troubleshooting rolling element bearing problems

7/28/2019 Troubleshooting Rolling Element Bearing Problems

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Troubleshooting Rolling Element Bearing Problems

Introduction to Bearing Failures

Troubleshooting rolling element bearing problems and determining their root

cause of failure is often difficult, because many failure types look very similar. This is

because bearing failures are almost always precipitated by spalling or flaking conditions

of the bearing component surfaces.

Spalling occurs when a bearing has reached its fatigue life limit, but also when

premature failures occur. For this reason, it is important for the troubleshooter to beaware of and able to recognize, all of the common failures of rolling element bearings.

This ability to correctly troubleshoot and recognize the root cause of bearing

problems will lead the analyst to the right conclusions with regard to the bearing failure.

How many times have we heard the comment, even by knowledgeable and well

meaning engineers and technicians, this bearing failed prematurely, because it was

defective. Manufacturing defects in rolling element bearings make up less than one

percent of the millions of bearings in use today around the world and this small defect

percentage is being reduced continually by improvements in manufacturing techniques

and bearing materials.

Bearing manufacturers use ultrasonic inspection devices to detect surface and

subsurface bearing material defects, eliminating poor quality products during the

production process. Eddy current testing is used to evaluate surface hardness and detect

cracks to ensure 100% product conformance to bearing specifications.

Only a small fraction of all the bearings in use fail because they have reached

their material fatigue limit. The vast majority of bearings outlive the machinery or

component in which they are installed. The first question which must be answered

therefore is, what constitutes bearing fatigue life limits?


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What Is Bearing Life Expectancy?

Rolling element bearing life expectancy is directly related to the number of

revolutions performed by the bearing, the magnitude of the load and the lubrication and

cleanliness of the lubricant, (assuming correct initial bearing selection and installation).

Fatigue is the result of shear stresses, referred to as elastic deformation, cyclically

appearing immediately below the load carrying surface, as the rollers or balls pass over

the raceway. After many revolutions, these stresses between the rolling element and

raceway surfaces will cause subsurface cracks to appear that will gradually extend to the

surface of either the rolling element, raceway or both. These cracks may cause surface

fragments of bearing material to break away. This condition is referred to as flaking or

spalling. The spalling continues until the bearing is no longer serviceable and it has now

reached its life limit.

Figure 1 Load zone in a typical bearing with a single defect on a rolling element.


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It should come as no surprise to experienced equipment troubleshooters, that

assuming proper design and application, rolling element bearings will fail sooner or later

due to their natural material fatigue life limit, but all bearings will fail prematurely

from abuse or neglect.

According to many bearing experts, the following statistics apply to rolling

element bearings failures, no matter in what type of rotating equipment they are installed

(electric motors, pumps, fans, gear drives, etc.):

10% Reach their natural fatigue life expectancy.

20%Fail prematurely due to inadequatelubrication.

20%Fail prematurely due to contaminatedlubricant, either oil or grease.

30%Fail prematurely due to improper selection

or faulty installation.

20%

Fail prematurely due to mechanicalvibration, excessive temperatures,electrical discharge caused by staticelectricity or current flow, or by operatingconditions which allow overloading and/orover speeding.

These bearing life percentages may vary from industry to industry depending on

operating conditions, maintenance practices and industry operational culture. For

example, in the pulp and paper industry, poor lubrication or contaminated lubricants are

the main causes of failure.

Bearing manufactures such as SKF, will provide its customers with bearing life

expectancy ratings, defined as the number of revolutions or number of operating


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hours at a given constant speed which a bearing is capable of, before the first sign of

fatigue spalling occurs on one of the rings or rolling elements.

SKF refers to its calculations as the basic life rating or the L10bearing life in

millions of revolutions which is the life that 90% of a sufficiently large group of

apparently identical bearings can be expected to attain or exceed under identical

operating conditions.

The challenge for troubleshooters is to learn to recognize the difference between

the 10% of bearings that display material fatigue spalling and the remaining 90% of

bearings that display premature spalling referred to earlier, because in many instances

they look similar to the untrained eye.

