Bearing life and load ratings
The individual life of a rolling bearing is expressed as the number of revolutions or the number of operating hours at a given speed that the bearing is capable of enduring before the first sign of metal fatigue (spalling) occurs on a raceway of the inner or outer ring or a rolling element.
However, under controlled laboratory conditions, seemingly identical bearings operating under identical conditions have different individual endurance lives. A clearer definition of the term “bearing life” is therefore essential to calculate bearing size. All information presented by SKF with regard to load ratings is based on the life that 90% of a sufficiently large group of apparently identical bearings can be expected to attain or exceed.
The rated life based on the above definition has to satisfy the requisite life expectations of the bearing application. In the absence of previous experience, guidelines regarding specification life of different bearing applications are provided in table 1 and table 2.
Due to the statistical nature of bearing life, it must be pointed out that the observed time to failure of an individual bearing mounted in an application can be related to its rated life only if the failure probability, of that particular bearing, can be determined in relation to the general population of bearings running under similar conditions. For instance, if a bearing failure is observed in a bearing fan application counting a total of two hundred mounted bearings working under similar conditions, this represent a failure probability of just 0,5%, thus a reliability for the installed application of 99,5%.
Several investigations performed throughout the years regarding the failures of bearings used in a variety of applications have shown that in a very large population (several million bearings), the observed failures are a relatively rare event and not directly related to typical raceway spalling. This shows that the design guidelines based on 90% reliability and the use of static and dynamic safety factors can lead to robust bearing solutions in which typical fatigue failures are in general avoided. Indeed, observed field failures are mostly related to abrasive wear, moisture, corrosion, improper mounting, improper shaft/housing fits, skidding of rolling elements, unforeseen contamination or related to failure of the cage, of the sealing or of the lubrication system.