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Irregularities in the winding and motor geometry can cause constant passage of electrical current through the bearings. Current flashover resulting from voltage spikes often occurs in motors controlled by frequency converters. Current densities of more than 0.1 A/mm² can damage the raceways, balls and grease and even cause the bearing to fail.
Welding beads caused by current flashover
Fluting caused by continuous passage of electrical current
To prevent such damage, the Schaeffler Group offers current-insulated bearings :
Current-insulated bearings and hybrid bearings
|Disruptive Voltage||500 VDC||1000 VDC||3000 VDC||3000 VDC|
|Coating Thickness||<100 μm||>200 μm||≈200 μm||≈200 μm|
|Applicable Dimensions||70…1400 mm
inner ring bore
|*) Preferably used for bearings with an outside diameter of at least 500 mm|
Current-insulated bearings have the same dimensions and load carrying capacities as those without insulation, which makes them perfectly interchangeable.
The most common type of insulation used in electric motor applications is the J20AA coating. It offers excellent protection against passage of electrical current, even in damp atmospheres.
Bearings with ceramic coating are available with seals also.
Bearings with J20C coating on the inner ring are used in special applications as well as applications where the outer ring rotates. Z shields can be used as well.
Hybrid bearings (HC): Hybrid bearings are characterized by the fact that the rolling elements are made of ceramic. They offer maximum protection against passage of electrical current. At the same time, hybrid bearings are suitable for higher speeds and have better dry-running characteristics.