Equivalent dynamic bearing load, P
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).
If a bearing is loaded with simultaneously acting radial load Fr and axial load Fa that are constant in magnitude and direction, the equivalent dynamic bearing load P can be obtained from the general equation
|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 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 influence the equivalent load and have to be considered.
The same general equation also applies to spherical roller thrust bearings, which can accommodate both axial and radial loads.
Certain thrust bearings, such as thrust ball bearings and cylindrical and needle roller thrust bearings, can only accommodate pure axial loads. For these bearings, provided the load acts centrically, the equation is simplified to
P = Fa
Information and data required for calculating the equivalent dynamic bearing load for the different bearing types is provided in the relevant product sections.
Mean load within a duty intervalWithin each loading interval, the operating conditions can vary slightly from the nominal value. Assuming that the operating conditions, such as 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:
Rotating loadIf, 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 unbalanced load, the mean load can be obtained from
Fm = fm (F1 + F2)
Values for the factor fm are provided in diagram 3.
Peak loadHigh loads acting for short times (diagram 4) may not influence the mean load used in a fatigue life calculation. Evaluate such peak loads against the bearing static load rating C0, using a suitable static safety factor s0. → Size selection based on static load
It is possible to calculate bearing loads based on the theory of elasticity, without making the above assumptions, but this requires the use of complex computer programs (→ SKF SimPro Quick and SKF SimPro Expert). In these programs, the bearings, shaft and housing are considered as resilient components of a system.
If external forces and loads – such as inertial forces or loads resulting from the weight of a shaft and its components – are not known, they can be calculated. However, when determining work forces and loads – such as rolling forces, moment loads, unbalanced loads and impact loads – it may be necessary to rely on estimates based on experience with similar machines or bearing arrangements.
With geared transmissions, the theoretical tooth forces can be calculated from the power transmitted and the design characteristics of the gear teeth. However, there are additional dynamic forces, produced either by the gear, or by the input or output shaft. Additional dynamic forces from gears can be the result of pitch or form errors of the teeth and from unbalanced rotating components. Gears produced to a high level of accuracy have negligible additional forces. For lower precision gears, use the following gear load factors:
- pitch and form errors < 0,02 mm: 1,05 to 1,1
- pitch and form errors 0,02 to 0,1 mm: 1,1 to 1,3
When calculating bearing loads for belt driven applications, “belt pull” must be taken into consideration. Belt pull, which is a circumferential load, depends on the amount of torque being transmitted. The belt pull must be multiplied by a factor whose value depends on the type of belt, belt tension and any additional dynamic forces. Belt manufacturers usually publish the values. However, should information not be available, the following can be used:
- toothed belts = 1,1 to 1,3
- V-belts = 1,2 to 2,5
- plain belts = 1,5 to 4,5
The larger values apply:
- where the distance between shafts is short
- for heavy or peak load type duty
- where belt tension is high
The importance of applying a minimum load is greater in applications where there are rapid accelerations or rapid starts and stops, and where speeds exceed 50% of the limiting speeds listed in the product tables (→ Speed limitations). If minimum load requirements cannot be met, potential improvements are: