Electrical Pitting

Electrical discharge through the oil film between the shaft and bearing in electrical machinery or in fans and turbines may occur due to faulty insulation or grounding, or due to the build-up of static electricity. This electric discharge can occur at very low voltages and may cause severe pitting of the bearing or shaft surfaces, or both.

In extreme cases, damage may occur very rapidly. When it does, the cause may be difficult to diagnose, as pitting of the bearing surface is followed ultimately by wiping, which may obscure the original pitting.

Addressing the Damage

Bearings that are severely pitted or wiped ought to be replaced. The shaft should also be examined and, if necessary, reground to eliminate pitting.

To prevent future damage, 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.

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.

The entire rotor support system should be considered in any evaluation so that curing the problem on one bearing does not merely shift the problem to the next bearing.

Figure 1: Close-up of severe electrical discharge pitting of whitemetal

Figure 2: Fine hemispherical pitting and scoring of whitemetal-lined generator bearing due to electrical discharge