Materials & Processes
4 Rolling element bearings have a significant impact on the energy-efficiency of most mechanisms, yet they have their limitations in terms of load capacity, speed and longevity. Jon Severn reports on recent developments that could lead to major advances in bearing steels.
Researchers make breakthroughs in rolling element bearing steels
R
olling element bearings are today found in a wide variety of products, from cars and aero engines to electric motors and bicycles. Often viewed as commodity components, rolling
element bearings are generally taken for granted - unless they fail prematurely due to incorrect specification, installation, lubrication or sealing, or because a user is sold a counterfeit product of very substandard quality. Bearing technology is relatively mature,
having evolved since, some say, Roman times, and then been developed more rigorously in the last hundred years or so. Nevertheless, manufacturers are continually seeking to improve their designs in order to stay one step ahead of their competitors. Drivers for product developments include higher speed and/or load capacity, longer life and reduced friction, with this latter factor being of particular interest to those designers seeking to improve the energy-efficiency of their products. The leading manufacturers of bearings are
certainly adept at improving their bearing designs. For example, SKF recently introduced the E2 range of energy-efficient bearings. The first E2 products launched in 2009 were single-row, deep-groove ball bearings for applications with light and normal loads. These benefit from features such as an optimised internal geometry, a new polymer cage and a low-friction grease; the result is that frictional moment is improved by 30 per cent or more compared with the company’s already efficient standard bearings,
plus the cooler running extends both grease life and relubrication intervals to reduce maintenance costs. Another product added to the E2 family in June 2011 is the double-row, angular-contact ball bearings (DRACBB) that deliver the same improvement in frictional moment, though in some cases the reduction can be 50 per cent or more (Fig. 1). However, SKF is not content
with making this level of progress, even though an improvement in the
Fig. 1. SKF’s E2 double-row, angular- contact ball bearings benefit from a reduction in frictional moment of, in some cases, over 50 per cent.
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order of 30 to 50 per cent in a particular characteristic is a substantial advance. For product innovation, SKF focuses on seven
core technologies: steel and heat treatment; non- metallic materials; sensorisation (combining sensors with bearings); tribology; modelling and simulation; lubrication; and sealing. Of course, there is some overlap between
these, so the research and development teams are encouraged to liaise with each other. In addition, sustainability and the environment are taken into account across all areas.
University research
As with rolling element bearings, the alloying of steel and its subsequent heat treatment might be considered mature technologies, with little scope for major developments. However, recent advances in computer-based modelling and simulation, in conjunction with electron microscopy, is enabling new steels to be developed with characteristics that are tailored to suit specific applications. One of the world’s leading research centres is the Department of Materials Science & Metallurgy at the University of Cambridge, and this has formed a strategic partnership with SKF in create the SKF University Technology Centre (UTC) for Steels. SKF has also established UTCs with: Tsinghua, Beijing, for non-metallic materials and sealing; Imperial College, London, for tribology and modelling and simulation; and Chalmers, Gothenburg, for sustainability and the environment. Dr Harry Bhadeshia, director of the SKF UTC
for Steels, has previously developed specialist steels for other applications (Fig. 2). For example, his work has resulted in special hard-wearing rails for use in the Channel Tunnel, and Super Bainite armour that is claimed to outperform all other types for use on vehicles and similar defence applications. Bearing components have to withstand
exceptional contact stresses as well as fatigue loading and environmental effects. And it has to be borne in mind that the strength of steel varies with temperature, strain and strain rate - all of which are important for typical bearing applications. Particular types of bearings also have their own special requirements; for example, bearings for aircraft jet engines have to tolerate vibratory stresses, bending moments, high rotational speeds in the order of 25 000 revolutions per minute,
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