Bearing Damage Analysis

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BEARING DAMAGE ANALYSIS:

Recognizing and preventing damage in automotive bearings can dramatically increase bearing lifeand decrease the potential for improper handling, installation and adjustment. It also reduces

instances of bearing failure, thereby increasing the safety of vehicle passengers.

The most common types of bearing damage that may result in a reduction of bearing or applicationlife are often caused by insufficient maintenance practices, mishandling, improper adjustment

practices or inadequate lubrication.

The following offers a quick reference to the common causes of bearing damage in automotive

applications.

Roller-end scoringMetal-to-metal contact from breakdown of lubrication

film.

Cone large rib and

roller large end scoringWelding and heatdamage from metal-to-metal contact.

Roller large end

deformationMetal flow from excessiveheat generation.

Total bearing lock-upRollers skew, slide sideways and lock-up bearing.

StainingSurface stain with no significant corrosion from moisture

exposure.

Etching

Rusting with pitting and corrosion from moisture/waterexposure.


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EXCESSIVE PRELOAD OR OVERLOAD

Line spallingRoller-spaced spalling from bearings operating after

etching damage.

Rapid and deep spalling

Caused by unusually high stresses. Full race width fatiguespalling is caused by heavy loads creating a thin lubricantfilm and possible elevated temperatures.

Geometric stress concentration

Spalling from misalignment, deflections or heavy loading.

Inclusion originSpalling from oxides or other hard inclusions in bearing

steel.

Point surface originSpalling from debris or raised metal exceeding thelubricant film thickness.


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IMPROPER FIT

Cone bore damage

Fractured cone due to out-of-round or over-sized shaft.

Cup spinning

Loose cup fit in a rotating wheel hub.

Abrasive wear

Fine abrasive particle contamination.

BruisingDebris from other fatigued parts, inadequate sealing orpoor maintenance.

GroovingLarge particle contamination imbedding into soft cagematerial.


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EXCESSIVE END PLAY

ScallopingUneven localized wear resulting from excessive end play.

Cage pocket wear

Heavy contact between the rollers and cage pocket surfaces causedby bearing operating too loosely.

RECOGNIZING AND PREVENTING DAMAGES OF BEARINGS:

Damage to bearings while handling before and during installation and damage caused by improperinstallation, setting and operating conditions are, by far, the causes of the largest percentage ofpremature trouble.

In the following, examples are shown of the most common types of damage and some of the causesof this damage.

In many cases the damage is easily identified by the appearance of the bearing, but it is not easy

and sometimes it is impossible, to determine the exact cause of that damage. As an example, abearing with scored and heat discolored roller ends and rib is easily identified as a burned up

bearing and damaged beyond further use. The cause of the burning or damage, however, might betraced to any one of a number of things such as insufficient or improper lubricant. It may be thewrong type of lubricant or the wrong system for supplying lubricant. Perhaps a lighter or a heavier

lubricant is needed or an extreme pressure type of lubricant rather than a straight mineral oil and acirculating oil system needed rather than an oil level or splash system. This type of damage could becaused by excessively tight bearing setting or a combination of too tight setting and inadequatelubrication.

From this it can be seen that simple examination of a bearing will not reveal the cause of thetrouble. It can reveal if the bearing is good for further service, but often it is necessary to make athorough and complete investigation of the mounting, installation and parts affecting the bearing

operation to determine the cause of the damage. Unless the true cause of the damage is found andcorrected, the replacement bearing will be damaged in the same manner and again there will bepremature trouble. This information is not an attempt to make "trouble shooters" or "bearingexperts" of all who read it. It is intended to caution users about possible causes of damage and alertthem to take preventive action. With proper precautions during the handling, assembly and

operation of bearings, almost all damage can be prevented. It is much easier, and a great deal lessexpensive, to prevent damage than to determine and correct the cause of damage after the machineor equipment is in operation.


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Typical modes of failures

Mode of contact fatigue

Geometric stressconcentrationGeometric stressconcentration fatigueresults from locally

increased stress at theends of roller/racecontact.

Point Surface Origin (PSO)PSO is fatigue damage that has its origin associated with surface

asperities, which act as local stress concentrations.

Peeling: This type of fatigue is characterized by a shallow < 2.5 m m (0.1 m in) deep, spallingwhich sometimes occurs locally around bruises, grooves, or ends of roller/race contacts wherethe EHD film is lost by leakage.

