Bearing answers to common questions

Plastic bearings are resistant to dirt, dust and chemicals, are self-lubricating and can endure high temperatures, heavy loads and high speeds.However, some engineers still hesitate to use them. Here, Matthew Aldridge of igus UK answers some of the more frequently asked questions

Where can I use polymer bearings? Plastic plain bearings can replace bronze, metal-backed and custom injectionmoulded 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 material property sheets with every material, detailing the temperature and pressure limits, as well as other important technical data.

How do you make polymer bearings? The standard method of manufacture is injection moulding, this process 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 onto the shaft to help lower the coefficient of friction (COF).

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 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 a 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 are now wanting 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, but 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.