The Calculation, Explained
The rating-life equation
Rolling bearings die by subsurface fatigue, and fatigue life follows a power law in load. ISO 281 defines the basic rating life:
C is the bearing's dynamic load rating — the load at which the catalogue promises a million revolutions of L10 life. P is the equivalent dynamic load the bearing actually sees, combined from radial and axial components (P = X·Fr + Y·Fa, factors from the catalogue). The exponent is p = 3 for ball bearings and 10/3 for roller bearings — which is why the C/P ratio is destiny: doubling the load divides ball-bearing life by eight.
What L10 actually promises — read this part twice
- L10 is a statistic, not a warranty. It is the life that 90% of a large population of identical bearings, identically loaded, are expected to reach or exceed. Ten percent fail earlier — by design of the definition. Median life (L50) is roughly five times L10; individual bearings scatter widely around both.
- P is where accuracy goes to die. C comes from a catalogue, exact. P comes from your estimate of real loads — belt pull, misalignment, unbalance, gear forces, mounting preload. A 20% error in P is a 73% error in ball-bearing life. Spend your effort there.
- The basic rating assumes good lubrication, cleanliness, alignment and mounting. ISO 281's modified rating life (aISO factor) adjusts for lubricant film and contamination — in dirty, wet or poorly lubricated service, real life is a fraction of the catalogue number, and the failure mode is often wear or corrosion long before fatigue gets its turn.
- Very low loads are not automatically safe either: below a minimum load, rolling elements skid and smear. Bearings have a load window, not just a ceiling.
Worked example (the defaults above)
C = 85 kN, P = 12 kN: C/P = 7.08; L10 = 7.08³ = 355 million revolutions; at 1,480 rpm, L10h = 355 × 10⁶ / 88,800 = ≈ 4,000 hours — under a year at 16 h/day duty. The lesson the numbers teach: this application is load-limited, and the cheapest life extension is not a bigger bearing but a smaller P — check the belt tension and the alignment before reaching for the catalogue.
References
ISO 281 — Rolling bearings: dynamic load ratings and rating life; manufacturer catalogues (SKF, Schaeffler, NSK) for C values and X/Y factors.
Related reading: The Maintenance Mix.
Related calculators: Vibration Severity · Pump & Fan Calculator