For the sake of simplification, when calculating the load components for bearings supporting a shaft, the shaft is considered as a statically determined beam resting on rigid, momentfree supports. Elastic deformations in the bearing, the housing or the machine frame are not considered, nor are the moments produced in the bearing as a result of shaft deflection. These simplifications are necessary if you are making bearing arrangement calculations without the aid of relevant computer software. The standardized methods for calculating basic load ratings and equivalent bearing loads are based on similar assumptions.
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.
Geared transmissions
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
Additional forces arising from the type and mode of operation of the machines that are coupled to the transmission can only be determined when the operating conditions, the inertia of the drive line and the behavior of couplings or other connectors are known. Their influence on the rating lives of the bearings is included by using an “operation” factor that takes into account the dynamic effects of the system.
Belt drives
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

Vbelts = 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
Requisite minimum load
In applications where the bearing size is determined by factors other than load – for example, shaft diameter constrained by critical speed – the bearing may be lightly loaded in relation to its size and carrying capacity. Where there are very light loads, failure mechanisms other than fatigue, such as skidding and smearing of raceways or cage damage, often prevail. To provide satisfactory operation, rolling bearings must always be subjected to a given minimum load. As a general rule, minimum loads of 0,01 C should be imposed on ball bearings and 0,02 C on roller bearings. More accurate minimum load requirements are given in the product sections.
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:

Use a bearing with a smaller dimension series.

Consider special lubrication or runningin procedures.

Consider NoWear coated bearings.

Consider applying a preload.