9/27/2016 Insulating Ball Bearings from Electric Arc Damage in

https://web.archive.org/web/20081202085656/http://www.ien.com/article/insulatingballbearings/7642 1/3

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Insulating Ball Bearings from Electric Arc Damage inElectric MotorsJay S. CarlsonMRC Bearing Services

Ball bearings in electric motors support and locate the rotor, keep the air gap smalland consistent, and transfer loads from the shaft to the motor frame. When astray current in a machine uses a bearing as its path to ground, the resultingdamage is referred to as electric arc bearing damage. The most common causes ofelectric arc bearing damage include asymmetry in the motor's magnetic circuit;unshielded power cables; and fastswitching variable frequency drives (VFDs).

Once electric arc bearing damage has begun, excessive vibrations,increased heat, increased noise levels, and the reduced effectiveness of thelubricant will contribute to shorten a bearing's service life. The extent of

damage to bearings will depend on the amount of energy and its duration.However, the effect usually will be the same: pitting damage to the rollers andraceways, rapid degradation of the lubricant, and premature bearing failure.

Why Arcing Occurs: Electric arcing will result if there is a difference in potentialbetween the shaft and the bearing housing. (Even a difference of a few volts inpotential can produce the effect.)

The voltage level when arcing occurs will depend on ball size, cage type, and sealdesign. For two bearings the same size, arcing occurs at a higher voltage level forthe open variant than for the sealed variant. If the bearing is equipped withpressed steel shields, the risk of arcing will be higher, because the insulating partis only the air gap between the electrically conducting shield and the bearing innerring.

How Damage Results: When an electric current passes through the contactzone of a bearing's rolling elements and raceway, the energy of the electricdischarge generates heat, causing localized melting of the surface. The effect on abearing is almost like a series of small lightning strikes, which melt and retemperinternal bearing surfaces. The outcome is that some surface material flakes awayand spalls out to create noise in the bearing and potentially shortened service life.

Warning Signs: Cratering is perhaps the most commonly experienced effect ofelectric arc damage. This type is characterized by molten pit marks (invisible tothe eye). A dull gray surface of the rolling element will send a visual warning signof cratering to telegraph that bearing deterioration is present.

Another telltale warning sign will present itself as characteristic fluting (orwashboarding) patterns in the raceways of bearings. Fluting is caused bythe dynamic effect of the rolling elements continually moving over themicro"craters" and etching a rhythmic pattern into the running surfaces of abearing's races. Noise and vibration from the bearing increases and, eventually,the deterioration will lead to complete bearing failure.

Should electric arc bearing damage be suspected, bearings should be replaced andproper insulation should be provided to prevent electric currents from passingthrough: Hybrid ball bearings (which substitute ceramic balls for steel rollingelements) offer an advanced and practical solution.

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9/27/2016 Insulating Ball Bearings from Electric Arc Damage in

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Hybrids incorporate rings made from bearing steel and rolling elementsmanufactured from bearing grade silicon nitride. Because silicon nitride has highresistivity, hybrid bearings provide ideal insulation from electric currents both in acand dc motors. Hybrid bearings further possess a higher speed capability and cansustain longer service life than allsteel bearings in most applications for a varietyof reasons.

Among the key characteristics of hybrid bearings compared with allsteelcounterparts:

Lower density: Silicon nitride balls are 40% less dense than similarlysized steel balls, reducing centrifugal force and friction. This means higherspeeds, less weight, lower inertia, and more rapid starts and stops. Inshort, the bearings can run faster and cooler.

Higher hardness: Ceramic balls are harder than both steel and mostpotential particle contaminants. This means the bearings can eliminatecontaminant particles either by crushing them or pressing them into the(softer) steel rings, where they can be rendered harmless.

Lower friction: Silicon nitride's low coefficient of friction enhances wearresistance to enable the bearing to run cooler even under poor lubricationconditions. This means better lubrication, less noise, and lower operatingtemperatures.

Higher modulus of elasticity: Ceramic rolling elements have a 50%higher modulus of elasticity than steel. This means increased bearingstiffness and reduced deflection under load to promote reliableperformance.

Lower coefficient of thermal expansion: Ceramic rolling elements havea thermal expansion of only 29% of similar steel rolling elements. Thismeans less sensitivity to temperature gradients for more accurate preloadcontrol.

All these characteristics can deliver key potential benefits to users:

Lower maintenance and energy costs: Maintenance costs can quicklymultiply if a bearing must be changed frequently and an extension in theservice life of a bearing without increasing maintenance costs cancontribute to reductions in the overall operating cost of equipment. Lessfriction adds up to lower energy costs.

Extended service life: Most bearings are designed into applications basedon loading conditions and do not take into account factors such aslubrication, contamination, and maintenance. Without proper attention tothese external factors, a steel bearing rarely reaches its optimized designand service life. The properties of ceramics combine to hold the promise ofservice life up to 10 times that of a standard steel bearing.

Extended grease life: In environments imposing high demands on thebearing lubricant, standard bearings experience surface wear due toinsufficient lubricant film and bearings can fail if the initial grease charge isnot replenished within an acceptable timeframe. Hybrid bearings run coolerand can operate with thinner lubricant films, so there is less aging of thegrease and relubrication intervals can be longer for increased service lifecompared with standard bearings in the same operating conditions.

Lower operating temperatures: The heat generated in bearings isattributed to viscous friction between the balls and raceways. The source ofthe loading is both external and internal, and little can be done to reducethe external loads. However, since ceramic balls have only 40% of thedensity of steel balls, less centrifugal load is generated by the balls and theinternal friction is lower. This translates to cooler running for the sameoperating conditions (or, if applicable, a higher rotational speed whilemaintaining the same temperature).

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Reduced wear from contamination: In contaminated environmentssolid particles create dents in the rolling surfaces and raised edges aroundthose dents, causing noise and premature wear as steel balls roll over thosesurfaces. The harder ceramic ball material puts contaminants in their place.

Reduced wear from vibration: In equipment exposed to static vibrationthere is an inherent risk of false brinelling (wearing away of the surfaceswithin the ball and raceway contacts), which can eventually lead to spallingand premature failure. Lighterweight ceramic balls minimize the potentialfor false brinelling.

Experienced product and service partners can serve as reliable resources to helpkeep users current about these and other remedial solutions for electric arcbearing damage.

MRC Bearing ServicesKulpsville, PA7166612727www.mrcbearingservices.comRequest Additional Information

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http://www.ien.com/article/insulatingballbearings/7642 Go NOV DEC

22007 2008

1 captures2 Dec 08 2 Dec 08