Saturday, March 30, 2019

BEARING FAILURE MODES


1.   Fatigue
Surface fatigue in bearings occurs due to interruptions in lubrication film or low oil viscosity. The bearing surfaces become hardened and then starts peeling and eventually gets cracked. This is a slow process and the bearing might become rough, hot and noisy during the operation. This can be avoided by proper lubrication of bearings. Care to be taken to check the oil/grease grades provided are as per the manufacturer.


2.   Spalling
Spalling occurs when the bearing reaches the end of fatigue life. The indentations or cracks developed will be deeper than fatigue cracks. High cyclic stress on bearings caused due to improper installation or fits might lead to spalling.


3.   Abrasion
Abrasive particles can contaminate bearing during installation, through lubricant or through the seals. These particles machine out the bearing surfaces, thus damaging the bearing. To avoid this, keep the work surfaces clean and proper care to be taken while installing the bearings.

4.   Corrosion
It indicates the oxidation damage of bearing components in the presence of air and moisture. Steel is prone to water attack at pressure point (where roller touches the race). During idle, the oil film will be pushed out of this point making it prone to corrosion. Good sealing of bearing housing and proper lubrication might prevent the corrosion.

5.   Fretting
Loose fits on mating parts of bearing might lead to relative movement between the two under load. Frequent movements might damage the mating surfaces by gradual wear. The surfaces might develop a copper brown colour in case of slight movements or even black in extreme cases. Maintaining proper fits as per manufacturer’s recommendation will help in solving this issue.

6.   True brinelling
True brinelling occurs due to high impact loads or static loads on bearings, poor mounting practices or dropping off equipment. Care to be taken during installation to apply load only to the parts which are to be press-fitted. Use bearings of higher static load rating if impact loads cannot be prevented. Plastic deformations with grinding marks in the dents indicate true brinelling.

7.   False brinelling
Metal removal due to high vibrations of bearings at rest produces false brinelling. Proper blocking of the shaft during shipping and isolation of base helps in preventing false brinelling.

8.   Electrical fluting
Sometimes referred to as Electric Discharge Machining (EDM) failure of bearings are caused due to electric discharge through the races and rolling elements of bearings. Motor winding defects and static electricity accumulation will lead to EDM failure. Proper grounding to be provided on the shaft or component generating the electricity to prevent such failures.

9.   Smearing
Removal of material from one surface and deposition on the other is termed as smearing. This occurs in bearing under heavy loads and insufficient lubrication. These are generally found in roller bearings.

10.  Scoring
Scoring or wire drawing is the removal of metal from bearing surface by abrasion. Bearing contamination by large foreign particles leads to scoring. The scored surface creates stress concentration which in turn leads to surface fatigue. This can be prevented by good sealing of bearing housing and proper lubrication.

11.  Debris denting
It occurs when the equipment is running. Abrasive materials inside bearing will be rolled into races by rolling elements, denting the surfaces which act as points of stress concentration thus leading to surface fatigue. This can be prevented by good sealing of bearing housing and proper lubrication.

12.  Assembly damage
Proper installation of the bearing is very important in getting the desired life from the bearings. One of the major assembly damage arises due to cocking of bearing race on shaft or housing. This will induce additional loads on bearing, eventually leading to failure. Cocking is the misalignment of bearing races with respect to the shaft or housing.

13.  Hot working
Hot working damage to bearings arises due to excessive heat generated by bearing due to lack of lubrication, reduced internal clearances, friction or excessive speed. The bearing geometry is changed due to excess heat and will lead to catastrophic failure. Even the heat might enter the bearings from the surrounding environment like furnaces etc.

14.  Fracture
Fracture of inner races occurs due to improper fits on the shaft or due to cocking. Proper fits to be maintained as per the manufacturer’s recommendation to avert failure.

15.  Retainer Failures
Retainer failures refer to the cracking and separation of bearing cage/retainers under random movements of bearing rollers due to misalignment. This can be eliminated by proper handling and installation of bearings, preventing contamination and proper lubrication.

2 comments:

  1. Hi karthKa ..nice to here from you ...and sharing your experience and observations on the bearing....

    Are you using sealed type bearing in your application....?
    Sealed bearings can avoid entry of foreign particles...

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    Replies
    1. Hello Manju. Yes we are using sealed bearings. Sealed/shielded bearings are generally grease lubricated. Seals are not provided for oil lubricated bearings. Most of the damages due to foreign particles in bearings are reported in oil lubricated bearings.

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