Plastic gears are slowly gaining popularity in industrial applications due to their numerous advantages over metal ones. These include being lightweight, non-rusting and quiet as compared to metal gears. They also do not require lubrication to function, which saves on cost and time. However, the decision to choose a plastic or metal gear should mainly be determined by the intended usage of the finished product. Despite the benefits, there are some situations where a metal gear is still a better choice.
The physical properties of gear plastics vary widely, and these variations make a significant difference in a gear’s performance. Standardized tabular data is available in most manufacturers’ literature, but this information should always be used with caution. It may be misleading since many of the standardized tables are based on a limited number of selected resins and their blends, which have varying fatigue life, load-carrying capacity, temperature resistance, lubrication characteristics and dimensional stability.
A critical consideration in designing a plastic gear is to ensure the resin selected has good properties for your application. Inexpensive commodity resins may not offer the strength, dimensional stability, temperature resistance and lubrication characteristics required for quality plastic gears. Engineering resins, such as acetal and nylon, are typically recommended. These resins are formulated with fillers, additives and processing conditions that provide the required performance for working gear applications.
Another consideration in plastic gear design is to take into account the nature and process of molding, which has a significant effect on the molded part. This is particularly true when the gear is a hollow one, such as a hub or rim. Injection molded plastic gears typically shrink during cooling, which can cause differential shrinkage between different regions of the gear. This can result in stress concentrations and a reduction in the flow of plastic material around sharp corners. To avoid these problems, corners should have radii to help spread the stresses and improve flow.
Other factors in the decision to use plastic or metal gears in a particular application are the speed of production and cost. For example, CNC machining is the fastest option for producing small quantities of plastic gears, whereas injection molding can produce large volumes with a low unit cost.
Injection molded plastic gears that are properly designed, manufactured and assembled can have very high accuracy. For example, a 48-pitch cluster gear made of acetal copolymer has been engineered to AGMA (American Gear Manufacturers Association) Quality Class Q9. This gear has a Total Composite Error of less than 0.075 inch and a tooth-to-tooth composite error of just 0.0015 inches. Even higher accuracies can be achieved with proper engineering, material selection and injection mold design. In addition, proper machining and tooling techniques can reduce the tolerances to the same level as traditional metal gears.