Gearing Up With Plastic

By Zan Smith and Maribeth Fletcher
Mechanical Engineering Magazine

For mechanical engineers, plastic gears are a powerful means of cutting drive cost, weight, noise, and wear. Plastic gears also open new opportunities for smaller, more efficient transmissions in many products. So, what are the payoffs when using plastic gears in place of metal? Where do they make the most sense? How are they specified, and which resins are best? These questions are timely, as more engineers turn to plastic gears in higher-power, high-precision applications. Some current examples illustrate the possibilities.

When Maytag engineers designed their new washer transmission around plastic gears, they effectively eliminated the noise of steel gears. They also saved 13 lbs. and did away with 42 parts compared with a previous metal gearbox. Gears that are injection-molded from unfilled and fiberglass-reinforced Celcon acetal copolymer keep their strength and tight tolerances, even in an oil-bath transmission. They also demonstrate the long-term durability essential in an appliance expected to have a long service life.

Hewlett-Packard and molder UFE took plastic gears to new standards of manufacturing quality in the DeskJet 660 color printer. Acetal copolymer cluster gears were specified to comply with the high quality standards of the American Gear Manufacturers Association (AGMA) Quality Class Q9. The accuracy was necessary for precise paper movement to prevent "banding"—obvious skipped lines or overprinting. For 48-pitch gears, 1.25 inches in diameter, AGMA Class Q9 denotes total cumulative error (TCE) of just 0.0015-inch, and tooth-to-tooth (TTT) error of 0.00071-inch.

To improve the reliability of the "world washer" that's manufactured in several countries, Whirlpool Corp. introduced a splined clutch, or "splutch," containing a spline and gears molded in acetal copolymer. The low-wear epicyclic gear assembly lasts four times the projected life of the washing machine. It also reduces the number of moving parts by 20 percent when compared with earlier designs using metal gears.

Injection-molded plastic gears have come a long way. Historically, they were limited to very-low-power transmissions, such as clocks, printers, and lawn sprinklers. Today's stronger, more consistent engineering polymers, and better control of the molding process, now make it possible to produce larger, more precise gears that are compatible with higher horsepower. For example, Whirlpool enhanced a 3/4-horsepower drive for another washing machine with a spin gear molded in fiberglass-reinforced acetal copolymer. The molded plastic gear cost about a fifth of what the original machined metal gear cost, and made the drive lighter.

As the experience base with plastic gears has grown, computer-aided-design (CAD) tools have advanced. For instance, CAD software can now optimize plastic gear designs based on temperature, moisture pickup, and other environmental factors. Ticona developed P-GEAR, the Plastic Gear Evaluation and Research tester, specifically to characterize gear resins in different service environments. The new data will allow design engineers to predict gear performance more accurately. And better predictions mean faster, shorter design cycles, since the development phase may be approached with greater confidence...

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