Which plastic for plain bearings? POM, PA66 and PEEK in comparison
Lars Butenschön | 20. August 2020
Once the first question “plastic or metal” has been answered in favour of plastics, one is often just as smart as before. As an apt comparison, my daily favourite question comes to mind: “What do we want to cook today?”. It would be great if the question could be broken down to “potatoes or noodles”. The agreement on “potatoes” limits the possibilities by about 50%. But then I still have to weigh things up: fried potatoes? Potato soup? Potato dumplings? Potato casserole? Potato salad? Which plastic to use for plain bearings? PEEK? Or POM? PA66 with 30% fibreglass? Or without?
Of course, in practice it is more complex and serious. The characteristics according to which the various options must be selected are of decisive technical and economic importance. This makes the decision all the more difficult. Both potato salad and potato soup fill you up. In case of doubt, the wrong plastic will be responsible for tens of thousands of customer complaints and ruined companies or even human lives. (Despite all the drama required for an exciting intro, the majority of these questions in bearing technology might, of course, be answered probably via an early squeaking somewhere in the machine)
Nevertheless, to get an idea of the differences within the most commonly encountered plastics for plain bearings, let’s take a closer look at some of them.
PA66 and POM – the most common representatives
… or to stick to the potato example: the chips and boiled potatoes. Goes with just about everything. But nothing special either. POM (polyoxymethylene) enjoys great popularity among plain bearing manufacturers and users alike. It is easy to process and work with and is therefore versatile and can be used for various types of bearings. In addition, it is very wear-resistant, dimensionally stable, reasonably temperature-resistant, quite creep-resistant and, in addition, relatively cost-effective. Plain bearings made of POM are used in many different applications. From a bearing in hand mixers to a packaging line. Users come up against the limits of the material for bearings with high loads or high temperatures. In addition, POM-based materials tend to emit small amounts of harmful formaldehyde. In most applications this is not a problem. However, POM is not used in vehicle interiors, for example. Here, emissions in confined spaces due to heat caused by solar radiation can become a problem – or at least lead to unpleasant odours.
PA66 (polyamide, consisting of 2 monomers with 6 carbon atoms each, pronounced PA-six-six) is even more stable than POM and somewhat more temperature-resistant, especially with the widespread additional reinforcement with carbon or glass fibres. Due to this, it is less easy to process. Production of large-volume parts and machining are more difficult here. PA66 is also quite cost-effective and is often used because of this and other advantages. However, PA66 shows all its strengths in terms of wear resistance, friction behaviour and mechanical strength, is therefore used frequently, and is available in a wide range of variants, especially in heavily stressed plain bearing applications. The addition of various fillers and reinforcing materials results in many different specifications and applications. Applications range from office chairs to the mounting of brake levers in mountain bikes.
PEEK – Sounds like “peak” not without reason
There are also a lot of other “higher-end” materials like PPS, PPS-U, PAI and so on. But in order not to go beyond the scope of this article, we will stay with PEEK for the time being, since this material is used quite often in plain bearings. Why is that? PEEK is a real problem solver when it comes to high temperatures above 200°C. It is extremely resistant to chemicals, offers good strength values and also good tribological specifications. The catch: the price. The pure kilo price can amount to twenty-fold the amount of simple POM or PA66. Just a problem solver. Accordingly, PEEK is used as a plastic for plain bearings where other plastics would not work and metallic bearings can only be protected from corrosion or their lubrication from dissipation at great expense.
And now? Figures, data, facts? What is better now?
“Good dimensional stability”, “extremely resistant to chemicals”. The engineer’s heart beats faster. Or does it? Of course there are also technical data to back up these statements. The problem is that this data alone is not enough to determine the best plain bearing material. (You can read why here). What is important to you for your plain bearing application? Heavy loads? Over 50 MPa? Then you better take a PA66. Preferably with fibreglass reinforcement. May it be something more then? Brand X from manufacturer Y with 35% fibreglass carries 80 MPa. But it is only “very wear-resistant”. The other is with 30% fibreglass, but is “extremely wear-resistant”. Nothing is easy here.
Spare yourself the comparisons – We will choose the right plastic for you
The best and, above all, the most cost-effective plastic for plain bearings depends on your bearing point. We will analyse the requirements resulting from the various environmental parameters and determine the right plain bearing for you in no time at all. Through decades of experience and hundreds of thousands of standardised wear tests in various scenarios, we can determine the right material for you based on actual wear results. So you do not simply use a plain bearing with “very good wear properties”, but the “most cost-effective plain bearing that works”.