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Bearing failure and its Causes and Countermeasures

A brief review about bearing and failure of its various parts due to other possibilities than design such as manufacturing, improper service and handling and other similar aspects.

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Bearing failure and its Causes and Countermeasures

  2. 2. Contents • Introduction • How Bearing life is defined ? • Why a Bearing fails ? • Bearing Failure Chart • Types of Bearing Damage and Failures • Research work • Some Other type of Common Failures. • References
  3. 3. Introduction • Bearings are the most important components in majority of machines . • Their demands are made upon their carrying capacity and reliability. • The purpose of a ball bearing is to reduce rotational friction and to support Radial and Axial loads.
  4. 4. How Bearing Life is defined ? • Bearing life is defined as no. of revolutions a bearing can perform before evidence of fatigue in any of its element. • It is generally defined in terms of Millions of Revolutions or in Hours. • It is generally based on 90 % Reliability. Lh = (106 /60N) * (C/P)a where, Lh = Life in terms of Hrs. N = RPM C = Dynamic load carrying capacity P = Load or Equivalent Load a = Constant.
  5. 5. Why a Bearing fails ? • Match 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 1. Heavier loading 2. Inadequate or unsuitable lubrication 3. Careless handling 4. Ineffective sealing or fits that are too tight 5. Insufficient internal bearing clearance.
  6. 6. Bearing Failure Chart
  7. 7. Life vs Dimension Stability
  8. 8. Types Of Bearing Damage and Failures 1. Primary Damage • Wear • Indentations • Smearing • Surface distress • Corrosion • Electric current damage 2. Secondary Damage • Flaking (Spalling) • Cracks • Cage damage
  9. 9. Wear Damage • Wear may, however, occur as a result of the ingress of foreign particles into the bearing or when, the lubrication is unsatisfactory. Vibration in bearings which are not running also gives rise to wear. • Three types of Wear Damage 1. Wear caused by Abrasive particles 2. Wear caused by Inadequate lubrication 3. Wear caused by Vibration
  10. 10. Wear Damage by Abrasive Particles • Appearance Small indentations around the raceways and rolling elements. Dull, worn surfaces. Grease discolored green. • Cause Lack of cleanliness before and during mounting operation. Ineffective seals. Lubricant contaminated by worn particles from brass cage. • Action Keep workshop clean and use clean tools. Check and possibly improve the sealing. Always use fresh, clean lubricant. Filter the oil.
  11. 11. Wear caused by Vibration • Appearance Depressions in the raceways. These depressions are rectangular in roller bearings and circular in ball bearings. The bottom of these depressions may be bright or dull and oxidized. • Cause The bearing has been exposed to vibration while stationary. • Action Secure the bearing during transport by radial preloading. Provide a vibration-damping base. Where possible, use ball bearings instead of roller bearings. Employ oil bath lubrication, where possible.
  12. 12. Indentations • Raceways and rolling elements may become dented if the mounting pressure is applied to the wrong ring, so that it passes through the rolling elements, or if the bearing is subjected to abnormal loading while not running. Foreign particles in the bearing also cause indentations. • Two types of Indentation Damage. 1. Indentations caused by faulty mounting or overloading 2. Indentations caused by foreign particles
  13. 13. Indentation due to faulty Mounting or Overloading • Appearance Indentations in the raceways of both rings with spacing equal to the distance between the rolling elements. • Cause Mounting pressure applied to the wrong ring. Excessively hard drive-up on tapered seating. Overloading while not running. • Action Apply the mounting pressure to the ring with the interference fit. Avoid overloading or use bearings with higher basic static load ratings.
  14. 14. Smearing Damage • When two inadequately lubricated surfaces slide against each other under load, material is transferred from one surface to the other. This is known as smearing. • Types of Smearing Damage 1. Smearing of roller ends and guide flanges 2. Smearing of rollers and raceways 3. Raceway smearing at intervals corresponding to the roller spacing. 4. Smearing of external surfaces 5. Smearing in thrust ball bearings
  15. 15. Hardness vs Time and Temperature • As per American Bearing Manufacturer’s association
  16. 16. Smearing of Roller end and Guide Flange • Appearance Scored and discoloured roller ends and flange faces. • Cause Sliding under heavy axial loading and with inadequate lubrication. • Action More suitable lubricant.
  17. 17. Raceway Smearing • Appearance Transverse smear streaks - spaced at intervals equal to the distance between the rollers - in the raceways of cylindrical roller bearings. • Cause Roller slipping due to excessive grease filling • Action Improve the preload Improve the bearing clearance Use a lubricant with good oil film formation ability Improve the lubrication method Improve the sealing mechanism
  18. 18. Smearing Of External Surfaces • Appearance Scored and discoloured ring bore or outside surface or faces. • Cause Ring rotation relative to shaft or housing. • Action Select heavier interference fits. Improve Lubrication method.
  19. 19. Surface Distress • If the lubricant film between raceways and rolling elements becomes too thin, the peaks of the surface asperities will momentarily come into contact with each other. • Small cracks then form in the surfaces and this is known as surface distress. • The surface distress cracks are microscopically small and increase very gradually to such a size that they interfere with the smooth running of the bearing. • Appearance Initially the damage is not visible to the naked eye. A more advanced stage is marked by small, shallow craters with crystalline fracture surfaces. • Cause Inadequate or improper lubrication. • Action Improve lubrication.