Transverse cracking fatigue

a) Non-propagatingspallInclusion origin spall

b) Spall propagated by hydraulic pressure

Damage by mechanisms other than contact fatigue

Abrasive Wear


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Spalling

Wear from foreign material. Debris bruises on all contact

surfaces due to hard particles in the lubricant

BrinellingBrinelling is the plastic deformation of bearing element surfaces due to extreme or

repeated shock loads.

False brinelling

False brinelling isrecognisable by thegrooves worn into theraceways by axial

movement of therollers duringtransportation.

Cage damage

Cage breakage


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Results of good practices

In the preceeding comments, the results of bad handling, improper assemblies, adjustments andoperating conditions have been stressed and the resulting damage shown. The following imageshows what happens when there is good lubrication, good assembly and maintenance and the

proper fitting practice for the bearing application has been followed. This bearing shows that, withreasonable care in machining the parts and in the assembly and maintenance, it is not difficult toget excellent life. This bearing operated for over 400,000 km (250,000 miles) in a bus and is still inexcellent condition and probably would run for many more kilometres.

HOW TO DETERMINE PROBABLE CAUSES


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BEARING SPEED CAPABILITIES:

The speed capability of a bearing in any application is subject to a number of factors including :

temperature bearing setting lubrication bearing design

The relative importance of each of these factors depends on the nature of the application. The effectof each factor is not isolated; each contributes, in varying degrees depending on the application, tothe overall speed capability of the design. An understanding of how each of these factors affectsperformance as speeds change is required to achieve the speed capabilities inherent in tapered

roller bearings.

Measuring speed

The usual measure of the speed of a tapered roller bearing is the circumferential velocity of the

midpoint of the inner race large end rib (fig. 5-2), and this may be calculated as :

Rib speed:

Vr= pDmn / 60000 (m/s), Vr= pDmn / 12 (ft/min)

where: Dm= Inner race rib diameter mm, in, n = Bearing speed rev/min

Fig. 5-2

Inner race rib diameter. The inner race rib diameter may be scaledfrom a print or approximated as the average of the inner race inside

diameter and the outer race outside diameter.

The rib diameter at the midpoint of the roller end contact can be scaled from a drawing of thebearing, if available, or this diameter can be approximated as the average of the bearing I.D. and

O.D.

DN values (the product of the inner race bore in mm and the speed in rev/min) are often used as ameasure of bearing speed. There is no direct relationship between the rib speed of a tapered roller

bearing and DN value because of the wide variation in bearing cross sectional thickness. However,for rough approximation, one metre per second rib speed is about equal to 16 000 DN for averagesection bearings. One foot per minute is equal to approximately 80 DN.


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Speed capability guidelines

Fig. 5-3

Speed capability guidelines for various types of lubrication systems

Fig. 5-3 is a summary of guidelines relating to speed and temperature. There are no clear-cut speedlimitations for tapered roller bearings regardless of the bearing design or lubrication systems. TheTimken Company recommends that testing be performed for all new high-speed applications.

Bearing design

Standard tapered roller bearings can operate at speeds up to about 30 m/s (6 000 ft/min orapproximately 500 000 DN) ; specially designed high speed tapered roller bearings can operatesuccessfully at speeds of over 200 m/s (40 000 ft/min or about 3 200 000 DN). These speeds can beachieved for either of these cases provided there is proper setting, adequate lubrication, no shock,

vibration or unusual loading, and there is adequate heat dissipation.

Bearing material limitations

Standard bearing steels cannot maintain the desired minimum hot hardness of 58 Rc much above135 C (275 F). Special steels that retain their hardness at elevated temperatures are available.Timken CBS 600 TM VIMVAR steel should be considered for temperatures between 150 to 230 C

(300 to 450 F) and Timken CBS 1 000 TM VIMVAR steel should be used for temperatures above230 C (450 F).

WARNING: Never spin a bearing with compressed air. The force of the compressed air may causethe rollers to be expelled with great velocity, creating a risk of serious bodily harm. Proper bearingmaintenance and handling practices are critical. Failure to follow installation instructions and failure

to maintain proper lubrication can result in equipment failure, creating a risk of serious bodily harm.

