bearing failure

ANALYSIS OF BEARING FAILURE

(1) INTRODUCTION Bearing are among the most important components in the vast majority of machines and exacting demands are made upon their capacity and reliability. Therefore it is quite natural that rolling bearings should have come to play such a prominent part and that over the years they have been subject of extensive research. Indeed rolling bearing technology has developed into a particular branch of Science among the benefits resulting from this research has been the ability to calculate the life of a bearing with considerable accuracy, thus making it possible to match the bearing life with the service life of the machine involved. Unfortunately it sometimes happens that a bearing does not attain its calculated rating life. There may be many reasons for this, heavier loading than has been anticipated, inadequate or unsuitable lubrication, careless handling, ineffective sealing, or fits that are to tight, insufficient internal bearing clearance. Each of these factors produces its own particular types of damage and leaves its own particular type of damage and leaves its own special imprint on the bearing. Consequently, by examining a damaged, it is possible, in the majority of cases, to form an opinion on the cases of the damaged and to take the requisite action to prevent a recurrence.

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(2) HOW BEARING LIFE IS DEFINED: Generally, a bearing cannot rotate for ever. Unless operating conditions are ideal and the fatigue load limit is not reached, sooner or later material fatigue will occur. The period until the first sign of fatigue appears is a function of the number of revolutions performed by the bearing and the magnitude of load. Fatigue is the result of shear stresses cause cracks which gradually extend up to the surface. As the rolling element pass over the cracks fragments of material break away and this is known as flaking or spelling. The flaking progressively increases in extent and eventually makes the bearing unserviceable. The life of a rolling bearing is defined as the number of revolutions the bearing can perform before incipient flaking occurs. These doses not mean to say that the bearing cannot be used after then. Flaking is relatively long, drown-out process and makes its presence known by increasing noise and vibration levels in the bearing. Therefore, as a rule, there is plenty of time to prepare for a change of bearing.

(3) TRUE BRINELLILNG: True brinellilng occurs when loads exceed the elastic limit of the ring material. Brinell marks are indentations at ball/roller frequently caused by any static overload or severe impact.

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Fig-1.1

Fig-1.2 \u2192 EXAMPLES OF TRUE BRIINELLING CAUSES:

\u2192 Using a hammer to install a bearing \u2192 Dropping a bearing \u2192 Pressing a bearing onto a shaft by applying force to the Non-rotating ring AGE NO 3

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These indentations are evident in the raceways and can increase bearing noise and vibration, leading to premature bearing failure.

(4) CONTAMINATION: Contamination is one of the leading causes of premature bearing failure. Symptoms of contamination are dents or scratches embedded in the bearing raceways and balls/rollers, resulting in undue bearing vibration and wear.

Fig-2 Contamination may include airborne dust, dirt or any abrasive substance that gets into the bearing. Principle sources are dirty tools, contaminated work areas, dirty hands and foreign matter in lubricant or cleaning solutions.

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(5) ELECTRICAL FLUTING: Electrical fluting occurs when a current is passed through the bearing, instead of to a grounded source.

Fig-3 Frequently seen in electric motors can be eliminated by ceramic-coating the OD of the bearing.

(6) FLASE BRINELLILNG FAILIURE: When the bearing is not turning, an oil film cannot be formed to prevent raceway wear.

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Fig-4 Wear marks are perpendicular to the line of motion, normally well-defined, and sometimes surrounded by debris occurs when there is small relative motion between the balls/rollers and raceways during non rotation times. Characterized by elliptical wear marks in the axial direction at each position.

(7) MISALIGNMENT FAILURE: Misalignment failure can be detected on the raceway of the none rotating ring by a rotating element wear path that is not parallel to the raceway edges. Excess misalignment can cause abnormal temperature rise and heavy wear in the cage pockets.

Fig-5

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→ The most prevalent causes of misalignment are: → Bent shafts

→ Burrs or dirt on the shaft or housing shoulders → Shaft threads not square with the shaft seats → Locking nuts with faces that is not square to the thread axis

(8) REVERSE LOADING: Occurs when loads shift direction in bearing that can only take axial loads in one direction angular contact ball bearings.

