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Bearings
Application Knowledge Menu
Bearings
and Bushings Products and Supply
Press fit or shrink fit bushing design and
installation is a common method of retaining bearings by
use of interference between the bushing and the
bushing hole. Bushings are available in standard sizes
with both inside and outside diameters surfaces finished,
and are commonly available in many sizes.
Typically, bushings are designed and
manufactured 0.002 to 0.003 inch over nominal on outside
diameter sizes for sizes 3 inches or less. Diameters
greater than 3 inches, the outside diameters are designed
0.003 to 0.005 inch over the nominal size. Because
interference material is built into the bushings, and the
bushing is normally manufactured to tolerances of +/-
.0002. The effective interference of the press fit is
designed and facilitated within the target hole which the
bushing is installed.
The process of press or shrink
fitting the bushing into the hole, causes the bearing
reduce in size due to the compressive forces applied.
Additionally, the material which the bushing has been
pressed into distorts some as well. In general, this
diameter changes are equal to 70 to 100 per cent of the
amount of the material interference or fit. Unfortunately,
precisely or mathematically predicting the size change due
to the effective compressive forces applied during
installation has not been shown to be accurate. Typical
installation compression forces and effective diameter
changes may be modeled or characterized by fabricating
several test cases and measuring the range and variations
of the actual fit. Often, bushings are designed with
additional diametrical material requiring a post press fit
installation machine process to accurately ream to the
desired diameter.
Shrink fits are facilitated by chilling
the bushing using a suitable median, such as liquid
nitrogen, dry ice, or placing the bushing within a
freezer. Cooling down the bushing is preferred rather than
heating the target housing. Normally, the bushing is of
significantly less mass than the target housing, though
heating the housing and simultaneously cooling the bushing
is no uncommon .
When a bushing or bearings are pressed
into a housing, the force driving the bushing / bearing
should be applied equally or uniform to the end of the
bearing to avoid galling, peening or damaging the bushing
/ bearing. The mating surfaces of both the bushing and
housing should be thoroughly cleaned and imperfections as
well as burrs removed.
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