Two numbers on every catalogue line decide the whole selection: the dynamic rating C that sets life, and the static rating C0 that sets the load a stationary bearing can survive without deforming. This page explains what they mean and how to use them.
C, the basic dynamic load rating, is the constant radial load at which 90 % of a large group of identical bearings reaches one million revolutions. It is a life statement, not a strength limit; the bearing can and does carry more, it simply has a shorter life.
C0, the basic static load rating, is the load that produces a defined permanent deformation in the most heavily loaded contact. It is used with the static safety factor s0 = C0 / P0, and it matters when the bearing is stationary, slow, or subjected to shock: a hammer blow on a stationary ring can brinell the raceway even though the running load is modest.
P, the equivalent dynamic load, converts a combined radial and axial load into the single radial load that would give the same life. That is the load you put into the life equation - not Fr on its own.
L10 = (C / P)^p · L10h = 10⁶ · L10 / (60 · n)
L10 is in millions of revolutions and L10h in hours. The exponent p is 3 for ball bearings and 10/3 for roller bearings. That difference is not cosmetic: a roller bearing rated the same as a ball bearing delivers roughly 1.6 times the life at C/P = 5, which is why roller bearings are chosen for heavy duty even when a ball bearing would physically fit.
Because the ratio is raised to a power, life is far more sensitive to load than to rating. Doubling the load cuts a ball bearing life by a factor of eight. Halving it multiplies life by eight. A margin that looks comfortable on paper can evaporate if the real load is understated.
| C / P | Ball bearing (p = 3) | Roller bearing (p = 10/3) |
|---|---|---|
| 2 | 8 | 10.1 |
| 3 | 27 | 46.8 |
| 4 | 64 | 127.9 |
| 5 | 125 | 278.6 |
| 6 | 216 | 516.7 |
| 8 | 512 | 1,529 |
| 10 | 1,000 | 4,310 |
L10 is a 90 % figure: one bearing in ten is expected to reach it. When a machine cannot tolerate that, the life is adjusted by a reliability factor a1.
| Reliability (%) | Life notation | a1 |
|---|---|---|
| 90 | L10 | 1 |
| 95 | L5 | 0.62 |
| 96 | L4 | 0.53 |
| 97 | L3 | 0.44 |
| 98 | L2 | 0.33 |
| 99 | L1 | 0.21 |
L10a = a1 · L10. Modern standards also offer a life modification factor aISO that accounts for lubrication, contamination and the fatigue load limit; it is applied instead of, not in addition to, the old a23 factor.
The relationship between Fr, Fa and P depends on the bearing type, because only some types can carry axial load at all and they do so with different internal geometry.
| Bearing type | Equivalent dynamic load P | Notes | |
|---|---|---|---|
| Deep groove ball | P = Fr when Fa/Fr ≤ e | P = 0.56 Fr + Y Fa when Fa/Fr > e | e and Y come from the f0 Fa/C0 table below; for pure radial load P = Fr. |
| Angular contact ball | P = X Fr + Y Fa | X and Y from the catalogue | Both factors depend on the contact angle and on Fa/C0; use the catalogue table for the exact part. |
| Self-aligning ball | P = Fr + Y1 Fa | Y1 from the catalogue | Y1 depends on the clearance class and the Fa/Fr ratio. Light axial load changes the value. |
| Cylindrical roller | P = Fr | Radial only | Cylindrical roller bearings take no axial load unless they are the HJ/HU thrust-rib design. |
| Needle roller | P = Fr | Radial only | Same rule as cylindrical roller bearings. |
| Tapered roller | P = 0.4 Fr + Y Fa | P = Fr when Fa/Fr ≤ e | Use the catalogue Y for the exact series; the induced axial load from the paired bearing must be added. |
| Spherical roller | P = Fr + Y1 Fa | P = 0.67 Fr + Y2 Fa | Which expression applies is decided by Fa/Fr against the catalogue e value. |
| Thrust ball | P = Fa | Axial only | Only valid while a minimum axial load keeps the balls loaded; running too light causes skidding. |
| Spherical roller thrust | P = Fa + 1.2 Fr | Permitted Fr is limited | The radial load is tolerated only up to a fraction of Fa, which the catalogue states. |
| f0 Fa / C0 | e | Y |
|---|---|---|
| 0.025 | 0.22 | 2.0 |
| 0.04 | 0.24 | 1.8 |
| 0.07 | 0.27 | 1.6 |
| 0.13 | 0.31 | 1.4 |
| 0.25 | 0.37 | 1.2 |
| 0.5 | 0.44 | 1.0 |
X = 1 and Y = 0 while Fa/Fr ≤ e; X = 0.56 and Y as tabulated when Fa/Fr > e. For static equivalent load use P0 = 0.6 Fr + 0.5 Fa, and never less than Fr. Higher clearance classes shift e upwards - a C3 bearing tolerates more axial load before the axial term starts to matter.
Load, speed, target life and the operating temperature are enough for us to size the bearing and confirm whether a ball or roller type is the right choice.