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Fig-6.1 When loaded in the opposite direction, the elliptical contact area on the outer ring is truncated b the low shoulder on that side of the outer ring.

Fig-6.2 The balls will show a band caused by the ball riding over the edge of the raceway. Failure mode is very similar to that of heavy interface fits. A thrust load applied to the wrong bearing face results in a wear band On the balls.

(9) HIGH TEMPARATURE FAILURE ANALYSIS: Creep occurs under load at high temperature. Boilers, gas turbine engines, and ovens are some of the systems that experience creep. An understanding of high temperature materials behavior is beneficial in evaluating failures in these types of systems. Failures involving creep are usually easy to identify due to the deformation that occurs. Failures may appear ductile or brittle. Cracking may be either transgranular or intergranular. While creep testing is done at constant temperature and constant load actual components may experience damage at various temperatures and loading conditions.

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→ CREEP OF METALS: High temperature progressive deformation of a material at constant stress is called creep. High temperature is a relative term that is dependent on the materials being evaluated. A typical creep curve is shown below.

Total Elongation Strain

Fracture

Stage 1

Stage 3

Stage 2

Time

Diagram 1 In a creep test a constant load is applied to a tensile specimen maintained at a constant temperature. Strain is then measured over a period of time. The slop of the curve, identified in the above figure, is the strain rate of the test during stage 2 or the creep rate of the material. Primary creep, stage1, is a period of decreasing creep rate. Primary creep is period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage 2. Secondary creep, stage 2, is a period of roughly constant creep rate. Stage 2 is referred to as steady state creep. Tertiary creep, stage 3, occurs when there is a reduction in cross sectional are due to necking or effective reduction in area due to interval void formation.

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(10) STRESS RUPTURE: Stress rupture testing is similar to creep testing except that the stresses used are higher then in a creep test. Stress rupture testing is always done until failure of the material. In creep testing main goal is to determine the minimum creep rate in stage 2. Once a designer knows the materials will creep and has accounted for this deformation a primary goal is to avoid failure of the component.

T1 T2

Stress T3 >T2 >T1

T3

Stress rupture time, hr Diagram-2

(11) CORROSION FAILURE: Corrosion results from the chemical attack on bearing material by hostile fluids or atmospheres.

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Fig-7 Symptoms include red/brown areas on rolling elements, raceways, or cages. Corrosion usually results in increased vibration followed by wear, with subsequent increase in radial clearance or loss of preload .

(12) EXCESSIVE LOAD FAILURE:

Fig-8.1

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Excessive load normally causes premature bearing failure. Symptoms are the same as normal fatigue, although showing heavier ball wear paths, greater evidence of overheating, and a more widespread and deeper spalling.

(13) LOOSE FIT FAILURE:

Fig-9 Caused by relative motion between mating parts which, in turn, causes fretting. Fretting occurs when fine metal particles oxidize, leaving a distinctive brown color. This normally occurs through outer ring slippage in the housing due to improper fits outer ring slippage caused by improper housing fits. Discoloration and scoring will appear on the outside of the outer ring.

(14) LUBRICATION FAILURE:

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Fig-10.1

Fig-10.2 Symptoms include discolored blue/brown raceways and balls/rollers. Restricted lubricant flow or excessive temperatures that degrade the lubricants properties typically cause failures. Lubricant failure will lead to excessive wear, overheating and subsequent bearing failure.

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(15) OVERHEATING: Symptoms of overheating are the discoloration of the rings, balls/rollers and cages from gold or blue. Temperatures in excess of 400 degrees C.

Fig-11.1

Fig-11.2 Extreme cases result in deformation of balls/rollers and rings. Primary indications are blue/black and silver/gold discoloration, and balls/rollers will usually be blue/black.

(16) PRELOAD FAILURE: AGE NO 14

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Preload is indicated by heavy rolling element wear paths in the bottom of the raceway around the entire circumference of the inner and outer ring. If interference fits exceed the internal radial clearance, the rolling elements become preloaded.

