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In the arrangement the bearing is subjected to generally acting forces in various magnitudes, at various rotational speeds and with different acting period. From the point of view of calculation methodology the acting forces should be re-calculated into the constant load, by which the bearing will have the same life as it reaches in the conditions of the actual load.
Such a re-calculated constant radial or axial load is called the equivalent load
P or P_r (radial) or P_a (axial). 

Combined Load

Constant Load

The outer forces acting on a bearing are not changed both from the point of view of size and time dependence.

Radial Bearings

If the radial bearings are simultaneously subjected to constant forces in radial and axial directions, the following equation is valid for calculating the radial equivalent dynamic load:

P_r = X.F_r + Y.F_a [kN] 

P_r - radial equivalent dynamic load  [kN]

F_r - radial bearing load  [kN]

F_a - axial bearing load  [kN]

X  - radial load factor

Y  - axial load factor

Factors X and Y depend on the ratio F_a/F_r. Values X and Y are shown in the dimension tables or in the introduction to each bearing type where closer information regarding bearing calculation of the respective type is given.

Thrust Bearings

Thrust ball bearings can carry only forces acting in axial direction and the following equation is valid for calculating axial equivalent dynamic load :

P_a = F_a [kN] 

P_a - axial equivalent dynamic load  [kN]

F_a - axial bearing load  [kN] 

Spherical roller thrust bearings can also carry some radial load, but only by simultaneous acting of axial load, when condition F_r ≦ 0.55 F_a must be fulfilled. Axial equivalent dynamic load is calculated from equation 

P_a = F_a + 1,2 F_r  [kN]

Fluctuating Load

Real fluctuating load, whose time course we know, is for calculation replaced by mean hypothetical load. This hypothetical load has the same influence on the bearing as the fluctuating load.

Change of Load Magnitude by Constant Rotational Speed

If the bearing is subjected to a load in a constant direction, whose magnitude is changed in dependence on time and the rotational speed is constant (Pict. 2), we can calculate the mean hypothetical load F_s according to the following equation

  [kN] 

F_s - mean hypothetical constant load  [kN]

F_i = F₁,...F_n - partial actual load  [kN]

q_i = q₁,...q_n - share of fractional load effects  [%]

If in dependence on time only the rotational speed is changed, the mean hypothetical constant rotational speed is calculated from equation

 [kN] 

If the actual load has a sine behaviour (Pict. 4), the mean hypothetical load is

F_s= 0,75.F_max [kN] 

Change of Load Magnitude by Change of Rotational Speed

If the bearing is subjected in time to a varying load and the rotational speed is being changed, the mean hypothetical load is calculated from equation

 [kN] 

n_i = n₁, ...n_n - constant rotational speed in time of partial F₁,...F_n acting [min^-1]

q_i = q₁, ...q_n - share of partial load and rotational speed acting  [%]

If in dependence on time only the rotational speed is changed, the mean hypothetical constant rotational speed is calculated from equation

[min^-1] 

n_s = mean rotational speed    [min^-1]

Oscillating Motion of Bearing

By oscillating motion with amplitude γ (Fig. 5) it is the simplest way of substituing the oscillating motion by hypothetical rotation, when the rotational speed equals the oscillation frequency. For radial bearings the mean hypothetical load is calculated from the equation

[kN] 

F_s - mean hypothetical load  [kN]

F_r - actual radial load[kN]

γ - oscillating motion amplitude [°]

p  - exponent p = 3 for ball bearings

for cylindrical roller, needle roller, spherical roller and tapered roller bearings