Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
SHAFT AND HOUSING CONSIDERATIONS
TAPERED ROLLER BEARINGS
In general, machining bearing seats and shoulders in spindles and
housings requires careful consideration of the following form and
orientation characteristics. The first four characteristics apply to
the seats of bearing rings.
• Circularity (roundness) of each seat at every cross section.
• Cylindricity of each seat. Cylindricity includes the taper,
roundness and other form characteristics of the seat.
• Coaxiality of the inner ring seats on the spindle and coaxiality
of the outer ring seats in the housing. Coaxiality includes
offset misalignment and angular misalignment between seats.
• Angularity of each bearing ring seat. This is a consideration
when an inner ring seat is tapered.
The following two characteristics apply to the shoulders
corresponding to each bearing seat.
• Perpendicularity (squareness) of each shoulder to its
corresponding bearing seat, or as a more practical measure,
perpendicularity of each shoulder to the spindle or housing
centerline established from the two bearing seats.
• Flatness of each shoulder. A practical way of assessing the
combined perpendicularity and flatness of each shoulder is to
measure the total runout of the shoulder relative to the spindle
or housing centerline. The runout of the face of the adjusting
nuts, if used, should also be measured.
The tolerances to which these characteristics should be held are
dependent upon the class, size and application of the bearing.
In general, these tolerances should be no greater than the total
indicator reading (T.I.R.) of the assembled bearing.
Some of the characteristics can be difficult to measure precisely.
The individual user may elect to measure a subset of these
characteristics (roundness and taper as an alternative to
cylindricity). The individual user must determine the degree of effort
and expense to be invested in the measurements. That determination
should be based on the intended application of the bearing and
the level of confidence in the machining process employed to
manufacture the spindle and housing.
Surface Finishes – Precision Bearings
Precision class bearings should be mounted on shafts and in
housings that are finished to at least the same precision limits as
the bearing bore or outside diameter.
Furthermore, high-quality surface finishes together with close
machining tolerances of bearing seats also must be provided. The
following tabulations give some guidelines for all these criteria.
Bearing class
All sizes C B A AA
3 0 00 000
μm
μin
μm
μin
μm
μin
μm
μin
Shaft - Ra 0.8 0.6 0.4 0.2
32 24 16 8
Housing - Ra 1.6 0.8 0.6 0.4
63 32 24 16
Table 19. Suggested tapered roller bearing shaft and housing finishes.
88 TIMKEN MACHINE TOOL CATALOG