Industrial gears are manufactured from a wide variety of materials, including metals like steel and aluminum. However, many industrial applications rely on plastics for their strength and durability. Whether it’s the resistance to heat, chemical exposure or the ability to retain their shape while under a heavy load, many of the properties that make plastics ideal for manufacturing gears are also the reasons that have made them increasingly popular.
Plastic gear are continuing to displace traditional metal gears in a widening arena of applications. This is because of advances in both the materials and the molding process.
Unlike metals, which are manufactured by hobbing and shaping, or alternatively by milling, plastic gears can be manufactured in one step through injection molding. This method eliminates machining operations and allows for the fabrication of gears with inserts or integral designs. This means that there is an increased potential for cost effectiveness and greater functionality.
The most common polymers used for injection molded gears are acetal resins such as DELRIN*, Duracon M90 and acetal copolymers such as CELCON* and nylon resins such as ZYTEL*, NYLATRON** and MC901. These polymers can be modified with fillers to increase their performance and provide more rigidity, strength and durability. In addition, they can be formulated to provide improved resistance to temperature, chemicals, and moisture. Standardized tabular data for the physical and mechanical characteristics of these polymers is available from various manufacturers.
When compared to metal gears, polymer-based plastic gears are more resilient and less dense. This can significantly reduce the vibration and noise that is produced when gears mesh together. Additionally, the lower level of density reduces the temperature rise and allows for more efficient power transmission with reduced friction losses. Additionally, most plastics can be operated with minimal or no lubrication because they have their own inherent lubricity.
Injection molded gears can be produced with high accuracy, if desired. This is especially true when using properly designed materials, mold design, tooling and quality control. This is a major improvement over the tolerances that can be achieved with highprecision machined metal gears. It is important to keep in mind, though, that this higher accuracy does negatively impact the load capacity of the gear.
Another issue that can arise when producing plastic gears is the need to rework them once they are molded. This can include splines, keys, set screws, and plain and knurled press fits. The choice of fastening method will depend on the design of the gear, the shaft size and type, and the intended application. For example, the use of splines or keys should be avoided for heavy-duty applications. In these cases, a set screw or knurled press fit may be a better option. Moreover, it is important to ensure that the spline or key is the same material as the gear, so that there are no differential thermal expansion problems. This can be accomplished by designing the gear with radii at corners to avoid stress concentrations and promote material flow.