When dealing with rolling bearings, precision is described by tolerance classes for running accuracy and dimensional accuracy. Table 1 shows a comparison of the tolerance classes used by SKF and different standards organisations.
Most SKF super-precision bearings are manufactured to P4A, P4C or SP tolerance classes. Standard and optional tolerance classes for SKF super-precision bearings are listed in table 2.
Each product section provides information about the tolerance classes to which the bearings are manufactured.
Running accuracyThe running accuracy of a shaft system depends on the accuracy of all the components within the system. Running accuracy of a bearing is mainly affected by the accuracy of the form and position of the raceways on the bearing rings.
When selecting the appropriate tolerance class for a particular bearing, the maximum radial or axial run-out (depending on the bearing type) of the inner ring is usually the determining factor for most applications.
Diagram 1 compares relative values of the maximum radial run-out of the inner ring for different tolerance classes.
Dimensional accuracyThe accuracy of the boundary dimensions of both a bearing and its mating components is very important to achieve the appropriate fit. The fits between the bearing inner ring and shaft or outer ring and housing influence the internal clearance or preload of the mounted bearing.
Cylindrical roller bearings with a tapered bore have slightly larger permissible dimensional deviations than other types of super-precision bearings. That is because the clearance or preload is determined during mounting, by driving the inner ring up on its tapered seat.