Phil Burge, country communication manager for SKF, looks at how improved component design and developments in the mechanical properties of Bainite steel have enhanced the capacity, resilience and lifespan of modern self-aligning roller bearings
Although designed in 1907, the operating principle of the first self-aligning bearing remains essentially the same in today’s components because, as in the case of so many great inventions, it is hard to improve upon. Beyond that operating principle, however, the bearing has benefited from many enhancements – such as the addition of electronic functionality and reductions in weight – and evolved into an increasingly robust, high performance component that today accounts for around 25% of all bearings sold worldwide.
Currently, the most significant developments in bearing technology have been the result of innovations in component design and developments in the mechanical properties of the Bainite steels used in the raceways. These have resulted in a 100% increase in operating life over previous equivalents, in the contaminated operating conditions that account for 75% of all bearing applications. The enhancements generate less friction, enable greater load carrying capacity and increase resistance to contamination.
Bainite hardening was developed as a heat treatment for metal by Edgar Bain in the 1920s, and SKF began to use this for bearing rings during the late 1950s. By the 1990s, adjustments in the transformation part of the Bainite hardening process added toughness, high dimensional stability, good wear resistance, positive surface compressive residual stresses and long fatigue life.
Today, we are enhancing the process yet further. Existing Bainite steel processes established that maintaining a low temperature during a significantly prolonged Bainite transformation was necessary to achieve the best results. However, SKF is now acting on the knowledge that, once the Bainite transformation has developed to a certain degree at an extremely low temperature, it will continue to develop the desired properties in the same manner, even if the temperature is increased in order to add further properties. Tests have shown that the finer microstructure now being achieved in the manufacture of Bainite steel offers improved wear resistance, while maintaining the favourable dimensional stability of the present Bainite steel.
High precision manufacturing techniques have enabled bearings to be produced to precise dimensions. As a result, internal roller and raceway geometries have been refined by manufacturers in order to ensure bearing osculation is kept to its optimum, while higher contact interface tolerances help bearings to run more smoothly and function more effectively with reduced friction and fretting corrosion.
The increased hardness now being achieved in the manufacture of Bainite bearing steel makes the raceways less prone to indentations and, when indentations are made, they are smaller, resulting in improved performance and extended life. Indeed, because the surfaces of the raceways are more resistant when it comes to metal-on-metal contact, bearings can provide twice the service life under poor lubrication conditions of their previous equivalents. This is a highly significant factor when, as research has shown, 36% of all bearing failures result from the use of incorrect lubricants or poor lubrication practices.
The use of a floating guide ring represents another important innovation in modern bearing design. Maintaining the correct roller position under load ensures a stable temperature within the bearing while simultaneously reducing friction and wear. The floating guide ring maintains the correct axial loading of the bearings while securing unloaded rollers in position. This critical area of bearing design is a contributing factor in reducing vibration and noise levels and enables safer, more efficient operation with extended life.
So, while today’s self-aligning bearing would still be recognised by SKF founder Sven Wingqvist as very similar to that he himself invented in 1907, innovations continue to optimise its performance.
Improvements in the structure of the roller and raceway geometries, the use of floating guide rings and the interaction between inner and outer raceways, plus developments in the mechanical properties of the Bainite steels, have enabled the manufacture of tougher, longer-lasting bearings.
The result is ever-more powerful and efficient products.
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