March 2020

30 n BELTS, BEARINGS AND CHAINS Choosing bearings for extreme environments M any engineers have at least a basic understanding of rotating bearing technologies. In some cases, they have become experts as they have worked on projects over the years. Some will also have hands-on experience, perhaps replacing wheel bearings on a car or headstock bearings on a motorcycle. But what happens when you are working with bearings that must survive extreme temperatures (above 400°C or below –200°C) for hours at a time? And what about equipment that is going into in space?What do you need to consider first? Materials? Rotating masses? Temperature capabilities? Lubrication? Of the various factors that are key to specifying the optimum bearing for extreme applications, the most important is data – and lots of it. Design engineers need to think long and hard about: the environment in which the bearing will operate; the axial and radial loadings it must cope with; potential variations in these loadings at different rotation speeds; ambient temperatures; the effects of thermal expansion and contraction; protection against the ingress of materials; lubrication and life expectancy – among other factors. The list is extensive. And this is before thinking about how the bearings will be manufactured. Some of the materials used in bearings for extreme applications need specialised manufacturing processes. So the more data that is available, the better. Once these variables have been either established or forecast accurately, the importance of materials selection will come to the fore. Designers will be aware that although martensitic steels have a high carbon content which helps to deliver hardness, they come in different grades and offer different balances of advantages and drawbacks. For example, while the 440C stainless alloy provides the highest strength and hardest wearing properties, its ability to combat corrosion is compromised compared to“lower”grades such as 440A and 440B. There is no escaping the fact that many standard austenitic grades of steel offer much better corrosion resistance. It really is horses for courses. There are other materials which, for some high-temperature applications, and for projects in extreme operating environments, are even better than 440C and which were developed originally for aerospace and defence applications. Here, substantial R&D programmes have delivered real breakthroughs in bearing technology. For example, some designers are now using bearings manufactured from specialist steels such as Cronidur30, XD15NW and BG42 that can outperform 440C in almost every respect, and are used extensively in space applications where a combination of exceptional corrosion resistance and greatly increased fatigue life are critical because they translate into extended service lives. Sometimes it isn’t corrosion that designers are trying to navigate around – it is extreme heat or cold. While there are off-the-shelf bearings that can cope with temperatures of up to around 350°C, applications involving extremely high temperatures (up to 800°C) need expert understanding and specialised designs. Here, ceramic materials, or hybrid ceramic bearings using dissimilar materials, come to the fore and can cope with temperatures as high as 800°C or as low as −269°C. Once again, materials selection is crucial at the design stage, and there is a surprisingly wide choice available, including hybrid bearings with mixed ceramic/steel combinations of cages and rollers. In addition to outstanding performance in extremes of temperature, ceramic bearings bring other advantages – for example, they are good electrical insulators as well. To add even more options when choosing bearings for extreme applications, there are new lubricants available for use in unusually hot, cold or aggressive environments. Some are solid and will work submerged in all sorts of liquids as well as in high-vacuum applications, and handily, cannot be washed out with water, steam, alkalis and most chemical cleaners. Furthermore, there is a wide choice of coatings and surface treatments available for special bearings, some of them vital to ensuring survival in arduous applications. Selecting bearings for use in extreme environments or applications is a specialist subject, so it is vital to engage with experts early in the design process. Time spent here will be rewarded with lengthy, trouble- free bearing service lives. Bearings are now available that can operate reliably for long periods in extreme temperatures – as high as 450°C, or as low as –200°C – and even in the emptiness of space. Mike Page, sales manager for Oxford-based Carter Manufacturing, examines some of the technologies that are making this possible. Bearings designed to survive in extremes of heat and cold can also be protected against corrosion March 2020 www.drivesncontrols.com