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How to Calculate Bearing Life (L10) — with a Worked Example

Bearing life is normally expressed as the basic rating life, L10 — the life that 90% of a group of identical bearings will reach or exceed under a given load and speed (so 10% may fail before it). It is calculated from the bearing's load rating and the load you apply, using one simple formula.

The Formula

L₁₀ = (C / P)^p

where:

• C = basic dynamic load rating (from the catalogue), in newtons.
• P = equivalent dynamic load actually applied, in the same unit.
• p = the life exponent: 3 for ball bearings, 10/3 for roller bearings.

The result is in millions of revolutions. To get hours, bring in the speed:

L_10h = (1,000,000 / (60 × n)) × (C / P)^p

where n is the speed in rpm.

A Worked Example

Take a deep-groove ball bearing (so p = 3) with a catalogue dynamic rating C = 42,000 N, carrying a steady radial load P = 4,000 N, at n = 1,500 rpm.

1. Load ratio: C / P = 42,000 / 4,000 = 10.5
2. L₁₀ = 10.5³ = ≈ 1,158 million revolutions
3. In hours: L_10h = 1,000,000 / (60 × 1,500) × 1,158 = 11.11 × 1,158 = ≈ 12,860 hours

That is roughly 1.5 years of continuous running — or much longer in intermittent service.

What About Combined Loads?

If the bearing carries both radial and axial load, you first work out the equivalent load P:

P = X·F_r + Y·F_a

where F_r and F_a are the radial and axial loads, and X and Y are factors from the manufacturer's catalogue (they depend on the bearing and the ratio of axial to radial load). For a purely radial load, P is simply F_r.

Adjusting for Reliability

L10 is the life at 90% reliability. If you need a higher reliability, multiply by a factor a₁:

So L5 (95% reliability) in our example = 0.64 × 1,158 ≈ 741 million revolutions. (These follow ISO 281:2007; some older references differ slightly.)

A Note on Real-World Life

L10 covers fatigue only — it assumes good lubrication, clean running and correct mounting. The fuller ISO 281 "modified" life adds factors for lubrication and contamination, because in practice those are what usually end a bearing's life early. In other words, the formula tells you the best case; how close you get depends on how well the bearing is looked after.

How to Get More Life from a Bearing

• Reduce the load, or choose a larger bearing (higher C) — life rises with the cube of C/P for ball bearings.
• Lubricate correctly and keep it clean — the biggest real-world levers.
• Get the fit, alignment and clearance right.

Frequently Asked Questions

What is L10 bearing life?

The basic rating life — the number of revolutions (or hours) that 90% of identical bearings will reach or exceed under a given load and speed.

What is the difference between the ball and roller exponent?

The life exponent p is 3 for ball bearings and 10/3 for roller bearings, reflecting the different way load is carried (point vs line contact).

Does doubling the load halve the life?

No — it has a much bigger effect. For ball bearings life varies with the cube of C/P, so doubling the load cuts life to about one-eighth.