Plastic gear is often used in industrial applications that require quiet operation and minimal vibration. It is also lightweight and less expensive than metal gears. However, plastic gears must be carefully designed to avoid failures and to ensure that they run lubrication free. The type of plastic, the molding process, and how the gear is sized will all have an effect on performance.
There are two main methods of making plastic gear: CNC machining and injection molding. CNC machining uses computer software to cut layers off of plastic rods to produce the desired gear shape. Injection molding, on the other hand, melts pellets of resin and forces them into a mold under high pressure to form the gear shape. The molded gear is then flash cooled and ejected from the mold as a finished part. Injection molded plastic gears generally have a higher accuracy than those machined from rod stock.
The plastics that are commonly used for molded gears are acetal and nylon. These are polymers derived from petroleum byproducts that manufacturers would otherwise discard as waste. These polymers are also able to resist corrosion, are chemical resistant, and have good tensile strength. These properties make plastic gears suitable for a variety of different applications including the drive system of office automation equipment, loaders of electric motors, and toy gears.
Because plastics are more flexible than metals, the meshing frequency of a plastic gear should be slightly lower than that of a similar metal gear. This allows the gears to absorb vibrations and shock loads. The lower meshing frequency also helps the gears to have a quieter operation.
In addition to the lower meshing frequency, a plastic gear should be sized for a larger amount of backlash than a comparable metal gear. This is due to the fact that plastics have much larger coefficients of thermal expansion and an affinity for absorbing moisture. Additionally, the tooth root stresses of a plastic gear are not as concentrated as those of a metal gear, which increases their life.
Finally, the design of a plastic gear should be careful to limit stress concentrations and avoid sharp corners. This will help to ensure that the gears do not experience a sudden crack propagation. It is also important to size the pinion appropriately. If the pinion is too small, it will not support the required load and will fail prematurely. In addition, a large gear should be sized to reduce the effects of vibrations and shock loads on the mating teeth. This will increase the lifespan of the mated pair. This is especially true if the mated gears are metal. Wear between a metal and plastic gear pair is highly dependent on the composition of the material, the temperature rise of the mated gears, the shaft speed, and the meshing frequency. Therefore, careful consideration needs to be given to the selection of the appropriate composite materials for each specific application.