Fig-12 If clearance is lost in a bearing, it results in rapid temperature rises accompanied by high torque. Continued operation can lead to rapid wear and fatigue.

(17) FATIGUE FAILURES: Metal fatigue is caused by repeated cycling of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe. The process of fatigue consists of three stages → Initial crack initiation → Progressive crack growth across the part → Final sudden fracture of the remaining cross section

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S Stress

N Cycles to failure

(Diagram-3.1)

→ STRESS RATIO The most commonly used stress ratio is R, the ratio of the minimum stress to the maximum stress (Smin/Smax).

→ If the stresses are fully reversed, then R = -1 → If the stresses are partially reversed, R= a negative number less then 1 → If the stresses is cycled between a maximum stress and no load, R=0 → If the stresses is cycled between two tensile stresses, R= a positive number less then 1

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Maximum stress

Minimum stress

(Diagram-3.2) Variations in the stress ratios can significantly affect fatigue life. The presence of a mean stress component has a substantial effect on fatigue failure. When a tensile mean stress is added to the alternating stresses, a component will fail at lower alternating stress then it does under a fully reversed stress.

(18) PREVANTING FATIGUE FAILURE The most effective method of improving fatigue performance is improvements in design. → Eliminate or reduce stress raisers by streamlining the part → Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes → Prevent the development of surface discontinuities during processing AGE NO 17

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→ Reducing or eliminate tensile residual stresses caused by manufacturing. → Improve the details of fabrication and fastening procedures

→ FATIGUE FAILURE ANALYSIS Metal fatigue is a significant problem because it can occur due to repeated loads below the static yield strength. This can result in an unexpected and catastrophic failure in use. Because most engineering materials contain discontinuities most metal fatigue cracks initiate from discontinuities in highly stressed region of the component. The failure may be due the discontinuity, design, improper maintenance or other causes. A failure analysis can determine the cause of the failure.

(19) INSTALL DAMAGE: Occurs when a sharp impact is applied incorrectly to a bearing during mounting or dismounting.

Fig-13 By using proper method to install the bearing we reduce it.

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(20) HYDROGEN EMBRITTLEMENT: When tensile stresses are applied to hydrogen embrittled component it may fail prematurely. Hydrogen embrittlement failures are frequently unexpected and sometimes catastrophic. An externally applied load is not required as the tensile stresses may be due to residual stresses in the material. The threshold stresses to cause cracking are commonly below the yield stress of the material. High strength steel, such as quenched and tempered steels or precipitation hardened steel is particularly susceptible to hydrogen embrittlement. Hydrogen can be introduced into the material in service or during materials processing.

→ HYDROGEN EMBRITTEMENT FAILURES: Tensile stresses, susceptible material, and the presence of hydrogen are necessary to cause hydrogen embrittlement. Residual stresses or externally applied loads resulting in stresses significantly below yield stresses can cause cracking. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. Very small amount of hydrogen can cause hydrogen embrittlement in high strength steels. Common causes of hydrogen embrittlement are pickling, electroplating and welding, however hydrogen embrittlement is not limited to these processes. Hydrogen embrittlement is an insidious type of failure as it can occur without an externally applied load or at loads significantly below yield stress. While high strength steels are the most common case of hydrogen embrittlement all materials are susceptible.

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(21) LIQUID METAL EMBRITTLEMENT: Liquid metal embrittlement is the decrease in ductility of a metal caused by contact with liquid metal. The decrease in ductility can result in catastrophic brittle failure of normally ductile material. Very small amount of liquid metal are sufficient to result in embrittlement. Some events that may permit liquid metal embrittlement under the appropriate circumstances are listed below → Brazing → Soldering → Welding → Heat treatment → Hot working → Elevated temperature service In addition to an event that will allow liquid metal embrittlement to occur, it is also required to have the component in contact with a liquid metal that will embrittle the component.

