Bearing know-howfor rewinders

Frequently asked questionswith answers

2Frequently asked questions with answers Contents Page

2. SKF rolling bearings in electric motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Bearing usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4. Bearing arrangements in electric motors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5. Dismounting bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6. Bearing failure analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

7. Lubrication of bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

8. Mounting bearings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Bearing know-howfor rewinders

The most common questions connected with bearings inelectric motors have been compiled together with theanswers. They form an easily accessible source of knowledge.The questions are collected for each chapter which makes iteasy to find a certain topic.

Question1. Why should the rust protectionon new bearings not be cleanedoff?

2. What is false brinelling?

3. If rolling bearings have such alow friction why do they need to belubricated?

4. Are deep groove ball bearingssuitable for the drive end of amotor with belt drive?

5. Can a cylindrical roller bearinge.g. NU 210 be replaced by a deepgroove ball bearing e.g. 6210,which has the same boundarydimensions?

6. Why shall a motor not be testedwithout load?

7. Do hybrid bearings need anylubricant?

Answer1. New bearings are clean and ready to be lubricated. Thelayer of rust protection should not be removed. Keep thebearings in the original package until they are to be mounted.

2. When a motor is at standstill it can be exposed to vibra-tions from machines in the surrounding area. The rollingelements in the bearings will transfer the vibrations fromthe outer ring to the inner ring. If the bearing does not ro-tate the rolling elements will be hammering on the samepoint all the time and that will cause corrosion and wear,forming shallow depressions in the raceways. This pheno-menon is also called false brinelling. By frequently rota-ting the shaft this can be avoided.

3. When metal surfaces are in contact under high loadthere will be wear and high temperature, which will rapid-ly make the bearing unusable. Proper lubrication separatesthe surfaces and wear is prevented. The lubricant also re-duces the friction further and power will be saved.

4. Normally not, as the radial loads could be high. The bestsolution is to use a cylindrical roller bearing or a CARBbearing.

5. Yes, but certain precautions have to be taken. If acylindrical roller bearing of the type NU or N is beingreplaced by a deep groove ball bearing, the outer ring fithas to be modified to allow axial movement of the outerring of the ball bearing. Also check that no mechanical stopwill restrict this movement.

This replacement is sometimes called for when a belt-drive motor is rebuilt for coupling-drive operation

6. In order to guarantee satisfactory operation all rollingbearings must always be subjected to a given minimumload.

Rotation without load can damage the bearing. Consultthe SKF General catalogue for advice about the size of theminimum load for the bearing in question.

7. Yes, they do. Open hybrid bearings should be lubricatedwith a high performance grease, e.g. SKF LGHP 2. SealedSKF hybrid bearings are supplied filled with a high per-formance grease (suffix WT) to obtain longest possible service life.

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2. SKF rolling bearings in electric motors

8. Why can hybrid bearings beoperated at such high speeds?

9. Should one bearing be insulatedonly or both?

10. Is the electrical insulating layera thermal insulator as well? How isheat conducted away?

11. Can an INSOCOAT bearing beused as an axially free bearing?

12. Why do all INSOCOAT bear-ings have brass cages?

13. Is there anything I should thinkabout when fitting INSOCOATbearings?

14. Is the insulation sensitive todamage?

15. I cant find the bearings I wantin the standard INSOCOAT range.What do I do?

8. In bearings that operate at very high speed the centrifu-gal forces from the mass of the rolling elements will beconsiderable. These forces will increase the total radialload on the bearing. Thus the life will be somewhat lowerand the heat higher. The mass of a ceramic rolling elementis 60 % lower than for a steel one and so the load and thetemperature will be lower. Therefore higher speed can beallowed.

9. That depends on the motor. Induction motors normallyneed to be insulated on the non-driven side only, but DCmotors should be insulated on both.

10. The layer does not conduct heat as well as steel does,but it is so thin that it makes no difference in practice.Experiments have shown that the working temperature ofthe bearing, measured at the outer ring, rises by no morethan 23 C.

11. Yes, its common to use it as the axially free bearing.The material in the bearing housing determines the frictioncoefficient.

12. We have chosen a type of cage that suits most ma-chines and working conditions. It would be prohibitivelyexpensive to double the range by offering two types ofcages. However, we can offer pressed steel cages for largerquantities (> 1 000 units a year).

