Rolling 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 30per 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 50per cent or more (Fig. 1).
However, SKF is not content with making this level of progress, even though an improvement in the 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.
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 25000 revolutions per minute, elevated temperatures and aggressive lubrication. Aero engine bearings are also unusual in that the thin section of the outer race (designed to reduce weight and increase fuel efficiency) and the high rotational speed result in high centrifugal hoop stresses. The bearing's inner ring also has to withstand high hoop stresses, as this is often press-fitted on the shaft to prevent relative movement and consequent fretting. Another application in which bearings suffer due to unfavourable operating conditions is wind turbines (see panel).
Bearing steel development
Enhanced understanding of steels, improved purity and sophisticated processing techniques have resulted in the current generation of bearing steels being far superior to those previously available, especially for challenging applications such as aero engine bearings. But, of the millions of tonnes of steel that are manufactured annually for bearing applications, almost all is based on compositions developed from those first used for tool steels, containing around 1 per cent carbon and 1.5 per cent chromium, with small additions of alloying elements to improve certain characteristics or counteract undesirable side-effects caused by other alloying elements. These steels have microstructures consisting of undissolved carbides in a matrix of either mildly tempered martensite or bainite generated by isothermal transformation (heat treatment at a constant temperature). Although it would be possible to make further improvements in purity and inclusion content to improve ductility, in fact there would be little gain because other factors would then determine the steel's brittleness.
Dr Bhadeshia's team at the SKF UTC for Steels has developed the theory for solid-state phase changes to create a new steel that contains no carbides. Very closely spaced interfaces result in high hardness and also maintain the separation between the ferrite and highly stable austenite. Furthermore, the new steel is cheap to manufacture, requiring no complex processing or heat treatment, is homogeneous and can be produced in quantity. Known as superbainite, the nanocrystalline steel has slender plates of ferritic bainite, just 20-40 nm in thickness, within a matrix of finely divided, high-carbon austenite (Fig. 3). Distortion caused by phase change is minimal.
At the same time, the researchers are investigating ways to capture hydrogen at carefully designed locations so as to render it harmless. This is important because hydrogen can readily diffuse into the steel during manufacture and operation through a number of different mechanisms, and as little as one part-per-million of hydrogen can cause embrittlement in steels.
Already Tata Steel has produced around 300 tonnes of superbainite to enable testing and microscopy to be used in evaluating the progress to date (Fig. 4). This steel has proven to be very strong, have uniform ductility, no residual stresses, and is uniform in large sections. Dr Bhadeshia is eager to see superbainite bearings manufactured and put into service so performance can be evaluated, but SKF will not say when this might happen. Indeed, SKF is content to let the research proceed to see where it leads, rather than driving it with product-related goals. SKF sees the UTC for Steels as a long-term partnership with the University of Cambridge, having committed to five years but stating that university-based research is five-times better value than in-house research.
Given the track records of both SKF and the Department of Materials Science and Metallurgy at the University of Cambridge, there is every possibility that truly 'disruptive' developments will emerge from the UTC for Steels, resulting in substantial progress in the design of rolling element bearings.