The result is that frequently, the troubleshooter will conclude that the bearing

failed due to a defect in manufacture or material and the root cause of failure may never

be determined.

What Causes Premature Spalling?

The existing literature available from bearing manufacturers and equipment

failure experts generally agrees that the primary causes of premature (and therefore

preventable) spalling of rolling element bearings includes the following list:

1. Misalignment; of either the bearing itself or the shafts upon which they may be

mounted. Misalignment can be traced as the cause of about 50% of the

breakdown of rotating machinery. A 20% load increase caused by misalignment,

can reduce the calculated bearing life by almost 50%.

2. Faulty Mounting or Installation Practices; including the careless use of

excessive or uneven heating of the bearing prior to the interference fitting to a

shaft or into a housing.

If heat is required to expand an inner ring, the temperature shouldnever exceed

255F (125C). If induction heaters are used, it is important to remember to


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demagnetize the bearing prior to installation. (A magnetized bearing will fail

very quickly due to its attraction of ferrous metal particles).

Sealed, pre-packed bearings frequently used in electric motors must never

be heated and unless approved by the manufacturer, bearings containing shields

should also not be heated.

Clean hands, clean tools and a thoroughly clean work area are absolutely

essential when tradesmen and technicians install new bearings. A small piece of

dirt or metal chip trapped in a newly installed bearing is an invitation to another

bearing failure.

When pressing bearings onto a shaft or into a housing, the use of adequate

presses or hydraulic tools must be used and hammers and punches must neverbe

used, if premature spalling failure of a new bearing is to be avoided.

Figure 2 This photo shows a bearing which was installed while magnetized. It failed within

hours of operation.


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Figure 3 This bearing shows clear evidence of hammer blows to the outer ring during

installation.

3. Defective bearing seats on shafts and in housings; factors that produce

defective seats include shaft seats and housing bores that are over or undersize,

tapered or oval. Oval or out of round housings or undersize shafts can cause a

condition called fretting corrosion, where the bearing ring will actually move on

its seat during operation. An oversized shaft can cause a bearings inner ring to

crack during the cooling period, after installation. An undersized or oval housing

can also cause the bearing outer ring to become pinched, causing premature

failure.


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Figure 4 This bearing failed as a result of several conditions. It was inadequately lubricated,

as is evidenced by the smearing on the roller ends and a frosted surface on the inner ring.

The inner surface of the ring shows clear signs of fretting corrosion caused by poor seating

on the shaft. In addition, water contamination is clearly evident due to the rusty

discoloration on the bearing surface. In cases like this, it is difficult to arrive at a root cause

of failure, because we may not determine which condition occurred first.

4. Improper Shaft or Housing Fits; The degree of tightness or looseness with

which a bearing is mounted on shafts or in housings is governed by the load and

speed to which the bearing will be subjected. If a bearing ring rotates with the

load, an interference fit is required.

i.e.: In an automotive front wheel bearing, the outer ring or cup rotates with the

wheel and therefore has an interference fit with the wheel hub. On the

other hand, the inner rings rotate relative to the load in a gear reducer or

electric motor and are therefore mounted on the shaft with an interference

fit.


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A too loose interference fit may cause a condition called creep,

resulting in scoring of the inner ring. If the lubricant can penetrate the loose fit,

the bore, as well as the shaft seat, will appear polished.

In contrast, an excessive interference fit may cause the bearing ring to

crack. The resulting creep in the first condition and the cracked inner ring in

the second condition will generate heat and wear particulate, both of which can

promote premature spalling and early bearing failure.

Either of these conditions may cause a far more serious problem such as

bearing seizure resulting in a catastrophic machine failure.

It is very important to remember that the degree of fit is governed by the

principle that heavier loads require greater interference. The presence of shock or

continuous vibration also requires a higher interference fit of the ring that rotates

with the load.

These concepts related to bearing fits, should make it clear that any plant

or facility which arbitrarily increases loads or speeds on industrial equipment

must be prepared to expect premature bearing failures!!