13. INSOCOAT bearings are fitted in the same way asstandard bearings. It is useful to have a 15 chamfer in thebearing seating to facilitate fitting. Every package containsa few suggestions on fitting.

14. You can handle an INSOCOAT bearing just like youwould an ordinary bearing hitting either with a hammeris not advisable.

15. Ask SKF! We may already manufacture them in thesize you need for someone else many different sizes areused in the railway industry. Provided that the quantity islarge enough, bearing sizes not in the standard range can beproduced.

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16. What decides permissible op-erating temperature for a bearing? 16. It is often the lubricant, which sets the temperature

limit. SKF rolling bearings can generally be used at oper-ating temperatures up to +120 C. For bearings with shieldsor seals on both sides the grease in the bearings decides thetemperature limits to 30 and +110 C.

If the operating temperature is above +120 C, the bear-ings must be specially heat treated. Of course high tem-perature grease has to be used in such cases.

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Question1. Why is it so important not tochange from C3 clearance toNormal?

2. You said Always use the samebearing as specified by the manu-facturer but also There is a trendto replace deep groove ball bear-ings with sealed or shielded ones.Is this not contradictory?

3. Why does a motor becomemore silent when the deep grooveball bearings are axially spring preloaded?

4. Too small a bearing clearancecan result in bearing failure, pleasedescribe how.

5. It is recommended that the shaftin a motor at standstill, and subjec-ted to vibrations, should be slowlyrotated in order to avoid damage tothe bearings. Please clarify.

6. When talking about heat stabil-ising it is said that: unwantedstructural changes in the materialcause dimensional changes in thebearing components. What doesthat mean?

Answer1. Because the clearance C3 is greater than Normal there isa risk that the clearance during operation can be too small ifclearance Normal is used. This will result in increased bear-ing temperature and deteriorated lubrication. In the worstcase it can lead to bearing failure.

2. For a motor used at temperatures below 100 C there is norisk in changing to sealed or shielded deep groove ball bear-ings of the same series. However, for high speed motors,check that the motor speed is not higher than the speed ratingof the actual sealed bearing.

For bearings operating at temperatures above 100 C boththe seals and the standard grease limit the operating temper-ature to +110 C. For motors operated above +110 C thechange to sealed or shielded bearing should not take place.

3. Due to the axial load all balls are loaded the whole timeand in constant contact with the raceways which gives thebearings a more silent running. The rotor is better guided andcentered, which will lead to reduced vibration and thus makethe motor more silent.

4. Too small a clearance means that the bearing can be radially preloaded with increased running temperature as aconsequence. When the temperature is increased the lubrica-tion is deteriorated. This means that the temperature will beeven further increased.

The process is in a vicious circle which, in the worst casecan end up with bearing failure.

5. When a bearing at standstill is subjected to vibrations therolling elements will be hammering on the same point of theraceway the whole time. By this the lubricant will be pushedaway and wear will occur forming shallow depressions in theraceway. By rotating the shaft, both the rolling elements andthe inner ring will change position all the time and the lubricant will stay in place. In this way this kind of damageis prevented.

6. When bearing steel is exposed to temperatures above+120 C for a long period of time the rings will grow. Thiscan e.g. mean that the interference fit on the inner ring be-comes loose and a relative movement occurs between theinner ring and the shaft which will generate wear and smearing. By a special heat treatment of the rings this process is prevented.

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3. Bearing usage

Question1. Why is it so important to haveone of the bearings on a shaft non-locating?

2. How can the cross-locationarrangement function when bothbearings are locating?

3. Can a cylindrical roller bearingbe replaced by a deep groove ballbearing?

4. What is the reason for changingfrom a cylindrical roller bearing toa deep groove ball bearing?

5. Is it possible to change from adeep groove ball bearing to acylindrical roller bearing?

6. What is the reason for changingfrom a deep groove ball bearing toa cylindrical roller bearing?

7. When should a toroidal rollerbearing, CARB, be used in electricmotors?

8. Why are deep groove ball bear-ings often spring preloaded inelectric motors?

Answer1. The bearing arrangement must accommodate the ther-mal expansion and shortening of the shaft respectively. Ifboth bearings are axially located these movements of theshaft can not take place and the bearings will be subjectedto internal thrust loads which may be heavy. Thus the bear-ing lives will be shortened and in the worst case result inearly bearing failures.

