Gears – The growing capability of plastic
Polymer development has led to the introduction of plastic gears with distinct performance benefits over their metal counterparts. The elasticity of the material allows them to absorb shock, vibration and reduce noise. They have a low coefficient of friction and, in many cases, they are self-lubricating. And of course, they are ideally suited to applications in wet environments and food preparation.
As with all materials though, plastic has its drawbacks too. The load carrying capacity of plastic gears is less compared to similarly sized metal gears, they are unable to hold the same high tolerances and it can be difficult to attach them to metal shafts. Some plastics are not as dimensionally stable, which means that temperature and humidity can cause variation. From a cost point of view, plastic can also be more expensive as its base constituents are often subject to price fluctuation.
That said, moulded gears substantially extend the choices open to the design engineer and the gap between what is possible with plastic, by comparison with metal, is narrowing in line with advances in material science.
The most common plastics used in the production of gears include acetal and nylon but within these groups there are many variations in material formulation that add specific performance qualities or characteristics. In most applications, acetal is used for moulded gears and nylon for machined gears.
Acetal is more dimensionally stable, is resistant to most chemicals and does not absorb moisture but it does require continuous lubrication when subjected to high loads. Nylon is less dimensionally stable and does vary in shape in some operating environments but is self-lubricating. Unlike acetal, cast nylon does eliminate the likelihood of the raw material containing voids.
Moulded acetal gears are inexpensive to produce yet durable under most environmental conditions. They are therefore commonly used in most household items such as paper shredders, ink-jet printers and home-use power tools. Applications that handle more torque and power are better suited to machined nylon gears. They are found on conveyor systems packaging equipment and industrial automation where they offer the additional benefit of noise and vibration reduction.
Nylon gears can also be introduced as a failsafe in a complex drivetrain. In the event of catastrophic failure, the sacrificial nylon gear can prevent damage to the rest of the drive, saving costly equipment replacement and potentially preventing injury.
Many designers are reluctant to use plastic gears due to the lower strength of material when compared to metals but new developments are addressing that. One of the ways to improve the strength of nylon gears is to cast the material in bar form and add some type of fibre to the mix to increase its compressive strength. For example, a gear made with 30% fibreglass fill that includes a carbon steel core can be bored, keyed and tapped for any shaft-mounting configuration, yet the outer rim has all the benefits of being a plastic gear.
So how does it compare performance wise? By setting the torque limit of a standard solid nylon spur gear at 100Nm, the maximum allowable bending strength torque for an equivalent stainless-steel hub gear with a standard nylon rim would also be 100Nm. The values are the same because the gear will fail at the tooth, so the material of the core is inconsequential.
However, the equivalent carbon-steel core gear with a 30% fibreglass fill nylon rim, will have a maximum allowable bending strength torque of 138Nm. This 38% increase in torque capacity is achieved without sacrificing any of the benefits of nylon gearing or increasing the design envelope; a clear benefit for many applications.
Thanks to these new materials and ongoing developments, plastic gears have greater application scope than ever.