Building Better Bearings
Ben - New materials are always needed to perform specific jobs and even the tiniest improvement in properties can mean the difference between an engineering success and an engineering failure, or it can make a previously expensive project economically viable. We are joined now by Dr. Alan Begg from the Swedish company, SKF. Now Alan, presumably, you rely on these sorts of step improvements all the time to make projects better, to make new products, and to remain competitive.
Alan - Absolutely. SKF is the world's largest bearings company. We've been established for over 100 years now. We were established on the back of an innovation when our founder invented what's called the self-aligning ball bearing, and SKF has led all of the developments, or most of the big developments, in the various industry ever since. You use bearings from the tiniest things that go round to the very biggest. Dental drills or disc drives in computers have bearings, right up to windmills, which I guess are topical at the moment. I think our latest bearings for the new big windmills that are being developed are nearly 4 meters in diameter.
Ben - Wow! When I think of bearings, I think of the constituents of my bike that I occasionally have to get replaced, I have to grease up. And those seem to basically just be tiny steel balls. What properties do you actually need in a metal that's going into a bearing?
Alan - Well, the steel balls are how most people think of bearings. And although SKF's name is associated with steel balls, we don't actually make our own steel balls anymore. They're pretty much commodity materials. The clever bits of a bearing are actually the two rings - the inner and the outer race ways that the balls rotate in. Basically, any bearing is a set of rolling elements separating those two inner and outer rings rotating to allow the inner and outer rings to rotate as freely as possible. The raceways, the inner and outer rings, the surface where the rolling element touches the raceway, can be incredibly highly loaded.
I was a metallurgist myself once, I actually studied metallurgy here at Cambridge and I remember being told then that steel was getting pretty close to its theoretical strength. The theoretical strength of steel, if you calculate it just in terms of the strength of a bond is about 20 gigapascals and I was told when I was studying material science here in Cambridge that we were getting to about 10% of that strength. As long as you did it in thin steel wire ropes, you could get to about 2 gigapascals. I now look at bearings and for bearings a 2 gigapascal load is nothing! In fact, the compressive stress that's put on a bearing at the point of contact where a ball touches the ring, routinely, it's about 4 gigapascals and at extreme conditions, it can be getting close to 8 gigapascals. Not for very long times, but it will stand it for a little while.
Ben - And so, what's changed? Why has the future that we saw a few years ago turned out so differently?
Alan - Well, people like Harry have been developing wonderful new grades of steel. I'd really like to give credit to Harry because when I joined SKF, about 5 years ago, I was quite surprised how much we relied on steel, and how little work was going on across the world to really generate new performances in steel. That's one reason why I really wanted to setup a major cooperation with Cambridge University and with Harry. SKF have setup what we call a University Technology Centre in Cambridge to exploit, amongst other things, super bainitic steels, but to also build our fundamental understanding of what really happens within a steel at that critical point of contact.
And we have other university technology centres, another one in the UK with Imperial College in London on what we call tribology. This is the study of the point of contact, the friction, wear and lubrication at that point of contact. We have some university centres in Sweden as well, right up in the north of Sweden, in Lulea on condition monitoring because we provide sensors and things with our more expensive bearings to actually tell you what's going on within them and start predicting potential problems that are going to arise either within the bearing or in the rotating equipment around the bearing. We've always described the bearing as being at the heart of any rotating plant. By adding condition monitoring you can think of it as the brain as well.
Ben - And going back to today's topic, super bainite, what is it about super bainite? What is it about these properties that makes it so attractive for you?
Alan - What we need in a steel is what's called fatigue resistance. A bearing, that rolling element rolls round and round, and round. So if you imagine any element on the ring underneath that point of contact, it's continually getting stressed and then relaxed, and stressed and relaxed over and over again. And when you do that to a piece of metal, you get what's called fatigue damage. Under this compressive load, you get what's called rolling contact fatigue damage, and that's quite a difficult thing to prevent happening. That's the way the bearing's loaded.
We've looked at many, many different microstructures created in many, many different ways and super bainite seems to us to be the first real step change in thinking in steel for a very long time. Will it give us all we want? I don't know. We're still very much at the stages of examining it. Are there still some interesting challenges? Yes, I think there are. One of them is that the structure has some slightly unstable nature about it, that some of the high temperature form of steel is left in between those very, very fine nanoscale structural parts and if that transforms during loading, it actually leads to a slight increase in volume. Now the intriguing bit is, first of all, will it happen? But if it does happen, could this growth, could this expansion, actually compensate for wear? Could we make a material that, as it wears down, it grows up and compensates in some ways for the wear? So, there's all sorts of intriguing science still to be done in super bainite. But as I say, as a material, it's got the right kind of hardness, strength, and toughness that we're looking for in a bearing steel. And so, we're very interested to see if we can exploit its properties to make stronger, better bearings for our vessel.