Passage of electric current
When an electric current passes through a bearing, i.e. proceeds from one ring to the other via the rolling elements, damage will occur. At the contact surfaces the process is similar to electric arc welding.
The material is heated to temperatures ranging from tempering to melting levels. This leads to the appearance of discoloured areas, varying in size, where the material has been tempered, re-hardened or melted. Small craters also form where the metal has melted.
The passage of electric current frequently leads to the formation of fluting (corrugation) in bearing raceways. Rollers are also subject to fluting, while there is only dark discolouration of balls.
It can be difficult to distinguish between electric current damage and vibration damage. A feature of the fluting caused by electric current is the dark bottom of the corrugations, as opposed to the bright or rusty appearance at the bottom of the vibration-induced fluting. Another distinguishing feature is the lack of damage to the rolling elements of bearings with raceway fluting caused by vibrations.
Both alternating and direct currents cause damage to bearings. Even low amperage currents are dangerous. Non-rotating bearings are much more resistant to electric current damage than bearings in rotation. The extent of the damage depends on a number of factors: current intensity, duration, bearing load, speed and lubricant.
The only way of avoiding damage of this nature is to prevent any electric current from passing through the bearing.
|Dark brown or greyish black fluting (corrugation) or craters in raceways and rollers. Balls have dark discolouration only. Sometimes zigzag burns in ball bearing raceways.||Passage of electric current through rotating bearing.||Re-route the current to bypass the bearing.
Use insulated bearings.
|Fluting, caused by the passage of electric current, in the outer ring of a spherical roller bearing (fig 1).
The outer ring of a self-aligning ball bearing damaged by electric current (fig 2).
Deep groove ball bearing with electric current damage in zigzag pattern. It is assumed that burns of this configuration arise when the momentary passage of high amperage current is accompanied by axial vibration (fig 3).
|Localised burns in raceways and on rolling elements.||Passage of electric current through non-rotating bearing.||Re-route the current to bypass the bearing. When welding, arrange earthing to prevent current passing through the bearing.
Use insulated bearings.
|A railway axlebox bearing damaged by the passage of high amperage current while the bearing was not running (fig 4) (fig 5).
Roller of a railway axlebox bearing as in previous fig (fig 6).