failed bearing evaluation

7/29/2019 Failed Bearing Evaluation

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Emerson Process Management - CSI

DoctorKnow Application PaperTitle: Failed Bearing Evaluation

Source/

Author:

Alan K. Pride

Product: GeneralTechnology: Vibration

Classification:

FAILED BEARING EVALUATION

Alan K. Pride

Preliminary Steps

Following bearing removal, the bearing should be engraved with a unique identifying serial

number on the same side of both the inner and outer ring. This serial number should be recorded in

the failed beating logbook and contain the following information:

1. Time and date of beating removal.

2. Reason for removal i.e., vibration, temperature, airborne noise, seized, other maintenance, etc.

3. Equipment removed from including position.

4. Bearing manufacturer, size, lot number, date of installation.5. Vibration level at time of removal.

An inspection of the general condition of the bearing should be performed and noted on the

bearing evaluation sheet. This general evaluation should consist of the following:

1. Ability to rotate the bearing by hand.

2. Condition of external surfaces such as rings, seals, shields, etc.

3. Condition of cage and lubricant, if possible without disassembly.

4. If seals or shields are present, they can be removed by using a screwdriver of the proper size topry them away from the inner ring and then removed.

5. Using a clean tongue depressor remove a grease sample from as close to the bearing races as

possible and place in a small plastic bag.

6. The grease sample should be worked to a thin film and held up to a bright light and the presence

of dirt, water, or other contaminates noted and recorded on the evaluation sheet.

7. The bearing can now be cleaned using a solvent or diesel fuel and a parts cleaning brush.

NOTE: Observe applicable safety precautions when handling solvents.

8. Following cleaning the bearing should rotate freely by hand. Any discrepancies should be noted

on the evaluation sheet.

9. Spray the bearing with CRC or similar substance to prevent corrosion from occurring if the

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bearing is not to be analyzed at this tune.

DISASSEMBLY

Angular Contact/Other Snap Together Bearings

Disassembly of beatings other than deep groove ball bearings cannot be accomplished without

minor damage to the beating components. However, this damage, nicks in the balls or fresh smearsin the race, are readily discernible from the damage which occurred during operation and should be

noted on the evaluation sheet.

To disassemble these type of bearings perform the following:

1. Place the bearing face (thin side) down inside a clean box on other suitable container in order to

prevent the balls/rollers from rolling away.

2. Use a small piece of wood as a spacer to support the outer ring of the bearing at sufficient height

to allow the cage and balls to clear the outer ring. Then strike the inner ring with a mallet at the

high point; the bearing should separate into individual components.

Disassembly of Deep Groove Bearings

In order to disassemble a deep groove or Conrad beating the cage must be first removed, the

difficulty of this task is dependent on the type of cage present. A snap in phenolic cage can be

removed by using a screw driver or punch and hammer, and a two piece phenolic cage can be

disassembled using the appropriate size drift to drive out the pins which hold the cage together.

However, disassembly of the two piece riveted metal cage can be difficult and is made much easier

if a drill press is utilized during the following procedure:

1. Place an indentation in the center of each rivet using a center punch and hammer.

2. Clamp the bearing to the drill press and drill the rivets with a twist drill bit of the same diameter

as the rivet. Only the head of each rivet needs to be removed and not the entire rivet.

3. Remove the cage halves and clean the bearing and cage assembly with a brush and solvent to

remove metal slivers.

4. Slide all of the balls to one side of the bearing and clamp bearing in a vise on the side opposite

from the balls.

Note: A face shield should be utilized during the next step as there is a chance the bearing may

fracture.

5. Slowly tighten vise until the rings are distorted enough to allow the inner ring to move past the

last ball. This will require a few mils of deflection and be accompanied by a snapping sound.

6. Release the tension on the vise and place the bearing components in a clean container.

Bearing Surfaces

The bearing consists of inner ( raceways, ball and cage raceways, shoulders) and outer ( outside

and inside diameters, faces/backs) surfaces. All of these surfaces must be carefully examined if an

accurate assessment of cause of failure is to be determined. In most cases, a good light and a 10X



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magnifying glass are all that is required to make a diagnosis. However, if failure rates seem

excessive and a failure pattern emerges, detailed analysis by a metallurgist may be warranted.

A detailed examination by an independent laboratory will cost several thousand dollars per bearing.

BEARING ANALYSIS

EXTERNAL SURFACES

This section on external surfaces describes the checks of the physical condition of the bearing andmachine interfaces where the root cause of the bearing failure is often determined. The information

obtained from the examination of the points of contact between the bearing and the machine should

be correlated with conditions found on the rollers and raceways to obtain the most probable cause

of bearing failure.

A review of beatings removed for cause performed by the U.S. Navy indicated that over one-half

of bearing failures were the result of contamination and improper installation. Contamination

problems are normally manifested on the internal surfaces while indicators of installation problems

are evident on internal and external surfaces.

Degradations and wear on external surfaces can be evaluated by observing the type and patterns of

simple and fretting corrosion, scoring marks, and color of components.

CORROSION

Simple Corrosion

Corrosion of bearing surfaces is very common and consists of simple corrosion or rust and fretting

corrosion which is a mechanical process. Simple corrosion is usually the result of water or

moisture coming into contact with an unprotected surface of the bearing and simply rusting the

steel. This type of corrosion is usually brown in color and takes place on the faces and shoulders

where there is no contact between the balls, races, shaft, or housing.

In some cases, the lubricant, water or other contaminates will interact with the lubricant and form

acids which result in dark brown and/or black stains on the balls and races. This type of corrosion

can take the form of very fine crack like lines in equipment which has not been in operation for

some time. In advanced states, severe pitting can take place which will form points for surface

initiated spalls to occur.

In general, simple corrosion is randomly scattered over the surface with no apparent pattern and

not associated with wear.

Fretting Corrosion

Fretting corrosion is the result of small amounts of relative motion between the external surfaces of

the bearing, the shaft, housing, and/or bearing outer caps. This relative motion, which can be

vibratory, causes adhesive wear to take place due to metal-to-metal contact at high points betweenthe two surfaces. The amount of fretting is a function of fit-up and load and thus is an excellent

indicator to the cause of bearing failure.



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Severe fretting can create weak spots in the bearing which will result in cracking and eventual

spalling of the bearing. In addition, heavy fretting on the guide or floating bearing outer ring may

cause the bearing to lock-up in the housing which will cause the bearing to become overloaded and

result in failure.

Unlike simple corrosion, fretting corrosion will often have a pattern associated with misalignment,

improper clearances, and excessive loading. Therefore, careful mapping of the fretting patterns will

greatly assist the analyst in accurately diagnosing beating failures.

The following photographs are examples of how classical fretting patterns can be utilized to

determine the root cause of the bearing failure:

NOTE: The photographs mentioned in this paper will be presented as a slide show during the

presentation. Unfortunately due to the need for high photographic quality reproduction in the

format of this paper is impossible.

1. The photograph shows normal fretting of the outer ring of a properly mounted thrust beatingfrom a vertical pump/motor. In vertical applications where no parasitic or radial forces exist the

fretting will be uniformly distributed near the center, usually offset by the contact angle of the

bearing in the same plane as the wear track created by the ball on the outer race.

Since the rings are flexible and the bearing generates pressures in excess of 300,000 PSI the ring

flexes in response to the resulting pressure wave creating the relative motion between the bearing

and the housing which causes the adhesive wear. Studies by the U. S. Navy have indicated that this

type of fretting can occur after as little as 100 hours of operation.

2. A radially loaded bearing, as found in a horizontal machine, will also show the same fretting

slightly offset from the center but will be heavy only in the load zone and tapper off to little or no

fretting in the upper half of the bearing on the outer ring. In both cases the inner ring of the bearing

will show signs of fretting the entire 360.

3. Excessive fretting can occur when any of the following conditions exist:

a. Heavy loads and severe vibration. b. Improper shaft geometry. c. Housing imperfections.

4. The following photograph shows the result of a combination of heavy load and vibration in ahorizontal machine.

5. In this example the fretting occurs at opposite sides of the inner and outer rings which indicates

a very high thrust load. If the wear track angle were measured it would probably indicate a higher

than designed contact angle.

6. These two photographs are classical examples of poor housing and bearing fitups where the

bearing was not fully supported by the housing which resulted in excessive flexing of the outer

ring in localized areas. This has the effect of increasing the amount of stress on the bearing andshortening bearing life.

7. Bearing misalignment is a leading cause of premature bearing failure and can easily be avoided



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by properly installing the bearing. These photographs show a skewed fretting pattern on the outer

ring but normal light fretting in the center of the inner ring bore. In order to avoid this type of

problem, squareness of the shaft shoulder should be confirmed as well as ensuring the bearing is

positioned snugly against the shoulder during installation.

8. Tapered housings produce fretting on the portion of the bearing in contact with the housing and

not in the area of no contact. Again, the inner ring shows no sign of high thrust loads.

9. A fluted housing where the outer ring is supported on the high points of the housing which does

not provide sufficient resistance to rotation to prevent the outer ring from chattering in the housing.

