Elec­tri­cal dan­gers of bear­ings

DEMM Engineering & Manufacturing - - CONTENTS -

Elec­tri­cal dam­age to rolling el­e­ment bear­ings is be­com­ing an in­creas­ingly com­mon cause of bear­ing fail­ure within in­dus­try, pri­mar­ily through the in­creas­ing use of vari­able fre­quency drives ( VFD).

When a stray cur­rent in an elec­tric mo­tor uses a bear­ing as its path to ground, bear­ing dam­age can oc­cur through mi­cro-cra­ter­ing in the race­ways of the in­ner and outer rings and on the rolling el­e­ment sur­faces.

The heat that is gen­er­ated by the dis­charges causes lo­cal melt­ing that cre­ates small craters and changes in the struc­ture of the metal. 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 change its com­po­si­tion, de­grade rapidly and fail pre­ma­turely.

Once bear­ing dam­age from elec­tric ero­sion has be­gun, in­creased noise lev­els, re­duced ef­fec­tive­ness of the lu­bri­cant, in­creased heat and fi­nally ex­ces­sive vi­bra­tion, all con­trib­ute to dras­ti­cally de­crease bear­ing ser­vice life.

The most com­mon causes of stray elec­tric cur­rents are: asym­me­try in the mo­tor’s mag­netic cir­cuit, un­shielded power ca­bles, and fast switch­ing pulse width mod­u­lated (PWM) fre­quency con­vert­ers used in mod­ern vari­able fre­quency drives ( VFDs).

Ad­dress­ing these po­ten­tial causes of elec­tri­cal dam­age can greatly im­prove the life of the bear­ings and lu­bri­cant. How­ever, these may not be able to elim­i­nate the fail­ure mech­a­nism com­pletely.

To over­come this prob­lem, SKF has de­vel­oped two elec­tri­cally in­su­lat­ing rolling bear­ing so­lu­tions, SKF hy­brid bear­ings and Inso­coat bear­ings. Se­lect­ing the best op­tion de­pends on the sever­ity and cause of the stray elec­tric cur­rent and the size of the bear­ing.

An SKF Inso­coat bear­ing is a stan­dard, all-steel bear­ing that is plasma-sprayed with a ce­ramic coat­ing that can be ap­plied in a range of thick­nesses to the in­ner or outer ring, depend­ing on the ap­pli­ca­tion. Re­sis­tance ranges from 50 to 150 MW at a break­down volt­age be­tween 1000 and 3000 V DC.

A hy­brid bear­ing com­bines steel rings with sil­i­con ni­tride rolling el­e­ments. Sil­i­con ni­tride, an en­gi­neered ce­ramic ma­te­rial, is an ex­tremely hard, non-con­duc­tive ma­te­rial that pro­vides a num­ber of ben­e­fits be­yond its in­su­lat­ing prop­er­ties.

As an in­su­la­tor, a hy­brid bear­ing pro­vides pro­tec­tion rel­a­tive to the size of the rolling el­e­ments. The small­est sealed SKF ball bear­ing in the elec­tric mo­tor range is es­ti­mated to have a re­sis­tance of more than 10GW at a min­i­mum break­down volt­age of 2.5kV DC.

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