Bearing L10 life, explained

Rolling bearings do not have a single lifespan — they have a failure distribution. The industry's standard point on that distribution is L10: the life that 90% of identical bearings will reach or exceed at a given load and speed (ISO 281 basic rating life). The formula is compact:

L10 = (C / P)^p  [million revolutions]   p = 3 (ball), 10/3 (roller)

L10h = L10 × 10⁶ / (60 × n)

where C is the catalog dynamic load rating and P the equivalent dynamic load — the radial-plus-axial combination collapsed to one number with the catalog X/Y factors. The bearing selection calculator runs the rating math; the bearing load calculator builds P from the shaft reactions first.

Worked example

A ball bearing with C = 30 kN carrying P = 5 kN at 1,500 rpm:

L10 = (30/5)³ = 216 million revolutions

L10h = 216 × 10⁶ / (60 × 1,500) = 2,400 hours

2,400 hours is about four months of continuous duty — fine for a seasonal machine, poor for a 24/7 fan. Now the cube law at work: drop the load to 4 kN and life jumps to (7.5)³ ≈ 422 million revs — 4,690 h, nearly double for a 20% load reduction. A roller bearing at the same C/P = 6 gives 6^(10/3) ≈ 392 million revs — the steeper exponent is one reason roller types carry heavy loads so much better.

What L10 does not include

• Lubrication and contamination. The basic formula assumes clean, properly lubricated operation. Modern catalogs offer a modified rating (a·ISO factors) that can swing life by 10× either way — dirty oil is a bigger enemy than load.
• Misalignment and fits. A cocked bearing or a spinning inner ring on a loose shaft fails long before fatigue would.
• Minimum load. Lightly loaded high-speed bearings skid instead of roll; catalogs specify a minimum P (often ~1–2% of C).
• Static overload. Brinelling from shock while stationary is governed by C0, not C — one forklift bump can spend years of fatigue life.

Choosing a target life

Customary L10h targets: 2,000–8,000 h for intermittently used machinery, 20,000–30,000 h for 8-hour industrial duty, 40,000–60,000 h for continuous process plant. Working backward from a target is the practical selection method: required C = P × (L10_target)^(1/p), then pick the next catalog size up. For the shaft side of the same problem, the shaft deflection calculator checks the slope at the bearing seats, and the overhung load calculator handles belt-and-pulley reaction loads that quietly dominate P.

Frequently asked questions

What does L10 life mean?

The life that 90% of a group of identical bearings will meet or exceed under the same load and speed — equivalently, a 10% failure probability. It is a statistical rating, not a promise for one bearing: the median bearing lasts roughly five times L10.

What is the L10 bearing life formula?

L10 = (C/P)^p in millions of revolutions, where C is the bearing dynamic load rating, P the equivalent dynamic load, and p = 3 for ball bearings or 10/3 for roller bearings. In hours: L10h = L10 × 10⁶ / (60 × rpm).

Why does a small load increase kill bearing life?

The cube law. For ball bearings, life scales with (C/P)³: 25% more load means (1/1.25)³ ≈ half the life; doubling load leaves an eighth. Load errors compound far faster than speed errors, which only scale linearly in hours.

What is the difference between C and C0?

C (dynamic load rating) feeds the fatigue-life formula — it is the load giving exactly one million revolutions L10. C0 (static load rating) limits permanent raceway denting while stationary or rotating very slowly. Both appear in every catalog row; the life formula uses C.