The advantages of plastic gears
Plastic gears vs metal gears:
Moulded-plastic gears have long provided alternatives to metal gears in lightly-loaded drives.
Due to the uncertainty around their response to moisture, temperature, and certain chemicals, their use was limited. Today, the full potential of plastic gears is being realised. Advances in polymers, scientific plastic moulding design, material processing and process control are creating plastic gears that are superior to metal gears in many applications.
As designers better understand their benefits and advantages over metal, plastic gears are continuing to displace metal gears in a widening arena of applications. Injection-moulded plastic gears are now being used in everything from food processors to windshield wiper drives, and from industrial and medical equipment to watches.
Why choose plastic gears?
Plastic offers many advantages over metal, including cost, design, processing, and performance.
- Lower cost: Generally, plastic gears are less expensive to produce than metal gears. As there is usually no need for secondary finishing, plastic gears typically represent a 50% to 90% saving relative to stamped or machined metal gears, according to Plastics Technology.
- Design freedom: Moulding plastic offers more efficient gear geometries than metal. Moulding is ideal for creating shapes, such as internal gears, cluster gears, and worm gears, where the cost for forming them in metal can be prohibitive.
- Accuracy: With consistent material quality and accurate moulding process control, plastic gears can achieve high levels of precision – now up to AGMA Quality 10.
- Larger, stronger parts: Because plastic gears can be wider than metal gears, they enable greater load-bearing capacity and more power transfer in a single stage. Thanks to better moulding controls and designs that better reflect environmental factors, plastic gear drive capacity has now been boosted from 1/4 to 10 HP.
- Corrosion-resistant: Unlike metal gears, plastic gears are immune to corrosion. Their relative inertness means they can be used in water meters, chemical plant controls and other situations that would cause metal gears to corrode or degrade.
- Lightweight: Plastic gears tend to be lighter in weight than similar gears made out of metal. The specific gravity of steel is 7.85, while the specific gravities of nylon and acetal are closer to 1.4.
- Good shock absorption: Plastic gears are more forgiving than metal because plastic can deflect to absorb impact loads. It also does a better job of distributing localised loads caused by misalignment and tooth errors.
- Reduced noise: The superior noise-dampening properties of plastics result in a quiet running gear. This has made plastics essential for the high-precision tooth shapes and lubricious or flexible materials required in the ongoing quest for quieter drives.
- Lubrication: The inherent lubricity of many plastics makes them ideal for computer printers, toys, and other low-load situations that require dry gears. Plastics can also be lubricated by grease or oil.
Limitations of plastic gears
Design engineers should be aware of the relative limitations of plastic gears as compared to metal.
Plastic gears can warp under strain. This can be a problem if the plastic needs to operate in extreme temperatures or if humidity levels change. And because plastic isn't as durable as metal, plastic gears can have their teeth worn down from repeated use – although this can be offset by adding a coating to the teeth.
Other limitations include:
- Lower load-carrying capacity
- Greater dimensional instabilities due to a larger coefficient of thermal expansion and moisture absorption
- High cost of initial mould to develop tooth form and dimensions
- Can be negatively affected by certain chemicals