- 1 Introduction To Bearing
- 2 Bearing Failures, Causes, And Preventive Actions
- 3 Conclusion
Introduction To Bearing
Bearing plays a very important role in all kinds of machinery, to make free movement between two contacting surfaces. Bearings are given importance in case of load carrying capacity and reliability. Bearing are designed to have a long and useful life.
Sometimes, a bearing does not attain its calculated rating life. More than 90% of all bearing failure occurs due to external factors. The bearings damages due to overload, inadequate lubricant or lubricant of the different type, improper handling, ineffective sealing or fits that are too tight, with resultant insufficient internal bearing clearance. All above-mentioned reasons provide its own particular type of damage.
Under proper operating conditions, a bearing should only fail from fatigue, which is predicted on the basis of bearing size and its application parameters. The damages due to above-mentioned reasons can be prevented by knowing how external factor affect the life of bearings.
Care Taken For Installation
- Bearing should be stored in dry and clean place.
- Do not remove bearings from its original packing before installing.
- Hands should be clean and dry. Gloves have to be used.
- Handling should be careful. If it falls down even from a lower height, it can cause rolling surface brinelling. Which act as starting point of damage.
- Avoid cleaning and degreasing a new bearing as it will remove rust preventing coating.
Correct Bearing Installation Techniques
- Use the appropriately sized tools for the bearings.
- The shaft and housing seating areas should be clean and free from contaminants.
- Proper lubricant type should be used for a particular application. Mixing of different lubricants is to be avoided.
- During press fitting alignment of the bearing should be checked.
- Avoid touching bearing surfaces with bare hands as this can lead to surface oxidation.
- Shock loads should not be given to the bearing during installation either through mishandling or direct use of a hammer. To avoid bearings from shock loads nylon hammers are used.
Bearing Failures, Causes, And Preventive Actions
The following graph shows different causes of bearing failures with its proportion:
1) Excessive Load
When the bearings are operated with speed and load more than predetermined then that bearing is said to be “overloaded”. The life of the bearing is decided by this overload. Higher the amount of overload, life will be less and vice-versa.
Change in production requirements or operator error results into unsuitable operations, this also causes to excessive load. Both bearings and equipment design may also offer excessive load.
Equipment and systems with longer warranty periods are generally more robust in design, taking into consideration potential overloads and environmental conditions.
Overload on the bearing causes surface damage, fracture, vibration, noise etc. Overloaded bearings should be repaired or replaced to prevent the bearing from further damage.
Even a small bearing failure or a small amount of overload is also going to create major problems. Another way of preventing bearings failure from overloading is to use solid lubricant.
Overheating results in discoloration of rings, balls, and cages as shown in the figure below. The excessive temperature may result in annealing of ring and ball material.
Further, it causes loss of hardness due to annealing, that reduces bearings capacity and that causes early failure. Overheating also degrades or evaporate lubricant.
When loads & speeds are excessive, the problems like electrical heat loads, inadequate heat paths & insufficient cooling or lubricant take place.
Inadequate heat path results in temperature rise as there is no heat dissipation. Another reason for overheating is the use of improper amount & unsuitable type of lubricant.
To avoid all these defects proper lubricant circulation is used, so that heat dissipation will take place and overheating is prevented.
As mentioned in overloading, here also solid lubricants can be used, as they have extreme temperature condition.
3) False Brinelling
Brinelling is nothing but the elliptical wear marks or spots in the axial direction at each ball position as shown in the figure below. The Brinell is the starting point of bearing damage because it results in a lot of vibration. Due to excessive vibration temperature increases and noise is also generated.
Due to external vibration, the clearance occurs between balls and races. This clearance causes to develop relative motion between balls & raceways ball bearings.
Due to Brinell marks some of the bearings will not rotate properly and when the bearing is not turning, generation of oil film will not take place to prevent raceway wear.
It can be prevented by keeping bearings away from external vibrations and using grease containing antiwear additives such as Molybdenum Disulfide.
4) True Brinelling
When loads exceed the elastic limit of ring material “True Brinelling” occurs. As discussed in false brinelling, the Brinell mark is an indentation in raceways which increase vibration and hence noise also. These indentations can cause fatigue failure.
Static overload is the example of true brinelling. For e.g. hammers used to remove or install bearings, dropping or striking assembled equipment and pressing a bearing on the shaft by forcing to an outer ring. This generates the indentation on the raceway.
To avoid this type of indentation formation, proper bearings installation technique like nylon hammer or heating bearing by induction heater should be used.
The care should be taken in induction heating that the temperature should not exceed 850 C. 850 C is the standard threshold value. Excessive load (static) should not be applied to bearing to avoid brinells. This will prevent bearing from vibration, noise generation, and fracture.
