Ongoing quest for better bearings

New developments are taking the performance of selfaligning roller bearings to new levels, as Phil Burge, communication manager at SKF, explains

Self-aligning bearings were first commercially developed in 1907 by Sven Wingquist, who went on to found SKF in the same year. His original design has long since evolved into a robust, high performance component that today accounts for around 25% of all bearings sold worldwide.

Despite its age and ubiquity, the self-aligning bearing is still being developed today. Indeed, just when many thought that the technology had been stretched as far as it was likely to go, engineers at SKF have made a breakthrough that can potentially double the operating life of self-aligning roller bearings in contaminated environments. As this type of environment accounts for about 75% of all self-aligning roller bearing applications, the new developments represent an important step forward.

The improvements have been achieved by a combination of factors: New component design, in terms of the structure of the roller and raceway geometries; the use of floating guide rings and the interaction between inner and outer raceways; plus, most importantly, developments in the mechanical properties of the Bainite steels used in the raceways and rollers.

The production process for manufacturing bearing steels has remained fundamentally unaltered for some time. A sequence of processes are used to create a material that has an optimised balance of properties derived from a combination of elements that include carbon, chromium, nickel, phosphorous or manganese. During manufacture, the bearing steel is heated to extreme temperatures, which alter its crystalline structure until it reaches a state known as Austenite. Rapid cooling then transforms the structure again, to Martensite, with the rate of cooling determining the mechanical properties of the steel. A further option is to hold the steel at a fixed temperature for a long period, which results in a far tougher material with excellent levels of hardness; this was discovered by Davenport and Bain in the 1920s and has since become known as Bainite.

Since its introduction on a commercial scale, Bainite hardening has been further developed, with modifications in the 1990s to the transformation stage of the hardening process helping to increase toughness, dimensional stability, wear resistance, surface compressive residual stresses and fatigue life.

At one point it was considered necessary to maintain a low temperature for prolonged periods during the process of Bainite transformation, to achieve optimum results. However, it has recently been recognised that once the Bainite transformation has developed to a certain stage at an extremely low temperature, it will continue to develop the desired properties in the same manner even if the temperature is increased to create additional properties. This creates a much finer microstructure that offers even greater wear resistance, yet retains the dimensional stability characteristics of existing Bainite steels.

Combining these newly modified materials with advanced manufacturing techniques and revised designs allows bearing components to be engineered to even greater levels of dimensional precision.

Among other benefits, this has resulted in refinements to the geometries of internal rollers and raceways. This has minimised bearing osculation, while giving higher contact interface tolerances and, in turn, helping bearings to run more smoothly and effectively with reduced friction and fretting corrosion.

The increased hardness now being achieved in the manufacture of Bainite bearing steel also makes the raceways less prone to indentations; when indentations are made, they are smaller, resulting in improved performance and extended life.

In practical terms, because the surfaces of the raceways are more resistant to metal-onmetal contact, it means that bearings can provide twice the service life under poor lubrication conditions of their previous equivalents. This is significant, as research has shown that 36% of all bearing failures result from the use of incorrect lubricants or poor lubrication practices, which dramatically increase metal-on-metal contact.

The use of a floating guide ring represents another important innovation in the latest design of bearing. This ensures that the correct roller position under load is maintained at all times, producing a stable operating temperature within the bearing while simultaneously reducing friction and wear. The floating guide ring also maintains the correct axial loading of the bearings while securing unloaded rollers in position.

This critical area of bearing design directly contributes to a reduction in vibration and noise levels and enables safer, more efficient operation with extended life.

Combined, these recent developments in self-aligning roller bearing technology have the potential to help OEMs and end users improve the performance, reliability and operating life of rotating systems. In particular, in demanding applications with high levels of contamination, where in the past failure rates have been proportionally higher, the advent of these recent developments will offer considerable benefits.