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Equivalent dynamic bearing load

If the calculated bearing load F obtained when using the above information is found to fulfill the requirements for the basic dynamic load rating C, i.e. the load is constant in magnitude and direction and acts radially on a radial bearing or axially and centrically on a thrust bearing, then P = F and the load may be inserted directly in the life equations.
In all other cases it is first necessary to calculate the equivalent dynamic bearing load. This is defined as that hypothetical load, constant in magnitude and direction, acting radially on radial bearings or axially and centrically on a thrust bearing which, if applied, would have the same influence on bearing life as the actual loads to which the bearing is subjected (fig 1).
Radial bearings are often subjected to simultaneously acting radial and axial loads. If the resultant load is constant in magnitude and direction, the equivalent dynamic bearing load P can be obtained from the general equation

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P=equivalent dynamic bearing load [kN]
Fr=actual radial bearing load [kN]
Fa=actual axial bearing load [kN]
X=radial load factor for the bearing
Y=axial load factor for the bearing
An additional axial load only influences the equivalent dynamic load P for a single row radial bearing if the ratio Fa/Fr exceeds a certain limiting factor e. With double row bearings even light axial loads are generally significant.
The same general equation also applies to spherical roller thrust bearings, which can accommodate both axial and radial loads. For thrust bearings which can accommodate only purely axial loads, e.g. thrust ball bearings and cylindrical, needle and tapered roller thrust bearings, the equation can be simplified, provided the load acts centrically, to

P = Fa

All information and data required for calculating the equivalent dynamic bearing load will be found in the introductory text to each product section and in the product tables.

Fluctuating bearing load

In many cases the magnitude of the load fluctuates. The formula for life calculation with variable operating conditions should be applied, see section Life calculation with variable operating conditions.

Mean load within a duty interval

Within each loading interval the operating conditions can vary slightly from the nominal value. Assuming that the operating conditions e.g. speed and load direction are fairly constant and the magnitude of the load constantly varies between a minimum value Fmin and a maximum value Fmax (diagram 1), the mean load can be obtained from:

Fm = (Fmin + 2Fmax)/3

Rotating load

If, as illustrated in diagram 2, the load on the bearing consists of a load F1, which is constant in magnitude and direction (e.g. the weight of a rotor) and a rotating constant load F2 (e.g. an unbalance load), the mean load can be obtained from

Fm = fm (F1 + F2)

Values for the factor fm can be obtained from diagram 3.
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