PITTING DUE TO ELECTRICAL DISCHARGE
Electrical discharge through the oil film between journal and bearing in electrical machinery, or on the rotors in fans and turbines, may occur due to faulty insulation or earthing or to the build-up of static electricity. This can occur at very low voltages and may cause severe pitting of bearings or journal surfaces, or both. In extreme cases damage may occur very rapidly and the cause is sometimes difficult to diagnose as pitting of the bearing surface is followed ultimately by wiping and failure which may obscure the original pitting.
Fig. 30 - Fine hemi-spherical pitting and scoring of whitemetal-lined generator bearing due to electrical discharge.
Fig. 31 - Close up of more severe electrical discharge pitting of whitemetal.
Scrap bearings if severely pitted or wiped. Examine and, if necessary, re-grind journal to eliminate pitting. Investigate earthing conditions of machine especially round any insulation, with particular attention to fittings such as guards, thermocouple leads, water connections, etc., which may be bridging insulation. Fit new bearings and run for only a short time, depending upon the period run to failure point on previous occasion, and examine.
|Scoring Due to Foreign Matter or "Dirt"
|Damage Caused by Faulty Assembly
|Black Scab / Wire Wool Damage
|Pitting Due to Electrical Discharge
|Wiping of Bearing Surfaces
|Fretting Damage from Vibration
|Damage Due to Oveheating
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