2. The bearings locate the shaft in one direction each andthere must be a small axial play to avoid internal thrustloading of the bearings. This arrangement is applicable onshort shafts only, which means that the shafts axial move-ments will be relatively small and within the sum of theaxial play in the housing and the internal axial clearance ofthe bearing.

3. Yes it can. If a cylindrical roller bearing of type N or NUis replaced by a deep groove ball bearing, the outer ring fithas to be modified to allow axial movement of the outerring of the ball bearing.

4. All bearings require a certain minimum load to attain along service life. If a belt drive motor is to be used forcoupling drive, where the load is much smaller, it is a riskthat the load on a cylindrical roller bearing is lower thenthe required minimum load. In such a case it is useful tochange to a ball bearing which requires a lower minimumload.

5. Yes it is if the deep groove ball bearing is non-locating.However, the outer ring of the cylindrical roller bearing mustbe axially located and the housing tolerance should bechanged to K7 by for instance metal coating and machining.

6. In the case a motor for coupling drive is to be used forbelt drive a stronger bearing might be needed to accom-modate the belt forces. In such a case a change from a deepgroove ball bearing to a cylindrical roller bearing could beadvisable.

7. When the loads are high there is a risk that the shaft willbe misaligned in relation to the housing. If the drive endbearing is non-locating CARB is the ideal solution.

8. The demand for low noise level is stringent on electricmotors and by the axial preloading the balls are loaded allthe time, which gives the bearing virtually silent running.

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4. Bearing arrangements

9. What are the most common reasons for high bearing noiselevels in electric motors?

9. It is often due to too low load on the bearings. In the caseof deep groove ball bearings axial spring preloading willgive the bearings virtually silent running.

The noise can also depend on misalignment of theshaft. In cases where the noise is caused by a non-locatingbearing the use of a CARB bearing offers a good solution.

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Question1. Why should the right tool beused when dismounting bearingswhich anyway should be replaced?

2. If the bearings are damaged andshould be exchanged, why shouldthey be carefully dismounted?

3. Why should large bearings belifted with a belt around the outerdiameter?

4. What should be observed when abearing is dismounted?

5. Should a damaged bearing bescrapped or should it be kept?

Answer1. The bearing has to be dismounted carefully in order notto damage the seatings which should be used for the newbearings.

2. In order to enable examination of the bearings and findout the cause of the damage they must be carefully dismounted. In this way, corrective actions may be taken toprevent a recurrence.

3. Because they are slender relative to the outer diameterand will be ovalised if the lifting strap is applied at onepoint only.

4. To make an analysis of a damaged bearing possible,information about the bearing must be available. Pleasenote the following: the position of the bearing in the motor (e.g. drive end,

non-locating etc.) the side of the bearing which was directed outwards how it was oriented in the housing (e.g. mark the outer

ring at 12 oclock) the appearance of the bearing collect a sample of the lubricant

5. In order to find the reason for the damage the bearinghas to be analysed and should therefore be kept until theproblem has been solved.

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5. Dismounting bearings

Question1. What is bearing fatigue?

2. Why does passage of currentoccur through the bearings?

3. Which motor types run the greatest risk of having circulating current through the bearings?

4. Why must a bearing alwayshave a minimum load?

Answer1. Normal fatigue is the life of the bearing that can be cal-culated with a certain probability by the life equations.Fatigue was formerly considered to be the result of stressescyclically appearing immediately below the load carryingsurface. It is observed as flaking or spalling of surfacematerial.

More recently, however, failures have increasinglybeen found to initiate from the surface rather than fromcracks formed beneath the surface. The failures occur dueto surface distress. This can be from indentations causedby over-rolled contaminants or due to lubrication prob-lems.

These new findings are considered in the SKF LifeMethod for calculation of bearing life where the clean-liness of the lubricant and the bearing surroundings play animportant role.

2. The oil film between rolling elements and raceways hasa certain insulating effect. If the voltage exceeds a criticalvalue, the threshold voltage, the current will start to flowthrough the bearing. The threshold voltage depends onseveral factors and can vary, but as typical values 0,5 to 1 V are normal.

3. Motors used with variable frequency drives have a highrisk of circulating current through the rotor and the stator,especially modern inverters using insulated gate bipolartransistors (IGBT). Experience has shown that AC motorsof approximate size 315 and above suffer from circulatingcurrent and motors below that size are at risk of capacitivedischarges. Of course, it also depends on the motor installa-tion and use.

