Plastic gears are becoming increasingly common in a wide variety of applications. They are a great alternative to metal gears, but they have their own unique set of characteristics.
They are durable and quiet, injection molding enables low production cost, they are resistant to chemicals and heat, and they don’t need to be lubricated. But these advantages can be offset by their poor accuracy and large dimensional change compared to metal gears.
While plastic gears are much lighter than metal gears, they must be designed with special considerations in mind. These factors include their thermal expansion, affinity for absorbing moisture, and swell. In addition, they must be sized properly so they don’t over-stretch or have excessive backlash compared to metal gears.
The most common thermoplastics used for gears are Polyacetal (POM) or MC Nylon (polyamid resin). These gears are often used in computer printers and similar low-load applications where dry materials are needed. They also work well in drone technologies due to their light weight and ability to be self-lubricating. However, they are most commonly used in a variety of machinery, including food production machines, medical equipment and consumer electronics, as well as automobile and electric motor-driven systems. They run lubrication free, are quiet and can be injection molded to lower costs and enable large scale production. They also do not rust and are chemical resistant. This makes them an excellent alternative to metal gears in a variety of applications.
Because plastic gears don’t have the stiffness of metal, they tend to have more compliance and can absorb vibration, noise and shock. They are also self-lubricating, which makes them well suited for applications in wet environments such as food preparation. But it’s important that engineers understand how these properties differ from metal gears. For example, experts recommend that engineers who use plastic gears run detailed load calculations and not rely on standard tabular data. This is because the data for machining and molding processes are less stable than those for metals.
Engineers who use molded plastic gears should be aware that the material used will vary in shape and dimensional stability in response to temperature, humidity and chemicals. Popular molded engineering plastics include acetal resins such as DELRIN* and Duracon M90 and nylon resins such as ZYTEL*, NYLATRON** and MC nylon. Hytrel thermoplastic polyester elastomer (TPC-ET) and DuPont Crastin and Rynite PBT are recommended for extra tough gears in applications that require high mesh noise reduction.
Plastic gears have a built-in natural lubrication that helps them resist forces that generate heat, pressure, and resistance. This means that they require no special lubrication, saving manufacturers time and money and keeping the gears safe from contaminants that can damage metal gearing.
The chemical resistance of plastic gears is particularly valuable in industrial settings, where gears may be exposed to harsh chemicals like acid washdowns that could rust or wear out metal. iglide plastics, including polyacetal resins such as Hostaform POM and Duracon M90, and nylons such as Zytel HTN and Minlon PA resist shrinking, heat, and chemical exposure without sacrificing strength or durability.
Additionally, the elastic compliance of plastic gears enables them to absorb shock stresses. This helps to distribute stress loads over a larger surface area and reduce the risk of tooth failure or misalignment. This is especially important in applications such as water meters and chemical plant controls. They also have a lower maximum allowable load-carrying capacity, which should be taken into account during design.
Thermoplastic gears are quieter-running and offer better vibration absorption than metal gears. They also resist corrosion, making them suitable for applications like water meters and chemical plant controls where rusting would be detrimental.
But these gears are not without issues. For example, they can swell when exposed to moisture and some can have a tendency to absorb and hold heat. They can also have larger coefficients of thermal expansion and backlash when mated with metal gears.
To mitigate these problems, engineers must first develop a true picture of the environment in which plastic gears will operate. Then they must be mindful of four often unconsidered variables. These include what type of material they are choosing, who their supplier is (there is wide variation from resin suppliers to final bar stock), how the gears will be molded, and how they will be processed after molding. With these factors in mind, engineers can get the most out of thermoplastic gearing.