11—Damage and Countermeasures

If a bearing is damaged, it is important to identify the cause to prevent recurrence.
Understanding changes in sound, vibration, and temperature prior to damage can help to determine effective countermeasures and extend life.

Bearing Damage

If handled correctly, bearings can typically be used until their predicted fatigue life. However, premature damage will render a bearing inoperable sooner than expected. Such damage is often due to improper handling, improper mounting, poor assembly, incorrect or insufficient lubrication, or inadequate consideration of the operating environment or temperatures during the design stage.

	Premature Failure	Incidental Failure	Wear Failure
Characteristics	Failure after only a short period of operation	Failure after a period of stable operation	Increased failures due to end of component life
Major Causes	Material defects, assembly defects, design flaws, operating errors	Poor maintenance, changes in use environment	Normal life (wear/fatigue, aging)
Countermeasures	Sufficient running-in, checking for abnormal noise, vibration, and temperature	Maintain maintenance quality or improve maintenance in response to changes, replace lubricants, and ensure proper operation	Preventive maintenance (replacement based on advance predictions)

Bathtub curve (failure rate)

Damage Causes by Timing

The primary cause of damage can be inferred based on its timing. Possible causes for damage are listed in the table below.

Timing	Probable Main Cause
Timing	Improper selection	Design or manufacturing error in shaft, housing, or surrounding parts	Improper lubrication method, type, or amount	Bearing flaws	Improper mounting	Entry of contaminants, moisture, or lubricant loss from faulty seals, wear, etc.
Shortly after mounting	●	●	●	●	●
After periodic inspection (disassembly)			●		●
After replenishing lubricant			●
After repair/ replacement of shaft, housing, or surrounding parts		●	●		●
During normal operation			●		●	●

Damage Causes by Operating Characteristics

Certain operating characteristics can suggest specific issues:

Characteristic		Cause
Noise	Low metallic noise	Dents on rolling surface
	High-pitched metallic noise	Excessively low clearance	Poor lubrication
	Irregular noise	Clearance too high Entry of foreign matter	Flaws on rolling element surface Improper lubricant
	Gradually changing noise	Change in clearance due to temperature rise	Increasing damage on the raceway surface
Abnormal temperature		Clearance too low Creep	Too much/little lubricant Excessive load
Loss of running accuracy		Damage to raceway surface or rolling elements due to contaminants or insufficient lubrication
Rough operation		Damage to raceway surfaces and rolling elements Entry of foreign matter	Clearance too high
Contamination of lubricant		Poor lubrication Entry of foreign matter	Wear

Running Traces and Applied Loads (Deep Groove Ball Bearings)

As a bearing rotates, dull areas develop on the raceway surface from rolling contact with the rolling elements. This coloration is called a “running trace” and is a normal phenomenon. Figures (a) through (d) below show normal running traces. However, the running traces in (e) through (h) suggest conditions that may negatively affect the bearing and lead to a shortened life.

Normal running traces

Running trace (a): A normal running trace along center of inner ring and bottom of outer ring.

Inner ring rotation
Radial load

Running trace (b): A normal running trace along top of inner ring and center of outer ring.

Outer ring rotation
Radial load

Running trace (c): A normal running trace along the left side of the inner ring and right side of the outer ring.

Inner or outer ring rotation
Axial load in one direction

Running trace (d): A normal running trace along the left side of the inner ring and the bottom right side of the outer ring.

Inner ring rotation
Radial and axial loads

Abnormal running traces (conditions may shorten life)

Running trace (e): A abnormal running trace along the left side of the inner ring and from the center to the top right of the outer ring.

Inner ring rotation
Axial load and misalignment

Running trace (f): An abnormal running trace along the center of the inner ring and from the bottom left and top right corners of the outer ring.

Inner ring rotation
Moment load
(misalignment)

Running trace (f): An abnormal running trace along the center of the inner ring and at the very top and bottom of the outer ring.

Inner ring rotation
Oval housing bore

Running trace (f): An abnormal running trace along the center of the inner ring and decreasing from the bottom towards the top of the outer ring.

Inner ring rotation
No internal clearance
(Negative operating clearance)

Running Traces and Applied Loads (Roller Bearings)

Dull areas also develop on the outer ring raceway surface of roller bearings from rolling contact with the rollers. Running traces are not an anomaly; rather, they provide excellent information on load conditions.

Running trace (i): A normal running trace along the bottom towards the center of an CRB outer ring.

Inner ring rotation
Radial load

Running trace (j): An abnormal running trace along the upper right and lower corners of an CRB outer ring from misalignment.

Inner ring rotation
Moment load (Misalignment)

Running trace (k): A normal running trace along the bottom center of a double-row TRB outer ring.

Inner ring rotation
Radial load

Running trace (l): A normal running trace along the right side of a double-row TRB outer ring.

Inner ring rotation
Axial load

Running trace (m): An abnormal running trace along the bottom left and upper right corners of a double-row TRB outer ring from misalignment.

