Gears transmit movement mechanically between machine components as their teeth mesh and turn each other. Like with many of today’s components, plastic gears are increasingly taking the place of their metal cousins inside various consumer products.
This is not new technology. Historians suggest that the Ancient Greeks started using gears in hundreds of years BC, and the earliest differential gear was used by them in a type of ancient computer used to predict astronomical positions and eclipses decades in advance. It is, therefore, a testament to how useful the humble gear is that they’re still being widely used today.
Let’s explore the benefits and drawbacks of plastic gears, and the types of engineering plastics commonly used to produce them when they’re being used as internal plastic mechanical parts.
Advantages of gears made from plastic
There are some key advantages of using plastic gears over metal, which include:
- Plastic is a great weight-saving option
- Noise reduction
- Ability to act as a vibration dampener as well as absorb any shock loading that could occur in a gear train
- Plastic has a lower coefficient of friction
- Plastic gears will not rust if subjected to or operated in wet environments
- Minimal lubrication is required due to the physical properties of plastic – some plastics being self-lubricating
Disadvantages
As with every material, there are also disadvantages of using plastic gears instead of metal ones, some of which are listed here:
- Plastic is generally weaker than metal, therefore, they are not able to withstand as much load or stresses
- Dimensional tolerances are not as tight as results able to be achieved with metal
- Plastic is susceptible to change when subjected to heat or moisture which can result in dimensional shifts and inconsistent fit between gear meshing
- Injection molding tooling costs are high (if the gears are produced in this way)
- The cost of raw polymer can fluctuate more than steel making the production cost less stable over a long period of time
- Limited operation at high and low temperatures
Polymers used in their production
There are generally two types of polymer used for plastic gear production, Polyamide (Nylon) used for machined gears and Acetal used for molded gears.
Both have different features:
Polyamides (PA)
These are the ‘Nylon’ plastics that are commonly used for machined plastic gears:
PA 6 (Polyamide 6)
- More suitable for larger diameter gears,
- Has good wear characteristics and shock absorbent properties
- Has low coefficient of friction values and can be run with minimal lubrication
- Offers a quiet running gear mesh
- High operating temperatures
PA 66 (Polyamide 66)
- Has better wear characteristics than PA 6
- It is more dimensionally stable than PA 6
- Suited to larger gears
- Has low coefficient of friction values and can be run with minimal lubrication
- High operating temperatures
Here you can see some nylon gears used inside a printer mechanism, perhaps for the paper feeding mechanism:
Acetal
Acetal plastics are commonly used for molded plastic gears:
Polyoxymethylene (POM)
- Acetal gears are more dimensionally stable than nylon gears
- Has good chemical resistance and can be operated in harsh environments
- Does not absorb moisture unlike nylon, therefore is more suitable for applications that can be subjected to moisture
- Most acetal gears will require lubrication in order to provide optimal performance. If left to run dry, there is a potential for degradation and drop-off in performance eventually leading to potential failure
Here’s an Acetal gear in a lubricated gearbox:
Summary: which to choose?
If you are going to run a mechanism that does not require lubrication or is an open mechanism where lubrication is not desirable, the right option for the gears would be polyamide, best known as nylon. You would need to do your mechanical calculations for gear design to determine stresses and forces. This will guide you on the specific nylon grade, bearing in mind that nylon can be altered by the use of additives during processing to improve its mechanical properties.
If, on the other hand, you are designing a gear train inside a sealed gearbox, then you may want to consider POM, better known as acetal. When lubricated, acetal gears will potentially outlast nylon gears as acetal is more dimensionally stable over time.
As a general rule of thumb, acetal is used for when molding gears and nylon is used for machined gears.
Each of these plastics can have additives to enhance their mechanical properties, including lubrication and other elements to make them more efficient on self-lubrication.
