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The ten things you most wanted to know about plastic bearings

01 February 2012

Over the past twenty years or so, igus UK has been asked many questions about its plastic bearings. Matthew Aldridge has sifted through them and picks ten of the more pertinent queries from engineers considering a polymer alternative to the more traditional metal bearing

For many years, engineers have trusted in the robust and reliable qualities of metals such as stainless steel and bronze to fulfil their bearing needs, with regular maintenance to prevent premature failure being paramount. This is especially true for bearings used in high cycle operations, where periodic lubrication and cleaning, as well as the occasional adjustment, are needed to minimise the effects of wear.

Over and over again, the main reasons cited as the cause of bearing failure are lubrication-related. Whether it is because of improper maintenance or contamination of the lubrication system, bearing failure often leads to significant periods of machine downtime – resulting in losses in production.

Plastic bearings have been around for a long while now; they are resistant to dirt, dust and chemicals, are self-lubricating and can endure high temperatures, heavy loads and high speeds. The only problem is that some engineers hesitate to use them in their designs because of the experience they have with the more traditional materials. As a leading provider of plastic bearings in the UK, igus is no stranger to these doubts and misgivings; so here, in a nutshell, are ten of the most common queries this manufacturer has received from engineers over the years, together with a response from the igus engineering team to help you with your decision:

Why would I consider using polymer bearings?
Plastic plain bearings can replace bronze, metal-backed and custom injection-moulded bearings in almost any application. Their resistance to dirt, dust and chemicals make plastic bearings a 'fit-and-forget' solution.

What is a polymer bearing made of?
In general terms, a polymer bearing material would consist of at least three components, a base polymer, blended with a solid lubricant and reinforced with a fibre. However, some bearings contain materials that have twelve separate components, so it can become extremely complex, which is why purchasers should always insist on full property sheets with every material, detailing the temperature and pressure limits, as well as other important technical data.

The standard method of manufacture is injection moulding, a process that allows mass production at low cost, and involves very little manual labour. It is also possible to have stock bar options available for popular blends and this allows the bearings to be machined as well as moulded, which is something we have increasingly seen.

How are igus bearings self-lubricating?
The solid lubricant is embedded in millions of tiny chambers in the polymer material. During operation, the bearing transfers lubricant to the shaft to help lower the coefficient of friction.

What is the biggest benefit of polymer bearings?
They don’t ‘wear out’ in the conventional sense, and they almost never suffer catastrophic failure. Instead, their life is determined solely by the amount of wear that is acceptable before replacement is advisable, a commonly adopted figure for plain bearings being 0.25mm. Note that even when this end-of-life condition is reached, the bearing has not failed; it is still working but with greater clearance.

What lead time can I expect?
Polymer bearings, especially when they are replacing traditional metal-based plain bearings, are usually needed quickly so you should not expect a long lead time at all. As an example, our aim is to deliver within “24 hours or today” for our main catalogue items, which is a standard range of parts that starts at 1mm, and goes all the way through to 150mm diameter. 

What help should I expect on which bearing to use in a specific application?
All your supplier should need is data on the load, speed, shaft material and temperature, then they can provide a detailed analysis of the application, and give a precise calculation of the bearing life. You should expect this service free of charge.

What if I only need a small quantity of parts?
This should not cause a problem as you should not expect a minimum order for catalogue parts, so if you need just one part, then you only need buy one part. For special parts, igus has two low-volume options. Firstly it can machine parts from stock bar to a customer drawing. Secondly, using a 3D CAD model from the customer, igus can manufacture a rapid tool, known as ‘speedigus’, which has no minimum order.

What quality accreditations should I look for?
Depending on your application you should, at the very minimum, want your supplier to be accredited to ISO 9001:2000 and for automotive applications the automotive standard TS16949. In addition to ISO9001:2000 many people now want the manufacturing facility they are dealing with to meet the environmental accreditation, ISO14001.

Should I get a sample?
Yes, if you need a sample of a standard catalogue part for technical evaluation then you should ask for free of charge samples.

What is the cost of a polymer bearing?
The cost varies considerably - literally from a couple of pence to hundreds of pounds. If, for example, a customer is buying hundreds of thousands of 1mm diameter iglidur G bearings, then the price would be just a few pence. However, if the customer needs a customised linear system, machined to a special drawing and fitted with a motor, then the price can run into hundreds of pounds. 

If, after reading this article, you still have questions about plastic bearings, then the igus UK team is there to help.

Matthew Aldridge is bearings product director at igus UK

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