BEARING DYNAMICS AND SOUNDS:

Preface

A growing awareness of noise pollution, prompted in part by government regulations, has beennoticeable during recent years. One is hard-pressed to single out an industry that has not been

affected, either as a user or a supplier.

In its role as a supplier, The Timken Company can look back on a long history --predating thecurrent emphasis on noise abatement by many years--of actively practicing noise control. This


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philosophy is exemplified not only by an extensive sound test program in its production facilities butalso by an ongoing commitment to research in both fundamental and practical aspects of bearingrelated sound.

It is very useful to picture the bearing as playing one of two very distinct roles. In one of these, its

passive role as a transmitter, the bearing merely provides a path for energy transfer between therotating and the stationary member, while in the second or active role, it causes its immediateenvironment to be excited by virtue of its rotation. It is important to recognize this distinction,particularly in situations calling for a diagnosis. Essentially, bearings play a significant role in the

transmission of vibration in rotating equipment, however, they usually are not the predominant

source of vibration.

Nomenclature

Symbol Description Units

d0 Cone raceway mean diameter mm, in

D0 Cup raceway mean diameter mm, in

DW0 Roller mean diameter mm, in

f Excitation frequency Hz

i Harmonic index of carrier frequency, 0, 1, 2, 3, ..

j Harmonic index of modulating frequency, 0, 1, 2, 3, ..

K1, K2, K3Geometry-related constants

Kbearing Bearing stiffness N/m, lbf/in

Khousing Housing stiffness N/m, lbf/in

Ksystem System stiffness N/m, lbf/in

S Rotational speed rpm

Z Number of rollers per row

a (alpha) included cup angle degree

(beta) included cone angle degree

n (nu) included roller angle degree

The bearing as a transmitter

Simply put, a bearing may be thought of as a massless spring/damper connecting a shaft to its

housing. Typically, the interest lies in determining how vibration is transferred from housing to shaftor vice versa. For example, the excitation of meshing gears is carried along the shaft, through thebearing and to the exposed housing surface, where some of the energy is converted to airbornenoise.

Tapered roller bearings enjoy an advantage not found in other types of rolling element bearings.Since two bearings typically are adjusted against one another, the setting will govern the axial force.This influences the stiffness of the bearings and thereby the stiffness of the system. By merely

varying the bearing setting, it may be possible to shift any unwelcome resonances out of thefrequency range of interest. Maximum stiffnesses of approximately 1.75 x 109N/m (10 x 106lbf/in)are common in tapered roller bearings.

In manipulating the system stiffness, it is essential that the stiffness of the bearing supports(housing) be taken into account. In simple conceptual terms:

1/Ksystem= 1/Kbearing+ 1/Khousing

While the prediction of stiffness in bearings is cumbersome at best, dealing with their damping

characteristics is even more elusive. It has been demonstrated, however, that bearing setting willaffect the amount of damping which can be realized.


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The bearing as an exciter

The excitation potential of any rolling element bearing is determined primarily by the topography ofits rolling surfaces. For example, a severe force pulse would result if a gross imperfection, such as aspall of sufficient size, were present in an operating bearing. Similarly, small imperfections such as

brinell marks, nicks and any other deviations from perfect roundness of the components, will causesmaller fluctuation in the dynamic force. Hertzian theory tells us that even minute deformations canresult in forces of significant magnitude. Therefore, this is the mechanism causing the bearing to actas an exciter.

Surface irregularities of various origins lead to dynamic forces. These forces do not remain localizedbut are transmitted quite readily into the supporting structure.

The dependence upon a number of rather unwieldy variables prohibits the mathematical

determination of the magnitude of these forces in any one bearing. Their frequencies can bedetermined very accurately, though, from the gross dimensions of the bearing and its operatingspeed. Three constants can be defined in terms of either the angles or the diameters of the bearing:

These constants, along with the operating speed (S), the number of rollers (Z) and a harmonic index(i), permit the calculation of certain frequencies. They, in turn, identify specific disturbances (Table

1).

