For more information you can download the document Lifetime Calculation here
Life time calculation
Even under suitable operating conditions, and even if the high precision rolling bearings have been mounted properly, a bearing may still fail. The service life of high precision rolling bearings is limited by various factors. The length of time during which a high precision rolling bearing works satisfactorily is called its “life time”. Material fatigue, vibrations, contamination or lubrication-failure may occur during this time span. In the following observations we will be ignoring the causes that can lead to the failure of a high precision rolling bearing without noticeable prior indication; these causes include faulty design, inadequate maintenance, faulty mounting, and wrong bearing dimensioning.
Fatigue
Fatigue of the material that high precision rolling bearings are made of is caused by the rising stress to the material that occurs when high precision rolling bearings rotate under load. Towards the end of the life of the bearing, “pittings” appear on the running surfaces or the rolling elements; an initial occurrence may be followed by progressive pitting of the material. Various factors are responsible for material fatigue; they can really only be covered with reference to statistics. The definition of a theoretical reference value for “fatigue life time” therefore refers to a very large quantity of rolling bearings running under the same conditions. The fatigue life time is reached as soon as 10% of all bearings have failed. The fatigue life time may be specified with reference to the number of revolutions or to the runtime in hours. In addition to fatigue life time, lubrication life time is another decisive factor in operating a high precision rolling bearing. Particular rules apply for the calculation of this factor. In the case of high precision rolling bearings that are permanently monitored, other parameters may also be crucial in determining their life time, including temperature, bearing noise and vibrations.
Basic rating life
In terms of pure definition, 10% of a great number of high precision rolling bearings of the same type, running under the same conditions, may fail within a basic rating life of L10. If a constant rotational speed is used, the basic rating life may also be specified in terms of a time L10,h.
Dynamic load rating
The dynamic load rating is the constant load of a high precision rolling bearing with a rotating inner ring for which 1 million revolutions will be achieved with a probability of 90%. It is considered as a central radial load that is constant with regard to magnitude and direction. It is based on numerous experiments and is a performance figure that may be calculated with the aid of empirical rules.
Static load rating
The static load carrying capacity of a high precision rolling bearing is defined by the static load rating C0. This is the load that a stationary high precision rolling bearing can bear with a maximum permanent plastic deformation of 1/10,000 of the rolling element diameter.
For more information you can download the document Lifetime Calculation here