20.12 Metal Inserts
Injection-molded metal inserts are used in
plastic gears for a variety of reasons:
(a) To avoid an extra finishing operation
(b) To achieve greater dimensional stability, because the metal will shrink
less and is not sensitive
to moisture, is also, a better heat sink.
(c) To provide greater load-carting capacity
(d) To provide increased rigidity
(e) To permit repeated assembly and disassembly
(f) To provide a more precise bore to shaft fit
Inserts can be molded into the part or subsequently assembled. By
subsequent insertion of inserts stress concentrations may be present which can
result in cracking of the parts. The interference limits for press fits must
be obeyed depending on the material used, also proper minimum wall thicknesses
around the inserts must be left. The insertion of inserts can be accomplished
by ultrasonically driving in the insert. In this case the material actually
melts into the knurling at the insert periphery.
Inserts are usually produced by screw-machines and made of aluminum or
brass, It is advantageous to attempt to match the coefficient of thermal
expansion of the plastic to the materials used for inserts. This will reduce
the residual stresses in the plastic part of the gear during contraction while
cooling after molding.
When metal inserts are used generous radii and fillets in the plastic gear
are recommended to avoid stress concentration, It is also possible to use
other types of metal inserts, such as self-threading, self-tapping screws, press
fits and knurled inserts. One advantage of the first two of these is that they
permit repeated assembly and disassembly without part failure of fatigue.
20.13 Attachment of Plastic Gears To Shafts
Several methods of attaching gears to shafts are in common use. These
include splines, keys, integral shafts, set screws, and plain and knurled
press fits. Table 1.53 lists some of the basic characteristics of each of
these fastening methods:
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TABLE 1.53 CHARACTERISTICS OF VARIOUS SHAFT-ATTACHMENT METHODS |
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