A Bushing is a type of bearing in which
the axle rotates in direct contact with the interior wall of the
bearing. Bushings are usually made from a material such as Oilite®
Bronze. These bearings are microscopically porous and will hold enough
lubricating oil so that the axle actually rides on an oil film, rather
than rubbing against the inner surface of the bearing. Non-detergent
20-weight oil is usually preferred by this type of bearing.
A mounted bearing is a
ball bearing or bronze bushing mounted in a frame to make it easy to
install. Rather than having to make a hole of precise size and position
for the raw bearing or bushing, the fabricator can simply drill two
bolt holes in the framework, finesse the bearing into place, then
tighten the mounting bolts.
Mounted bearings that are not self-centering, like the gray ones above,
are very difficult to use in pairs to hold a shaft. Tiny amounts of
misalignment between the bearings causes them to pinch the shaft. This
is called binding and makes
the shaft hard to turn. Turning the hand wheel on the demo with the two
gray bearings illustrates the problem of binding. (It's hard to turn.)
Note that the gray bearings are used quite successfully in the round
belt pulley, timing belt, and chain and sprocket demos, but that they
are not used in pairs on the same shaft.
Two points define a line. An axle is a line. Therefore an axle must be
supported at two points on its length. Usually this means two bearings.
In the belt demos, only one bearing is used per shaft because (1) the
length of the bearing is long in comparison to the length of the shaft
and (2) the bearing (bushing, actually) is rigidly mounted and
mechanically stable. Hand-fabricated mechanical devices which use
bearings in pairs to hold axles, are often built using self-centering bearings. In a
self-centering bearing, the actual bearing or bushing is made in such a
way that it can swivel a bit in its housing. This motion is not
designed for continuous use, only for aligning the bearings during
installation. Self-centering bearings are rare in bore diameters below
1/2 inch. Well-stocked hardware stores will sometimes have mounted
bearings. All industrial supply houses carry them. Mounted bearings
smaller than 1/2 inch bore are specialty items. Small Parts Inc., and
SDP-SI have some selection.
When end-to-end shaft alignment is required, flexible couplings are
useful. For instance, the axle of a large direct-drive pump is always
coupled to the drive motor with some sort of flexible coupling.
Universal joints and CV (Constant Velocity) joints in cars are flexible
couplings. The flexible couplings shown in the display are a helical beam coupling, and a spider coupling. The helical beam
coupling is so named because the shape of the aluminum connecting its
two ends is a small number of interleaved ribbons, or beams wound edge
wise around the center axis of the coupling. This type of flexible
coupling flexes back-and forth, but it won't "wind up" by compressing
or stretching. These couplings are suited to servo applications because
of their torsional rigidity. The smaller flexible coupling in the
display is a spider coupling. The spider coupling is named for the
rubber spider in its middle. Spiders are shaped something like an "x"
or an asterisk, "*". The arms of the x or * are what transfers motion
from the driving to the driven side of the coupling. Being rubber the
spider is elastic and can make a mechanism bouncy or jerky if this type
of coupling is misapplied.
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Mounted bearings and flexible couplings
are available at MSC, McMaster-Carr, and other industrial suppliers.
Smaller sizes will be available from SDP-SI and Small Parts Inc.
See the sources
page for
possible vendors.
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