A whipped cream making attachment for a 50-W food processor uses a geartrain to cut speed from 1,800 rpm to 200 rpm at the mixing bowl. Drive and driven gears are made from high mold flow polyester, and they slide in an acetal copolymer housing. An intermediate gear is made from a wear-resistant acetal copolymer.

Molded plastic gears have long provided alternatives to metal gears in lightly loaded drives. They transmit power quietly and often without lubrication in applications such as food processors, windshield wiper drives, and even watches. They also reduce the number of parts and resist chemicals in many applications.

Previously, plastic gears were limited to ¼-hp drives because of variations in their properties and uncertainties about how they respond to environmental conditions such as moisture, temperature, and chemicals.

Today, better molding controls combined with design practices that more accurately encompass environmental factors have boosted plastic gear drive capacity to ¾-hp.

Prepare first, then design

Though plastic gears give engineers more flexibility, designing them is more complicated. Their material properties and dimensions vary with changes in environmental conditions, thereby affecting speed reducer capabilities.

Few, if any, engineers have sufficiently broad expertise to design speed reducers with plastic gears. Therefore, the first step is to assemble a team of experts consisting of at least a gear engineer, plastics engineer, and plastics supplier. Other valuable additions may include a manufacturing engineer, quality control engineer, molder, and tool builder.

The team's objective is to design each gear set for nominal operating conditions, fine-tune the design for worst-case operation, and create detailed specifications for producing prototype and production gears.

Plastics aren't metals

Before starting the design process, engineers accustomed to designing metal gears need to understand the major properties of plastics that affect gear performance. These include strength, elastic modulus, thermal expansion, moisture absorption, and friction characteristics.

Plastics have much lower strengths than metals. For example, bending strength ranges from 12,000 to 45,000 psi, depending on the specific material. This means larger gears to carry the same load. However innovative designs, such as those that split the torque between two or more gears operating in parallel, can minimize the size of a gear set.

Windshield wiper drive transmits power from a metal worm via two spur gears (worm wheels) connected to a third larger gear (not shown). The two spur gears are molded from acetal copolymer. Opposite leads on the two worm segments cancel axial forces and simplify bearing requirements. Drive torque capacity is 700 Nm at 1,860 rpm.

Gear teeth have a lower elastic modulus and mesh stiffness, so they deflect more under load. Consequently the designer generally needs to increase backlash and tip relief to prevent interference between mating teeth.

Strength and stiffness vary widely with temperature extremes and exposure to water and chemicals. For example, the elastic modulus of plastic drops as much as 60% with a 90oF temperature rise, whereas the modulus of steel remains nearly constant. Nylons absorb moisture, causing some of them to lose as much as 50% of their modulus.

Gear dimensions also change with environmental conditions. The coefficient of thermal expansion for unreinforced plastics is three to twenty times that of metals. However, certain reinforced plastics offer thermal expansions close to that of metals. The thermal effects of molding, particularly gating and cooling, affects the long-term dimensional stability of gears.

Moisture absorption causes plastic parts to swell, especially those made from nylon. Swelling reduces gear tooth clearances, causing tightly meshing gears to jam. Therefore, the designer needs to compensate by increasing clearances or by selecting a material with less swelling tendency. Some chemicals cause plastics to either shrink or swell.

Friction and wear characteristics depend largely on geometry, load, speed, surface finish, material combination used in mating gears, and environment conditions. The coefficient of friction typically ranges from 0.10 to 0.40 for plastic-on-steel and 0.12 to 0.60 for plastic-on-plastic. These characteristics tend to be poorly defined.

Using two different materials in mating, dry-running gears tends to reduce wear and noise. Conversely, using the same material in both gears gives better dimensional control.

Additives such as PTFE, silicone, or graphite improve the natural lubricity of plastics, and reduce wear. But be sure the lubricants are compatible.

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