Damage caused by the 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.
| Appearance |
Dark brown
or greyish black fluting (corrugation) or craters in raceways and rollers. Balls have dark
discolouration only. Sometimes zigzag burns in ball bearings raceways.
Localised burns in raceways and on rolling elements. |
| Cause |
Passage of
electric current through rotating bearing.
Passage of electric current through non-rotating bearing. |
| Action |
Re-route the
current to by-pass the bearing.
Use insulated bearings.
Re-route the current to by-pass the bearing. When welding, arrange earthing to prevent
current passing through the bearing.
Use insulated bearings. |
 |
 |
| Figure 1 – Fluting caused by
the passage of electric current, in the outer ring of a spherical roller bearing. |
Figure 2 – The outer ring of
a self-aligning ball bearing damaged by electric current. |
Figure 3 -
Deep grove 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.
Figure 4 - A
railway axlebox bearing damaged by the passage of high amperage current while the bearing
was not running.
Figure 5 -
Roller of a railway axlebox bearing damaged by electric current (same bearing as in fig
4).
Back to types of bearing damage
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