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Equivalent dynamic bearing load

The information in Dynamic bearing loads can be used to calculate the bearing load F. When the bearing load fulfills 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 into the life equations.
In all other cases, the equivalent dynamic bearing load must be calculated first. The equivalent dynamic bearing load is defined as a hypothetical load, constant in magnitude and direction that acts radially on radial bearings and axially and centrically on thrust bearings. This hypothetical load, when 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

Equivalent dynamic bearing load
Perform calculation

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. Other thrust bearings, like thrust ball bearings and cylindrical and needle roller thrust bearings, can accommodate pure axial loads only. For these bearings, provided the load acts centrically, the equation can be simplified to

P = Fa

Information and data required for calculating the equivalent dynamic bearing load is provided in the relevant product section.

Fluctuating bearing load

In many cases, the magnitude of the load fluctuates. The formula to calculate fluctuating loads can be found in Calculating life 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 like 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 Mean load

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, such as the weight of a rotor, and a rotating constant load F2, such as an unbalance load, the mean load can be obtained from

Fm = fm (F1 + F2)
Values for the factor fm are provided in diagram 3.
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