Plastic fantastic

The use of plastic bearings is increasingly being looked at for low cost and maintenance free applications. Justin Cunningham reports

Plastic bearings have been around since the 70's and all the major metallic bearing suppliers have, at some point, flirted with plastic bearing technology. But, Igus has never focussed it attention on anything else.

"The thing that sets us apart is the research and development we do on our materials," says Matthew Aldridge, director of Igus UK. "We run thousands of tests to get the best material for the application."

And the company is on a continuing mission to pass this knowledge over to its customers. As a result it has recently launched an on-line selection system to allow design engineers to quickly and easily see if a plastic bearing can be a direct replace for a metallic equivalent. By entering the wear-rate and the life requirement, the web-based expert system will make recommendations about the most suitable plastic alternative.

"Plastic bearings can be seen as one of the remaining black arts of engineering," says Aldridge. "In the past the selection has come from experience and estimation work.

"But we want to bring in wear rates, and other variables, so we can get the right bearing, the right price for the applications every time. We don't want a bearing to be over engineered, over priced or outlast the product by years; because that is not what the design engineer wants."

The plain bearings, or bushes, are made up of a base polymer to allow the components to be mass produced. They are then reinforced with fibres to add strength and additionally have a solid lubricant added to make the material suitable for bearing applications. The mix is homogeneous so the properties stay the same when the part begins to be worn.

As a component supplier Igus, like many companies, has seen a reduction in orders, particularly from the automotive sector. But it has found growth in the medical and food industry and is working hard to find new applications for its product. Around 80% of its current work is with new projects.

"People are surprised as they often think they will lose something when they go to plastic from a steel," says Aldridge. "But these are like for like replacements. And you actually are able to eliminate lubrication and reduce costs, and they can increase the life of a product."

The company has been able to cast a wide net and supply its bearings to a plethora of industries. It recently fitted a Rickshaw with polymer bearings on the vehicles anti-dive mechanism that prevents the vehicle toppling over during braking.

The vehicle, the weight of the passengers and the inertia forces load the bearing, which is estimated to total 3tonnes. A bronze bushing with lubrication grooves and punched holes with graphite filling at the edge was being used. But drivers reported that lubrication regularly failed resulting in the bearings quickly being worn out.

Unsurprisingly, plastic bearings biggest enemy is heat. This is defined as a PV limit; P being the pressure or load of the bearing and V being speed. The combination makes up the temperature likely to be exerted.

"Although advances in material means the plain bearings are able to handle increasingly higher temperature applications, some 250ºC, plain bearings are not designed to be used for high speed rotational applications," says Aldridge.

The newest innovation from the company, in terms of product range, is polymer ball bearings. These are being for rotational applications but are not designed to directly replace every metallic ball bearing application. What they can provide is a corrosion and maintenance free alternative. And, perhaps more importantly, they are much lower cost than metallic alternatives and as such, has been the fastest growing product range at the company. But there is an obvious thermal barrier, although the high temperature variant can work up to 150ºC.

René Achnitz, a product manager added: "The need of our customers for maintenance-free bearing solutions always lead us to develop new materials and products.

"The topic of the plastic ball bearing hasn't been exhausted. With our long years of experience in tribologically optimised plastics and our testing potentials, we believe polymer ball bearings can advance a lot further."

Justin Cunningham

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