Timken Super Precision Bearings for Machine Tool Applications

BALL BEARINGS
Shaft Fits
The main purpose of the shaft fit is to assure a proper attachment
of the inner ring to the shaft. Under normal conditions of shaft
rotation, a loosely fitted inner ring will creep on the shaft, leading
to wear and fretting. This condition will be further aggravated by
increase of load or speed. To prevent creeping or slipping, the
inner ring should be mounted firmly in place and held securely
against the shaft shoulder. However, it is important that the shaft
fit should not result in any undue tightening of the bearing. An
excessive interference fit of the bearing bore with the shaft could
result in a proportionate expansion of the bearing inner ring which
could disturb the internal fit of the bearing and lead to heating and
increased power consumption.
As a general rule, it is suggested that the shaft size and tolerance for
seating super precision ball bearings (ABEC 7/ISO P4 and ABEC 9/
ISO P2) be the same as the bearing bore. In the case of
preloaded bearings, the suggested shaft fit is line-to-line, since
an excessively tight fit expands the bearing inner ring and
increases the bearing preload, which can lead to overheating.
For example, a duplex pair of 2MM9111WI DUL bearings with
16 kg (35 lbs.) built-in preload, when mounted on a shaft that
provides an interference fit of 0.010 mm (0.0004 in.), will increase
the preload to approximately 86 kg (180 lbs.), which could result in
elevated operating temperatures.
Bore size Shaft diameter Resulting mounting fit
Max Min. Max. Min. Loose Tight
mm
in.
mm
in.
mm
in.
55.000 54.994 55.000 54.994 0.004 0.006
2.1654 2.1651 2.1654 2.1651 0.0002 0.0003
mm
in.
Table 20. Shaft fit example: MMV (ABEC 7/ISO P4).
mm
in.
mm
in.
Housing Fits
ENGINEERING
Under normal conditions of rotating shaft, the outer ring is stationary
and should be mounted with a hand push to a light tapping fit.
Should the housing be the rotating member, the same fundamental
considerations apply in mounting the outer ring as in the case of
an inner ring mounted on a rotating shaft. Contact your Timken
representative for outer ring rotation requirements.
As a general rule, the minimum housing bore dimension for super
precision ball bearings may be established as the same as the
maximum bearing outside diameter. If the bearing O.D. tolerance
is 0.008 mm (0.0003 in.), the maximum housing bore should be
established as 0.008 mm (0.0003 in.) larger than the minimum housing
bore dimensions.
Resulting
Outside diameter Housing bore
Average fit
mounting fit
Max Min. Max. Min. Loose Tight Loose Loose
mm
in.
mm
in.
mm
in.
mm
in.
90.000 89.992 90.007 90.000 0.015 0.000 0.006 0.006
3.5433 3.5430 3.5436 3.5433 0.0006 0.0000 0.0003 0.0003
Table 21. Housing bore fit example: MMV (ABEC 7/ISO P4).
Tables covering suggested shaft and housing seat dimensions for
super precision (ABEC 7/ISO P4) ball bearings are shown with part
numbers in the product pages.
To accomplish the optimum mounting condition, it is important
to follow the tabulated tolerances, except when deviations are
suggested by your Timken representative. It is equally important
that all shaft and housing shoulders be square and properly relieved
to assure accurate seating and positioning of the bearings in the
mounting.
On high-speed applications where nearby heat input is along
the shaft, it is extremely important that the floating bearings can
move axially to compensate for thermal changes. Ball bearings
cannot float axially if they are restricted by tight housing bores or
by the radial expansion of the bearing itself due to temperature
differentials. Therefore, in such cases, the suggested housing
mounting fit for the floating bearings is slightly looser than the
tabulated average fit.
Likewise, in spring-loaded ball bearing applications, the housing
mounting fit must be free enough to permit axial movement of
the bearings under the spring pressure during all conditions of
operation. The suggested housing dimensions to ensure proper
“float” of the bearings under average conditions are listed in the
product pages.
mm
in.
mm
in.
mm
in.
mm
in.
A
TIMKEN MACHINE TOOL CATALOG 89