Rolling-element bearings do not fail on a fixed clock — they fail on a distribution. L10 life is the number of revolutions 90% of a batch will survive before the first signs of fatigue. If you have ever wondered where a catalog “rating life” number comes from, this is the calculation behind it, worked end to end.
The core equation
The ISO 281 basic rating life is deceptively simple:
- C = basic dynamic load rating (from the bearing catalog, in kN)
- P = equivalent dynamic bearing load (the load your application actually applies, in kN)
- p = life exponent — 3 for ball bearings, 10/3 for roller bearings
Notice the exponent. Because life scales with the cube of the load ratio for ball bearings, halving the load does not double life — it multiplies it by eight. That single fact drives most bearing-selection decisions.
Step 1 — Find the equivalent dynamic load P
A bearing rarely sees pure radial load. The equivalent load rolls radial and axial components into one number:
X and Y are the radial and axial load factors, and they switch values once the axial-to-radial ratio crosses a threshold e that depends on the bearing geometry. For pure radial load, X = 1 and Y = 0, so P = Fr. When you have combined loading, the Bearing Equivalent Load calculator pulls the correct X, Y, and e values for the series so you are not interpolating catalog tables by hand.
Step 2 — A full worked example
Take a 6205 single-row deep-groove ball bearing carrying a purely radial load on a 1,500 rpm shaft.
| Quantity | Symbol | Value |
|---|---|---|
| Basic dynamic load rating | C | 14.0 kN |
| Applied radial load | Fr | 1.5 kN |
| Equivalent load (pure radial) | P | 1.5 kN |
| Life exponent (ball) | p | 3 |
| Shaft speed | n | 1,500 rpm |
Plug into the rating-life equation:
Revolutions are hard to feel. Convert to hours with the shaft speed:
= (106 × 813) / (60 × 1500)
≈ 9,030 hours ≈ 376 days of continuous running
So under a steady 1.5 kN radial load, 90% of these bearings will run roughly a year non-stop before fatigue becomes probable. Push the load to 3.0 kN and — thanks to that cube — L10 collapses to about 102 million revolutions, roughly 1,130 hours. Same bearing, one-eighth the life.
Step 3 — From L10 to the adjusted life L10m
The basic L10 assumes a clean, well-lubricated, 90%-reliability world. Real applications rarely match that, so ISO 281:2007 introduces the modified rating life:
The reliability factor a1
a1 adjusts for a survival probability other than 90%. It equals 1.0 at 90% reliability, drops to about 0.64 at 95% (L5), and to roughly 0.25 at 99% (L1). Demand more survivors and you must accept a shorter guaranteed life.
The life-modification factor aISO
aISO captures lubrication quality and contamination. It is built from the viscosity ratio κ (actual oil film versus what the contact needs) and a contamination factor ec. Clean, generously lubricated bearings can see aISO above 1 — sometimes far above. Starved or dirty ones fall below 0.1, quietly erasing most of the catalog life. Because aISO spans two orders of magnitude, it is often the single biggest lever on real-world bearing life, and it is the factor engineers most often skip. The Bearing L10 Life calculator computes κ, pulls aISO from the ISO 281 chart, and returns L10m directly.
Don’t forget speed and fit
Two more checks decide whether your L10 number is even meaningful. First, confirm you are under the bearing’s limiting speed with the Bearing Speed / DN calculator — exceed it and lubrication, not fatigue, sets the life. Second, get the shaft and housing tolerances right: an interference fit that is too tight preloads the raceway and can cut life dramatically. The Bearing Fit calculator handles the ISO shaft/housing tolerance stack so the mounted internal clearance lands where it should.
If you are still choosing a bearing rather than checking one, start with the Bearing Selection Wizard, which is vendor-neutral and sizes on your actual load, speed, and life target.
A note on measurement
All of this assumes you actually know the load. On real machinery that means a calibrated setup — a bearing puller, a dial indicator, and often a decent digital caliper to confirm shaft and housing diameters before you trust any tolerance calc. Cheap measuring tools quietly poison good math; a mid-range Mitutoyo caliper pays for itself the first time it keeps you off the wrong fit column.
Dynamic vs. static load — a common trap
The C in the rating-life equation is the dynamic load rating, and it only means something for a rotating bearing. A bearing that mostly sits still under load is governed by the static load rating C₀ and its permissible static safety factor s₀ instead. Confusing the two is a classic error: sizing a slow-oscillating or frequently-stopped bearing on C alone can badly overstate its life. If your duty cycle includes long dwell under high load, check the static rating separately with the Bearing Load calculator before you trust any L10 figure.
Variable load? Use an equivalent cubic-mean load
Real machines rarely hold one steady load. When the load varies over a known duty cycle, you do not average it — you take the cubic mean, which respects that same load-cubed relationship:
where U is the fraction of time (or revolutions) spent at each load. Because the heavy portions of the cycle are cubed, a brief spike load can dominate the answer — another reminder that peak loads, not averages, drive bearing life.
Run the numbers on your own bearing
Drop your load, speed, and lubrication conditions into the Bearing L10 Life calculator and get the adjusted L10m in seconds — no ISO chart interpolation required.
Stop looking it up twice — run the numbers and keep the record.
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