While metal was historically the primary gear material, plastic has started to gain ground. As an alternative to metal, molded thermo plastic gear offer benefits such as lower weight and less costly materials and manufacturing processes. They also require no lubrication, which eliminates maintenance and reduces downtime in machinery. In addition, they are not subject to corrosion like their metal counterparts, which saves the end user time and money in sanding, polishing and coating activities.
The most popular plastics used for gears are crystalline resins such as nylon and acetal copolymer. Generally, these polymers have excellent fatigue resistance and can be used over a wide temperature range. For many gear applications, nylon with glass reinforcement has been successful. MC nylon with a blue color (polyacetal resin) or Duracon with a milky white color have been the choice of most manufacturers. However, there are several other resins with good mechanical properties such as polypropylene and ethylene tetrafluoroethylene (ETFE) which have gained some popularity.
Gears made of crystalline resins tend to be more resistant to corrosion than those of amorphous plastics, but the load capacity and life of gears in either type are governed by bending fatigue at the tooth root. The fatigue resistance of a plastic gear increases with the number of teeth and the load on the teeth. It is therefore important to size the gears correctly. A well-designed sized gear will achieve a high load capacity with a minimum of stress.
Unlike metal gears, which are often designed with an external ring and hub to fasten set screws to the shaft, plastic gears are typically built as a solid-body unit. This allows the design of gear geometries that would be impossible or impractical in metal, but are easy to mold in plastic. For example, a spur gear with a large ratio requires a very wide center distance that could be difficult to machine in metal, but is easy to mold in plastic.
Plastic gears are usually molded to high accuracy, which improves the strength, cost, weight and precision of the part. For injection molded gears, the Total Composite Error can readily be held to a tolerance of roughly 0.075 – 0.125 mm and a corresponding Tooth-to-Tooth Composite Error of about 0.025 – 0.050 mm with the right blend of material, mold design, tooling and quality control.
A key issue for all gears is the ability to withstand ambient temperatures and operating conditions, including moisture. Metal gears can corrode in damp or humid environments, and specialized surface treatments are required to prevent such degradation. Plastic gears, on the other hand, are impervious to moisture and resist degradation even in harsh chemical washdown environments.
A significant advantage of plastic gears is that they do not require lubrication to function, but their load capacity and life is dependent upon the dimensional changes that occur over time due to ambient temperature and climatic conditions. A housing that matches the thermal and moisture expansion of the plastic gears will help to maintain precise center distances. Moisture absorption of some plastic resins can cause them to swell, which can also stall the gear mesh and require additional adjustment. CAD tools are available that can help the designer to incorporate these parameters in their design.