Plastic gears are recognized for their quiet operation and rust resistance, properties that are important in the food processing, medical device and chemical processing industries, as well as in consumer applications. But plastic gears offer many other benefits, and advances in materials and manufacturing processes are closing the gap between the performance of plastics and metals in gear applications.
The most critical factor in determining the suitability of plastic gears for a particular application is the environment. Plastics are not as dimensionally stable as metals, and their strength and rigidity characteristics are heavily dependent on temperature and may be altered by exposure to water or chemicals (moisture causes many plastic materials to swell, and chemical exposure may cause them to shrink or swell, depending on the chemical and the plastic.)
Plastic gears and metal gears also differ in the type of contact they make under load. Metal gears are primarily line contact, meshing one tooth at a time. Plastic gear teeth, however, have involute surfaces that deform under load, distributing the contact pressure over a larger surface and allowing contact between neighboring teeth. This provides load distribution between gears and helps to improve the life of plastic gears in some applications, particularly those with high shock loads and relatively low continuous loads.
The lighter mass of plastic gears means they have lower inertia than their metal counterparts, which is essential for aerospace and some military applications. Most plastic gears operate without lubrication or only require embedded lubrication materials (such as graphite, silicone or PTFE). However, some operating conditions still require consideration of lubrication influences. When selecting a lubricant for plastic gears, conditions such as the gear's operating environment, load and speed should be taken into account. If the lubricant is not compatible with the plastic material, the performance and life of the plastic will be affected.
While there is a wide range of plastics suitable for gear applications, common ones include nylon, acetyl, polycarbonate, polyphenylene sulfide and polyurethane. While the addition of glass fibers can increase the stiffness and thermal conductivity of some materials, it can also reduce their fatigue resistance. Plastic gears can be injection molded for better gear strength and lower cost than machined.
