Preventing electrical erosion in bearings
Bearing life calculations according to ISO suggest that under ideal operating conditions bearings can operate nearly indefinitely. In reality bearings often fail within several years as a result of contamination, overloading, vibration, improper lubrication and poor fitting.
Although variable frequency drives ( VFDs) provide many benefits to industry, electrical related bearing damage is a common root cause for failure in electric motors through both damage to the bearing surfaces and lubricant degradation from electrical arcing.
The most common root cause for stray electric currents to occur within electric motors are variations in the magnetic circuits, unshielded power cables, and fast switching pulse width modulated (PWM) frequency converters used in VFDs.
When stray currents pass through a bearing, damage occurs to the raceways of the inner and outer rings and on the rolling element surfaces. The heat which is generated by the discharges causes local melting that creates small craters and changes in the structure of the metal, which can be seen as a dull grey discolouration of the surfaces. 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 fail prematurely.
SKF Insocoat bearings are designed to protect against electric current passage. By integrating the insulating properties into the bearing, Insocoat bearings can improve reliability and increase machine uptime by virtually eliminating the damaging effect of electrical erosion.
Insocoat bearings have an aluminium- oxide coating on the external surfaces of the outer ring or the inner ring to provide an insulation layer. The coating is sealed with a resin to provide stable electrical properties in humid environments which can cause a reduction of the insulation properties.
It is generally recommended to install an Insocoat on the nondrive end of an electric motor in combination with an effective shaft grounding mechanism on the drive end to provide a safe path for any stray electrical current. This helps to prevent possible circulating currents, as well as current leakage through the bearings.
Bearings with an electrically insulating coating on the external surfaces of the outer ring are the most common Insocoat bearings and are identified by the suffix VL0241 and are recommended for medium-size motors, that use bearing sizes smaller than 6226 and 6324.
Insocoat bearings with an outer ring coating are suitable for all types of housings and outer ring coating can be applied to bearings with an outside diameter > 80 mm.
Bearings with an electrically insulating coating on the external surfaces of the inner ring provide enhanced protection against electric current damage. The enhanced protection results from the increased impedance due to the smaller coated surface area.
Bearings with a coated inner ring are identified by the suffix VL2071 and are recommended for larger- size motors or other applications where the bearings risk being subjected to high shaft voltages. Inner ring coating can be applied to bearings with a bore diameter > 70 mm.
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