5. Ineffective Sealing; The use of incorrect seal materials incompatible with

process fluids or the lubricant used, improper seal installation or improper

operation or maintenance of mechanical seals, or the use of seals which cannot

effectively operate under the existing temperature or contamination conditions are

just a few of the considerations which must be reviewed when troubleshooting

bearings for premature spalling.


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Figures 5 & 6 Water contamination is clearly evident on these bearings. Excessive water

contamination will cause severe corrosion, while small amounts of water will stain the surfaces with a

brown discoloration.


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Figure 7 The surface of this bearing shows clear evidence of scoring (note the scratches along the

surface) caused by contaminated lubricant. Some contaminants actually became embedded inthe bearing surface as evidenced by the areas of discoloration. This failure shows spalling and

flaking which is frequently mistaken for a lubrication failure.

6. Incorrect Initial Bearing Selection;

All rolling element bearings must have some internal clearance between

components in order to compensate for slight variances in housing and shaft fits

and to allow for thermal expansion due to normal operating temperatures.

Reduced levels of internal clearance caused by improper initial bearing

selection (or incorrect selection of replacement bearings), excessive operating

temperature or out of round housings which place excessive loads on bearing

components, will all increase bearing loads, causing premature failure which

frequently is accepted as a fatigue spalling condition.


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i.e. Internal radial clearance classifications for spherical roller bearings:

C1 has the least internal clearance, approximately 412 ten thousandths of an inch.

C2 clearance of 1220 ten thousandths of an inch.

C0 clearance of approximately 2129 ten thousandths of an inch.

C3 clearance of approximately 3043 ten thousandths of an inch.

C4 clearance of 4457 ten thousandths of an inch.

C5 has the most clearance, approximately 5770 ten thousandths of an inch.

It is a serious mistake to simply select a C3 classification if it should be C5.

7. Unacceptable Operating Conditions; The operating conditions which will

cause premature bearing failure include excessive vibration, overloading, over

speeding, high temperatures and electrical discharge.

If a typical bearing loadis doubled, the bearing life may be reduced by up

to 90%. Doubling the rated speed will reduce bearing life by about 50%. These


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are principles which must be kept in mind when production increases are

demanded without increasing equipment capacity! (See Figure 8)

Figure 8

Electrical discharge is becoming a serious problem in some equipment.

V-belt drive systems build up high levels of static electricity during operation and

this current can dissipate through the bearings to ground causing pits or fluting to

form on the bearing.

Stray magnetic fields in electric motors, both AC and DC, can generate

currents that will pass through bearings. To eliminate these potential problems,

grounding brushes should be used to ground motor shafts and V-belts.

Silicone greases contain electric insulation properties and these greases

might also be considered for some applications.

In many of todays machines, insulated bearings are used to eliminate the

problem of electrical discharge causing pitting or fluting of the bearing surface.


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Figure 9 Typical fluting pattern.

Vibration in a bearing while stationary can cause damage called false

brinelling. The damage may be either brightly polished depressions, or the

characteristic reddish stain common to fretting. These marks left by false

brinelling will be equal to the distance between the rolling elements, just as it is in

cases of true brinelling, so these two conditions are often difficult to distinguish.

Operating bearings at higher temperatures than those recommended by the

manufacturer will dramatically shorten the life of bearings, no matter what type,

quality or amount of lubricant is used. To illustrate the importance of this point;

consider the fact that a good quality, well-refined mineral oil will begin to oxidize

at 160F (71C). The same result will occur in greases where such oils are used

as the lubricating agent.

What this illustrates is that excessive temperatures; that is, temperatures

continually exceeding 160F, will have a detrimental effect on both the bearing

and the lubricant used. In fact, mineral oils have a high temperature limit of

around 550F (300C), at which point the oil decomposes to a soot or tar like

substance.


