Fluting & Bearing Inspection

EDM, Fluting, and Bearing Inspection

Motors controlled by VFDs are subject to two kinds of shaft voltage: capacitively coupled shaft voltage, and inductively coupled end-to-end shaft voltage. Regardless of the type, once the shaft voltage gets high enough to overcome the dielectric properties of the bearing grease, it will discharge along the path of least resistance — typically the motor bearings — to the motor housing. When the shaft voltage discharges by arcing through the bearing, the arc’s temperature is hot enough to melt bearing steel and severely damage the bearing lubrication.

These discharges are so frequent (up to millions per hour) that before long the entire bearing race becomes marked with countless pits known as frosting. A phenomenon known as fluting may occur as well, producing washboard-like ridges across the frosted bearing race. Fluting causes excessive noise and vibration, and in HVAC systems, it is magnified and transmitted by the ducting. Regardless of the type of bearing or race damage that occurs, the resulting motor failure often costs many thousands or even tens of thousands of dollars in downtime and lost production.

How to Inspect a Bearing

Inspect the bearing and bearing cavity and retain a sample of the lubricant if further analysis is warranted. Look for:

  • Contamination
  • Signs of excessive heat
  • Hardening of grease
  • Abnormal coloration (blackened grease)
  • Excess grease escaping the bearing

Cut the outer race into halves.

  • Follow established safety precautions and use personal protective equipment including eye protection, hearing protection, face shield, gloves and protective clothing.

Inspect the grease and contamination in the bearing. You should be looking for:

  • Burnt Grease: Continuous electrical arcing in the motor bearings will often rapidly deteriorate the lubricating capability of the grease and cause bearing race damage. When an arc occurs, the oil component of the grease is heated beyond its temperature capacity.
  • Contamination: In addition to the burnt grease, the arcing causes small metal particles to loosen from the bearing races/balls which are distributed in the grease. These particles are abrasive and intensify the bearing wear.

Download Bearing Protection Handbook