spark errosion and remedies

A problem that can arise from currents generated in the rotor shaft due to stray magnetic fields, under fault conditions these stray fields can be very large, these currents if allowed to flow, would arc across the bearing surface and cause small craters which would eventually destroy the bearings.remedies: shaft earthing device and continuously monitoring the value of current flowing between shaft and hull. another reason Spark erosion is caused by voltage discharged between the main bearings and their respective journals. This voltage originates from the development of galvanic action between the ship’s steel hull and the propeller shaft, with the seawater acting as an electrolyte. This is then transferred to the main crankshaft where, due to dissimilar metals, erosion can occur between the white metal main bearing and its journal. Spark erosion can only occur if the current is not grounded. Checks:The checks consist of a visual check for white metal fragments around the main bearings and respective journals and checking for any electric current between the main bearing white metal and journal. This should be carried out using a micro-amp current meter or similar device for measuring small amperage and voltages. This should read no more than 50mV; any higher than this indicating that shaft grounding is not working. Grounding device:Grounding is carried out by fitting a cathodic protection system to the main propeller drive shaft, consisting of a set of slip rings on the shaft and carbon pick-up brushes. The brushes are wired and grounded to a good earth on the ships structure close by the slip rings.