Bearing Defects & Preventive Measures

Bearings must be carefully handled, mounted, and maintained in order to operate properly. Care and attention must be used during and after installation to increase the work life of the bearing, prevent bearing defects, and reduce the cost of repair and replacement.

In case of poor performance or breakdown, the cause of failure must be determined to prevent repetition. There are three types of data which should be gathered to enable the correct diagnosis of bearing problems:

• Time of occurrence
• Symptoms during operation
• Condition of Bearings

Time of Occurrence 

Time of Occurance / Cause	Bearing Selection	Design or Manufacture of other drive parts	Lubricant Type, System or Amount	Defective Bearing	Bearing Insallation	Seal Failure
Soon After Installation	✔	✔	✔	✔	✔
Soon after periodic disassembly			✔		✔
Soon After Re-lubrication			✔
After replacement or repair of other parts		✔	✔
During normal operation					✔	✔

Symptoms During Operation

Operation Condition	Cause	Remarks
Ever-changing sound	Change of clearance by temperature rise Defect in progress on raceway
Low Level Metallic Sound	Impression on Raceway
High Level Metallic Sound	Loss of Clearance, Poor Lubrication
Irregular Sound	Excess clearance, contaminants, defect of rolling element surface, improper lube
Abnormal temperature rise	Loss of clearance, creep, insufficient or excess lubricant, excess load	Use a surface thermometer.
Reduction in accuracy	Raceway or rolling element broken by impurities, or insufficient lubricant	Lathe: stick-slip marks Grinder: wavy pattern Cold roll mill: occulting wave pattern
Contaminated lubricant	Poor lubrication, foreign matter, wear
Unstable operation	Broken raceway, rolling element Foreign matter Excess clearance	Example: vibration

Condition of Bearings & Prevention Methods

(1) Premature Flaking
The repeated heavy stress cycle between the bearing raceway and rolling element surface results in fatigue cracks which become loosened from bearing materials.
Causes	Counter Measures
Abnormal axial load or excessive load caused by expanded shaft.	Clearance fit for the outer ring of one bearing to allow the floating end to move freely axially.
Deflection or misalignment of shaft.	Correct alignment of shaft and bearing housing.
Poor parallelism of inner and outer rings.	Improve mounting and alignment.
Poor lubrication.	Carefully clean and handle shaft and housing.
Rusting, Nicks, Galling from dirt, etc.	Review type and quantity of lubricant.
(2) Seizure
Bearing is seized up by excessive heat. Discoloration, softening and fusion of raceway and rolling element.
Causes	Counter Measures
Loss of clearance.	Review fitting and bearing clearance.
Operating over limiting speed.	Review type of bearing.
Poor or improper lubricant.	Select a proper lubricant, and feed it in proper quantity.
(3) Breakage
Splits and cracks in the inner/outer ring or rolling element.
Causes	Counter Measures
Excessive interference fit.	Check fits. Finish shaft and sleeve to higher accuracy.
Bearing seat has larger corner radius than bearing.	Make shaft corner radius smaller than that of the bearing.
Excess clearance during operation.	Check fits and bearing clearance.
Excess impact load.	Re-check load conditions.
(4) Indentation
Brinelling, indentation and pear skin of bearing raceway and rolling
element.
Causes	Counter Measures
Impact applied during mounting.	Carefully handle the bearing.
Impact from dropped bearing.	Clean shaft and housing.
Contamination	Improve the sealing.
Load applied to bearing at rest in excess of static load rating.	Re-check load conditions.
(5) Fretting
Occurs when a small relative motion is repeatedly caused in non-rotating bearing. Fretting surface wear produces red colored particles at fitting surface.
Causes	Counter Measures
Vibration applied to bearing at rest (e.g. during shipment).	Fit the shaft and housing during shipment.
Swing with smaller amplitude.	Apply a preload. Use oil for lubrication.
Minute clearance on fit surface.	Increase the interference.
Slight sliding during operation as a result reduced interference under a load.	Apply oil.
(6) Scoring
Metal to metal contact of bearing raceway, flange face, rolling element and retainer due to destruction of oil film.
Causes	Counter Measures
Incorrect lubrication.	Review type and quantity of lubricant.
Contamination by foreign matter.	Improve sealing and component cleaning.
Deflection or misalignment of shaft. Excessive axial load.	Correct mounting errors. Review load conditions.
Sharp speed increase at startup.	Change to smooth acceleration.
(7) Smearing
Metal to metal contact due to the destruction of oil film. Sliding motion between outer/inner ring and rolling element.
Causes	Counter Measures
Excess axial load.	Correct mounting errors.
Misalignment of bearing.	Review the load condition.
Poor lubrication.	Select a proper lubricant, and feed it in proper quantity.
Intrusion and galling caused by foreign matter.	Improve the sealing.
High acceleration on start-up.	Clean shaft and housing.
	Avoid sharp acceleration.