Even if the operating conditions are appropriate and the high- precision rolling bearings had been installed correctly, failure may occur. The service life of high-precision rolling bearings is limited by a wide range of influences. The period of time in which the high- precision rolling bearing can perform its task satisfactorily is described as the lifetime. Material fatigue, vibration, contamination or a lubrication failure can occur within this period of time.
This does not take into account causes that can result in a failure of high-precision rolling bearings without significant advance notice. For example, this can be due to design errors, inadequate servicing, installation errors or incorrect bearing selection.
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.
Numerous factors affect the fatigue of the material, so this can only be determined statistically. The definition of a theoretical comparison value "fatigue life" therefore relates to a very large number of similar rolling bearings that run under the same conditions. The fatigue lifetime has been reached when 10% of the rolling bearings have failed. This can be related either to the number of revolutions or the running time in hours. The lifetime of the
lubricant is also crucial for high-precision rolling bearings in addition to the fatigue life. Particular rules apply for this calculation. Other parameters can also be important for the working life in the case of high-precision ball bearings that are monitored constantly. For example, these include the temperature, bearing noise or vibration.
Nominal service life
Purely in terms of a definition, 10% of a large number of high-precision rolling bearings of the same type will fail within the nominal lifetime L10 when operated under the same conditions. The nominal service life can also be given as a time value L10,h when running at a constant speed.
Dynamic load rating
The dynamic load rating C is the constant load of a high-precision rolling bearing with a rotating inner ring which achieves 1 million rotations, with 90% probability. 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.