Materials • Processes • Finishes
simulation; and Chalmers, Gothenburg, for sustainability and the environment. Dr Harry Bhadeshia, director of the SKF UTC for Steels,
Fig. 2. Dr Harry Bhadeshia, director of the SKF University Technology Centre 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,
Photo: Steve Penny.
high rotational speeds in the order of 25 000 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
Fig. 3. Superbainite, the world’s first bulk nanocrystalline metal, has slender plates of ferritic bainite within a matrix of finely divided, high-carbon austenite.
Bearings for wind turbines
is proving problematic. The main locations for bearings within a turbine are where the blades mount (for pitch control), on the main shaft, within the gearbox, and where the nacelle mounts on the tower (for yaw control, to ensure the turbine faces into the wind). The precise causes of failures in these bearings are not fully understood, but bearing steels in these applications are susceptible to microbial attack, erosion and corrosion - though protection is normally provided by seals. Another problem arises when the turbine is not operating; static loads on the yaw and pitch bearing races are very high, so lubricant can be forced out of the gap
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ind power is the fastest growing source of alternative energy, yet turbine bearing reliability
between the races and rolling elements, resulting in metal-to-metal contact. If the races are deformed at the points of contact, they can then suffer further damage when the bearing is in motion (a wear process known as false brinelling).
Main shaft bearings appear to suffer premature failure due to micropitting that leads to fatigue spalling. If the micropits are caused by sliding shear instead of rolling contact, then the solution to the problem is likely to be found through mechanical design rather than materials technology. However, other factors that have been suggested as contributing to micropitting are the contact area between the raceway and the rolling element, the distance between the asperities, and the Stribeck
parameter (the ratio between the thickness of the lubrication film and the composite surface roughness). In addition, because the bearing rings for wind turbines tend to be large, they are not subjected to as much deformation during manufacture as would be the case for smaller bearings. Because the deformation can help to refine the grain structure, enhance chemical homogeneity and break up inclusions, the lack of deformation in larger bearings can leave inclusions that might contribute to the premature failure.
While new bearing steels are unlikely
to be sufficient to solve the problem of premature bearing failures in wind turbines, they could still have an important role to play. l
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