Recognizing shaft damage types

Technical notes of interest to Marine Engineers

Shaft current damage types

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1) Frosting

By far the most common type of shaft current damage.Parts affected are bearings, seals, thrust collars, journals, and to a lesser extent, gears. The appearances is that of a sand blasted surface, characterized by overlapping, molten, shiny pit marks. If the entire available surface is affected, the damage may not be noticeable to the naked eye due to its satin-like appearance. When viewed microscopically, however, the frosted surface is seen as very small and individual "craters." The bottoms of the craters are round and shiny, indicative of the melting that had occurred. This frosting occurs during voltage discharge and its commonly referred to as "Electrical Discharge Machining" (EDM), or Electrolysis. As EDM occurs, material is removed. Sometimes chemical attack gives a similar appearance to frosting; however, the marks are smaller, not as deep, and appear dull.

2) Spark Tracks

The initial appearance of these tracks is that of scratches in the babbitted surface resulting from foreign particles in the lubrication or seal oil. However, a closer examination reveals a continuous, meandering, narrow track depleted of babbitt, ranging in length from 1/8 in. to 7 in. They may run askew or concentric to the direction of rotation. Under magnification, the track is shiny and molten, ranging in depth from 2 mils to 1/16 in. Spark tracks are usually associated with an electromagnetic source as a large amount of power is needed to develop the continuous voltage discharge. This damage is often misdiagnosed as mechanical scouring.

3) Pitting

This damage is listed separately from frosting as it is generally much larger (from 1/32 in. to  in.) since its source is extremely powerful. It often occurs in gear teeth, on the backs of bearings or seals, and sometimes between frame splits. As opposed to frosting where the entire surface might be affected, pitting occurs more randomly and it is sometimes possible to count the number of discharges, which is impossible to do with frosting. The appearance of the pits is similar to the individual frosted craters; that is, they often have round shiny bottoms. Sometimes pitting is confused with frosting-type corrosion. In this case, a qualified metallurgist is needed to distinguish the difference. Electrical pitting usually stems from an electromagnetic source, as a much more concentrated amount of energy is needed to form the larger pits. However, very high voltage electrostatic sources have been known to cause pitting.

4) Welding

Welding of parts such as frame splits, bearing pads and seals, have occurred due to a great amount of current (hundreds of amperes) passing through them. They are easily evident to the naked eye as spot welded marks and quite often have to be separated by sledge hammers or other mechanical means. Welding can only occur from an electromagnetic source, as an extremely large current is generated, causing fusion between two components. In turbomachinery, this damage is usually the result of an upset in the process, allowing a rotor to momentarily contact the stator, thereby producing the large flow of current.