What it is. Every rolling bearing converts sliding into rolling — but the price of that efficiency is that the entire load passes through contact areas the size of a fingernail, generating enormous compressive pressures (1.5 to 3+ GPa), well above the yield strength of ordinary steel. The material only survives because the stress is compressive, confined and cyclic — every revolution is another stress cycle beneath the surface. After millions to billions of such cycles, microcracks form below the surface (at microscopic inclusions in the steel), grow slowly, and eventually reach the surface: a piece of metal breaks loose, leaving a crater — the spall. This is the bearing's natural wear-out death, and the reason no bearing lasts "forever."
How to recognize it in the field. Rising vibration at specific frequencies (the so-called BPFO/BPFI/BSF — each tied to a different part of the bearing), rhythmic metallic noise that worsens over time, and — at an advanced stage — elevated temperature and audible noise. The good news: this process is slow and progressive, giving weeks to months of warning before functional failure — enough time to plan the replacement instead of an unplanned stoppage.
What it costs if left untreated. A bearing that sheds a large spall can seize, generate extreme heat and damage the surrounding shaft and housing — a foreseeable maintenance issue turns into an expensive unplanned outage. The right strategy here is not fixed-interval replacement (real life varies widely bearing to bearing, even identical ones) — it is condition monitoring, replacing when the signal appears.