  20. 20. Damage due to Corrosion • Rust will form if water or corrosive agents reach the inside of the bearing in such quantities that the lubricant cannot provide protection for the steel surfaces. This process will soon lead to deep seated rust. • Appearance Grey-black streaks across the raceways, mostly coinciding with the rolling element spacing. At a later stage, pitting of raceways and other surfaces of the bearing. • Causes Entry of corrosive gas or water Improper lubricant Formation of water droplets due to condensation of moisture High temperature and high humidity while stationary Poor rust preventive treatment during transporting Improper storage conditions Improper handling • Actions Improve the sealing mechanism Study the lubrication method Anti-rust treatment for periods of non-running Improve the storage methods Improve the handling metheod
  21. 21. Damage due to Electric Current • When an electric current passes through a bearing, i.e. proceeds from one ring to the other via the rolling elements, damage will occur. At the contact surfaces the process is similar to electric arc welding. • Appearance Dark brown or grey-black fluting (corrugation) or craters in raceways and rollers. Balls have dark discoloration only. Sometimes zigzag burns in ball bearings raceways. Localized burns in raceways and on rolling elements. • Cause Passage of electric current through rotating bearing. Passage of electric current through non- rotating bearing • Actions Design electric circuits which prevent current flow through the bearings Insulation of the bearing
  22. 22. Flaking (Spalling) Four stages of Flaking.Fatigue is the result of shear stresses cyclically appearing immediately below the load carrying surface. After a time these stresses cause cracks which gradually extend up to the surface. As the rolling elements pass over the cracks fragments of material break away and this is known as flaking or spawling.
  23. 23. Flaking (Spalling) • Appearance Heavily marked path pattern in raceways of both rings. Flaking usually in the most heavily loaded zone. Rough and Coarse texture. • Cause Excessive load Poor mounting (misalignment),Entry of foreign debris, Poor lubrication, Improper lubricant, Unsuitable bearing clearance Improper precision for shaft or housing Progression from rust, corrosion pits, smearing, dents. • Actions Improve the mounting method, Improve the sealing mechanism. Use a lubricant with a proper viscosity. Improve the lubrication method , Check the precision of shaft and housing Check the bearing internal clearance
  24. 24. Cracking Damage • Cracks may appear in Bearing due to so many reasons. • The most common cause is rough treatment when the bearings are being mounted or dismounted. • Hammer blows, applied direct against the ring which results in breaking of the parts of Bearing. • Cracks may appear due to 1. Rough treatment 2. Fretting 3. Smearing 4. Excessive Drive-up.
  25. 25. Cracking Damage • Appearance Cracks or pieces broken off, generally at one face of the bearing. • Causes Excessive interference Excessive load, shock load Progression of flaking Heat generation and fretting caused by contact between mounting parts and raceway ring Heat generation due to creep Poor taper angle of tapered shaft Poor cylindricality of shaft • Actions Correct the interference Check the load conditions Improve the mounting method Use an appropriate shaft shape
  26. 26. Cage Damage • There are certain main causes of cage failure 1. Vibration 2. Excessive speed 3. Wear 4. Blockage.
  27. 27. Cage Failure • Appearance Cracks, cage deformation, Fracture. • Causes Poor mounting (Bearing misalignment). Poor handling. Shock and large vibration. Excessive rotation speed, sudden acceleration and deceleration. Poor lubrication. Temperature rise. • Actions Check the mounting method. Check the temperature, rotation, and load conditions. Reduce the vibration and use vibration damping base. Select a cage type properly. Select a lubrication method and lubricant.
  28. 28. Research work Failure analysis of bearing in wind turbine generator gearbox. Presented By:- Sanskar S., Nataraj M. and Prabhu Raja V. • This research paper describes the failure analysis of bearing in Wind Turbine Generator (WTG) gearbox. The two-stage filter element and the gearbox were examined at turbine tower top called nacelle to find out the reason for filter choke alarm in the turbine controller. Drive train alignment was ensured between the asynchronous generator shaft and the gearbox shaft to conclude the mode of bearing failure.
  29. 29. Research Work • Conclusion The visual observation and the microstructure examination confirmed that an excessive damage of the roller as well as the surface of the race is due to the debris collected at the filter unit resulted from contact wear (Scoring) followed by surface and sub-surface wear (Spalling) on rollers and races of an intermediate non-drive end bearing. It is obvious from failure analysis that the failure of bearing is due to high cyclic fatigue fracture. It is clear that the failure of the bearing has happened due to continuous peak power generation of the WTG during high wind season and the presence of bauxite element such as aluminum oxide, calcium oxides and silicon oxides in the gearbox oil.
  30. 30. Some Other types of Common Failures • Failure due to Creep • Failure due to Pitting • Failure due to False Brinneling. • Failure due to Seizure or Excessive Heating. • Failure Due to Reverse Loading.
  31. 31. References • “Failure Analysis Of Bearing In Wind Turbine Generator Gearbox” by Sanskar S., Nataraj M. and Prabhu Raja V. in Journal of Information Systems and Communication Vol-III, 2012. • “Tech Talk Roller bearing life at High Temperatures” Published by NSK Americas. NSK Bearings, NSK America Corp. • “ New Bearing Doctor” Published by NSK Europe. NSK Bearings. NSK Europe corp. • “Bearing Failure Causes and Curses” Published by Wilcoxon Research. • An Article on “Bearing Life” and catalogue Published by SNR bearings. • “Bearing Failures and its Causes” published by SKF bearings Sweden.
  32. 32. THANK YOU