Bearing life, load rating and equivalent load

Definitions

• Bearing life: the number of revolutions (or hours at constant speed) that a bearing completes before the first sign of fatigue (spalling) of the rolling surfaces.
• Rated life $L_{10}$: the life, in million revolutions, that 90 percent of a large group of identical bearings will reach or exceed under the same conditions. The median life is about five times $L_{10}$.
• Basic dynamic load rating $C$: the constant radial load that a group of bearings can carry for a rated life of one million revolutions with 90 percent reliability.
• Equivalent dynamic load $P$: the single radial load that would give the same life as the actual combination of radial and axial loads.

Relations

$$L_{10} = \left(\frac{C}{P}\right)^k \quad \text{million revolutions}$$

with $k = 3$ for ball bearings and $k = 10/3$ for roller bearings. Life in hours:

$$L_{10h} = \frac{10^6\, L_{10}}{60\, n}$$

Equivalent load:

$$P = X V F_r + Y F_a$$

where $V = 1$ when the inner ring rotates (1.2 when the outer ring rotates), and $X$ and $Y$ are radial and axial factors from the catalogue, depending on $F_a/F_r$ compared with $e$. If $F_a/(VF_r) \le e$, then $X = 1$ and $Y = 0$. A service (shock) factor multiplies $P$ for rough loads.

Selection steps

1. Find the radial and axial loads, speed and required life in hours.
2. Choose the type of bearing from the load direction, speed and space.
3. Assume $X$, $Y$ and calculate the equivalent load $P$ including the service factor.
4. Calculate $L_{10}$ in million revolutions and the required $C = P\,L_{10}^{1/k}$.
5. Choose from the catalogue a bearing of suitable bore with $C$ not less than the required value; check the static rating $C_0$ and the limiting speed.
6. Recheck $X$, $Y$ with the actual $C_0$ and repeat if necessary.