Electrical damage to rolling element bearings is a common cause of failure – common enough to have a whole category listed under the ISO standard for Bearing Damage (ISO 15243:2004, from www.iso.org.) It's technically called electrical erosion. So, where does the damage come from, and what can you do to prevent it from happening in your machines?
There are basically four cases to consider: Is the machine moving or not; and are you getting current leakage, or a high amperage discharge through the bearings?
Electrical current passage through bearings can happen when machines are not moving. Example: Your friendly contractor does some welding at your plant and decides to attach her ground via your nearby electric motor. This would be a high amperage discharge, and might "weld" bearing parts together internally. Low-intensity current would likely produce many tiny craters that aggregate into darker areas of damage. If you have burnt-smelling lubricant as well, it's another confirmation of current leakage. This leakage might be from static discharge (are you making plastic or paper products?) or from high-voltage current spikes caused by variable frequency drive (VFD) converters.
When the machine is moving, the current passage often produces a very regular pattern of fine, dark lines across the bearing raceway. These can grow in size and depth as the damage continues. If you don't have vibration monitoring, eventually an operator will hear it, hopefully before something catastrophic happens.
What can be done to eliminate these unwanted currents? If you suspect static discharge, inspect your grounding: straps get corroded or come loose. If you suspect VFD's as the source, have an expert look at your cabling, especially cable quality, lengths and termination.
Some motors and generator sets simply produce stray currents that can't be eliminated. In these cases, change to a bearing with Insulation (SKF Insocoat, for example) or a bearing with ceramic rolling elements to allow the current to go safely to ground. Whatever you'd pay for these parts is a small cost considering the price of your downtime.
Find out more information about Ceramic and Insocoat bearings through the link or visit www.skf.com.