Inner ring rotation
Radial and moment loads (Misalignment)

(i), (j): Outer rings of N-type single-row cylindrical roller bearings
(k), (l), (m): Outer rings of KBE-type double-row tapered roller bearings

Damage and Countermeasures

Flaking (Spalling)
Condition	As a bearing rotates under load, pieces of the raceways of the inner/outer rings and the rolling surfaces of the rolling elements peel off in an scaly pattern due to rolling fatigue.
Cause	Excessive load Poor mounting (misalignment) Moment load Entry of foreign matter/moisture Poor lubrication, improper lubricant Unsuitable bearing clearance Poor accuracy of shaft/housing, uneven housing rigidity, or large shaft deflection Further damage from rust, corrosion pits, smearing, or dents
Countermeasures	Review the bearing application and check load conditions Improve mounting and maintenance Improve the sealing mechanism, prevent rust when idle Use a lubricant with the proper viscosity, improve lubrication methods Check accuracy of shaft/housing Check bearing internal clearance
Example	Inner ring of spherical roller bearing Flaking of only one raceway row over its entire circumference Caused by excessive axial loads

Scale-like flaking damage on bottom portion of SRB inner ring.

Photo

Fretting
Condition	Fretting is a type of wear caused by repeated and relative micro-slips between two surfaces. It occurs at the contact area between the ring and rolling elements and at fitting surfaces. It is sometimes called fretting corrosion due to the reddish brown or black wear particles produced.
Cause	Poor lubrication Oscillating motion with a small amplitude Insufficient interference
Countermeasures	Use proper lubricants Apply preload Check the interference Apply lubricant to fitting surfaces
Example	Inner ring of deep groove ball bearing Fretting on bore surface Caused by vibration

Reddish-brown fretting damage around the bore surface of a DGBB inner ring.

Photo

Dents
Condition	Fine particles or foreign matter trapped in the bearing create indentations on the raceway/rolling contact surfaces. Impacts or shocks during mounting may also cause dents at the rolling element pitch interval.
Cause	Contact with embedded metal powder or other foreign matter Impacts during mounting or transport Excessive load
Countermeasures	Clean the housing Improve the sealing mechanism Filter the lubricant Improve mounting and handling methods.
Example	Rollers of a tapered roller bearing Small and large indentations over the rolling surface Contact with foreign matter

Dents on tapered rollers from contact with debris.

Photo

Electrical Erosion
Condition	Electrical erosion (sometimes called electric corrosion) occurs when an electric current flows through the oil film in the contact area between the raceways and rolling elements, causing sparking/arcing that melts the surfaces locally and makes them uneven. Visible dull areas are made up of small depressions called craters or pits. These may develop into an uneven axial pattern of ridge-like marks (fluting) with circumferential grooving.
Cause	Electric potential difference between the outer and inner rings Electric potential difference from high frequencies generated by nearby instruments or circuit boards
Countrermeasures	Design electrical circuits to prevent current flow through bearings Insulate the bearing
Example	Inner ring of cylindrical roller bearing Belt-like pattern with pits on the raceway surface

Dark belt of electrical erosion and pits on a CRB inner ring.

Photo

Seizure
Condition	During rotation, the rings, rolling elements, and cage heat up rapidly. If heat becomes excessive, parts may become discolored before softening, deforming, and melting, causing eventual seizing.
Cause	Poor lubrication Excessive load (excessive preload) Excessive rotational speeds Clearance too low Entry of foreign matter/moisture Poor accuracy of shaft/housing, large shaft deflection
Countermeasures	Review lubricant and lubrication methods Review bearing selection Examine fit, bearing clearance, and preload Improve the sealing mechanism Check accuracy of shaft/housing Improve mounting
Example	Inner ring of spherical roller bearing Discoloration and melting of raceway surface. Wear particles from cage bonded to other surfaces Caused by insufficient lubrication

Discolored and melted raceway on inner ring of SRB.

Photo

Discoloration
Condition	Temperature rise and lubricant reactions/deterioration can cause changes in the color of the rings, rolling elements, and cage.
Cause	Poor lubrication Oil staining from lubricant reactions Large increase in temperature
Countermeasures	Improve the lubrication method and use proper lubricant
Example	Inner ring of angular contact ball bearing Bluish-purple discoloration on raceway surface Caused by heat generation due to poor lubrication

Blue and purple streaks on ACBB inner ring.

Photo

Scoring
Condition	Scoring is a kind of surface damage caused by localized bonding (adhesive wear) between sliding surfaces. On the raceway and rolling surfaces, linear damage occurs circumferentially. Cycloidal-shaped damage also appears on roller end faces and ribs in contact with the end face.
Cause	Excessive load, excessive preload Poor lubrication Contact with embedded contaminants Inclination of inner/outer ring, shaft deflection Poor accuracy of shaft/housing
Countermeasures	Confirm the amount of load Adjust preload Change lubricant and/or lubrication method Check accuracy of shaft/housing
Example	Inner ring of tapered roller thrust bearing Scoring of inner ring rib surface Caused by breakdown of oil film from wear particles in lubricant and excessive load

Linear damage (scoring) on large rib face of SRB inner ring.

Photo

Mounting Flaws
Condition	Linear scratches in the axial direction on raceways and rolling surfaces caused by improper mounting, dismounting, or handling.
Cause	Inclination of inner/outer ring during mounting or dismounting Impact loads during mounting/dismounting
Countermeasures	Use appropriate jigs and tools. Avoid impact loads by using apress machine Ensure proper centering when mounting
Example	Inner ring of cylindrical roller bearing Axial scratches on raceway surface Caused by inclination of inner and outer rings during installation