How to use this calculator

Enter the bearing’s basic dynamic load rating C from the manufacturer’s catalog, the radial and axial loads it carries, and the shaft speed. For a purely radial load leave X = 1 and Y = 0; with thrust, take X and Y from the catalog for your bearing and load ratio. Raise the reliability above 90% to see how life shrinks when fewer failures are acceptable, and enter a target life to get the load rating you need.

Formulas

Basic rating lifeL₁₀ = (CP)^p million revolutions p = 3 (ball), 10/3 (roller)

In hours and equivalent loadL_10h = 10⁶ L₁₀60 n P = X F_r + Y F_a

Reliability factor (ISO 281:2007)L_n = a₁ L₁₀ a₁ = 0.95 (ln(100/R)ln(100/90))^2/3 + 0.05

L₁₀ is the life that 90% of a large group of identical bearings will reach or exceed. The reliability factor comes from the Weibull distribution that fatigue failures follow; it gives a₁ = 1 at 90%, 0.64 at 95% and 0.25 at 99%.

Worked example

A ball bearing with C = 14 kN, 2 kN radial load, 1,500 rpm (the default)

P = 1 × 2 + 0 = 2 kN, so C/P = 7.
L₁₀ = 7³ = 343 million revolutions.
L_10h = 343 × 10⁶ / (60 × 1500) = 3,811 hours.
For a 20,000-hour target: C_req = 2 × (60 × 1500 × 20,000 / 10⁶)^1/3 = 2 × 1800^1/3 = 24.3 kN, so a larger bearing is needed.
At 99% reliability, a₁ = 0.248 and the life drops to 946 hours.

Typical life targets

Designers pick a target life to suit the machine: a few hundred to a few thousand hours for household appliances and hand tools, 8,000 to 12,000 hours for machines used intermittently, 20,000 to 30,000 hours for machines running 8 hours a day, and 40,000 to 60,000 hours or more for continuous 24-hour service such as pumps, fans and compressors. Because life scales with (C/P)³, a bearing with 26% more load rating doubles the life.

Assumptions and limits

The ISO 281 basic rating life: standard bearing steel, good lubrication and normal cleanliness. It does not include the a_ISO factor for lubrication film and contamination.
Constant load and speed. For a duty cycle, use a cube-weighted average load.
Valid for P well below C, roughly P < 0.5C. Very heavy loads also need a static check against C₀.
Reliability from 90% up to 99.95%, the range the ISO formula covers.

Frequently asked questions

What is the difference between C and C₀?

C, the dynamic load rating, is the load that gives an L₁₀ of one million revolutions and is used for fatigue life. C₀, the static rating, limits permanent dents under load when the bearing is stationary or turning very slowly.

Why are roller bearings p = 10/3?

Rollers touch the races along a line rather than at a point, which changes how contact stress grows with load. The exponent comes from fitting fatigue test data.

Will my bearing really last this long?

Half of a population of bearings typically lasts roughly four to five times L₁₀. But contamination, misalignment and poor lubrication cause most real failures, and those can shorten life far below the rating.