Preventing electrical pitting damage in bearings

Electrical pitting damage, caused by intermediate arcing between stationary and rotating machine components, erodes the babbitted surface of a bearing, causing it to fail. With the right precautions, however, this type of damage can be prevented.

In an example of a steam turbine application, the consequences of erratic thrust pad temperatures were not obvious until the bearing was removed from the machine. In this case, the normal pad temperature was 95° C for a 10½", 6 x 6 LEG thrust bearing operating at 5700 RPM. The pad temperatures had been rising steadily for several days when suddenly the temperature of one pad shot up to 118° C and the other to 103° C. A few days later, the pad temperatures dropped back to 111° C and 92° C.

When the bearing was removed from the machine, all the thrust pads showed some signs of distress.

The original babbitt is only present at the leading edge of the shoe. Evidence of a wipe can be seen toward the outer edge of the shoe. The areas at the trailing edge of the shoe and toward the inner diameter at the leading edge have clusters of small pitted marks.

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