Recently we examined how corrosion contributes to bearing failure. Now we’ll examine electric current damage.
Random electrical currents, which come from multiple sources, such as wiring, insulation, static electricity from operating equipment, and so on, will damage bearings. The damage usually appears on the surface of rings and rolling elements and is similar to that produced by electric arc welding. Either visible regular patterns (flutting) or irregular patterns (pitting), as well as dark or discolored areas, can be signs of electrical current damage. This type of damage will increase bearing noise or vibration levels. If severe enough, the bearings are rendered useless.
The type of electrical current does not matter; alternating current, direct current, and low amperage can damage bearings. How much damage occurs depends on current intensity, duration of the current, bearing load, speed if the bearing is rotating, and lubrication. Rotating bearings are more prone to this type of damage than non-rotating bearings.
The currents tend to heat bearing material to temperatures that melt that portion of the surface, resulting in discolored areas where bearing material was re-hardened or melted, or that produced small craters.
To prevent this type of damage, you must prevent electric current from passing through the bearing. There are several options for blocking electric current. One is the use of shunts. A shunt can be formed into a slip ring assembly to encourage a random current to go around the bearing.
Random currents also arise from wiring issues, unsound insulation, or poorly maintained rotor windings. Proper, regular maintenance can prevent future damage.
Proper grounding of companion equipment is also important to prevent stray currents. Bearings can be insulated, for example, with nonconductive material used either between the rings, housing, and shaft.
Another alternative is to use electrically conductive grease, which can provide a path for the current, protecting the bearings.
Factors that contribute to bearing failure include material fatigue; operating with a heavier loading than specified; inadequate or unsuitable lubrication; careless handling; poor sealing; and bearing balls or rollers installed too tightly, with insufficient internal bearing clearance.
Each of these contributors leaves known clues on the bearings or the races, so it is fairly easy to determine what happened. However, multiple contributors can be at play in each cause, so it’s useful to have an understanding of each contributor and each cause. Primary types of damage are: