vib - failed bearing

8/3/2019 Vib - Failed Bearing

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835 Innovation DriveKnoxville, TN 37932Phone: (865) 675-2110 Fax: (865) 675-3100Customer Support: (865) 671-4274

DoctorKnowApplication PaperTitle: Failed Bearing EvaluationSource/Author:Alan K. Pride

Product: General

Technology: Vibration

FAILED BEARING EVALUATION

Alan K. Pride

Preliminary Steps

Following bearing removal, the bearing should be engraved with a unique identifying serialnumber on the same side of both the inner and outer ring. This serial number should berecorded 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, othermaintenance, 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 thebearing 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 theproper size to pry 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 lightand the presence of dirt, water, or other contaminates noted and recorded on theevaluation sheet.7. The bearing can now be cleaned using a solvent or diesel fuel and a partscleaning brush.

NOTE: Observe applicable safety precautions when handling solvents.

8. Following cleaning the bearing should rotate freely by hand. Any discrepanciesshould be noted on the evaluation sheet.9. Spray the bearing with CRC or similar substance to prevent corrosion from

occurring if the bearing is not to be analyzed at this tune.


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DISASSEMBLY

Angular Contact/Other Snap Together Bearings

Disassembly of beatings other than deep groove ball bearings cannot be accomplishedwithout minor damage to the beating components. However, this damage, nicks in the balls

or fresh smears in the race, are readily discernible from the damage which occurred duringoperation 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 suitablecontainer 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 atsufficient height to allow the cage and balls to clear the outer ring. Then strike theinner ring with a mallet at the high point; the bearing should separate into individualcomponents.

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 cagecan be removed by using a screw driver or punch and hammer, and a two piece phenoliccage can be disassembled using the appropriate size drift to drive out the pins which holdthe cage together. However, disassembly of the two piece riveted metal cage can bedifficult 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 notthe entire rivet.3. Remove the cage halves and clean the bearing and cage assembly with a brushand solvent to remove metal slivers.4. Slide all of the balls to one side of the bearing and clamp bearing in a vise on theside opposite from the balls.

Note: A face shield should be utilized during the next step as there is a chance thebearing 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 accompaniedby a snapping sound.6. Release the tension on the vise and place the bearing components in a cleancontainer.

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 carefullyexamined if an accurate assessment of cause of failure is to be determined. In most cases,a good light and a 10X magnifying glass are all that is required to make a diagnosis.

However, if failure rates seem excessive and a failure pattern emerges, detailed analysis bya metallurgist may be warranted.


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

BEARING ANALYSISEXTERNAL SURFACES

This section on external surfaces describes the checks of the physical condition of the

bearing and machine interfaces where the root cause of the bearing failure is oftendetermined. The information obtained from the examination of the points of contactbetween the bearing and the machine should be correlated with conditions found on therollers 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 overone-half of bearing failures were the result of contamination and improper installation.Contamination problems are normally manifested on the internal surfaces while indicatorsof installation problems are evident on internal and external surfaces.

Degradations and wear on external surfaces can be evaluated by observing the type andpatterns 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 andfretting corrosion which is a mechanical process. Simple corrosion is usually the result ofwater or moisture coming into contact with an unprotected surface of the bearing and simplyrusting 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 andform acids which result in dark brown and/or black stains on the balls and races. This typeof corrosion can take the form of very fine crack like lines in equipment which has not beenin operation for some time. In advanced states, severe pitting can take place which will formpoints for surface initiated spalls to occur.

In general, simple corrosion is randomly scattered over the surface with no apparent patternand not associated with wear.

Fretting Corrosion

Fretting corrosion is the result of small amounts of relative motion between the externalsurfaces 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 contactat high points between the two surfaces. The amount of fretting is a function of fit-up andload and thus is an excellent indicator to the cause of bearing failure.

Severe fretting can create weak spots in the bearing which will result in cracking andeventual spalling of the bearing. In addition, heavy fretting on the guide or floating bearingouter 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


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misalignment, improper clearances, and excessive loading. Therefore, careful mapping ofthe 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 todetermine 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 reproductionin the format of this paper is impossible.

1. The photograph shows normal fretting of the outer ring of a properly mountedthrust beating from a vertical pump/motor. In vertical applications where no parasiticor radial forces exist the fretting will be uniformly distributed near the center, usuallyoffset by the contact angle of the bearing in the same plane as the wear trackcreated by the ball on the outer race.

