L10 life is the most cited number in bearing engineering and the most misunderstood. It’s not how long a bearing will last. It’s the number of revolutions (or hours) at which 90% of a population of identical bearings are still running. Ten percent have already failed. That’s the definition. Everything else flows from there.
To run the math without the spreadsheet, use the L10 Bearing Life Calculator. For dynamic equivalent load and selection, see the Bearing Load Calculator and the Bearing Selector.
The Basic Equation
From ISO 281, the basic rating life of a rolling-element bearing is:
Where:
- L10 — basic rating life in millions of revolutions
- C — basic dynamic load rating (from the bearing catalog)
- P — equivalent dynamic load on the bearing
- p — exponent: 3 for ball bearings, 10/3 for roller bearings
Converting to Operating Hours
Engineering departments live in hours, not revolutions. The conversion is:
Where n is rotational speed in RPM. The 10⁶ is because L10 is in millions of revolutions; 60n converts revolutions to revolutions per hour.
Equivalent Dynamic Load P
If only radial load is present, P = Fr. With combined radial and axial load:
X and Y come from the bearing catalog and depend on the Fa/(C0 · e) ratio, where C0 is static load rating and e is a constant for that bearing series. For a 6206 deep groove ball bearing with C = 19.5 kN, C0 = 11.2 kN, Fr = 3 kN, Fa = 1.5 kN: Fa/C0 = 0.134 → e ≈ 0.32, Fa/Fr = 0.5 > e, so X = 0.56, Y ≈ 1.31. P = 0.56(3) + 1.31(1.5) = 3.65 kN.
Worked Example: 6206 at 1,750 RPM
With C = 19,500 N, P = 3,650 N, p = 3, n = 1,750 RPM:
That’s only 1,452 hours of basic rating life. For an industrial application running 16 hours/day, 250 days/year, that’s about 4 months. For most applications, you’d downsize the load, upsize the bearing, or apply life adjustment factors.
The Adjusted Life: Lnm
ISO 281 introduced the modified rating life equation:
The two adjustment factors are where most real-world life prediction lives:
- a1 — reliability factor. a1 = 1 for 90% reliability. For 95%, a1 = 0.62. For 99%, a1 = 0.21. Higher reliability means shorter rated life.
- aISO — combined factor for lubrication, contamination, and load. Ranges from 0.1 (poor) to 50 (ideal).
Computing aISO
The aISO factor depends on viscosity ratio κ (actual viscosity / required viscosity), the contamination factor eC, and the load ratio Cu/P, where Cu is the fatigue load limit. The full formula is in ISO 281:2007 Annex A and is non-trivial to evaluate by hand. In practice:
| Condition | eC | Typical aISO |
|---|---|---|
| Extreme cleanliness, κ > 4 | 1.0 | 10–50 |
| Normal cleanliness | 0.5 | 1–5 |
| Slight contamination | 0.3 | 0.5–2 |
| Heavy contamination | 0.05–0.1 | 0.1–0.3 |
Why Two Identical Bearings Have Different Lives
Bearing fatigue is a statistical phenomenon driven by sub-surface inclusions in the steel — tiny ceramic particles that nucleate spalls. Modern vacuum-degassed steels (VIM-VAR for premium grades) reduce inclusion size and frequency, which is the only meaningful way to extend life beyond aISO‘s upper bound. This is why two bearings from the same lot, on the same shaft, with the same load can fail 5x apart. The L10 number captures the 10th percentile of that distribution.
Practical Rules
- If P / C > 0.1, expect short life. Halve P and life increases 8x (for ball bearings).
- If you’re running with poor lubrication, sizing for life is pointless — fix the lube first.
- L10 of 30,000 hours is the typical industrial design target. Mining and crushing is 8,000. Aerospace is 100,000+ with redundancy.
- For variable loads, use the cube-mean load rule: Pm = (Σ Pi³ ti / Σ ti)1/3.
Pick the Right Bearing First
Computing L10 for the wrong bearing wastes time. Run the Bearing Selector to narrow down to deep groove, angular contact, tapered roller, or spherical, then drill into our 6206 spec sheet (and similar pages for other series) for the C, C0, and dimensional data. Verify shaft and housing tolerances with the Bearing Fit Calculator and check operating speed with Bearing Speed (DN) Calculator.
Stop looking it up twice — run the numbers and keep the record.
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