In this particular case, the bearing could have been noisy in service and removed prematurely.

SCORING

There are two types of scoring, axial and circumferential, both of which are usually indicative of

poor bearing fitup. While axial scoring can be created during bearing removal a determination of

the relative age of the marks can normally be made and a conclusion reached as to whether or notthere was excessive interference between the bearing and either the shaft or the housing.

Furthermore, insufficient heating of the bearing, hard particle contamination or a lobed shaft can

cause scoring of the inner ring and damage to the shaft. Excessive interference will expand the

inner ring, taking up all the internal clearance and overloading the bearing.

Circumferential scoring is caused by the shaft being undersized or the housing being oversized and

is indicative of the bearing turning on the shaft or in the housing. In some cases excessive load may

cause the bearing to spin or creep relative to the machine mounting surface and/or fastening

devices. In addition, damage to the bearing face can result due to rubbing of the bearing and theshaft shoulder and lock nut/washer. In both cases this may result in sufficient damage to cause

cracks and spalls to occur in the bearing.

Excessive scoring of either type will result in premature bearing failure and can be avoided by

carefully measuring all fitup dimensions and installing the bearing in accordance with the vendors

recommendations.

COLOR

The color of the bearing, overall and in specific locations, provides information on the operating

environment especially temperature. As the operating temperature increases the beating

components will change from a bright shiny or steel gray color to a straw, reddish brown, blue and

finally black with increase in operating temperature. As a result of operating at excessive

temperature the steel hardness is reduced and operating life significantly shortened.

The following photograph depicts the change in harness and color caused by increases in

temperature. The specimens are cut sections of bearing steel which have been heated to the

indicated temperature and their Rockwell C hardness checked.

A determination should be made as to the cause of the overheated bearing components. For

example, the bearing could be the cause of the excessive heat generation due to a reduction of

internal clearance by a tight fitup or by failure of the lubricant. In addition, an external source of



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heat could be conducted to the bearing which would cause premature failure. Thus, the bearing and

surrounding components should be examined for signs of thermal distress.

INTERNAL SURFACES

The internal surfaces consist of the rolling elements, raceways, internal shoulders or lands, and the

cage or retainer. In normal operation all of the load should be carried by the rolling elements and

the raceways and no signs of excessive abrasive wear should be apparent on the cage or theshoulders. For phenolic or other non-metal cages, a minor amount of rubbing on the shoulder is

acceptable since the cage uses one of the shoulders as a guide. Steel cages ride on the rolling

elements and should never come into contact with the inner or outer ring.

CAGE

The color and presence of any signs of abrasive wear should be noted. The color of the cage, steel

or phenolic, will darken with increases in temperature. As previously mentioned, with the

exception of a phenolic cage which uses a land as a guide there should be no signs of abrasive wearor cracks in the cage.

The cage pockets will often show signs of scoring due to the fact sliding and not rolling takes place

between the cage and the rolling elements. In the ideal situation, the condition of each cage pocket

will show uniform wear patterns and any pocket to pocket deviation should be noted and the cause

determined. The most common cause of excessive scoring of the pockets is hard particle

contamination.

RACEWAYS

The raceways are the portions of the inner and outer rings between the lands where the rolling

elements support the load. The area of contact is referred to as the ball path and is the portion of

the bearing which sees the cyclic stress which limits bearing life. As previously mentioned,

pressures in this region can exceed 300,000 PSI.

This high pressure can cause bearing failure through the following two mechanisms:

1. Surface initiated spalling

2. Fatigue spalling

Surface initiated spalls are caused by hard or soft particle denting and/or electrical pitting of the

raceways. These indentations create stress rises which cause the metal to fracture and spallout. Soft

particle dents can be caused by thread, tobacco, cage material, and lubricant impurities. Hard

particle denting can be caused by the metal oxide particles formed do to simple and fretting

corrosion. Electrical pits are often the result of improperly grounding arc welders as well as the

presence of stray currents in the rotating assembly of electrical motors.

Fatigue spalls occur when the beating has work hardened the raceway, resulting in embrittlementof the material, and finally loss of material.

The observable difference between the two types of spalls are 1) the surface initiated spall has an



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arrowhead shape often with a point at the tip of the arrow and 2) a fatigue or subsurface initiated

spall is usually oval and has steeper sides than a surface initiated spall.

Conclusion

A macroscopic examination of failed beatings can be used to determine the true or root cause of

beating failures, identify problems with lubrication, bearing design, installation and operation, and

evaluate the accuracy of the vibration analysis program.

All contents copyright 1998 - 2006, Computational Systems, Inc.

All Rights Reserved.

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