Since the rings are flexible and the bearing generates pressures in excess of 300,000 PSIthe 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 ofoperation.

2. A radially loaded bearing, as found in a horizontal machine, will also show thesame fretting slightly offset from the center but will be heavy only in the load zoneand 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 andvibration in a horizontal machine.5. In this example the fretting occurs at opposite sides of the inner and outer ringswhich indicates a very high thrust load. If the wear track angle were measured itwould probably indicate a higher than designed contact angle.

6. These two photographs are classical examples of poor housing and bearing fitupswhere the bearing was not fully supported by the housing which resulted inexcessive flexing of the outer ring in localized areas. This has the effect of increasing

the amount of stress on the bearing and shortening bearing life.

7. Bearing misalignment is a leading cause of premature bearing failure and caneasily be avoided by properly installing the bearing. These photographs show askewed fretting pattern on the outer ring but normal light fretting in the center of theinner ring bore. In order to avoid this type of problem, squareness of the shaftshoulder should be confirmed as well as ensuring the bearing is positioned snuglyagainst the shoulder during installation.

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


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housing which does not provide sufficient resistance to rotation to prevent the outerring from chattering in the housing. In this particular case, the bearing could havebeen noisy in service and removed prematurely.

SCORING

There are two types of scoring, axial and circumferential, both of which are usuallyindicative of poor bearing fitup. While axial scoring can be created during bearing removal adetermination of the relative age of the marks can normally be made and a conclusionreached as to whether or not there was excessive interference between the bearing andeither the shaft or the housing. Furthermore, insufficient heating of the bearing, hard particlecontamination 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 andoverloading the bearing.

Circumferential scoring is caused by the shaft being undersized or the housing beingoversized and is indicative of the bearing turning on the shaft or in the housing. In somecases excessive load may cause the bearing to spin or creep relative to the machinemounting surface and/or fastening devices. In addition, damage to the bearing face canresult due to rubbing of the bearing and the shaft shoulder and lock nut/washer. In bothcases this may result in sufficient damage to cause cracks and spalls to occur in thebearing.

Excessive scoring of either type will result in premature bearing failure and can be avoidedby carefully measuring all fitup dimensions and installing the bearing in accordance with thevendors recommendations.

COLOR

The color of the bearing, overall and in specific locations, provides information on theoperating environment especially temperature. As the operating temperature increases thebeating components will change from a bright shiny or steel gray color to a straw, reddishbrown, blue and finally black with increase in operating temperature. As a result ofoperating at excessive temperature the steel hardness is reduced and operating lifesignificantly 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 tothe 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 areduction of internal clearance by a tight fitup or by failure of the lubricant. In addition, anexternal source of heat could be conducted to the bearing which would cause prematurefailure. Thus, the bearing and surrounding components should be examined for signs ofthermal 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


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elements and the raceways and no signs of excessive abrasive wear should be apparenton the cage or the shoulders. For phenolic or other non-metal cages, a minor amount ofrubbing 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 inneror outer ring.

CAGE

The color and presence of any signs of abrasive wear should be noted. The color of thecage, 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 nosigns of abrasive wear or cracks in the cage.

The cage pockets will often show signs of scoring due to the fact sliding and not rollingtakes 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 deviationshould be noted and the cause determined. The most common cause of excessive scoringof the pockets is hard particle contamination.

RACEWAYS

The raceways are the portions of the inner and outer rings between the lands where therolling elements support the load. The area of contact is referred to as the ball path and isthe portion of the bearing which sees the cyclic stress which limits bearing life. Aspreviously 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 spalling2. Fatigue spalling

Surface initiated spalls are caused by hard or soft particle denting and/or electrical pitting ofthe raceways. These indentations create stress rises which cause the metal to fracture andspallout. Soft particle dents can be caused by thread, tobacco, cage material, and lubricantimpurities. Hard particle denting can be caused by the metal oxide particles formed do tosimple and fretting corrosion. Electrical pits are often the result of improperly grounding arcwelders as well as the presence of stray currents in the rotating assembly of electricalmotors.

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

The observable difference between the two types of spalls are 1) the surface initiated spallhas an arrowhead shape often with a point at the tip of the arrow and 2) a fatigue orsubsurface 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 rootcause of beating failures, identify problems with lubrication, bearing design, installation and

operation, and evaluate the accuracy of the vibration analysis program.

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