# Tolerances for tapered shaft seats

When a bearing is mounted directly onto a tapered shaft seat, the tolerance grade of the diameter of the shaft seat can be wider than the tolerance grade for a cylindrical seat. Fig. 1 shows an IT9 grade diameter tolerance, while the stipulated geometrical tolerance is the same as for a cylindrical shaft seat. For rolling bearings mounted on tapered shaft seats, SKF recommends:
• The permissible deviation for the slope of the taper should be a ± tolerance in accordance with IT7/2, and is based on the bearing width B (fig. 1). For design purposes, the tolerance value has to be expressed in degrees. The value can be determined using

The permissible deviation for the slope of the taper can be determined using

where
 Δk = the permissible deviation of the slope of the taper Vk = the permissible range of dispersion of the slope of the taper B = bearing width [mm] IT7 = the value of the tolerance grade, based on the bearing width [mm] k = factor for the taper = 12 for taper 1:12 = 30 for taper 1:30

• The straightness tolerance grade IT5/2, based on the diameter d, is defined as: “In each axial plane through the tapered surface of the shaft, the tolerance zone is limited by two parallel lines a distance “t” apart.”

• The roundness tolerance grade IT5/2, based on the diameter d, is defined as distance “t” in each radial plane between two concentric circles along the tapered surface of the shaft. In applications where a high degree of running accuracy is required, IT4/2 should be used instead.

Only dimensional and geometrical tolerances of the taper are indicated in fig. 1. To locate the taper axially, individual specifications have to be provided.
To check whether a shaft taper is within its recommended tolerances, SKF recommends measuring it with a special taper gauge, based on two saddles. More practical, but less accurate measurement methods include ring gauges, taper gauges and sine bars. For information about SKF measuring devices such as RKM, 9205, GRA 30 series ring gauges and DMB taper gauges, contact the SKF application engineering service.