4. In an unloaded bearing the rolling elements and thecage are free to move within the bearing clearance and therolling elements will touch the raceways in an uncontrolledmanner. These contacts will give rise to smearing whichafter a certain time will result in noise.

By applying the requisite minimum load the rolling elements and raceways are in continuous contact with eachother and smearing will not occur.

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6. Bearing failure analysis

5. What is false brinelling?

6. Which are the most commoncauses of bearing failure in electricmotors?

7. In the case of a bearing failurewhat can be done to prevent thesame damage from recurring?

5. When a bearing at standstill is exposed to vibrations e.g.from surrounding machines small relative motions betweenthe rolling elements and the raceways will take place. Dueto the absence of an oil film, which cannot be formed whenthe bearing is not rotating, wear will occur, forming shal-low depressions in the raceways.

The appearance of the marks is somehow similar to theindentations at brinell testing of steel hardness and there-fore it is called false brinelling. However, the false bri-nelling marks have a polished appearance.

6. The most common causes are: inadequate lubrication wrong fits mounting errors contamination incorrect bearing clearance passage of current vibration damage

7. Each failure mode has its own type of action, which shouldbe taken to avoid recurrence. This will be too long to reporthere, but in the Manual for bearing health these actions aredescribed in the chapter Bearing failure analysis.

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Question1. When the rolling friction is solow why do rolling bearings needto be lubricated?

2. Could the same relubricationintervals be applied for deep grooveball bearings as for roller bearings?

3. When should greases with dif-ferent stiffness be used?

4. Have greases of the stiffnessesNLGI 2 and 3 the same lubricatingability?

5. How could the very thin oil filmseparate the rolling elements fromthe raceways?

6. Why is cleanliness so import-ant?

Answer1. When metallic surfaces are in contact under heavy pres-sure wear will occur since the rolling elements are drivenby the friction between them and the rotating ring.

Furthermore, all bearings with curved raceways, e.g.deep groove ball bearings, have only two points wherepure rolling occurs. Between these points and outside themthere is sliding between the rolling element and the race-way in opposite directions. Proper lubrication separates thesurfaces and wear is prevented.

2. Roller bearings require shorter relubrication intervalsthan deep groove ball bearings. To have the same relub-rication interval for both bearings the shorter roller bearingrelubrication interval must be chosen.

3. The SKF grease LGMT 2 is a soft grease with the stiff-ness NLGI 2. It is used in most small and medium-sizedelectric motors where the speed is moderate to high.

The SKF grease LGMT 3 is a thicker grease with thestiffness NLGI 3. It can be used for vertical shafts as, dueto its stiffness, it will stay longer in the bearing comparedto an NLGI 2 grease .

4. It is the base oil in the grease that lubricates the bearing.The base oil viscosity should be chosen with respect to thebearing speed and expected operating temperature. Seealso the answer to question 3.

5. The viscosity of the oil film increases dramaticallywhen it is exposed to the very high pressure in the contactbetween the rolling elements and the raceway. This meansthat the oil is solidified and will not be pressed out from thecontact. When the rolling elements roll on, the viscosityreturns to its original condition.

The demand for smoothness of the rolling element andraceway surfaces is therefore very stringent.

6. Consider that the oil film is about 0,0005 mm thick.Only particles with a size less than 0,0005 mm can passthrough the contact without damaging the surfaces. If ahard particle with the size of a few microns is over-rolledit will make an indentation in the raceway and sooner orlater a crack is initiated that shortens the bearing life.

This is reflected in the SKF Life Method where the cleanliness plays a main role in achieving long bearinglives.

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7. Lubrication bearings

7. My company uses only lubric-ant of a certain brand. How shouldI find an equivalent to the SKF grease e.g. LGMT 2?

8. What is meant by compatiblegreases?

9. What happens if water penet-rates into the grease in a bearing?

10. What water content can beallowed in a lubrication grease?

11. What can I do if water haspenetrated into the grease?

12. If a bearing has failed due toinsufficient lubrication and hasbeen replaced with a new one, whatshould be done in order to avoid thesame failure happening again?

13. Why cannot only one universalgrease be used for all motors?

7. Ask the lubricant supplier to propose an equivalent grease to SKF LGMT 2 of their brand.

8. They contain the same soap base oil and have the sameconsistency and can therefore be mixed with each other.

9. There is a great risk of corrosion on the rolling contactsurfaces of the rollers and rings. The lubricating ability ofthe grease is reduced and the bearing service life will beshortened.

