GEARS & GEARBOXES FEATURE
HAVE YOU THOUGHT ABOUT specifying plastic gears?
Designers may be reluctant to use plastic gears, however material developments mean these have greater application scope than ever before. Darren Reynolds, operations director at R.A. Rodriguez (UK), explains why
P
olymer development has led to the introduction of plastic gears with
distinct performance benefits over their metal counterparts. Not only does the elasticity of the material allow them to absorb shock, vibration and reduce noise, but they have a low coefficient of friction and, in many cases, are self- lubricating. And, of course, they are ideally suited to applications in wet environments and food preparation. However, the load carrying capacity of
plastic gears is less compared to similarly sized metal gears. They are also unable to hold the same high tolerances, it can be difficult to attach them to metal shafts, and some plastics are not as dimensionally stable, so temperature and humidity can cause variation. Plastic can also be more expensive as its base constituents are
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
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 BENEFITS 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. This material 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 it is self-lubricating. Unlike acetal, cast nylon eliminates the likelihood of the raw material containing voids. Being inexpensive to produce yet durable under most environmental conditions, moulded acetal gears are 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 – such as 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.
IMPROVING THE STRENGTH While many designers are reluctant to use plastic gears due to the lower strength of material when compared to metals, 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.
R.A.Rodriguez
www.rarodriguez.co.uk
ABSOL DRIVEN
UTEL
For further information and sales contact Greg Smith on 01484 600200 or
greg@labtex.co.uk
Y NEW RANGE OF DRIVES AND VARIABLE SPEED GEARBOXES
www.labtex.co.uk DESIGN SOLUTIONS | JUNE 2018 29
ATEX compliant to EC Directive 94/9/EC Distributed in the UK by:
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