DEMM Engineering & Manufacturing - - MAINTENANCE PLANNING -

ROLLING BEAR­INGS in elec­tric mo­tors have one fail­ure mode which is com­mon to this ap­pli­ca­tion: The prob­lem of elec­tric cur­rent pass­ing through the bear­ings caus­ing dam­age in the con­tact area be­tween rolling el­e­ments and rings.

The lu­bri­cant film present in the bear­ing pro­vides sep­a­ra­tion be­tween rolling el­e­ments and race­ways, and the lu­bri­ca­tion film also cre­ates a small in­su­lat­ing layer. When an elec­tric cur­rent reaches a suf­fi­cient level how­ever, it passes be­tween the com­po­nents mak­ing a burn mark both on the rolling el­e­ments and in the ring race­ways and dam­ages the lu­bri­cant.

Each elec­tric dis­charge causes lo­calised melt­ing and re­hard­en­ing that em­brit­tles the steel sur­face which leads to spalling over time. The cur­rent leak­age can also dam­age the lu­bri­cant, and in the case of grease lu­bri­cated bear­ings sig­nif­i­cantly re­duce the grease life, caus­ing pre­ma­ture bear­ing fail­ure. The typ­i­cal ap­pear­ance of cur­rent leak­age is a dull race­way and ball sur­faces with a flut­ing pat­tern on the race­way.

The phe­nom­e­non has been ob­served in AC and DC mo­tors for al­most 100 years, and coun­ter­mea­sures have been de­vel­oped to ad­dress the dif­fer­ent causes by means of in­su­la­tion, bet­ter de­sign of mo­tors, im­proved ca­bles etc. The types of cur­rent found in in­duc­tion mo­tors can gen­er­ally be cat­e­gorised into three main areas. Mo­tor asym­me­tries

Be­cause of lim­i­ta­tions and tol­er­ances in man­u­fac­tur­ing, it is im­pos­si­ble to ob­tain per­fect elec­tro­mag­netic sym­me­try in an elec­tric mo­tor. This asym­me­try leads to the gen­er­a­tion of a flux of al­ter­nat­ing mag­ni­tude, in­duc­ing shaft volt­age. Large mo­tors, es­pe­cially 2-pole mo­tors, are more likely to suf­fer from this type of is­sue and the use of an SKF INSOCOAT bear­ing to break the cur­rent flow has proven to be a re­li­able so­lu­tion in most in­stances.

Mo­tor ca­bling

Cur­rents i nduced by un­sym­met­ri­cal, non- shielded ca­bling can com­monly be re­solved by sym­met­ri­cal shielded ca­bling and a l ow i mpedance ground­ing of mo­tor f rames along with SKF INSOCOAT bear­ings fit ted i n both po­si­tions.

Fre­quency in­vert­ers

Most mod­ern fre­quency in­vert­ers try to sim­u­late a sine wave sup­ply with a se­ries of square pulses that op­er­ate with high switch­ing fre­quency and very fast volt­age switches, how­ever th­ese very steep- edged volt­age pulses cre­ate high fre­quency cur­rent tran­sients.

The am­pli­tude of th­ese High Fre­quency cur­rents varies with mo­tor or gen­er­a­tor sizes, con­verter type and cable pa­ram­e­ters. Most of this HF cur­rent re­turns to the con­verter through the cable Power Earth lead and shield, but the re­main­ing cur­rent may find a path to earth through the mo­tor bear­ings.

SKF So­lu­tions

Iden­ti­fy­ing the source of the cur­rent and the ap­pli­ca­tions en­vi­ron­men­tal fac­tors is key to in­stalling the best so­lu­tion. En­vi­ron­men­tal fac­tors such as mois­ture and hu­mid­ity can af­fect the elec­tri­cal re­sis­tance of some ce­ramic in­su­lat­ing lay­ers, and im­prove­ments to SKF INSOCOAT bear­ings have fur­ther in­creased both the break­down volt­age of the layer and re­sis­tance to mois­ture. In most cases SKF INSOCOAT bear­ings in ei­ther one or both bear­ing lo­ca­tions is a suit­able so­lu­tion.

SKF Hy­brid bear­ings com­bine steel rings and rolling el­e­ments of bear­ing grade sil­i­con ni­tride (Si3N4) and can sub­stan­tially im­prove re­li­a­bil­ity and ro­bust­ness par­tic­u­larly in ap­pli­ca­tions where the bear­ings are of­ten ex­posed to po­ten­tially dam­ag­ing abra­sive par­ti­cles, in­ad­e­quate lu­bri­ca­tion, vi­bra­tion or higher fre­quency ca­pac­i­tive dis­charge cur­rents.

Bear­ing grade sil­i­con ni­tride, an en­gi­neered ce­ramic ma­te­rial, has a uni­form and clean mi­crostruc­ture, which is ex­tremely hard and tough. The ma­te­rial can be used as an in­su­la­tor and is chem­i­cally in­ert. Due to the hard­ness and low den­sity, th­ese bear­ings can run at higher speeds and with lower fric­tion than equiv­a­lent steel bear­ings, also re­sult­ing in ex­tended grease life.


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