Stray electrical current can rapidly reduce bearing lifespan and inflate maintenance costs. If you are concerned about the effects of electrical discharge, it may be time to start investing in ceramics, says Phil Burge, Communication Manager at SKF.

Ceramic ball bearings were first used commercially in the 1960s and 1970s in high performance applications such as jet engines, where their combination of strength and lightness offered significant advantages. Today, following steady development, ceramic ball bearings are found in a wide range of equipment, from F1 engines to machine tools.

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. In each case, it is the characteristics of the ceramic or silicon nitride materials that are so attractive to engineers.

Demonstrating these characteristics in real-world conditions depends on extensive testing of ceramic balls, which in turn supports further product developments; for example, a detailed understanding of factors such as bearing surface properties, endurance and rolling contact fatigue has led to the recent introduction of large ceramic balls over 47mm in diameter, which are now being adopted in wind turbine generators.

Hybrid ceramic ball bearings have a higher speed capability than standard steel bearings and will provide longer service life than all-steel bearings in most applications. They are also excellent electrical insulators. When electrical current passes across bearings, a washboard or fluting pattern appears on the raceways, in addition to a darkened grey appearance. This damage usually results in excessive noise, which subsequently requires the bearing to be replaced. Besides the surface damage, premature aging of the lubricant also occurs. In contrast, the natural insulating properties of ceramic material eliminate this type of damage.

Stray currents inside motors can come from a number of sources, including rotor asymmetries, unbalanced windings and variable speed drives. The latter are increasingly popular and therefore a growing cause of stray currents. In many motor installations, the bearings provide the path of least resistance for stray currents; the lubricant film in a rolling element bearing acts as a capacitor, and when the voltage difference that builds up between the bearing casing and the rolling elements overcomes the insulation provided by the lubricant a discharge occurs. As a result heat generated by the discharge causes local melting of the bearing metal surface. Bearings with ceramic rolling elements can be particularly useful in eliminating the electrical pathway through the motor bearings, providing perfect electrical insulation between stator and rotor.

It is easy to see how hybrids can increase bearing life in some applications, but some engineers have naturally questioned the cost of a hybrid bearing, which is typically higher than that of a standard steel bearing. In reality, the price difference is soon recovered in maintenance savings, which escalate rapidly when bearings are repeatedly damaged by stray electrical currents.

Of course, not all applications will require hybrid ceramic bearings; it all depends on the specification. Steel bearings that are designed into applications based solely on loading conditions without taking into account factors such as lubrication, contamination and maintenance may well be wrongly specified and will not reach their potential lifespan. With the right specification, a hybrid ceramic bearing can offer a lifespan up to ten times that of a standard steel bearing, reducing the need for machine maintenance and costly interruptions to production.

And there are further benefits; 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 re-lubrication intervals are longer.

Hybrid ceramic bearings have proved so effective that they are now offered in extra-large bore sizes to combat the dangers of stray electrical currents in a relatively new and demanding application, where the need to prevent maintenance work is especially high: wind turbines. The 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 costly, is relieved by hybrid ceramic ball bearings, which are now available in sizes specifically designed to fit these turbines.