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Spark Erosion in bearing and shaft earthing devices

Spark erosion is micro pitting related to main bearings and thrust bearings where the bearing material (HMO7 or AlSn40) is corroded by electrical sparks from the bearing shell to the main bearing journal due to electro-potential difference between the crankshaft and the bearing. Spark erosion in AlSn40 bearings may develop faster than in HMO7 bearings. micro pitting marks on the sliding surfaces of main bearings and journals might result in a slight transfer of white metal stripes to the journal and faster wear down of bearing material.

Pitting Corrosion

Without a device, voltage differences of up to 350-450 mV can be measured between shaft and bearing. It has proved that sparks are normally not generated at a voltage below 80 mV. To avoid spark erosion, the volt¬age have to be reduced to a value below 50 mV by a shaft earthing device placed on the propeller shaft close to the engine. This can most easily be done by using a solid silver track as a slip ring, and silver/graphite material as brushes. The silver track material is superior because of its low electrical resistance, and the silver/graphite brush ensures that the system is self-lubricating and self-cleaning. The strip is connected to the hull via brush system to reduce the potential between shaft and bearing.

shaft earthing device

 A voltmeter is installed to ensure a continuous display of the potential difference. Readings in excess of 50 mV are a sign of worn brushes or poorly maintained brush gear and/or slip rings. The functioning of the earthing and measuring device is checked, first by releasing the earthing bushes from their contact with the shaft slip ring, which should then give a "high" voltage pulse of 100 - 400 mV, read at the meter. This indicates that the monitoring circuit is in order. Secondly, when the brushes once more are brought into contact with the slip ring, the reading should again fall below the 50 mV limit. The silver track and the silver/carbon brushes should not need any regular cleaning, but if oil is found on the track, it should be cleaned off carefully, using an electro-cleaner or similar solvent'

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