Bearing Issues in Electric Motors

The increased use of Variable Frequency Drives (VFDs), in recent years, has aided in reducing electricity costs substantially but has also caused increased numbers in premature bearing failure. The rising numbers of failures can be attributed to electrical fluting or, an issue also known as, electrical erosion.

Electrical fluting occurs when a stray current is passed through the bearing instead of a grounded source. These electric currents harm many aspects of the bearing including the rolling elements, the raceways and even the lubrication. The discharges can cause fluting, pitting, decreased grease integrity and vibrations all of which are signs of advanced stages of fluting damage.

The damage caused by these stray currents is not always visible to the naked eye. Caused by electrical sparks, the first stage of damage is characterized by micro craters that are formed due to a localized melting of the material from the rolling elements or the raceways. The remaining particles and debris that are remaining are washed away leaving a small crater shape on the material.

The second stage of damage that occurs as the craters continue to progress is known as electrical fluting. Over time the stress of the rolling elements on the craters causes a series of parallel lines to form along the raceways. Once bearing damage from electrical fluting has begun, reduced effectiveness of the lubricant, increased heat and excessive vibration all decrease the life of the bearing.

One solution to combat the issue of stray electric currents is through the use of ceramic hybrid bearings. Silicon nitride ceramic balls offer little to no conductivity significantly reducing the likelihood of electrical fluting. Ceramic hybrids also offer many other advantages including longer bearing life, higher operating speeds, lower operating temperatures, and increased bearing stiffness.

Detecting the root cause behind bearing failure is also critical in repeatable application success. Gathering all the clues associated with the failure allows for repeated success and the correct action to be implemented. Through a complete Failure Analysis Program Napoleon Engineering Services can determine whether the cause of failure is bearing or application related and provide solutions to your bearing problems.