Through the microscope: Electrical discharge in a ball bearing

Electrical discharges in metallic components caused by stray currents are a well-known phenomenon in technical damage analysis. They are particularly critical in components subject to contact stresses, such as ball bearings. Here, electrical discharges often initiate wear mechanisms that progress during further operation. As the raceways are increasingly destroyed, this secondary damage can eventually obscure the original cause – the electrical discharge.

Comparing intact and damaged raceways

The scanning electron microscope (SEM) images provide a direct comparison between an intact raceway and a surface damaged by electrical discharges. The intact raceway is characterised by numerous fine, parallel machining marks originating from the manufacturing process.

The damaged surface looks very different: the original machining marks have disappeared completely, and the surface appears irregularly scarred. This is caused by the extremely high local temperatures generated by individual electrical discharges.

Each electrical discharge can melt the surface locally. The material then solidifies again very quickly because the underlying component rapidly conducts the heat away. This creates rounded melt craters.

Millions of repeated electrical discharges

The dimensions illustrate the extent of the damage: the individual melt craters are only around 2 µm in diameter. Compared with a ball bearing’s total raceway area of several square centimetres, this suggests that millions of electrical discharges must have occurred in this component.