Calculation of Theoretical Life is not only the Factor of Perfect bearing

Maintenance engineers and Machine design engineers have to deal with bearings in their rotating machines a various stages, firstly the selection and after maintenance. The main criteria for selecting the bearings have been the calculation of  bearing life basic rating life formula:

L10=(C/P)e . 100000/60*N

C is the dynamic load   

P is the equivalent load on bearing

e is the exponent depending on ball or rollers

n is the RPM for constant speed machines

 L10 life is calculating, with 90% reliability, how many hours a bearing will last under a given load and speed. There is a 10% probability that at the applied load and speed, 10% of a population of identical bearings would suffer a fatigue failure, typically pitting on the raceway of the bearing. Note that this does not address failures caused by other conditions such as contamination, wear, misalignment, and improper lubrication. Average life, sometimes called Mean Time Between Failure, is about five times the calculated L10 life.

But it is important to remember that the complete bearing can be viewed as a system in which the life of each component i.e. rolling elements, raceways, cage, lubricant, and seals, when present, contributes equally and in some cases dictates the effective endurance of the bearing. Basically, the life of a bearing is determined by the life of the different components that make up the bearing system.

Lifebearing  = F(Liferings ,Lifecages ,Liferolling element ,Lifeseals ,Lifelubricant )

In addition to the bearing rating life, there are key factors you must consider when putting together the bearing specifications for an application, including:

When selecting bearings for any purpose, ultimately you want to be certain of achieving the required level of equipment performance and at the lowest possible cost. Robustness is also very important because the conditions in which your equipment is assembled, operated, and maintained may not be precisely known and may, in fact, vary over time.

Due to the wide variety of possible installation and operating conditions, it is not possible to precisely predetermine the operating life. The most reliable way of arriving at a close estimate is by comparison with similar applications.