Why Are Plastic Gears Used in Geared Motors?

Why are some metal gears replaced by plastic gears in geared motors? This is usually due to five reasons: noise and vibration reduction, weight reduction, cost reduction, increased lubrication and corrosion resistance, and ease of design. Using Fengtech geared motors as an example, we’ll explain the importance of plastic gears.

Reduce Noise

The engineering plastics used inside the motor have inherent damping and shock absorption capabilities, absorbing the impacts and vibrations generated during motor start-up, stopping, and load changes. This makes the gear transmission process smoother and quieter.

For example, in our motor production, to typically keep noise levels below 50 dB, replacing metal gears with plastic gears is a good solution.

Lightweight

The density of plastic is much lower than that of metal, which has a significant impact on the overall weight of the motor.

From the perspective of our motor factory’s production, this can effectively reduce the weight of goods during transportation, thereby reducing transportation costs.

For some household appliances and small devices, such as drones and handheld tools, weight control is also very important.

* The density of POM is approximately 1.4 g/cm³, while that of steel is approximately 7.8 g/cm³. For gears of the same volume, the weight difference is approximately 5 times.

Reduce Costs

Plastic gears are injection molded, allowing for mass production and reducing raw material costs.

Metal gears require additional cutting and secondary processing, typically making them much more expensive.

Lubricity and Corrosion Resistance

Self-lubricating: Many engineering plastics possess a low coefficient of friction and self-lubricating properties. This means that in some applications, they can operate with little or no lubrication, avoiding the contamination, dust accumulation, and regular maintenance associated with lubricating oils. This is crucial for food, medical, and clean environments.

Corrosion resistant: Plastics exhibit better resistance to water, chemicals, and acids/alkalis, and do not rust like metals. This makes them ideal for use in humid or corrosive environments.

The limitations of plastic gears (why not use plastic entirely?)

Plastic gears also have significant drawbacks, which prevent them from completely replacing metal gears:

Lower strength and load capacity: The strength, stiffness, and heat resistance of plastics are far lower than those of metals. Under conditions of high torque transmission, high impact loads, or high temperatures, plastic gears may deform, wear, or even break.

High coefficient of thermal expansion: Plastics expand more significantly when heated than metals, which can affect gear meshing accuracy and clearance at high temperatures.

Dimensional stability: Some plastics (such as nylon) absorb moisture from the air and expand, affecting dimensional accuracy.

Case: Taking one of our previous projects as an example, the client strongly requested that all the gears in the 50ktyz motor be replaced with metal gears to ensure the stability of the motor’s operation.

Common Engineering Plastics for Gear Motors:

POM : High rigidity, high strength, and low coefficient of friction; one of the most commonly used gear materials.

PA (Nylon, such as PA66 or reinforced nylon): Good toughness and wear resistance, but high water absorption.

PBT : Dimensionally stable and heat-resistant.

PC : High impact strength, but slightly lower wear resistance.

PEEK : High-performance plastic with excellent strength and heat resistance, but expensive; used in extreme environments.

Conclusion

The choice of plastic gears is usually a result of weighing noise, weight, cost, self-lubrication, and operating environment. They are well-suited for applications with low to medium loads, high speeds, sensitivity to noise and weight, and the need for maintenance-free operation. For demanding industrial applications with high loads, high torque, and high temperatures, metal gears remain the irreplaceable choice.

In many gear set designs, a combination of metal and plastic gears is used to balance strength and quiet operation.