→ LIQUID METAL EMBRITTLEMENT FAILURE: The liquid metal can not only reduce the ductility but significantly rebuke tensile strength. Liquid metal embrittlement is an insidious type of failure as it can occur at load below yield stress. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component. Intergranular or transgranular cleavage fracture is the common fracture modes associated with liquid metal embrittlement. However reduction in mechanical properties due to decohesion can occur. This result in a ductile fracture mode occurring at reduced tensile strength. An appropriate analysis can determine the effect of liquid metal embrittlement on failure.

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(22) REDUCING BEARING FAILURE: → BEARING SELECTION → Procure the correct bearing for the application. Often, the replacement bearing is not compatible with the equipment where it is to be installed. Depending upon the age of the equipment, advances in bearing technologies may exist that make the OEM bearing obsolete. Knowing the limits of the equipment and what bearing best suit the application will save time and money. → determine the maximum load for the bearing. This is important both vertically and horizontally. → Determine the minimum and maximum running speeds for the bearing. Thus will help determine the correct lubricant and bearing for the application. → Determine all possible environmental conditions to which the bearing will be exposed. Very hot or cold environments often require require varied bearing specifications. This may, in turn, change the type of lubricant and relubrication requirements as well. Bearings exposed to wash ups or moistureheavy environments need to stay well sealed and seals must be kept in proper condition to protect the rolling elements. Bearing that operate in caustic environments may require special seals and care. Pay special attention to the seal manufacturer’s recommendations regarding handling and care.

→ BEARING HANDLING AND STORAGE → If possible, determine when a bearing was manufactured and if it

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was properly stored before being purchased. Ask the bearing distributor about his storage and handling procedures. It might be prudent to have a representative from your company personally visit the bearing distributor to confirm how bearing are being stored. For example, a tapered roller bearing should be stored with the taper down and never stacked, one on top of another. → Store bearing in an attitude angle that will reduce or eliminate the possibility of damage to rolling elements and raceway. It may be weeks or months before the bearing is called into service. Reducing the risk of the startup damage begins with proper storage. → Bearing are manufacture with the extremely tight tolerances and therefore require special care when moving or handling. Consider them fragile at all times and make the efforts to treat them as such. → Consider the proximity of the storeroom to areas of the plant that are affected by vibration. Could a rail rode main line affect the storeroom? Does the plant have equipment that vibration nearby building? Bearing subjected to even minor daily vibration can become damaged while in storage. Take the necessary steps to insulate stored bearings from the vibrations. → always store bearings in a clean and sterile environment. Keep them free of moisture, dust, and chemicals.

→ BEARING INSTALLATION AND HANDLING → Take care when removing old or damaged bearing from their shafts and housings. Be careful to not damage holders or surfaces where the new bearings will be installed. → Clean all housings, shafts, holders, keyways, etc. Before attempting to install a new bearing inspect the shafts and equipment for damage. Install new bearing in a clean and dry environment as possible. If possible, use sterile gloves to prevent contamination. Contamination at this stage will ensure a shorter bearing life cycle.

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→ Carefully inspect the new bearing for any obvious damage that may have occurred during shipping, storage, or manufacture. Inspect bearing to determine if all parts are present. Bearing have been known to ship from the factory missing roller elements and other parts. Also, check for factory lubricant. Lack of lubrication from factory can cause rust. → Properly align bearing with shafts. Do not assume the original bearings were properly aligned even in motors. → Never push or pound on bearing surfaces. Use only safe insulation methods accepted and approved by the manufacturer.

→ INITIAL LUBRICATION PROCEDURES → Never assume the manufacturer has properly lubricated the bearing. The new bearing may have been shipped with a limited amount of the lubrication inside. This level may not be enough to form necessary film between the inner race and rolling elements. → Determine lubrication level by using sound analysis or vibration monitoring methods. Remember, a day or under lubricated bearing will sound louder or scratchier then a quiet or smooth sounding properly lubricated bearing.

→ ON GOING BEARING LUBRICATION → Your lubrication supplier and bearing supplier should have the most current data and be able to recommend the proper lubricant for the application. As in selecting the proper bearing for the application, the conditions to which the lubricant will be considered. → How grease waiting for future use is treated and stored will be a key factor in the life expectancy of equipment. Lubricants should be stored in moisture and temperature controlled environments, free of dust and chemical exposure.

