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BEARING FAILURE ITS CAUSES AND COUNTERMEASURES Prepared By : Dutt Thakar
32

Bearing failure and its Causes and Countermeasures

Aug 19, 2014

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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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Page 1: Bearing failure and its Causes and Countermeasures

BEARING FAILURE ITS CAUSES AND COUNTERMEASURES

Prepared By : Dutt Thakar

Page 2: Bearing failure and its Causes and Countermeasures

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

Page 3: Bearing failure and its Causes and Countermeasures

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.

Page 4: Bearing failure and its Causes and Countermeasures

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.

Page 5: Bearing failure and its Causes and Countermeasures

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.

Page 6: Bearing failure and its Causes and Countermeasures

Bearing Failure Chart

Page 7: Bearing failure and its Causes and Countermeasures

Life vs Dimension Stability

Page 8: Bearing failure and its Causes and Countermeasures

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

Page 9: Bearing failure and its Causes and Countermeasures

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

Page 10: Bearing failure and its Causes and Countermeasures

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.

Page 11: Bearing failure and its Causes and Countermeasures

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.

Page 12: Bearing failure and its Causes and Countermeasures

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

Page 13: Bearing failure and its Causes and Countermeasures

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.

Page 14: Bearing failure and its Causes and Countermeasures

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

Page 15: Bearing failure and its Causes and Countermeasures

Hardness vs Time and Temperature

• As per American Bearing Manufacturer’s association

Page 16: Bearing failure and its Causes and Countermeasures

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.

Page 17: Bearing failure and its Causes and Countermeasures

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

Page 18: Bearing failure and its Causes and Countermeasures

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.

Page 19: Bearing failure and its Causes and Countermeasures

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.

Page 20: Bearing failure and its Causes and Countermeasures

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

Page 21: Bearing failure and its Causes and Countermeasures

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

Page 22: Bearing failure and its Causes and Countermeasures

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.

Page 23: Bearing failure and its Causes and Countermeasures

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

Page 24: Bearing failure and its Causes and Countermeasures

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.

Page 25: Bearing failure and its Causes and Countermeasures

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

Page 26: Bearing failure and its Causes and Countermeasures

Cage Damage

• There are certain main causes of cage failure

1. Vibration

2. Excessive speed

3. Wear

4. Blockage.

Page 27: Bearing failure and its Causes and Countermeasures

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.

Page 28: Bearing failure and its Causes and Countermeasures

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.

Page 29: Bearing failure and its Causes and Countermeasures

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.

Page 30: Bearing failure and its Causes and Countermeasures

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.

Page 31: Bearing failure and its Causes and Countermeasures

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.

Page 32: Bearing failure and its Causes and Countermeasures

THANK YOU