In operation Under normal conditions, the working surfaces of a bearing are subjected to very high alternating stresses due to the continuous action of the rolling elements, as a result of fatigue of the material and is called the fatigue failure.
The bearing life is defined as the number of revolutions achieved by the bearing before the first signs of fatigue failure appear.
When a group of identical bearings operate under identical load conditions, the life of the individual bearings show a considerable dispersion. Therefore when selecting a bearing it is more practical to adopt the life that the majority of the bearings will attain or exceed. For this reason the basic rating life of a group of bearings is defined as the number of revolutions (or hours at some given constant speed) that 90% of the group of bearings will attain or exceed before the first avoidance of fatigue develops. This basic rating life is in line with ISO definition.

The basic dynamic load rating "C" is defined as the constant bearing load which will give an ISO basic rating life, as defined above, of one million revolutions. It refers to pure radial load for radial bearings and to pure axial load for thrust bearings.

Similarly the basic static load rating "Co" is defined as the static load which corresponds to a total permanent deformation of the rolling elements and race ways at the most heavily stressed contact point of 0.0001 times the diameter of the rolling elements. It refers to pure radial load for redial bearings and to pure axial load for thrust bearings.

The relationship between the basic rating life, the basic dynamic load rating and the bearing load can be presented in the ISO equation as

L 10 = (C/p)p
L 10 = Basic rating life in millions of revolutions
C     = Basic dynamic load rating, N
p      = Equivalent dynamic bearing load, N
p     = Exponent for the life formula
        = 3 for ball bearings
        = 10 / 3 for roller bearings

Until now we have only considered the pure radial load or the pure axial load. In practice however, both radial and axial loads may act on a bearing simultaneously. It is therefore necessary to calculate the equivalent dynamic bearing load.

This is defined as that hypothetical load, constant in magnitude and direction, which when applied would have the same influence on bearing life as the actual loads to which the Bering is subjected.

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