Bearings need protection from electric currents

Phil Burge looks at the increase in electric-current-related bearing damage and failure, and introduces the latest generation of bearings designed to counter the problem.

The phenomenon of electric currents passing through rolling bearings with detrimental effects on raceways is not new, but the problem is becoming of increasing concern to designers and manufacturers.

A significant cause of this problem is the growing popularity of variable speed drives throughout a diverse cross section of industries.

These devices are allowing significant improvements in productivity and flexibility for manufacturing, processing and packaging companies alike, offering improved efficiency of motor driven equipment by matching speed to changing load requirements, and enabling accurate and continuous process control over a wide range of speeds.

However, combined with the effects of using larger and more powerful motors and generators, this technology is leading to bearings and lubricants being frequently subjected to strong electrical currents, leading to reduced performance and in some cases complete bearing failure.

Damage to bearings from electric currents has been recognised as a problem for some time and is generally caused by asymmetries in the magnetic circuits of drive motors, with asymmetric flux distribution inducing axial shaft voltages, which lead to low frequency circulating currents flowing through the bearings.

Bearing current can also be generated by asymmetric, nonshielded motor cabling.

Both these conventional causes are a particular problem in large motors with low numbers of pole pairs, which typically have larger flux asymmetries than small motors or motors with many poles.

However, this problem is now becoming more widespread, with the growth in the use of variable frequency motors with pulsewidth modulation (PWM) convertors.

The high frequency currents originating from the common mode voltage of the convertors can cause significant damage to bearings, and so too can currents originating from the high switching speed of the integrated gate bipolar transistors (IGBTs) used inside each convertor.

Further damage can be caused by the way in which frequency convertors simulate a sine wave supply by PWM signals that have a high switching frequency and very steep edged pulses, which cause capacitive discharge currents.

As they pass through the contact zone of the rolling elements and raceways of bearings, these currents generate heat, which in turn leads to localised melting of the metal surfaces and the formation of craters in the contact area as particles of molten metal break loose.

The crater material is then rehardened but brittle, with a layer beneath it that is softer than the surrounding material.

This type of microcratering is by far the most common effect of electric current.

Multiple microcraters, typically between 5 and 8um in diameter and visible only as a dull finish, cover rolling elements and raceway surfaces, and result in impaired performance, and in some cases failure.

Electric currents can also cause fluting or washboard effects in bearings, which can be seen as patterns of multiple shiny or molten grey lines across the raceways.

Rather than being caused directly by electric currents acting on the bearing, fluting takes the form of secondary damage, resulting from a mechanical resonance vibration caused by the dynamic effect of the rolling elements when they roll over smaller craters.

Rapid grease blackening can also occur, where lubricant in the bearings changes its composition and degrades rapidly.

This is due to localised high temperatures that cause additives and the base oil to react, leading to burning or charring of the base oil, and the lubricant becoming almost hard and blackened.

Although it is possible to protect bearings and lubricants by insulating either equipment housings or each rotating shaft, both of these solutions are relatively expensive and require additional components, leading to increased maintenance costs.

A far more efficient solution is offered by the latest generation of Insocoat bearings, which feature specially developed surface coatings, typically just 50um in thickness, which act as electrical insulators to provide protection against electrically induced flashovers of up to 500V, with thicker coatings also being available to withstand discharges of up to 1000V.

The coating is applied to the inner or outer ring of a bearing through plasma spraying, by injecting aluminium oxide powder into a high temperature gas stream.

This then heats the powdered coating material to a molten state and sprays it onto the surface of the bearing at high speeds.

The resulting finish is both durable and consistent, and is unaffected by extremes of temperature or humidity.

Protected bearings are also now available that incorporate silicon nitride rolling elements to provide excellent insulation properties, making them ideal for isolating housings from shafts in motors and generators.

These new bearings, such as SKF's Hybrid range, offer extended service life over conventional bearings when used in conjunction with variable frequency drives.

As silicon nitride rolling elements have a lower thermal expansion than steel rolling elements of similar size, more accurate preload control can be achieved due to lower sensitivity to temperature gradients within the bearing.

With these latest specifically designed bearings, the negative effects of electric currents resulting particularly from the use of variable frequency drives can be countered, allowing designers, manufacturers and end users to utilise the benefits of the new technology without the risk of premature bearing damage or failure.

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