T-194
SECTION 18 DESIGN OF PLASTIC GEARS
18.1 General Considerations Of Plastic Gearing
Plastic gears are continuing to displace metal gears in a widening arena of applications.
Their unique characteristics are also being enhanced with new developments, both in
materials and processing. In this regard, plastics contrast somewhat dramatically with
metals, in that the latter materials and processes are essentially fully developed and,
therefore, are in a relatively static state of development.
Plastic gears can be produced by hobbing or shaping, similarly to metal gears or
alternatively by molding. The molding process lends itself to considerably more economical
means of production; therefore, a more in-depth treatment of this process will be presented
in this section.
Among the characteristics responsible for the large increase in plastic gear usage, the
following are probably the most significant:
1. Cost effectiveness of the injection-molding process.
2. Elimination of machining operations; capability of fabrication with inserts and
integral designs.
3. Low density: lightweight, low inertia.
4. Uniformity of parts.
5. Capability to absorb shock and vibration as a result of elastic compliance.
6. Ability to operate with minimum or no lubrication, due to inherent lubricity.
7. Relatively low coefficient of friction.
8. Corrosion-resistance; elimination of plating, or protective coatings.
9. Quietness of operation.
10. Tolerances often less critical than for metal gears, due in part to their greater
resilience.
11. Consistency with trend to greater use of plastic housings and other components.
12. One step production; no preliminary or secondary operations.
At the same time, the design engineer should be familiar with the limitations of plastic
gears relative to metal gears. The most significant of these are the following:
1. Less load-carrying capacity, due to lower maximum allowable stress; the greater
compliance of plastic gears may also produce stress concentrations.
2. Plastic gears cannot generally be molded to the same accuracy as high-precision
machined metal gears.
3. Plastic gears are subject to greater dimensional instabilities, due to their larger
coefficient of thermal expansion and moisture absorption.
4. Reduced ability to operate at elevated temperatures; as an approximate figure,
operation is limited to less than 120°C. Also, limited cold temperature
operations.
5. Initial high mold cost in developing correct tooth form and dimensions.
6. Can be negatively affected by certain chemicals and even some lubricants.
7. Improper molding tools and process can produce residual internal stresses at
the tooth roots, resulting in over stressing and/or distortion with aging.
8. Costs of plastics track petrochemical pricing, and thus are more volatile and
subject to increases in comparison to metals.
18.2 Properties Of Plastic Gear Materials
Popular materials for plastic gears are acetal resins such as DELRIN*, Duracon M90;
nylon resins such as ZYTEL*, NYLATRON**, MC901 and acetal copolymers such as
*
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.
