Ball Bearing

While not a common occurrence, sometimes wheel and other bearings fail well before their expected lifespan. The most common types of bearing damage that may result in a reduction of bearing or application life are often caused by:

• insufficient maintenance practices

• mishandling

• improper installation and fitting practices

• inadequate lubrication

• excessive application loads

• contamination

The following from Timken offers a quick reference to the common types/causes of bearing damage.

Signs of Overload

Softening and deformation of the thrust shoulder caused by excessive loads. The discolouration on the inner ring is the result of elevated temperatures.

Signs of Inadequate lubrication

Metal-to-metal contact from lubricant film breakdown resulting in a: 1) polished appearance on the inner race followed by an orange peel texture, and 2) raised edge on the thrust shoulder.

Signs of Improper Installation

Bearing supported by only one- third of its outer ring width within housing.

Distorted seal due to interference with adjacent mating parts.

Seal damage due to improper installation procedure and/or tools. Irregular marks on the outer ring caused by improper seating in the housing. Loose inner ring fit on a rotating shaft resulting in discolouration from heat generation.

Axial crack on inner ring due to oversized or out-of-round shaft.

Inner ring cracking due to excessive taper on the shaft.

Cage Damage

Metal cage damage due to misalignment or excessive speed. Polymer cage damage due to improper installation procedure, where the bearing seal was pressed into the cage.

Fretting Corrosion

Fretting on inner ring due to out-of-round shaft. Fretting on the outer ring due to out-of-round housing bore.

Foreign Material/Contamination

Fine particle abrasive wear on the inner race, commonly known as “frosting.”

Gross contamination resulting from harsh operating conditions and/or seal failure.

Lubrication characteristics of the grease are compromised by water ingress.

Fatigue Spalling

Inclusion origin: spalling from oxides or other hard inclusions in bearing steel.

Geometric stress concentration (GSC): spalling from misalignment, system deflections, or heavy loading. Point surface origin (PSO): spalling from contamination in the ball path or raised metal exceeding the lubricant film thickness.

Grease Etching

Chemical reaction of grease and moisture in a prolonged static condition, resulting in pitting and corrosion. A “witness mark” is generated at each ball location on the inner race.

True Brinnelling

Damage from shock or excessive loads.

False Brinnelling

Wear caused by vibration or axial movement between the balls and races in a static condition.

Special thanks to The Timken Company for the information used here. Full

PDF versions of these failure analysis signs can be found at www.timken.com.