Table 1

Type of Disturbance and Resulting Excitation Frequencies

Disturbance Frecuency, Hz

Eccentricity of Rotating Member f0= S/60

Out-of-Round of Rotating Member f1i= i * f0

Roller Irregularity, e.g., nick or spall f2i= 2 * k1* k3* f1i

Cone Irregularity, e.g., nick or spall f3i= Z * k1 * f1i

Cup Irregularity, e.g., nick or spall f4i= Z * k1 * k2 * f1i

Roller Size Variation (Rotating Cone) f5i= k1 * k2 * f1i

Roller Size Variation (Rotating Cup) f6j= k1* f1i

Measurement considerations

The frequencies listed in Table 1 are applicable whenever a bearing is evaluated. A typical approach

employs an accelerometer attached on or near the bearing. By performing a narrow band frequencyanalysis of the acceleration signal, one can usually determine if the bearing is damaged or meets auser established vibration criterion.

To avoid ambiguity when identifying the acceleration spikes occurring at the above frequencies, the

bandwidth must be sufficiently narrow. For example, as the operating speed decreases, so shouldthe bandwidth.

It is not uncommon to observe modulation, particularly when the signal is obtained in a direction

perpendicular to the axis of the bearing. Under these circumstances, the predominant evidence willbe found at the frequencies f2i f5jor f2i f6jwhere i and j denote harmonic indices.


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Up to this point it has been assumed that the bearing operates with a 360 load zone. If this is notthe case, such as when operating with radial load and end play, the rollers moving in and out of theload zone cause a spectrum that tends to have a "smeared" appearance.

As one of the final steps in its quality assurance program, The Timken Company subjects its

bearings to a vibration analysis in highly specialized, accelerometer-equipped test machines.

In addition, the following rationale is employed: "The vibration (dynamic force) level of a bearing,operating at a specific speed and under a specific preload, is compared to and must meet an

established standard. If this is the case, then by implication the geometric imperfections are of such

small magnitude that the bearing's potential to act as an exciter is considered acceptable." Note thatthis implies that the merit of the bearing is strictly a function of the geometric imperfections, notone of speed and/or load and/or the bearing supports. The vibration signature may, of course, differ

under other combinations of speed and load.

Acoustic implications

The mechanical energy in the bearing-generated dynamic forces and those presented to the bearingfrom the rotating member for transmission to the stationary member, will first be transferred to the

structure supporting the bearing. The energy then permeates the structure and will be partiallyconverted to acoustic energy upon arriving at an air/solid interface. Depending upon the mass,

stiffness, geometry and boundary crossings characterizing the structure, the mechanical energy willundergo modifications. As a result of this transfer function, the prevailing acoustic energy (or

airborne sound) will be a function not only of the mechanical vibration of the bearing but also theattenuation/amplification characteristics of each particular structure.

One such structure is the quality assurance equipment employed by The Timken Company. Bearingsare tested for vibration in a relatively unenclosed configuration, i.e., one in which a large percentageof the bearing surface is exposed. Clearly, this condition is acoustically quite different from one inwhich the bearing is fully enclosed, as for example, in a machine tool.

The structure greatly influences the outcome of an acoustic measurement. Since sound is mainlycaused by transverse vibration of the housing walls, a stiffer housing tends to be less noisy than onethat is less rigid. Thus, any comparisons made or conclusions drawn between dissimilar structures

are at best haphazard. The design of the structure can profoundly affect the overall noisecharacteristics of the system. This is the most important reason for not attaching sound level

specifications, dB(A), to bearings.

Design considerations

Usually, resonances can be shifted or minimized by selective design, i.e., the shrewd manipulationof mass and/or stiffness. Where possible, impedance mismatches should be part of the design. Forexample, the vibration path between some electric motors and their bases is interrupted by rubber-

like inserts. Also, consideration should be given to damping, either in the form of visco-elastic layersor mechanical discontinuities.The latter is realized wherever bolts, rivets or interference fits.

Within this context, the excitation potential of the bearing can be optimized by a variety of differenttechniques. An increase in the operating speed of a bearing causes an upward shift toward the

frequency range of maximum hearing sensitivity. Simultaneously, the overall vibration levelincreases. A variation in preload/end play of the bearing can be utilized to bring about a "most

favorable" condition. Run-in will typically result in some "quieting". The same effect can be observedby going from a condition of marginal lubrication to one of "adequate" lubrication, but there is apoint where additional lubricant flow no longer produces a benefit. It is good practice to fully enclosethe bearing to minimize the direct acoustic path.

Assistance is readily available from Timken Company sales engineers. Their experience can assistthe user in selecting the proper bearing.

Documents

Bearing Damage Analysis