bonus module understanding bearing failures failures.pdf · 2020. 5. 30. · lubricant failure :...

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ROOTCAUSE FAILURE ANALYSIS

Maintenance Excellence RSA 2004

Bonus Module

Understanding Bearing Failures

2004 All Rights Reserve

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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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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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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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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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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


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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


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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 :


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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.


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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


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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.


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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


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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


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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


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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


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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


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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


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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


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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)


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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


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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

bonus module understanding bearing failures failures.pdf · 2020. 5. 30. · lubricant failure :...

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