SKF bearing life theory and formula
For modern high-quality bearings, the nominal or basic rating life may be very different from the actual life in a particular application. In a particular application, the service life is affected by various factors, including lubrication, pollution degree, misalignment degree, correct installation and environmental factors.
Therefore ISO 281:1990/Amd 2:2000 contains a revised life equation to supplement the calculation of the basic rating life. This life calculation method uses a correction factor to account for bearing lubrication, contamination and material fatigue limits.
ISO 281:1990/Amd 2:2000 also allows bearing manufacturers to recommend appropriate methods to calculate available bearing life correction factors based on operating conditions. SKF correction factor aSKF uses the concept of fatigue load limit Pu, similar to the algorithm used for other machine components. The fatigue load limit values are listed in the product list. In addition, the SKF life correction factor aSKF also uses lubrication conditions (viscosity ratio) and a coefficient ηc indicating the degree of contamination to reflect the operating conditions in use.
The SKF rating life equation is consistent with ISO 281:1990/Amd 2:2000
Lnm = a1 aSKF L10 = a1 aSKF (C/P)p
If the speed is constant, the life span can be expressed in terms of working hours using an equation
Lnmh = a1 aSKF 106/(60n) L10
Lnm = SKF rated life (reliability is 100-n1) %, million revolutions
Lnmh = SKF rated life (reliability 100-n1) %, operating hours
L10 = basic rating life (90% reliability), million revolutions
a1 = life reliability factor
aSKF = SKF life correction factor
= Basic dynamic load rating, KN
P = equivalent dynamic load, KN
n = rotation speed, r/min
p = exponent of the life calculation equation
3 For ball bearings
10/3 for roller bearings
1) The coefficient n represents the probability of failure, for example: the difference is between the required reliability and 100%.
In some cases, instead of millions of revolutions or hours, it is preferable to use other units to express bearing life. For example, the bearing life of axle bearings used in road and railway vehicles is usually expressed in terms of operating kilometers.