Industrial plastic gears are typically made from acetal and nylon. These polymers are dimensionally stable, resistant to chemicals and have lower shrinkage rates compared to other engineering plastics.
In addition, they have a much lower weight than their metal counterparts. These benefits make them a popular choice for manufacturers with cost constraints.
Material
The material used in plastic gear is a wide variety of engineering thermoplastics, most commonly acetal resins like DELRIN* and Duracon M90 or MC nylon. These plastics vary with regard to strength, rigidity, dimensional stability, lubrication requirements and moisture absorption. Standardized tabular data is available in manufacturers’ catalogs.
Some plastics can operate without a lubricant, but even those that can require constant lubrication when mated with metals. The lubrication prevents temperature rise from meshing friction and dampens vibration. Plastic gear teeth can also absorb and distribute the stress load, but the load-sharing capability may not eliminate static stresses that lead to tooth breakage. Therefore, the same calculation methods that apply to steel gears must be used for plastics. The only significant difference is the allowance for material-specific properties.
Strength
Due to their high strength, plastic gears can handle more power/torque than traditional metallic gears and therefore are preferred in many applications. They are also much lighter and have better dimensional stability.
Until recently, injection-moulded plastic gears were used primarily in lightly-loaded applications. Uncertainty about their response to temperature, moisture and certain chemicals prevented them from being considered as replacements for metal gears in more heavily-loaded applications.
However, with advances in polymers and scientific plastic moulding design, these gears are able to perform as well as metal gears in heavy-load applications. This is made possible by the use of accurate load calculations, including tooth surface stress. In addition, these gears are self-lubricating and have low heat transfer rates, so they don’t overheat or require greasing.
Durability
Using polymers like linear polyphenylene sulfides and nylon as well as thermoplastics like polyacetals, engineers can design gears that are self-lubricating. They are also able to handle much higher levels of heat and pressure than metal alloys.
These are only some of the reasons why injection-moulded plastic gears are rapidly displaceing metal ones in a wide range of applications. Advances in engineering plastic resin development, precision plastic molding and material processing have given plastic gears the ability to handle heavier loads and withstand environmental conditions that once limited their use.
Engineers designing molded plastic gears should take into account their larger coefficient of thermal expansion and moisture absorption rates. For a long gear life, they should also design teeth with large root fillets and minimize abrupt changes in cross section to avoid molded-in stresses.
Adaptability
Plastic gears are unaffected by corrosion, which makes them suitable for applications such as water meters and chemical plant controls. They also are lightweight and can withstand vibration, which helps reduce noise. In addition, plastic gears deflect to absorb shock stresses, which help spread them over a larger surface area than metal gears.
Despite this, plastic gears must be designed carefully to handle the load. The Lewis equation commonly used to calculate traditional metal gears does not apply, so engineers must learn new design principles. For example, they must determine whether the gear material will tolerate fluctuations in dimensional size or shape caused by environmental conditions such as temperature and moisture. This is especially important for molded plastics like polyacetal and MC nylon. Other plastics, such as cast nylon, have lower tolerances and require lubrication.
Self-lubricating
While metal gears must be lubricated at regular intervals, plastic ones can go for longer periods without needing additional lubrication. This saves time and money in the long run, especially for equipment that may be hard to reach for servicing.
Plastic gears have lower resistance values than AISI 8620 steel gears at nominal temperatures, but the latter have a much higher pulse decay. The reason behind the difference is that shearing causes different shear rates on the tip and root surfaces of teeth, whereas in plastic gears, this happens less often.
Custom manufacturer of gears in aluminum, acetal, cast nylon and PEEK with corrosion, moisture and wear resistance. Engineering and design assistance, prototypes and low to high volume production. Serves analytical instrumentation, sample preparation, marine, food processing and material handling industries.