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→ Contamination entering grease will likely happen during transfer from one point to another. Failure to exercise care in this process will nullify the attention given previously. There are a number of ways to properly refill grease guns. Using a scoop or paddle from a container and tamping it into the grease gun to remove air bubbles. This method is most likely to introduce contaminants into the grease, especially when performed in the field. It is not a recommended method except in the most dire of circumstances. → using tube refills is the most common method of refilling a grease gun. It involves removing the empty tube and installing new, compatible tube of grease into the grease gun. Take care to clean dirt and old grease. Perform this task in as clean and dry an environment as possible. → When refilling from a storage container using mechanical or hydraulic pumps, grease is pumped mechanically from the storage container directly into the portable grease gun. When care is taken to clean off the port on the grease gun and delivery hooks up from the pump, this is the fasted and safest method of grease transfer. → Assuming that the correct grease is introduced into bearings may involve coding systems, labels, numbers, tags or color coding on bearing housings that indicate what type of grease is being used can be very helpful to the lubrication technician. Ensure that grease guns are matched up with the coding system on equipment. New employees should be trained on the matching system before any lubrication task is performed. This is an easy system to implement and minimizes the chances of introducing non compatible greases into the bearing.

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→ When it become necessary to switch delivery tube from one grease gun to another make certain to clean &grime from he tube & then purge all the grease from the tube to prevent mixing of incompatible grease types. Clean & purge grease zerk fitting connectors as well.

→ Different grease gun manufacturing allow varying amounts of grease be applied by a pump or short of grease. The amount of pressure each grease gun or grease delivery system contains also may vary dramatically. This lake of an industry standard has made it difficult to determine the amount of grease actually being delivered and there fore create problems using a time and amount based lubrication schedule. It is important to calibrate each grease gun and note the volume of grease each gun delivers with one full pump. → To properly relubricate a bearing, certain information must be obtained. To help determine the correct time and amount based schedule of relubrication, interval must be combine with reliability knowledge and experience. → Traditionally, the job function of lubrication is an entry level position in the maintenance. Much was required of these important individuals with little or no specific training provided. Fortunately, this is changing. Companies have invested in maintenance technologies and training to prevent and predict machinery failure. Companies are learning to invest in standard training for lubrication practices and in the tools necessary to performing lubrication tasks have not changed, awareness of the importance of the individual performing these tasks is changing. As skill and training criteria standard evolve, the oilier becomes a skilled lubrication technician and analyst. Also called lubrication engineers, these individuals are being provided with the necessary resources to perform their job function. Reliability and predicative maintenance groups are

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increasingly relying upon the lubrication technician’s knowledge and skills.

→ Acoustic analysis or sonic analysis is a rapidly growing method in preventing over lubricated and under lubricated bearings. This equipment uses sonic sound technology and listens to the noise generated by the vibration of the bearing in sonic range (20 Hz-20 kHz) to decipher when and if a bearing requires greasing. By listening to the voice of the bearing, the lubrication technician is able to make a direct determination of the grease requirements of the bearing.

→ As grease is slowly injected into a bearing, the change in sound or lack of sound change informs the technician when sufficient grease is present. Thus eliminates the need to calibrate a grease gun as the amount of grease the bearing requires is determined as it is being lubricated. By implementing this proactive method of greasing, lubrication technicians are able to customize existing time/amount based lubrication schedules. → For example, a bearing that had a previous schedule of two shots of grease every two weeks was over greasing the bearing. Customizing or adjusting the lubrication schedule to fit the actual bearing requirements slashes bearing failures.

(23) CANCLUSION:

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By analysis we can say that each of these factors produces its own particular type of damage and leaves its own special imprint on the bearing. Consequently, by examining a damaged bearing, it is possible, in the majority of cases, to form an opinion on the cause of the damage and to take the requisite action to prevent a recurrence.

(24) REFERANCES → WWW. RELIABILIT. COM → WWW. EMERSONBEARING.COM → WWW. MBFYS.KUN. NL → WWW. NATIRIALSENGINEER.CO

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bearing failure

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