AEGIS® Bearing Protection - Why Shaft Grounding?


Bearing Inspection


Cutting & Inspecting a Bearing

Two primary sources of bearing currents

  1. Clean the bearing’s components using a degreaser or solvent.

Follow all safety precautions.

  1. Inspect for evidence of Electrical Discharge Machining (EDM): EDM are millions of microscopic electrical pits created when current discharges through the motor’s bearings. The electrical voltage overcomes the dielectric of the bearing lubrication and instantaneously arcs through the inner race, through the ball and to the outer race. The individual pit is usually between 5 and 10 micron diameter.

Voltage arcs through the bearing

  1. Frosting: This will appear to be a grey discolored line around all or part of the bearing race and may be evident in the inner and outer race. The discoloration may be caused by wear or by EDM. Examination under a microscope may be required to determine if the line is EDM or not. If the motor was operated on a VFD with no bearing protection there is a high likelihood that the frosting is from EDM.

A third source of bearing currents

  1. Fluting Damage: Identified by a distinctive washboard pattern. Fluting can be identified with the naked eye or with 10x magnification. Fluting is sometimes confused with mechanical bearing damage so care should be taken to correctly assign electrical fluting damage to the pattern observed.

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