INSOCOAT® – electrically insulated rolling bearings

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INSOCOAT® – the SKF solution to prolong bearing life in electricmachinery

What are INSOCOATbearings?SKF provides electrically insulatedbearings, called INSOCOAT, to protectagainst damages caused by electriccurrents. These bearings have anelectrically insulating coating on theouter ring (VL0241 execution), or onthe inner ring (VL2071 execution). Theinner ring coating can be made tobore diameters above and including

70 mm. The coating consists of anominal 100 µm thick aluminium oxidelayer and is applied by a unique coat-ing process using plasma-sprayingtechnology.

INSOCOAT bearings with outer ring coating – VL0241 execution

Industrial motors

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Traction motors

Generators

Customer value INSOCOAT bearings• provide two features in one

solution: – a bearing function; – electrical insulation.

• reduce the number of failures andimprove the machine uptime.

• reduce maintenance costs. • are the most economical solution in

comparison with other insulatingsolutions.

• have global availability thanks toSKF presence in more than 130countries and at 7 000 distributionlocations worldwide.

INSOCOAT bearings provide an economical solution in comparison with other insulating methods

INSOCOAT Housing Shaftbearing insulation insulation

INSOCOAT bearing

Housing insulationincluding bearingcost

Shaft insulationincluding bearingcost

Total cost

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Technical benefits• The standard 100 µm layer thick-

ness prevents most current passageproblems.

• SKF does 100 % testing to guaranteea breakdown voltage larger than 1 000 V DC. Lab tests show that electrical breakdown occurs above 3 000 V DC.

• SKF guarantees a minimum ohmicresistance of 50 M Ω at 1 000 V DC.

• The application process using plasma spraying technique in com-bination with sophisticated pre- andafter preparation of the bearing ringsgives an outstanding coating quality.

• INSOCOAT bearings are treated witha unique sealant ensuring insulationin humid environments. It is alsoinsensitive to heat and chemicals.

• INSOCOAT bearings provide a betterelectrical and mechanical perform-ance than other insulation methods.

• SKF can supply values for relevantelectric parameters for the bearing(capacitance, impedance).

Additional benefits• Simple mounting and dismounting.

INSOCOAT bearings should behandled with the same care asstandard bearings.

• Shaft fits up to and including p6 canbe applied for both coating variants(VL2071 and VL0241).

INSOCOAT bearing with inner ring coating – VL2071 execution

Plasma spraying process and equipment

The inner ring coating provides anenhanced protection of the bearingagainst electric current damages. Theunderlying improvements are:

• Improved insulation at high frequen-cies compared to outer ring coatingthanks to a smaller coated surface.

• No risk of damaging the housing inapplications which use a loose fit tothe outer ring.

• Better protection of the coatingduring handling.

• Standard boundary dimensionsaccording to ISO 15:1998.

• Environmentally friendly • INSOCOAT bearings are suitable for

all types of housings.• SKF has experience with ceramic

coatings since more than 20 years.

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1

0

-1 0 0,005 0,01 0,015 0,02 0,025 0,031

0

-1 0 0,005 0,01 0,015 0,02 0,025 0,03

1

0

-1 0 0,005 0,01 0,015 0,02 0,025 0,031

0

-1 0 0,005 0,01 0,015 0,02 0,025 0,03

Vu (Udc)

Vcommon (Udc)

Vv (Udc)

Vw (Udc)

time (s)

1Fig

The three phases and their sum – common mode voltage

The coating process –plasma spraying technologyThis unique technology has the follow-ing benefits:

• Plasma spraying is the most versatilethermal coating process – it pro-duces high-performance coatingsthat deliver durability and reliability.

• The process applies coatings by injecting aluminum oxide powder intoa high temperature gas stream. Theplasma gas heats the powdered coat-ing material to a molten state andsprays it onto substrates at highspeeds.

• Plasma spraying is extremely flexibleand is ideal for producing a widevariety of functional coatings.

AssortmentYou can find the living assortment forelectrical machinery and railway ap-plications in the enclosed assortmentlists.

If you cannot find the item that youneed in the standard range, pleasecontact your local SKF representative.

Shields and sealsShields and seals are available onrequest for some bearings sizes.

LubricationIt is important to ensure proper lubrica-tion by frequent regreasing.

Sources of electric currentThe problem of electric current pass-ing through rolling bearings and caus-ing damage in the contact area ofballs or rollers and raceways of innerand outer rings has been known foralmost 70 years. In addition to thedamage to bearing elements, it wasalso supposed that the structure of thelubricant itself might change under theinfluence of a current passage. Allrotating machines, both AC and DCmotors as well as generators, poten-tially suffer from this phenomenon.

