Electrical dangers of bearings
Electrical damage to rolling element bearings is becoming an increasingly common cause of bearing failure within industry, primarily through the increasing use of variable frequency drives ( VFD).
When a stray current in an electric motor uses a bearing as its path to ground, bearing damage can occur through micro-cratering in the raceways of the inner and outer rings and on the rolling element surfaces.
The heat that is generated by the discharges causes local melting that creates small craters and changes in the structure of the metal. As a result of this initial damage, a “washboard pattern” may be found on the raceways and rolling elements for roller bearings.
This secondary damage is wear caused by the dynamic effect of the rolling elements when they roll over the smaller craters along with the electrical discharge causing the lubricant in the bearing to change its composition, degrade rapidly and fail prematurely.
Once bearing damage from electric erosion has begun, increased noise levels, reduced effectiveness of the lubricant, increased heat and finally excessive vibration, all contribute to drastically decrease bearing service life.
The most common causes of stray electric currents are: asymmetry in the motor’s magnetic circuit, unshielded power cables, and fast switching pulse width modulated (PWM) frequency converters used in modern variable frequency drives ( VFDs).
Addressing these potential causes of electrical damage can greatly improve the life of the bearings and lubricant. However, these may not be able to eliminate the failure mechanism completely.
To overcome this problem, SKF has developed two electrically insulating rolling bearing solutions, SKF hybrid bearings and Insocoat bearings. Selecting the best option depends on the severity and cause of the stray electric current and the size of the bearing.
An SKF Insocoat bearing is a standard, all-steel bearing that is plasma-sprayed with a ceramic coating that can be applied in a range of thicknesses to the inner or outer ring, depending on the application. Resistance ranges from 50 to 150 MW at a breakdown voltage between 1000 and 3000 V DC.
A hybrid bearing combines steel rings with silicon nitride rolling elements. Silicon nitride, an engineered ceramic material, is an extremely hard, non-conductive material that provides a number of benefits beyond its insulating properties.
As an insulator, a hybrid bearing provides protection relative to the size of the rolling elements. The smallest sealed SKF ball bearing in the electric motor range is estimated to have a resistance of more than 10GW at a minimum breakdown voltage of 2.5kV DC.