There is not much to think about when designing a plain bearing – or is there?

Such is the nature of the plain bearing – a piece of material to wrap around a shaft – that engineers can become familiar and reuse a particular part.

“You have a commodity element where people are used to using these types of bearings and they know where to use them,” said Paul Mitchell, MD of Bowman International. “It’s when you have someone who’s maybe not used to using it or the application is something they’re not designed for, that’s when they need to ask about the right material or bearing style for that application. There are many more options to go for in plain bearings than there are for rolling elements. It’s more important that you get the right one for the right application.”

The problems come when the same bearing is being relied on when the applications start to become more demanding – the loads get bigger or the speeds get faster.

Mitchell observed: “There’s a limit to that bearing. You use it in an application that works, and then another one. But then you start to push the boundaries and you can get problems.”

Problems in the bearing represent problems in the system. It is quite often the most important part of a system in as much as it is the part that keeps everything running.

“As we are developing materials or developing people’s knowledge of applications, they can be the better option [compared to roller bearing] as well as maybe a more cost-effective option,” added Mitchell.

The guiding principle for an engineer is that the bearing is the wear element – the shaft must be made of a harder material than the bearing. It is therefore important that before the bearing is specified the shaft material and its properties are known. Having a bearing material made of the hardest available material is going to be counterproductive if it results in the shaft being damaged.

The main parameter when specifying plain bearings is the pressure velocity ratio (PVR), the relationship between load and speed, which can give an indication of the wear that is likely. It does not tell the whole story however, as Mitchell explained.

“There are all sorts of lifetime calculations which, because you’ve got a shaft that hasn’t been rated by ourselves and might be of different materials, is running against the bearing and there’s a huge variable,” he said. “So it’s educating customers to use the right shaft, it’s as important as the bearing to get the shaft material right. After that we primarily look at the load and speed, the type of load – is it an intermittent load, is it a continuous load, is there a shock load?There’s all sorts of factors within the load and with speed as well.”

Other factors that come into consideration are the coefficient of friction and the lubrication in the system.

Getting it right

It is clear from the above that no single bearing technology is going to be suitable for all applications. In fact, despite the company’s claims that its latest BowMet bearings are ‘the biggest advance in plain bearings in over 70 years’, Mitchell admits it is not the best solution for every application.

However, it is significantly harder than ordinary bronze bearings and can accommodate four or five times the loading. The ToughMet material that these bearings are made from is a spinodal bronze, a structure that allows a very high nickel content of 15%. Most bearings have a PV of between 1.5 and 3, bronze is about 2. ToughMet is at 9.6.

“It is very useful in high-load, low-speed applications,” claimed Mitchell. “For example in construction, mining, marine, on crane systems, on diggers, on conveyance systems. There’s lots of applications in these of high stress applications because it’s harder, it’s very resistant to ingress of materials. You can imagine if you’ve got sand or coal dust getting into the system, it’s going to be very abrasive and destroy the bearing in no time. So if you’ve got a material that’s significantly harder, more resistant, but still has the bearing properties that a bronze-based material gives you, then it has to be an advantage. And it’s proven to be.”

There are other options that broaden the capabilities of the bearing. As an example Bowman’s Oilite range that has oil impregnated into the structure of the surface to act as a ‘lubrication for life’.

Mitchell added: “We have other bearings where you have PTFE coatings, acetyl linings, bronze coatings – all manner of different bearing materials which can be all the way through the thickness of the bearing or lining.

“But we tend to look at the base materials as much as anything. If we can improve those base materials, it puts less reliance on other lubrication or surface treatment innovations because they tend to be only adding very small percentages and they tend to be even more application-specific as well.”

Such have been the recent developments in plain bearings, Mitchell said that they have increasing applications in place of the more sophisticated roller bearing.

“That’s a market for looking at when using plain bearings can improve the design and improve the way that it can functions,” he concluded. “On top of that there’s always developments in, say, packing composites and things like that. There’s people coming up with new materials and then there’s supplementing all of those with new technologies like additive manufacture and laser printing, and we’re looking at those as well within our designs.”