Classical failuresWith sinusoidal line voltages, the bear-ing current is generated due to asym-metries in the motor’s magnetic circuit.The asymmetric flux distribution insidethe motor induces an axial shaft volt-age, which further leads to a low-fre-quency circulating current flowingthrough the bearings. Bearing currentis also generated by asymmetric, non-shielded motor cabling. These ‘clas-sical’ bearing currents are a problemespecially for large motors with lownumbers of pole pairs (e.g. 2-polemotors). They have larger flux asym-metries than small motors or motorswith many poles.

High frequency currentsApart from the classic voltages andcurrents generated by the motor itself,new effects have been observed whenthe motor is supplied from a pulsewidth modulation (PWM) converter(frequencies of 3 to 16 kHz). It hasbeen discovered that the bearingdamages are caused by a high fre-quency (several kHz – MHz range)current flow. The current originatesfrom the so-called common mode voltage of the frequency converter.Another origin of current is the highswitching speed of the integrated gatebipolar transistors (IGBTs) used insidethe converter.

Problems arise because of threetypes of currents:

• High frequency shaft grounding currents

• High frequency circulating currents• Capacitive discharge currents

The reason for the first two types ofcurrents is the common mode voltageat the converter output. The commonmode voltage is produced due to thefact that the sum of the three phasevoltages is not equal to zero ( fig ).

Furthermore, the frequency convertertries to simulate a sine wave supply byPWM signals, which have a high switch-ing frequency and very steepedgedpulses, which cause capacitive dis-charge currents.

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Effects of electric current going throughthe bearingDue to the passage of electric currentin the contact zone of rolling elementsand raceways heat is generated caus-ing local melting of the bearing metalsurface. Craters are formed in thecontact area and particles of moltenmaterial are transferred and partlybreak loose. The crater material is re-hardened and much more brittlethan the original bearing material.Below the re-hardened layer there is a layer of annealed material, which issofter than the surrounding material.

Micro-crateringBecause frequency converters aremore and more used micro-crateringis by far the most common effect ofelectric current passage. The damagedsurface appears dull, characterised bymolten pit marks ( fig ). Multiplemicro-craters cover the rolling elementand raceway surfaces. Crater sizesare small, mostly from 5 to 8 µm indiameter, disregarding whether it is onthe inner ring, outer ring or a rollingelement. The real shape of these craters can only be seen under amicroscope in very high magnification.

Fluting or washboardThey are patterns of multiple greylines across the raceways ( fig ).They appear shiny and molten. Thereason for this fluting is a mechanicalresonance vibration caused by thedynamic effect of the rolling elementswhen they are over-rolling smaller craters. This means that fluting is not aprimary failure mode produced by thecurrent flow through the bearing itself.It is a secondary bearing damage thatbecomes visible only after time andhas its initial point from craters.

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2

2Fig

4Fig

3Fig

Grease-blackeningCurrent discharges also cause thelubricant in the bearing to change itscomposition and degrade rapidly.The local high temperature causesadditives and the base oil to react, andit can cause burning or charring of thebase oil. Additives will be used upmore quickly. Thus the lubricant gets

Black discolouredgrease affected by currentdischarges

Fluting or wash-board in raceway

The dull surfaceof the ball is asign of micro-cratering

almost hard and blackened ( fig ).A rapid breakdown of the grease is atypical failure mode that results fromcurrent passage.

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Electric behaviour ofINSOCOAT bearingsOne has to distinguish between DCcurrent and AC current applications.In DC applications the insulating coat-ing acts as a pure resistor where theohmic resistance R of the aluminiumoxide layer is the important quantity.The breakdown voltage of the stand-ard layer is larger than 1 000 V DCand the resistance is larger than 50 MΩ, which provides efficient insulation of the bearing.

In AC applications, especially invariable speed drives (VSDs) one hasto consider the impedance of the ce-ramic coating. The impedance describesthe voltage-current relationship in ACcircuits. The value of the impedancedepends mainly on two electrical char-acteristics of the coating: the ohmicresistance and the capacitance. Thecapacitance should be as small aspossible to counteract the effects ofhigh frequency electric currents.

The impedance of the aluminiumoxide coating can be modelled as aparallel connection of a resistor and a capacitor.

Impedance and capacitanceFig shows a typical graph of theimpedance of a coated deep groove ballbearing (outer ring coating). Fig shows a typical graph of the capacit-ance of a coated deep groove ballbearing (outer ring coating).

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DC AC

Description of voltage- Resistance R Impedance Zcurrent relationship

Electrical behaviour Resistor Resistor and capacitor in parallel

Unit Ohm [Ω] Ohm [Ω], Farad [F]

DC – Resistance Guaranteed more –than 50 MΩ

AC – Impedance – The value depends on the bearingsize and the electric current frequency

AC – Capacitance Constant over the frequency – range; the absolute value is

depending on the bearing size

Electrical behaviour of INSOCOAT bearings in DC and AC applications

100 1000 10000 100000 1000000 0,01

0,1

1

10

100

1000

6316 VL0241

100 1000 10000 100000 1000000

6,6

6,8

7

7,2

7,4

6316 VL0241

Measured impedance as a function of frequency

Measured capacitance asa function offrequency

6Fig

7Fig

Frequency [Hz]

Frequency [Hz]

Impe

danc

e [k

Ω]

Cap

acita

nce

[nF

]

5Fig

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SKF has a range of specific bearinggreases to meet allapplication needs

For this reason SKF, the leader notonly in bearing design and manufactur-ing but in global service, and the SKFdistributors offer customers the mostcomprehensive range of tools to ensure optimal bearing performance.

Induction heaters and mountingtools to preserve the quality, pullers for easy removal, equipment and soft-ware for the condition monitoring ofbearings and machinery.

In addition, a wide range of greasesof the highest quality for bearing lub-rication, consistent in quality, for gen-eral and specific requirements, avail-able worldwide through the SKF distri-bution network.

SKF bearings add value to yourmachine, our tools make your lifeeasy, our distribution network, provid-ing replacement bearing, tools andgreases ensures the commitment ofSKF to your business.

Your products should run faultlessly. At least foras long as planned – preferably even longer. Toensure this, there are three basic requirements:• Use the right bearings. Preferably from SKF.• Mount them correctly. Appropriate skills and the

right tools for the job are imperative.• Ensure proper maintenance. Here, the essentials

are know-how, plus the appropriate tools andlubricants.

More products for trouble-freeoperation

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SKF measuringequipment

SKF inductionheaters and hotplate

SKF pullers

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The SKF Group– a worldwide corporation

SKF is an international industrial Groupoperating in some 130 countries and isworld leader in bearings.

The company was founded in 1907following the invention of the self-align-ing ball bearing by Sven Wingquist and,after only a few years, SKF began toexpand all over the world.

Today, SKF has some 45 000 em-ployees and around 80 manufacturingfacilities spread throughout the world.An international sales network includesa large number of sales companies andsome 7 000 distributors and retailers.Worldwide availability of SKF productsis supported by a comprehensive technical advisory service.

The key to success has been a con-sistent emphasis on maintaining thehighest quality of its products and services. Continuous invest-ment in research and

development has also played a vitalrole, resulting in many examples ofepoch-making innovations.

The business of the Group consistsof bearings, seals, special steel and a comprehensive range of other high-tech industrial components. The experi-ence gained in these various fieldsprovides SKF with the essential know-ledge and expertise required in orderto provide the customers with the mostadvanced engineering products andefficient service.

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The SKF Group is the first major bearingmanufacturer to have been granted approvalaccording to ISO 14001, the internationalstandard for environmental managementsystems. The certificate is the most com-prehensive of its kind and covers more than60 SKF production units in 17 countries. The SKF Engineering & Research Centre

is situated just outside Utrecht in TheNetherlands. In an area of 17 000 squaremetres (185 000 sq.ft) some 150 scientists,engineers and support staff are engaged inthe further improvement of bearing perform-ance. They are developing technologiesaimed at achieving better materials, betterdesigns, better lubricants and better seals – together leading to an even better under-standing of the operation of a bearing in itsapplication. This is also where the SKF LifeTheory was evolved, enabling the design of bearings which are even more compact and offer even longer operational life.

SKF has developed the Channel concept infactories all over the world. This drasticallyreduces the lead time from raw material toend product as well as work in progressand finished goods in stock. The conceptenables faster and smoother informationflow, eliminates bottlenecks and bypassesunnecessary steps in production. TheChannel team members have the know-ledge and commitment needed to share theresponsibility for fulfilling objectives in areassuch as quality, delivery time, productionflow etc.

SKF manufactures ball bearings, roller bearings and plain bearings. The smallestare just a few millimetres (a fraction of aninch) in diameter, the largest several metres.SKF also manufactures bearing and oilseals which prevent dirt from entering and lubricant from leaking out. SKF's subsidiaries CR and RFT S.p.A. are amongthe world's largest producers of seals.

® SKF and INSOCOAT are registered trademarks of AB SKF.

© Copyright SKF 2002The contents of this publication are thecopyright of the publisher and may notbe reproduced (even extracts) unlesspermission is granted. Every care hasbeen taken to ensure the accuracy ofthe information contained in this pub-lication but no liability can be acceptedfor any loss or damage whether direct,indirect or consequential arising out ofthe use of the information containedherein.

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