Ball-bearing fatigue life scales with 1/load³.
The ISO 281 L10 relation: L ∝ (C/P)³. Even a 26% load increase halves the life — margins on load matter enormously.
The cube law describes rolling-contact fatigue in a clean, properly lubricated bearing. Below a fraction of the fatigue load limit, life is effectively unlimited and the exponent stops meaning anything; with contaminated grease or a marginal oil film, bearings die far faster than any load calculation predicts, and the failure is not fatigue at all.
At 2× the rule says 0.125× and the exact answer is 0.0992× — 26% high. At the point most people use it the shortcut is already outside its own 10% tolerance. It only holds between 1× and 1.3×, which is not where it gets used.
The cube is right for ball bearings and wrong for roller bearings, which use 10/3 - so the rule is optimistic for exactly the parts that carry the heaviest loads.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Bearing Calculator →Ball-bearing fatigue life scales with 1/load³. The ISO 281 L10 relation: L ∝ (C/P)³.