There are several types of motor or roller bearing failures. These include:
- Fluting due to shaft currents
- Pitting due to shaft currents
- Smears due to debris
- Fretting corrosion due to lose fit or vibration
- Water intrusion
- Over temperature indicated by colour variations
Shaft current routing and pitting are looking usually associated with the drive or cabling. The drive manufacturer has guidance on cabling or the use of insulated bearings to control shaft currents.
An excellent article on motor bearing faults including photos is in IEEE Industry Applications, Vol.18 No. 6, "Root Cause Methodology For Induction Motors" by Bonnet, Austin H.