How to calculate bearing life: L10, L10h and a worked example
Bearing life is calculated from the dynamic load rating C, the equivalent load P and the speed. Here is the formula, a worked example and what the answer really means.
The basic rating life formula
The basic rating life L10 is the life that 90 percent of a group of identical bearings reach or exceed under the same conditions.
| Quantity | Formula |
|---|---|
| L10 in million revolutions | L10 = (C / P)p |
| L10h in operating hours | L10h = 1,000,000 / (60 × n) × (C / P)p |
| Symbol | Meaning |
|---|---|
| C | Basic dynamic load rating, from the bearing data table |
| P | Equivalent dynamic bearing load (same units as C) |
| n | Speed in revolutions per minute |
| p | 3 for ball bearings, 10/3 for roller bearings |
Worked example
A deep groove ball bearing with C = 55 kN carries an equivalent load P = 8 kN at 1,500 rpm.
| Step | Calculation | Result |
|---|---|---|
| 1. Load ratio | C / P = 55 / 8 | 6.875 |
| 2. L10 (ball, p = 3) | 6.875³ | about 325 million revolutions |
| 3. L10h | 1,000,000 / (60 × 1,500) × 325 | about 3,600 hours |
That is about five months of continuous running. The bearing is too small for a machine that should run for years, so a larger bearing or a lower load is needed. Because life depends on the load ratio cubed, a modest cut in load gives a large gain: halving P multiplies L10 by 8 for a ball bearing.
Equivalent load P
When a bearing carries both radial load Fr and axial load Fa, use P = X × Fr + Y × Fa, with the factors X and Y from the bearing data for your bearing type. For a purely radial load, P = Fr. The real load on the bearing is the hard part, because belt pull, gear forces and weight all add up. The Load Calculator works the shaft loads out from your machine.
Adjusted rating life
ISO 281 adds factors for the reliability you need and for lubrication and contamination: Lnm = a1 × aISO × L10. The reliability factor a1 is:
| Reliability | 90% | 95% | 96% | 97% | 98% | 99% |
|---|---|---|---|---|---|---|
| a1 | 1 | 0.64 | 0.55 | 0.47 | 0.37 | 0.25 |
What life should I aim for?
| Type of machine | Typical L10h target |
|---|---|
| Light duty or intermittent use | 4,000 to 8,000 hours |
| 8 hours a day, not fully used (motors, gear drives) | 14,000 to 20,000 hours |
| 8 hours a day, fully used (general machinery) | 20,000 to 30,000 hours |
| Continuous, 24 hours a day | 40,000 to 60,000 hours |
| Continuous, failure very costly | 100,000 hours and more |
Typical guide values. Your machine standard or your bearing supplier's recommendation takes priority.
Questions
What does L10 life mean?
It is the life that 90 percent of a group of identical bearings reach or exceed. Ten percent can fail earlier, and many last much longer.
Does a bearing fail when it reaches its L10 life?
No. L10 is a statistical life, not a guaranteed failure time. Many bearings run far beyond it, and some fail sooner, especially if lubrication or contamination is poor.
What is the difference between L10 and L10h?
L10 is expressed in millions of revolutions. L10h converts that to operating hours using the shaft speed.
Why is p different for ball and roller bearings?
The contact geometry differs. The life exponent is 3 for ball bearings (point contact) and 10/3 for roller bearings (line contact).
How do I find C for my bearing?
Look up the basic dynamic load rating in the bearing data table for your designation. The bearing finder on this site lists it for 435 common bearings.