Subsurface-initiated fatigue

The raceway fails from the inside out. Urgency

The steel simply wore out from repeated stress, or was pushed there early by poor lubrication.

What subsurface-initiated fatigue looks like

• Flakes or craters where the surface has broken away, usually growing from one small pit
• On the stationary ring they sit inside the load zone; on the rotating ring they can be anywhere round the track
• Balls and rollers can spall as well, often as a cap-shaped chip

Rotec AI schematic illustration. It shows the typical pattern and is not a photograph of a real bearing.

How subsurface-initiated fatigue develops

Every revolution squeezes a small patch of steel under each rolling element. After millions of cycles, hidden cracks open a few tenths of a millimetre below the surface, where shear stress peaks. They creep upward and a flake lets go. The bearing keeps running, but noise and vibration climb as the spall spreads.

How fast it progresses: From first flake to failure can be days or months depending on speed, load and the machine. Plan the replacement; do not wait for the trip.

Causes of subsurface-initiated fatigue

Bearing selection

• Bearing too small for the real load (include shocks, belt pull, gear forces)

Design & seats

• Soft foot, distorted housing or poorly supported seats raising local stress

Operating conditions

• Overloads, shocks, heavy vibration or temperature spikes that cut the steel's strength

Environment & sealing

• Water, stray current or hydrogen from broken-down lubricant weakening the steel, which brings failure on early

How to confirm it is subsurface-initiated fatigue

• Compare running hours with the calculated life: if it is close, this is a natural end of life, not a defect
• Note where the spall sits: load zone only is normal, one-sided suggests misalignment, two zones 180° apart suggests an oval seat
• A failure at a small fraction of rated life points to overload or weakened steel

How to fix and prevent subsurface-initiated fatigue

• Recheck the true loads in the Load Calculator and step up in size if needed
• Make sure seats and housings are round, square and properly supported
• Use the correct fits and mounting method, and keep alignment within limits
• If life was reached normally, simply reorder: this is the system working as designed

Easily confused with

• Surface-initiated fatigue (surface distress): Surface fatigue starts as a dull, finely pitted band; subsurface fatigue starts as discrete flakes.
• Misalignment & distorted seats (edge loading): If the flakes cluster on one edge or in two zones 180° apart, suspect misalignment or an oval seat.

Worth knowing

Three ways a spall starts

Not every spall is the same. Point-surface origin starts at a nick, dent, debris mark or etch pit and often shows arrowhead-shaped spalls travelling in the direction of rotation: it is the most common kind. Geometric stress concentration starts at the extreme edge of the contact path, from misalignment, deflection or machining errors in the seats. Inclusion origin starts at a tiny hard particle inside the steel and gives a localised elliptical spall: it is now uncommon because bearing steel is much cleaner than it used to be.

Many bearings never reach their calculated life

In field analysis by one major maker, about half of the bearings sent in had not reached their calculated life, and in roughly nine of ten of those the cause was not fatigue at all. It was contamination, corrosion, poor lubrication, rough handling or bad running conditions. Check those before you blame the bearing.

Catching subsurface-initiated fatigue early

A spall on the outer ring shows in a vibration spectrum as a peak at BPFO (typically about 3–5× shaft speed). On the inner ring it appears at BPFI (about 5–7×), usually with 1× sidebands as the defect passes through the load zone. Outer-ring defects are the most common and the easiest to catch. Run the bearing through Bearing Selector to get its estimated BPFO and BPFI.

Other fatigue modes

Questions about subsurface-initiated fatigue