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Dynamic bearing loads and life

The basic dynamic load rating C is used for calculations involving dynamically stressed bearings, i.e. a bearing that rotates under load. It expresses the bearing load that will give an ISO 281:1990 basic rating life of 1 000 000 revolutions. It is assumed that the load is constant in magnitude and direction and is radial for radial bearings and axial, centrically acting, for thrust bearings.
The basic dynamic load ratings for SKF bearings are determined in accordance with the procedures outlined in ISO 281:1990. The load ratings given in this catalogue apply to chromium steel bearings, heat-treated to a minimum hardness of 58 HRC, and operating under normal conditions.
SKF Explorer class bearings account among others, for improvements in material and manufacturing techniques applied by SKF and apply update factors to calculate the basic dynamic load ratings according to ISO 281:1990.
The life of a rolling bearing is defined as
  • the number of revolutions or
  • the number of operating hours at a given speed

which the bearing is capable of enduring before the first sign of metal fatigue (flaking, spalling) occurs on one of its rings or rolling elements.
Practical experience shows that seemingly identical bearings operating under identical conditions have different individual endurance lives. A clearer definition of the term "life" is therefore essential for the calculation of the bearing size. All information presented by SKF on dynamic 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.
There are several other types of bearing life. One of these is "service life", which represents the actual life of a bearing in real operating conditions before it fails. Note that individual bearing life can only be predicted statistically. Life calculations refer only to a bearing population and a given degree of reliability, i.e. 90%, furthermore field failures are not generally caused by fatigue, but are more often caused by contamination, wear, misalignment, corrosion, or as a result of cage, lubrication or seal failure.
Another "life" is the "specification life". This is the life specified by an authority, for example, based on hypothetical load and speed data supplied by the same authority. It is generally a requisite L10 basic rating life and based on experience gained from similar applications.
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