10. In principle nothing at all. Even such a small contentof water in the grease as a fraction of one percent will givea considerable reduction of the bearing service life.

11. If the bearing has seals it should be replaced with anew one. If the bearing is open, dismount carefully andclean it. Inspect the raceways with the bearing stillassembled. If no signs of corrosion could be detectedregrease the bearing and mount it back into the motor.

If the motor is equipped with grease valves and thewater ingress is early detected regrease the bearing untilfresh grease purges through the valve opening.

In all cases check the external seals and exchange themif they are damaged.

12. Check the external seals and exchange them if they aredamaged. If the seals are not effective enough for the envir-onment the motor is operated in, replace them with moreeffective ones to prevent the ingress of contamination.Check that the right quality of grease is used. Check alsothe relubrication intervals so they are in accordance withthe manufacturers recommendation. If such information ismissing use the general relubrication diagram in theManual for bearing health to find the correct relubrica-tion intervals.

Always relubricate the bearing when it is rotating.

13. It will not give the best lubrication under all the differ-ent conditions the motors are operated at. In most cases itwill be a compromise and will in the worst case lead todrastically reduced bearing service life.

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14. The environmental issues arevery much in focus. Does anygreen grease exist which can beused in electric motors?

14. The SKF LGGB 2 is a green grease with low tox-icity, it is biodegradable and recommended where the en-vironment is a concern.

It can replace the SKF LGMT 2 and LGEP 2 greasesand due to the synthetic oil in SKF LGGB 2 it can be usedat low temperatures as well.

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Question1. Is it really necessary to use mark-ing blue paint when checking theshaft?

2. In connection with fits you talkabout rotating and stationary loadsetc. Which is the most commoncase for electric motors?

3. In the manual you give anexample of tolerances for a bear-ing seating and the requirement forcylindricity is more stringent thanfor the dimension of the diameter.Why is it so?

4. If a motor has been stored forten years could it then be put intooperation? Have the bearings beendamaged?

5. To which temperature can bear-ings be heated?

6. Which method is preferredwhen bearings shall be heated?

7. Could bearings be heated withan open flame if it is carefullydone?

8. The use of sealed bearings inelectric motors is increasing buthow long will the grease in thesebearings last?

Answer1. This is the best way to do it but if marking blue paint isnot available you can check that there are no narrow lightslots between the shaft and the ruler.

2. The most common case is stationary load on the outerring and rotating load on the inner ring. For this case wechoose a tight fit for the shaft and a loose fit for the hous-ing seating.

3. The bearing rings are slender and will take the form ofthe seating. A bearing with an oval ring will be noisy andthere is a risk that the rolling elements are squeezed betweenthe rings with high loading and short life as consequence.

4. If the bearings have not been exposed to moisture they arenot damaged but the grease is too old. It is difficult to removethe old grease from the bearings and there is a great risk ofcontaminating the bearing during this procedure.

Over a ten year period, bearing development has beenconsiderable. The performance, in all aspects, is improvedsignificantly e.g. lower noise, longer service life etc. The bestsolution is therefore to dismount the bearings and replacethem with new ones, which should be greased with fresh andclean grease. In the case of deep groove ball bearings, con-sider exchanging them to sealed or shielded ones.

5. Open, ungreased bearings, can be heated up to 125 Cas a maximum. Sealed or shielded bearings should, in prin-ciple, not be heated because of their grease fill. However,if an induction heater is used the bearings can be heatedduring a very short period of time and not over 110 C.

6. The use of an induction heater is the best and easiestmethod to heat bearings. Note that it is not possible to usean induction heater for sensor bearings.

7. Under no circumstances should bearings be heated withan open flame.

8. They are greased for life and will in most cases outlastthe motor. The grease in them is clean from the beginningand is, due to the seals, kept clean which ensures the long-est possible life for the bearings. An opinion of the greaselife can be obtanied from the relubrication diagram in theManual for bearing health.

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8. Mounting bearings

SKF 2001The contents of this publication are the copyright of the publisherand may not be reproduced (even extracts) unless permissionis granted. Every care has beentaken to ensure the accuracy ofthe information contained in thispublication but no liability can beaccepted for any loss or damagewhether direct, indirect or con-sequential arising out of the use of the information contained herein.

Publication 5074 Ewww.skf.com

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