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Figure 10 This catastrophic bearing failure was caused by a combination of factors. There is

evidence of overloading or excessive thrust (as indicated by the offset ball path of the inner

raceway). This could indicate that the bearing was either not installed correctly or was

installed in the wrong position. There is also evidence that the balls reached such

temperatures that they turned blue-black in color softening the material, causing them toskid in the raceway. There is evidence of melting, skidding and skewing on both the balls

and ball paths in the raceways indicating that these components reached temperatures of

over 550F (300C).

8. Improper or Inadequate Lubrication; As already illustrated, about 70% of

bearing failures occur for reasons other than their lubrication quality or

quantity, yet users of industrial equipment will very often blame the lubricant

used when a bearing failure occurs.

We often hear the term lubrication failure, implying that there was no oil orgrease in the bearing. In most cases the answer is not that simple, because the

question that should be asked is, why did the lubricant fail to prevent damage

to the bearing? The answer to this question is not so obvious, because the

answer involves investigating much more than the lubricant, as we have seen by

the discussions of the many other causes of bearing failure.


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Lubrication related failures occur primarily as a result of three possible

situations. The lubricant used was either unsuitable, inadequate, or

excessive.

1) Unsuitable Lubricant; is a lubricant which, when used in a particular

bearing application, does not contain the suitable additives, is of an

incorrect viscosity, or may not be designed for use in such an application

or temperature range.

i.e. Grease should not be used where oil is recommended (and vice versa).

2) Inadequate Lubricant; Viscosity of the oil, either as oil itself or as the oil

content in greases, is the most important property of any lubricant. The

viscosity/temperature relationship is critical to the quantity of lubricant

which any bearing might require at a given time. If the viscosity is too

high (thick) relative to the temperature, insufficient oil will flow to (or

through) the bearing.

If the viscosity is too low (thin), the oil will not be sufficient to

maintain a separating film between the rolling elements and raceways ofthe bearings. In either case, the asperities (microscopic machined high

points) of the bearing component surfaces may contact each other, initially

causing a frosted or smearing condition, followed by adhesion at the

contact points. Failure of the bearing will be inevitable.


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Figure 11 The frosted appearance of this bearing race illustrates what happens when the oil viscosity

is too low (thin) and metal to metal contact occurs. This type of premature failure occurs during

initial start-up of heavily loaded bearings. This particular damage occurred after only 15 seconds of

operation.


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Figure 12 This bearing has failed due to continual welding contact between asperities on the metal

surfaces, eventually causing metal to be pulled out as the surfaces adhere to each other during

rotation. This condition may have been caused by using oil of the wrong viscosity, excessive loads or

speeds, incorrect internal clearances, or a combination of these problems. Even an increase of aslittle as 4 or 5 in temperature may have contributed to this failure, due to unacceptable thinning of

the lubricants viscosity. When analyzing the root cause of a failure, all of the possible contributing

causes must be considered.

As a general guideline, non-vibrating, lightly loaded bearings operating at

temperatures of 70C or less and running at high speeds can operate very effectively

using an anti-wear or R & O oil with a viscosity range of ISO 3246 cSt. Bearings

running at higher temperatures may require higher viscosity oil of 68 cSt, particularly at

heavy loads. These applications may also require oils with EP (extreme pressure)

additives. For applications where the ambient temperatures are at 0 or less, viscosity

index improved oils of ISO 15 or 22 cSt should be used.

Minimum oil viscosities for low and medium speed bearings at normal operating

temperatures should not be less than 1320 cSt and not less than 30 cSt for rolling


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element thrust bearings, with the generally accepted optimum viscosity in the range of

1350 cSt.

This optimum viscosity range depends upon bearing RPM, size, type and load.

For high speed bearings such as spindle bearings, minimum viscosity is 610 cSt. There

are several methods of calculating the ISO viscosity selection for bearing lubrication.

These methods include the following formulae.

