Rolling back the years

The design improvements for cylindrical roller bearings can be traced back to the 1870s, when the first bicycle factories were established. From the early days of the industry, their energy-saving credentials were clear.

In a research project conducted in New York in 1905, WP Graham of Syracuse University compared the energy consumed by two equivalent streetcars: one was equipped with sliding bearings and the other one with roller bearings. The test showed that 52 per cent of energy could be saved when roller bearings were used.

In 1909, Dr Josef Kirner, an engineer at the Norma Compagnie in Germany, was granted a patent for his invention of a modern cylindrical roller bearing. The bearing design had crowned raceways to prevent damaging edge stresses at the roller ends.

Bearings developed in the following years were equipped with short rollers that were accurately manufactured and exactly guided between flanges. Kirner recognised the benefits of these bearings and introduced them in Germany, against strong resistance from fellow experts.
In 1914, SKF in Gothenburg, Sweden, acquired half the shares of Norma Compagnie where Kirner worker and the firm was incorporated into the SKF Group. Since then, the history of the single row cylindrical roller bearing has been intertwined with that of SKF and there have been a number of developmental milestones.

From the early days of the roller bearing industry, their energy-saving credentials were clear

The period from 1920 to 1925 is characterised by the initial recognition of the cylindrical roller bearing as an important design. Ten years later came the standardisation of the boundary dimensions of cylindrical roller bearings, which were identical to those of standard ball bearings.

Around the same time, the first theoretical descriptions of the ideal profile of rolling elements and crowned rollers with a circular arc profile emerged. Named the Lundberg theory, this formed the basis of the crowned profile of cylindrical roller bearings and finally yielded the B-roller, which had a straight profile in its central part and was slightly crowned toward the roller end.

The introduction of E-type bearings in the 1960s was a milestone in the history of the cylindrical roller bearing. The E-type bearing design enabled optimisation of the bearing cross section to incorporate a reinforced roller complement. This resulted in increased load-carrying capacity by an average of 35 per cent and an extended bearing life.

At the time, the E-type bearing was a challenge to manufacture, as roller dimensions changed and new cages were introduced. All bearings in the 2, 22, 3 and 23 dimension series were redesigned to the E-type. For the other series, only some selective items were redesigned.

1981 saw the introduction of the EC cylindrical roller bearings with modified roller end and flange contact surface. The geometry of the flange had an opened design. The roller ends were surface treated to improve the effectiveness of the lubricant, reduce friction and lower operating temperatures, resulting in less need for maintenance. The improved roller end/flange contact resulted in higher axial load capability for those bearings that have flanges on the inner and outer rings.

Several sizes of SKF high-capacity cylindrical roller bearings have been running successfully on wind turbines

Progress did not stop there. For some applications, such as the mining industry and industrial gearboxes, particularly for the windmill industry, high load-carrying capacity crucial to providing high operational reliability. SKF developed the high-capacity cylindrical roller bearing to suit these operations.

These bearings have an increased number of rollers compared with the standard model, while maintaining the same boundary and main internal dimensions. The wall thicknesses remain unchanged, in keeping with the E-type bearing design that has proved successful for 45 years.

The innovative design is in the cage development. In a high-capacity cylindrical roller bearing the cage is arranged far above or below the bearing pitch circle diameter and is ring shoulder-guided. The window-type steel cage is very strong comparable with that of a two-piece solid-brass cage, although it has more rollers.

Following theoretical studies and simulations, this design was tried and tested for a year. The final validation test took 1,000 hours. Several sizes of SKF high-capacity cylindrical roller bearings have now been running successfully for years in wind turbine applications. Inspections have supported their effective performance in these applications.

Cylindrical roller bearings have evolved steadily over the past 100 years. Recent engineering efforts in bearings are characterised by higher carrying capacity, longer life coupled with the opportunity for downsizing and significantly reduced friction. Further progress is now leading to more energy-efficient designs that support its use in a broad range of applications, as well as meeting the demands of more recent industries such as wind energy.