Ceramic bearings: Demand grows

Hybrid ceramic bearings have proved so effective in challenging applications that ranges have expanded to serve ever-more demanding environments, as Phil Burge, country communication manager at SKF, explains

It was the need to provide bearing solutions for high performance gas turbines that kicked-off the development of ceramic rolling elements. However, as their combination of strength and lightness has become more widely appreciated, ceramic rolling bearings have been applied to more common applications, such as machine tool spindles, electric motors and generators.

One reason behind the success of ceramic ball bearings is that they offer a higher speed capability than standard steel bearings and typically provide a longer service life than all-steel bearings. They also offer a high degree of resistance to high temperatures; the operating conditions for the main shaft bearings within a jet engine include shaft speeds in excess of 30,000 r/min and predicted bearing temperatures higher than 650°C.

Ceramic ball bearings are proven to withstand these conditions above steel bearings and, indeed, where gas temperatures rise above 1100°C, only high-performance ceramic materials are considered.

However, there is a further quality to consider that has made ceramic rolling bearings the component of choice in many applications, and that is the capability to insulate against electrical current. When stray currents pass across bearings, it leaves behind a fluting pattern and a darkened appearance on the raceways. The excessive noise that results from this damage causes premature aging of the lubricant and early bearing replacement. In contrast, the natural insulating properties of ceramic material eliminate this type of damage.

One cause of stray currents is an increasingly popular device used to enhance efficiency: the variable speed drive. In motor installations, bearings can provide the path of least resistance for electrical currents. To high frequency currents, the lubricant film in a rolling element bearing acts as a capacitor.

A voltage difference builds up between the bearing casing and the rolling elements until it becomes strong enough to overcome the insulation provided by the lubricant. At that point, discharge occurs. As a result, heat generated by the discharge causes local melting of the bearing metal surface.

However, bearings with ceramic rolling elements eliminate the electrical pathway through the motor bearings, providing efficient electrical insulation between stator and rotor.

More recently, hybrid rolling bearings, comprising ceramic balls and a split inner bearing ring, have been introduced to an expanding range of applications, including rail and renewable energy. Despite the fact that the cost of a hybrid bearing is typically higher than that of a standard steel bearing the difference is soon recovered in maintenance savings, which would otherwise be caused by problems such as stray electrical current damage.

Hybrid ceramic bearings have proved effective in extra-large bore sizes to combat the dangers of stray electrical currents in a demanding application: wind turbines. There is always pressure to minimise maintenance visits to remote and hard-to-access wind turbines, where premature bearing failure, generator breakdown and unplanned turbine downtime can prove extremely costly.

This pressure is relieved by hybrid ceramic ball bearings, which are now available in sizes specifically designed to fit these turbines. These robust hybrid bearings can measurably increase machine lifespan, while providing a highly efficient performance.

Without regular and correctly measured lubrication, standard bearings wear and fail prematurely, but hybrids require less lubricant and therefore the margin of error is wider. Hybrid bearings run cooler and can operate with thinner lubricant films, so there is less aging of the grease and required relubrication intervals are longer.

With so many benefits to offer, ceramic rolling bearings and hybrid rolling bearings are enabling operators in an increasing range of applications to decrease downtime and increase profitability.