Static load rating ($C_o$)

The load carried by a non-rotating bearing is called a static load. The basic static load rating is defined as the static radial load (in case of radial ball or roller bearings) or axial load (in case of thrust ball or roller bearings) which corresponds to a total permanent deformation of the ball (or roller) and race, at the most heavily stressed contact, equal to 0.0001 times the ball (or roller) diameter.

Basic dynamic load rating (C)

The basic dynamic load rating is defined as the constant stationary radial load (in case of radial ball or roller bearings) or constant axial load (in case of thrust ball or roller bearings) which a group of apparently identical bearings with stationary outer ring can endure for a rating life of one million revolutions (which is equivalent to 500 hours of operation at 33.3 r.p.m.) with only 10 percent failure.

Static equivalent load

The static equivalent load may be defined as the static radial load (in case of radial ball or roller bearings) or axial load (in case of thrust ball or roller bearings) which, if applied, would cause the same total permanent deformation at the most heavily stressed ball (or roller) and race contact as that which occurs under the actual conditions of loading.

Dynamic equivalent load

The dynamic equivalent load may be defined as the constant stationary radial load (in case of radial ball or roller bearings) or axial load (in case of thrust ball or roller bearings) which, if applied to a bearing with rotating inner ring and stationary outer ring, would give the same life as that which the bearing will attain under the actual conditions of load and rotation.

Life of a bearing

The life of an individual ball (or roller) bearing may be defined as the number of revolutions (or hours at some given constant speed) which the bearing runs before the first evidence of fatigue develops in the material of one of the rings or any of the rolling elements.

The rating life of a group of apparently identical ball or roller bearings is defined as the number of revolutions (or hours at some given constant speed) that 90 percent of a group of bearings will complete or exceed before the first evidence of fatigue develops (i.e. only 10 percent of a group of bearings fail due to fatigue).

The term minimum life is also used to denote the rating life. It has been found that the life which 50 percent of a group of bearings will complete or exceed is approximately 5 times the life which 90 percent of the bearings will complete or exceed. In other words, the average life of a bearing is 5 times the rating life (or minimum life). It may be noted that the longest life of a single bearing is seldom longer than the 4 times the average life and the maximum life of a single bearing is about 30 to 50 times the minimum life.

The life of bearings for various types of machines vary from 500 to 200000 hours depending upon the nature of the application.

Reliability Factor or Probability of survival (p)

Reliability factor the rating life is the life that 90 percent of a group of identical bearings will complete or exceed before the first evidence of fatigue develops. The probability (p) is defined as the ratio of the number of bearings which have successfully completed L million revolutions to the total number of bearings under test. Sometimes, it becomes necessary to select a bearing having reliability of more than 90%. According to Weibull, the relation between the bearing life and the reliability is given as

$$\frac{L}{L_{90}}=\frac{\log _{e}(1 / p)}{\log _{e}\left(1 / p_{90}\right)}$$

Temperature factor $\quad$ PSG 4.2

While the operating temperature of the bearing is individually defined in accordance with material and structure, the bearing is capable to be used at temperatures higher than 110°C by applying a special treatment for thermal resistance. However, this will cause a reduction of basic dynamic load rating as a result of the reduction of permissive contact stress. Basic dynamic load rating with consideration for temperature increase is given by the following formula.

$$C^{\prime}=k_{t} \cdot c$$

$\mathrm{C}^{\prime} :$ Basic dynamic load rating with consideration for temperature change

$k_{\mathrm{t}} :$ Temperature factor

$\mathrm{C} :$ Basic dynamic load rating