5) Improper Lubrication
The most important point of bearings failure is improper lubrication. Bearing fails to work properly when too much or too less amount of lubricant is used.
Use of the wrong type of lubricant, incorrect lubrication frequency, mixing of lubricants, using old, contaminated or deteriorated grease or oil are the reasons of improper lubrication.
To use the bearing with its maximum life of working, recommendations given from bearing manufacturer or some research organization regarding the use of lubricant amount and type should be followed.
The lubricant recommendations given by research organizations are usually based on type, grade, viscosity, additives, amount, storing, inspection and application interval. When one fails to follow these recommendations or guidelines, it will reduce the life or warranty period of bearing and equipment.
The life of lubricants varies depending upon the lubricant type, area of application, additives and storage conditions. Lubricants should be maintained with proper labeling and date.
When changing lubricant or with a new bearings start-up, always monitor the bearings for signs of trouble. Check bearings temperature continuously or at least periodically. If the bearings are operating at more than 180°F, considerably lubricant service life will decline.
Hence, improper lubrication results in wear and tear of bearings, heat generation, less load carrying capacity. If lubricant is contaminated with dust it will create the blockage of bearings and its rotation is stopped, which may create a hazardous condition.
If lubricant is contaminated with water, corrosion may take place. To prevent all above problems right type of lubricant at the right place in right time should be used.
6) Proper Handling & Storage
During handling, removing and installing bearings proper methods and tools are to be used. During handling, even though the bearing falls down from a small height, it may destruct bearing properties. Even the most minuscule contaminant or the smallest dent or nick will reduce their performance and reliability.
Improper storage is another reason for bearing damage. Bearings should not be taken out from its original packaging before use for any reason.
Because the bearings are stored with a protective coating in anticorrosive wrapping and they should be unwrapped in a clean environment. Otherwise, oxidation will take place and bearing will get corroded.
All bearings should be stored flat in a vibration-free, dry area with a cool, steady temperature and relative humidity conditions at 75 % at 20°C (68°F), 60 % at 22°C (72°F) or 50 % at 25°C (77°F).
7) Environmental Influence
Even with the best design, proper lubrication, and installation, failures will occur if the operating environment is not taken into consideration. Following environmental issues should be taken into consideration:
- Dust and dirt will affect the bearings to damage. When dust or dirt enters the bearing area it creates difficulty in the path of bearings which may create surface scratches, blockages may also occur. To avoid entry of dirt and dust proper sealing method should be used.
- Aggressive media or water. This causes scoring. As in case of dirt and dust, here also sealing is an important factor. The use of special type of seals that do not score the shaft is preferred.
- External heat. The ambient operating temperature mandates many choices in radial internal clearance, high-temperature lubricants, intermittent or continuous running and other factors that affect bearings life.
- Current or electrolytic corrosion. The current in the bearings generate sparks, which can create pitting or flutes on bearings surfaces. Pitting can be prevented by creating a bypass circuit for the current or by using insulation on the bearing.
It can be detected on the raceway of the non-rotating ring by a ball wear path that is not parallel to the raceway edges.
If misalignment exceeds 0.001 inch an abnormal temperature rise in the bearing and/or housing take place and heavy wear in the cage ball pockets.
The bent shafts, burrs or dirt on shaft or housing shoulders, shaft threads that are not square to the thread axis are the reasons to create misalignment. The maximum allowable misalignment varies with different applications, decreasing.
Misalignment results in vibration, that causes heat and noise generation. Due to this fracture of bearing surface will also take place.
To avoid above-mentioned defects inspection of shafts and housings for run out of shoulders and bearings seats; use of single point turned or ground threads on non- hardened shafts and ground threads only on hardened shafts, and using precision grade locknuts must be preferred.
The first step in the overall prevention of bearing failure is the consideration of bearing technologies that are most suitable to the application with regard to specifications, recommendations, maintenance strategies, fatigue life and wear resistance of the bearing.
The bearing failure occurs due to one or more of the causes discussed (lubrication, mounting, operational stress, bearing selection or environmental influence etc), which can and should be corrected in order to avoid future bearing failures and additional cost.
1.Barden Bearings Failure: Causes and Cures
2.Rolling Bearings Life Prediction, Theory and Application, Erwin V. Zaretsky, Glenn Research Center, Cleveland, Ohio.
3.SKF Bearings Failures and their Causes.
4.KOYO Rolling Bearings: Failures, Causes, and countermeasures.
5.HYATT- Heavy Duty Truck and Trailer, Bearings Failure Analysis.
6.Bearings Investigation, Extract from the Railway technical Handbook, volume 1, chapter 6, page 122 to 135.