Plastic gear are increasingly replacing metal gears in industrial applications. They are easy to make, cost effective and provide a noise dampening benefit. In addition, they are often more durable and can withstand harsh conditions that would destroy metal gears.
Gears are generally made from unfilled polymers such as nylon (POM), polyphenylene sulfide (PPS) or acetal (DELRON* or Duracon M90). These materials are available in extruded rods which can be hobbed with conventional gear cutters or molded by injection molding. Injection molded gears require sophisticated design to achieve high accuracy. The design must take into account the material shrinkage, thermal expansion, lubrication requirements and backlash characteristics. The gears can also be annealed after molding to reduce residual stresses.
The choice of plastics is based on their mechanical properties, resistance to stress, heat and corrosion and cost. For example, nylon resins offer a good balance of stiffness, dimensional stability, resistance to chemical and fatigue and torsional rigidity. Unfilled nylon molded to DIN 543 standards can be made with teeth as thin as 0.066 mm or two-thirds the diameter of a human hair. For heavy loads and high speeds, gears can be reinforced with glass fiber or other high-strength polymers to enhance strength and durability.
Injection molded gears have the advantage of eliminating costly machining. However, they are more difficult to produce than die-cast aluminum or steel gears. Molding a gear requires a great deal of skill, particularly for helical gears. A mold maker must be able to design the pathway or “runner” system to ensure consistent flow of plastic into each cavity and avoid bubbles, warps and other defects.
He must also understand the operating environment of the gear so that he can engineer the gear to be resistant to brief impact loads, vibration and non-continuous temperatures which may not be accurately represented in the data sheets. Finally, he must design the gear to have sufficient rigidity and support fasteners or bearings that will hold up to vibration, shock and rough handling.
Gear designers should be careful not to overdesign plastic gears. They can operate with very little or no lubrication and have a higher degree of backlash than metal gears, which should be compensated for in the design. Also, plastics have different coefficients of thermal expansion and an affinity to absorb moisture – both factors which can affect performance.
Adding lubrication or designing the gear with solid-state lubricants can further improve performance. Molybdenum disulfide is an excellent lubricant for nylons and acetals, while graphite, colloidal carbon or other carbon-based lubricants work well for the more demanding acetal, PPS, and phenolic gears.
For plastic gears with external lubrication, the lubricant must be compatible with the thermoplastic, to avoid degradation or contamination. Plastics fortified with a solid-state lubricant are another option. This is done by blending the polymer with other additives such as molybdenum disulfide, zinc oxide, barium sulfate or titanium dioxide. This can be particularly useful for e-mobility applications where there is a focus on low NVH and emissions, lightweighting, and reduced energy consumption.