Bearing life L10: what it says and what it does not, with load, speed and reliability

Bearing life calculation invites one confusion above all: the "life" figure in the catalogue versus the real life in the field. L10 says that 90% of identical bearings will last at least that long; 10% fail earlier. It is not an average, it is a lower bound.

1. Basic rating life

• L10 = (C / P)^p million revolutions; p = 3 for ball bearings, p = 10/3 for roller bearings.
• L10h = L10 × 10⁶ / (60 × n) hours; n in revolutions per minute.

Example: dynamic load rating C = 25.5 kN (from the catalogue), equivalent dynamic load P = 3.1 kN, speed 1,450 rpm, ball bearing.

• C / P = 8.23
• L10 = 8.23³ = 556.6 million revolutions
• L10h = 556.6 × 10⁶ / (60 × 1,450) = 6,398 hours

With the same C, P and speed, a roller bearing (p = 10/3) gives 12,914 hours; the difference in the exponent shows why roller bearings are preferred under heavy load.

2. A small change in load changes life a lot

Because the exponent is 3, a 25% increase in load cuts life by nearly half:

• load × 0.8 → P = 2.48 kN → L10h = 12,495 hours
• load × 1.0 → P = 3.10 kN → L10h = 6,398 hours
• load × 1.25 → P = 3.88 kN → L10h = 3,276 hours

This is where the most common loss of life in the field comes from: belt tension, misalignment or unbalance raise the equivalent load, the catalogue life looks unchanged, but the real life halves. Vibration measurement does not see this load directly; it sees unbalance and misalignment, and the load is inferred from them.

3. Reliability adjustment

If you want 95% reliability instead of 90%, a1 = 0.64 (the value commonly given in ISO 281; verify against the standard or the catalogue before publishing):

• L5mh = 0.64 × 6,398 = 4,094 hours

Asking 95% of the same bearings to survive pulls the planning life from 6,398 to 4,094 hours. On critical equipment, spares and maintenance intervals are planned on this figure, not on L10.

4. The bearing needed for a target life

If 40,000 hours are targeted on this machine (at 95% reliability), the required dynamic load rating is C = 54.5 kN, 2.14 times the current bearing. The target therefore calls for a larger bearing or a lower load; it cannot be closed by maintaining more often.

Questions L10 does not answer

• Lubrication, contamination and temperature: the a_ISO factor in ISO 281 (a23 in the tool) carries them; without a catalogue value it is taken as 1 and the result is optimistic.
• Mounting errors and electrical erosion: these are not fatigue and sit outside the calculation; most early failures come from here.
• Heavy loads with P/C > 0.5: the model loses reliability in this region and the tool warns.
• When one particular bearing will fail: L10 is the lower 10% of a distribution and gives no date for a single part. The closest thing to a date is the vibration trend.

Try it with your own bearing

The bearing life L10 calculator works from C, P (or Fr, Fa with X, Y), speed and reliability; it holds no bearing data, you enter it from your catalogue. To interpret it together with the vibration trend, get in touch.