Occasionally, a form of damage is seen consisting of long tracks which appear to be made up of numerous small pits, each of which has a rounded ‘molten’ appearance. Usually the tracks are less than 1 mm (0.04") in width. This is caused by a form of electrical discharge different from that described under electrical pitting. Usually, the cause of spark tracking is that the machine shaft has become magnetized, causing stray currents to be generated as it rotates.
Degauss the machine and investigate the grounding of the rotor and the insulation at each bearing, with particular attention to fittings such as guards, thermocouple leads, water connections, etc., that may be bridging insulation. Bearings showing damage from spark tracking should be replaced.
Electrical discharge will seek out the point of least resistance; this is likely to be the thrust bearing, where the oil film is usually thinner than in the associated journal bearings. In an existing machine, it can be relatively simple to replace metallic pads with polymer-lined ones to provide electrical insulation. The electrical insulating properties of polymer linings are increasingly viewed as a significant benefit in original equipment design as well.