Guide to Rolling Element Bearing Oil ISO Viscosity Grade Selection

Bearing/Oil Temperature (C)

DN

Value

ndm 95

170K 10 22 46 68 150

DN Value =Shaft diameter or bearing bore (mm) X RPM

ndm Value = (Shaft diameter +outside diameter of bearing (mm) X RPM

2

Notes:

i. Verify the viscosity selection with the bearing manufacturer.

ii. If vibration or shock loading will occur during bearing operation, use a higherviscosity grade with anti-wear or extreme pressure additives and confirm with the

bearing manufacturer. (Use the higher viscosity grade only if the temperaturedoes not increase).

In grease applications, the results will be the same if the

viscosity/temperature relationship is such that oil will not flow from the grease


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thickener in sufficient quantities to protect bearing surfaces under all operating

conditions. For this reason grease consistency grades are critical in these

applications, in addition to the viscosity of the oil contained in the grease.

The National Lubricating Grease Institute (NLGI) has established nine (9)

consistency grades, based on the worked penetration of greases under test

conditions.

These grades run from triple zero (000), which has a consistency similar to

a high viscosity oil for typical use in a centralized lubrication system, to a number

six (6), which is a block grease. Common greases used in machinery applications

with ambient temperatures in a range of 6075 F (1624C) might require a

consistency of two (2) or three (3) with the appropriate oil viscosities, to ensure

sufficient separation of the oil from the grease thickener under these operating

conditions. These grease selections would also depend upon bearing speeds.

Typical Maximum Bearing ndm Speed Factors by Type of Bearing;

and When They Should be Oil or Grease Lubricated

Bearing Type **Oil Lubr icated Grease Lubr icated

Radial Ball 500,000 340,000

Cylindrical Roller 500,000 300,000

*Spherical Roller 290,000 145,000

Thrust; Ball or Roller 280,000 140,000

* Grease lubrication is not recommended for spherical roller thrust

bearings.

** When oil lubricant is used, the oil level in the bearing housing should beno higher than the centre of the lowest rolling element of the bearing.

Grease Consistency Grade Selection

Determined by Dispensing Method


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Dispensing Method NLGI Grade Worked Penetration

gear box 000 445-475 (semi-fluid)

central system 00

0

400-430

355-385

grease gun 1

2

3

310-340

265-295 (common)

grease cup 4

5

175-205

130-160

block 6 85-125 (hard)

3) Excessive Lubricant; is frequently the cause of higher than normal

bearing operating temperatures. Excessive grease or oil quantities cause

internal friction within the lubricant, which in turn promotes excessive

temperatures causing oxidation and premature lubricant and bearing

failure.

Oil levels that are too high and excessive quantities of grease in

bearings cause a churning action within the rotating components and the

result will always be an increase in temperature.

Oil of too high viscosity, or grease with a too high consistency will

also increase operating temperatures. Care must be taken therefore, that

when investigating high temperatures, the troubleshooter must consider

not only the possibility of excessive lubricant, but that the correct lubricant

for the application is in use.

As a rule of thumb, if the troubleshooter cannot hold his or

her hand comfortably on the bearing housing, whether on an electric

motor or gear reducer, the temperature is too hot.


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Another quite common error made by some inexperienced

technicians is to fill new sealed bearings with grease using a syringe under

the bearing seal! This is a serious mistake! Churning will occur, internal

temperatures will rise, oxidation will take place within the lubricant and

premature bearing failure will result.

Sealed bearings are shipped from the manufacturer with

approximately 20% of the bearing cavities grease filled. No more

lubricant is required. Many bearings fail as a direct result of excessive

lubrication.

The standard tube of grease used in the common grease gun

contains 400 grams of grease and typical cylindrical roller bearings with a

6 inch OD and 4 inch ID operating at 1800 RPM only require about 35

grams of grease applied every two and one half (2) months or 1825

hours, when the bearing is operated in ambient room temperatures of 65F

(18C). However, when determining re-lubrication intervals, bearing

operating temperatures must be considered.

If we use the example noted above and the actual operatingtemperature of the cylindrical roller bearing is 130F (54C), our re-

greasing interval should be reduced to about 900 hours (or every five (5)

weeks) using 35 grams of grease.

