20.2 Properties of Plastic Gear Materials
Popular materials for Plastic Gears are acetal resins such as DELRIN*, nylon
resins such as ZYTEL*
and NYLATRON** and acetal copolymers such as CELCON***. The physical
and mechanical properties of a these materials vary with regard to strength,
rigidly, dimensional resistance, fabrication requirements, moisture absorption
etc. Standardized tabular data is available from various manufacturers catalogs.
In general, the information and data is less simplified and fixed than for the
metals. This is because plastics are subject to wider formulation variations and
are often regarded as proprietary compounds and mixtures. Tables 1.38 through 1.43A are representative listings of physical and
mechanical properties of gear plastics taken from a variety of sources.
It is common practice to use plastics in combination with
different metals and materials other than plastics. Such is the case when gears
have metal hubs, inserts, rims, spokes, etc. In these cases one must be
cognizant of the fact that plastics have an order of magnitude different
Coefficients of Thermal Expansion as well as Density and Modulus of Elasticity. For
this reason TABLE 1 .43A is presented.
Other properties and features that enter into considerations
for gearing are given in Table 1.44 (Wear) and Table 1.45 (Poisson’s Ratio).
Moisture has a significant impact on plastic properties as
can be seen in Tables 1.38 thru 1.43. Ranking of plastics is given in Table
1.46. In this table, rate refers to expansion from dry to full moist condition.
Thus, a 0.20% rating means a dimensional increase of 0.002 inch per inch. Note
that this is only a rough guide as exact values depend upon factors of
composition and processing, both the raw material and gear molding. For example,
it can be seen that the various types and grades of nylon can range from 0.07%
to 2.0%.
Table 1.47 lists safe stress values for a few basic plastics
and the effect of glass fiber reinforcement
| TABLE 1.38 PHYSICAL PROPERTIES OF PLASTICS USED IN GEARS | ||||||||||||||||
|
Reprinted with the permission of Plastic Design and
Processing Magazine; see Raf. 11.
________________
*Registered trademark. E.I. du Pont de Nemours and Co., Wilmington, Delaware
19898.
**Registered trademark, The Polymer Corporation, P.O. Box 422,
Reading Pennsylvania, 19603 ***Registered trademark, Celanese Corporation, 26
Msin St., Chaitham, N.J. 07928
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