A spalled raceway is the most common thing a bearing investigator is handed, and the least self-explanatory. Two spalls of similar size and appearance can carry opposite implications: one is the ordinary end of a bearing's fatigue life, the other evidence that something entered the rolling contact that should never have been there. The distinction separates a component that performed as rated from a system that failed to keep its lubricant clean, and the answer sits at the origin of the spall rather than on its visible surface.
Two spalls, one appearance
By the time a bearing is removed, a spall has usually grown well past its point of initiation. Rolling elements pass over the damaged zone thousands of times a minute, breaking down its edges and enlarging the crater until the original geometry is obscured. An examination drawn to the largest damage is looking at consequence rather than cause.
ISO 15243 supplies the working vocabulary, classifying bearing damage into fatigue, wear, corrosion, electrical erosion, plastic deformation and fracture, with sub-categories separating subsurface from surface initiation. Used consistently it keeps an examination from drifting into impressionistic description, and gives opposing experts a shared set of terms to disagree within.
Where classical fatigue begins
Under clean, adequately lubricated Hertzian contact the maximum shear stress occurs below the surface, at a depth set by load and contact geometry. Cracking nucleates there, frequently at a non-metallic inclusion, propagates roughly parallel to the raceway and eventually turns upward to liberate a flake of material. The resulting spall has steep walls and an origin that a cross-section places beneath the original surface.
This is the mechanism rating life models, and a bearing that spalls this way after an appropriate number of revolutions has not malfunctioned so much as been consumed. Steel cleanliness is itself specified: ASTM A295 and ISO 683-17 cover bearing-quality steels, and ASTM E45 gives the method for rating inclusion content.
What a contaminant leaves behind
A hard particle rolled through the contact dents the raceway. The dent carries a raised rim, and that rim concentrates stress on every subsequent pass. Cracking initiates at or just beneath the surface at the trailing edge of the dent, and the spall growing from it originates shallow, often with the parent dent still visible at one end or with a field of similar indentations surrounding it.
The population of dents is informative in its own right. An isolated indentation may be a mounting artefact; a distributed field across the loaded arc points to circulating debris, and therefore to filtration, sealing or ingress rather than to the bearing as supplied.
Why the cross-section usually settles it
Surface examination narrows the field; metallography decides it. Sectioning through the origin and preparing the specimen in accordance with ASTM E3 shows whether the crack network runs upward from a subsurface inclusion or downward from the surface. Microindentation hardness traverses under ASTM E384 establish whether case depth and core hardness meet specification and whether the material was tempered by heat generated in service.
Where the section reveals altered microstructure — white etching areas, or a tempered band beneath a discoloured raceway — the thermal history becomes legible in a way no surface photograph conveys.
The lubricant carries the other half
Debris suspended in the grease or oil is the same evidence viewed from another direction. Analytical ferrography under ASTM D7690, and particle characterisation under ASTM D7684, separate fatigue chunks from cutting wear, sliding wear and non-metallic contaminant. Elemental methods such as ASTM D6595 identify silica, iron migrating from another component, or an additive package inconsistent with the specified lubricant.
For circulating oil systems, a particle count coded under ISO 4406 against the cleanliness target the system was designed to hold often demonstrates that a contamination pathway existed before the failure rather than because of it.
Where the damage sits on the raceway
Position matters as much as morphology. Damage confined to the expected contact path is consistent with ordinary loading. Damage displaced toward one shoulder suggests misalignment or edge loading. Damage in what should be the unloaded zone is difficult to explain by service loading at all and more often reflects handling, mounting force routed through the wrong ring, or vibration while the shaft was stationary. That relationship survives only if the wear band is mapped before disassembly.
Comparing the finding against expected life
Neither conclusion is complete without the arithmetic. Basic rating life under ISO 281, calculated from the load and speed the bearing actually carried, gives the statistical expectation against which observed service life is measured; ABMA Standards 9 and 11 carry the parallel North American treatment for ball and roller bearings.
A subsurface fatigue spall arriving at a plausible fraction of rating life is unremarkable. The same spall at a small fraction of it means either the load assumption was wrong or something accelerated the mechanism, which returns the enquiry to contamination, lubrication or alignment.
Where the distinction gets contested
The challenges are predictable: that the origin was inferred from a photograph rather than a section, that the section was cut in a plane missing the initiation site, that the load spectrum used in the life calculation was nominal rather than measured, and that indentations attributed to service debris arrived during removal or shipping.
Work that locates the origin dimensionally, states the sectioning plane and the reason for choosing it, and separates pre-existing damage from handling damage by reference to as-found photography holds up. A conclusion resting on surface appearance alone rarely does.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.