Service life
If the bearings are used in ideal operating conditions, their service life is determined by metal fatigue, which means that the term "life" is the service period limited by the phenomena of fatigue.
In tapered roller bearings that have operated under clean and well lubricated conditions, as a consequence of surface stress cycles, the symptom of the end of their service life will be the appearance of pits on the surfaces.
Seeing as fatigue is a statistical phenomenon, the service life cannot be precisely determined, and is expressed as the number of revolutions that 90% of a group of similar bearings will surpass before scaling and chipping problems appear on the surfaces.
The practical determination of the service life for tapered roller bearings is calculated using the formula:
L10 = (Cr/Pr) 10/3
Where L10 = nominal service life in millions of revolutions, Cr = radial dynamic load in Newton, Pr = equivalent radial dynamic load in Newton.
For certain applications, it may be desirable to calculate the service life adjusted to other levels of reliability, for which the following formula is used:
Lna = a1.a2.a3.L10
Where Lna = service life adjusted to the characteristics of the material and non-conventional operating conditions and for a reliability of (100-n)% in millions of revolutions, a1 = correction factor as a function of reliability, a2 = correction factor as a function of material, a3 = correction factor as a function of lubrication and environment.
Load capacities
The nominal capacity of the radial dynamic load Cr is the radial load with constant intensity and direction that a bearing can theoretically support for a nominal duration of 1 million revolutions (ISO 281). Its value is determined for each bearing in the A&S Fersa® catalogues.
In most cases, the loads applicable to the bearings are a combination of radial and axial loads which, moreover, fluctuate in magnitude and direction.
Due to this, in order to calculate the service life of a bearing, an equivalent dynamic load must be calculated using the following formula:
Pr = X Fr + Y Fa
Where Fr = radial load in Newton, Fa = axial load in Newton, X = radial dynamic load factor and Y = axial dynamic load factor.
When a bearing is subject to an excessive load, or to a large impulse load which surpasses the elastic limit, permanent local deformations may be produced on the raceway surfaces.
The value that regulates this possibility is the nominal capacity of the radial static load Cor, and is defined as the radial static load that corresponds to the calculation of a reaction in the centre of the roller contact / most loaded path equal to 4000 MPa (ISO 76).
There also exists an equivalent static load, due to the variety of possibilities of load application, with the formula:
Por = XoFr + YoFr
Where Xo = radial load factor, Yo = axial load factor and Por = equivalent static radial load in Newton.
There are different restrictive factors to be kept in mind in more extreme situations such as: high temperatures, reduction due to hardness of shafts and housings, of impact, of safety, etc.