ability to support
the load and form a varnish residue, this will destroy
the ability of the grease or oil to lubricate the bearing
and introduce particles in the lubricant
• Leading bearing manufacturer states that the life of bearing oil is directly
related to heat. Non contaminated oil cannot wear out and has a useful
life of about 30 years at 30 deg
centigrade (86 deg F) & states
that the life of the bearing oil is
cut in half for each 10 deg C rise
in the temperature of the oil
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
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Fatigue Failure :
• Fatigue fractures are generally considered the most serious type of fracture
in machinery parts since they can occur in normal service w/o excessive
over-loads and under normal operating procedures.
• Fatigue is a phenomenon leading to fracture under repeated or fluctuating
stress having a maximum value less that the tensile strength of the mate-
rial. Fatigue fracture are progressive w/c begins as micro or mini cracks
that grow under the action of fluctuating stress. When stress exceeds the
strength of the material then we have a fatigue.
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
UNDERSTANDING FATIGE FAILURE
1) Straighten out a standard paper clip
When a bearing is pressed on a rotating shaft the load passes from the
inner race through the balls to the bearing’s outer race and the ball carries
a portion of the stress as the ball rolls under the load
2) Flex it a little and then let it go. You will notice
that it returns to the straightened position. You
may repeat this cycle many times (many years
actually) without breaking (fatiguing) the metal
because you are cycling the metal in its elastic
range
3) Now, Let’s bend the paper clip a lot further & you
will note that it did not return to the straighten
position. This means that you have stressed
the metal way beyond its elastic range
4) If you bend the metal back and forth in its elastic range it will crack and break in
less than 20 cycles. The metal fatigue more quickly because it work hardened
and it became brittle. Hence fatigue is a function of stress and cycles
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
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Fatigue Failure :
• Other terms used such as spalling which is a fracture of the running
surface & subsequent removal of small discrete particles of material.
Spalling can occur in the inner ring, outer ring or balls. Fatigues are
progressive which start as micro cracks until its final rupture and a
marked increase in the machine’s vibration.
Moderately spalled areas indicate that the bearing has
reached the end of its useful life.
• Bearing life is deter-
mined by the number
of hours it will take
the metal to fatigue
and that is a function
of the load on the
bearing, number of
rotation & the amount
of lubrication the bea-
ring receives
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
Fatigue Failure :
Symptoms are the same as normal fatigue, although showing heavier ball wear paths, greater evidence of overheating, & a more widespread and deeper spalling
By examining the path patterns in a dismantled
bearing that had been in service, it is possible
to gain a good idea on the conditions under
which the bearing had operated and we may
know if the bearing had been running under
normal or abnormal working loads
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
Fatigue Failure :
4 POINTS IN UNDERSTANDING FATIGUE
• Without Stress fluctuations fatigue
cannot happen
• Fatigue happens at stress levels well
below the tensile strength of the material
• Where corrosion is present, the fatigue
strength of metals continuously decrease
• The cracks takes measurable time to
progress across the fractured face
Excessive loads usually cause premature
fatigue. Tight fits, brinelling and improper
pre-loading can also bring about an early
fatigue failure
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
CONTAMINATION
• Contamination is one of the leading causes of
premature bearing failure. Symptoms of contami-
nation are dents or scratches embedded in the
bearing raceways and balls/rollers, resulting in
undue bearing vibration and wear. Contaminants
may include airborne dust, dirt or any abrasive
substance that gets into the bearing. Principal
sources are dirty tools, contaminated work areas,
dirty hands and foreign matter in lubricant.
• Contamination includes ingress of solids or fluids
into the bearing. Solid contamination can enter a
bearings cavity under extremely dirty and dusty
environment even with the most effective seals
• Poor lubrication and contamination can account
for a large number of bearing failures and thus
can be prevented through the application of Total
Contamination Control Program besides the
routine concept of lubrication schedules performed
during Preventive Maintenance
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
POOR or IMPROPER INSTALLATION
Misalignment failure
can be detected on the
raceway of the non-ro-
tating ring by a rolling
element wear path that
is not parallel to the
raceway edges. Excess
misalignment can cause
abnormal temperature
rise or heavy wear in the
cage pockets.
• The most prevalent causes of misalignment are,
bent shafts burrs or dirt on the shaft or housing
shoulders shaft threads not square w/ the shaft
seats locking nuts with faces that are not square
to the thread axis
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
POOR or IMPROPER INSTALLATION
What exactly is misalignment? Misalignment occurs when the centre
lines of rotation of two machinery shafts
are not in line with one another. There
are two types of misalignment: parallel
and angular. In most cases, machine
misalignment is actually caused by
a combination of these two types.
Misaligned shafts can cause: • Increased bearing load
• Reduction in bearing life
• Increased seal wear
• Increased vibrations
• Increased noise
• Increase in energy consumption
• Correct shaft alignment will
minimize these effects
Misalignment
Proper alignment
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
False Brinelling :
• Occurs when there is small relative motion between
the balls/rollers and raceways during non-rotation
times. Characterized by elliptical wear marks in the
axial direction at each ball/roller position. When the
bearing isn't turning, an oil film cannot be formed to
prevent raceway wear. Wear marks are perpendi-
cular to the line of motion, normally well-defined,
and sometimes surrounded by debris.
• Correct by isolating bearings from external vibrations
and using greases containing anti-wear additives.
• Consider the storeroom to
areas in the plant that are
affected by vibration, rail-
road main line. Does the
plant have equipment that
vibrates. All of these can
contribute to false brinelling (Start-up damage)
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
False Brinelling :
• If a bearing is subject to vibration when
stationary, fretting damage occurs at the
points of contact, giving rise to what is
known as false brinelling
• Standby machines and machines in store
should be rotated slightly once per week
to change the points of contact. Where
there are installed standby machines, it
is good practice to run the machines
alternately to reduce the risk of fretting
UNDERSTANDING BEARING FAILURE
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ROOTCAUSE FAILURE ANALYSIS
Maintenance Excellence RSA 2004
BEARING SELECTION
When selecting a bearing :
• Determine the maximum load for the bearing.
This is important both vertical & horizontal.
(Axial and Radial loads) • Determine the minimum & maximum running
speeds for the bearing. This will help us to
determine the correct lubricant and bearing
for the application • Determine all possible environmental condi-
tions to which the bearing will be exposed.
Very hot or cold environments often require
valid bearing specifications which may affect
the type of lubricant requirements as well • If possible determine when a bearing was
manufactured and if it was stored before
being purchase. Ask bearing vendor about storage & handling procedures