Bearing damage, and ultimately failure, can be caused by a variety of conditions, including improper assembly, poor lubrication, and overloading, to name a few. The damage pattern – what actually happened to the bearing due to damaging conditions – is characterized by visible features, such as discoloration, wear or corrosion, on the rolling element and raceway surfaces. However, different types of damage can produce visually similar results, although their causes and long-term effects may not be the same. This is why it is important to understand operating conditions when investigating bearing damage, as they can provide more clues as to the root cause of the damage.
ISO 15243:2017, Bearings – damage and failure – terms, characteristics and causes , classifies the failure modes for bearings made from standard bearing steels. The standard defines six primary damage/failure modes, along with several secondary modes.
Bearing damage/failure modes according to ISO 15243.
Image credit: SKF
When talking about bearing damage, especially in the context of linear bearings, three terms that come up quite frequently are brinelling, spalling and fretting. Let’s look at their causes and the differences between them.
Brinel
Brinelling is a type of plastic deformation often caused by static overload.
A type of plastic deformation caused by repeated, localized impacts or static overloading, brinelling causes damage to a track's surface in the form of regularly spaced indentations. (These indentations are similar to those produced in a Brinell hardness test, hence the name “brinelling.”)
Note that brinelling is not explicitly listed in the ISO damage/failure modes shown above, but occurs in the “Plastic Deformation” subcategories of “Overload” and “Indentation Handling” (due to falling materials, for example).
Peeling
Spalling is the result of surface or subsurface fatigue, which causes fractures to form on sliding surfaces. As the rolling elements pass through these cracks, pieces or slivers of material break off. (Chipping is also known as “flaking,” “peeling,” or “corrosion.”)
In ISO damage/failure modes, flaking occurs in the “Fatigue” category, both “Fatigue Initiated Below the Surface” and “Fatigue Initiated at the Surface”. Chipping damage is progressive and may indicate that a bearing has reached the end of its fatigue life.
Surface fatigue (chipping) in recirculating ball bearings typically begins with a V-shaped crack (A). The chip grows (B) until the material peels off the surface (C).
Concern
Friction generates fine metallic particles, similar to rust.
A type of contact corrosion, friction is caused by microscopic oscillating movements (vibrations) between bearing surfaces. Attrition is a subtype of the “Corrosion” damage/failure mode and is often called “fretting corrosion” because it produces fine metal particles that can oxidize, such as rust. Because of these metallic particles, the friction is abrasive and will quickly worsen the symptoms of the damage.
False brinelling, considered a type of friction, produces distinct depressions in the bearing race that appear similar to true brinelling. It is normally caused by vibrations that occur when the bearing is in a static state and therefore the depressions caused by false brinelling will be the same distance from the rolling elements.