The general rule of thumb states; the service life of grease

lubricated bearings is reduced by half for every 27F (15C) increase

in temperature above 160F (70C).

If for example, the calculated re-lubrication interval for a given

bearing is 1000 hours at 70C, this interval must be cut in half to 500

hours, if the actual operating temperature is 85C (185F).

To calculate the required amount of grease in ounces to

re-lubricate a bearing in service, use the following calculation.

Bearing OD (inches) X Bearing Width (inches) X the constant .114


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i.e.: 3 inch OD X .75 Width X .114 = .25 ounces of grease

or 3 X .75 X .114 = .25 ounces of grease

Where bearings are specified in metric dimensions the following

calculation may be used.

Bearing OD (mm) X Bearing Width (mm) X the constant .00018 = grease quanti ty in ounces

(The initial grease pack for a bearing before installation should be

3 X either of the above results).

Tips for Troubleshooting Bearing Lubrication Problems

The first action that should be taken by the troubleshooter when

investigating the cause(s) of a bearing problem is to familiarize oneself with the

following conditions, regardless of the bearings location or the type of machine in which

it is installed.

1. What is the recommended operating temperature of the bearing? Compare this

with the actual operating temperature using an accurate testing device, such as an

SKF thermopen or a hand-held Thermocam infrared camera.

2. Measure the noise level using a device such as the UE systems ultrasound tester.

If noise levels are increasing above those normally experienced, it could indicate

insufficient lubricant, vibration, premature spalling, or reduced internal clearance,

due to higher than normal temperatures or poor bearing installation. (Keep in

mind that noise is frequently accompanied by high temperatures).

If insufficient lubricant is suspected, determine if the bearing has the

correct amount of oil in the housing and add if necessary. If grease lubricated,


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pump a shot or two of grease into the bearing. If the noise level does not change

after a few minutes, insufficient lubricant is not the cause.

3. Often noise is associated with mechanical looseness or some other condition

which may cause vibration at or near the bearing. A stroboscope will very

quickly indicate whether or not a vibration is present.

4. Vibration or noise may also be the result of an overloaded bearing or a bearing

rotating at excessive speeds; using a digital tachometer, speeds can quickly and

accurately be determined, then compared with specifications.

5. Often noise is associated with defective seals which may be rubbing on the

bearings shaft. This may also increase the operating temperature (near the lip of

the seal) combined with lubricant leakage or seepage past the seal. A groove at

the seal lip may be observed.

6. If a leaking seal is obvious or suspected, contamination may have entered the

bearing causing premature damage. An oil or grease sample should be obtainedand analyzed for higher than normal wear metals and contaminants, particularly

dirt or water.

7. In a grease-lubricated bearing, a leaking seal combined with high temperatures

might indicate that the grease has reached or exceeded its dropping point.

Confirm that the grease in use is the recommended product and that it has not

been mixed with an incompatible type of grease. Incompatibility also might lead

to a seal leak, as the two incompatible greases react with each other and oil

separates from the thickening agents in either (or both) greases.

8. On machines using ring oiling of the bearings, ensure that the rings are in fact

rotating. If they are worn or not rotating, oil will not be picked up and distributed


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by the ring. High temperatures, noise and eventual premature bearing failure will

be the result.

9. On centralized lubrication or oil mist systems, ensure that the system is calibrated

properly and distributing the correct amount of grease or oil to the affected

bearings.

10.Finally, the troubleshooter should be thoroughly familiar with the machine itself,

its overall operating conditions and the processes or applications for which the

machine is used. Above all, remember that about 70% of bearing failures are

not lubricant or lubrication related, although they may appear to be!

R e f e r e n c e s

The Practical Handbook of Machinery Lubrication, 3rd

Edition; L. Leugner.

SKF Bearing Maintenance Handbook; The SKF Manufacturing Group.

Care and Maintenance of Bearings; The NTN Bearing Corporation.

Failure Atlas for Hertz Contact Machine Elements, 2nd

Edition; T.E. Tallian.

troubleshooting rolling element bearing problems

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