Pre­vent­ing elec­tri­cal ero­sion in bearings

DEMM Engineering & Manufacturing - - MAINTENANCE MATTERS -

Bear­ing life cal­cu­la­tions ac­cord­ing to ISO sug­gest that un­der ideal op­er­at­ing con­di­tions bearings can op­er­ate nearly in­def­i­nitely. In re­al­ity bearings of­ten fail within sev­eral years as a re­sult of con­tam­i­na­tion, over­load­ing, vi­bra­tion, im­proper lu­bri­ca­tion and poor fit­ting.

Although vari­able fre­quency drives ( VFDs) pro­vide many ben­e­fits to in­dus­try, elec­tri­cal re­lated bear­ing dam­age is a com­mon root cause for fail­ure in elec­tric mo­tors through both dam­age to the bear­ing sur­faces and lu­bri­cant degra­da­tion from elec­tri­cal arc­ing.

The most com­mon root cause for stray elec­tric cur­rents to oc­cur within elec­tric mo­tors are vari­a­tions in the mag­netic cir­cuits, un­shielded power ca­bles, and fast switch­ing pulse width mod­u­lated (PWM) fre­quency con­vert­ers used in VFDs.

When stray cur­rents pass through a bear­ing, dam­age oc­curs to the race­ways of the in­ner and outer rings and on the rolling el­e­ment sur­faces. The heat which is gen­er­ated by the dis­charges causes lo­cal melt­ing that cre­ates small craters and changes in the struc­ture of the metal, which can be seen as a dull grey dis­coloura­tion of the sur­faces. 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 fail pre­ma­turely.

SKF Inso­coat bearings are de­signed to pro­tect against elec­tric cur­rent pas­sage. By in­te­grat­ing the in­su­lat­ing prop­er­ties into the bear­ing, Inso­coat bearings can im­prove re­li­a­bil­ity and in­crease ma­chine up­time by vir­tu­ally elim­i­nat­ing the dam­ag­ing ef­fect of elec­tri­cal ero­sion.

Inso­coat bearings have an alu­minium- ox­ide coat­ing on the ex­ter­nal sur­faces of the outer ring or the in­ner ring to pro­vide an in­su­la­tion layer. The coat­ing is sealed with a resin to pro­vide sta­ble elec­tri­cal prop­er­ties in hu­mid en­vi­ron­ments which can cause a re­duc­tion of the in­su­la­tion prop­er­ties.

It is gen­er­ally rec­om­mended to in­stall an Inso­coat on the non­drive end of an elec­tric mo­tor in com­bi­na­tion with an ef­fec­tive shaft ground­ing mech­a­nism on the drive end to pro­vide a safe path for any stray elec­tri­cal cur­rent. This helps to pre­vent pos­si­ble cir­cu­lat­ing cur­rents, as well as cur­rent leak­age through the bearings.

Bearings with an elec­tri­cally in­su­lat­ing coat­ing on the ex­ter­nal sur­faces of the outer ring are the most com­mon Inso­coat bearings and are iden­ti­fied by the suf­fix VL0241 and are rec­om­mended for medium-size mo­tors, that use bear­ing sizes smaller than 6226 and 6324.

Inso­coat bearings with an outer ring coat­ing are suit­able for all types of hous­ings and outer ring coat­ing can be ap­plied to bearings with an out­side di­am­e­ter > 80 mm.

Bearings with an elec­tri­cally in­su­lat­ing coat­ing on the ex­ter­nal sur­faces of the in­ner ring pro­vide en­hanced pro­tec­tion against elec­tric cur­rent dam­age. The en­hanced pro­tec­tion re­sults from the in­creased im­ped­ance due to the smaller coated sur­face area.

Bearings with a coated in­ner ring are iden­ti­fied by the suf­fix VL2071 and are rec­om­mended for larger- size mo­tors or other ap­pli­ca­tions where the bearings risk be­ing sub­jected to high shaft volt­ages. In­ner ring coat­ing can be ap­plied to bearings with a bore di­am­e­ter > 70 mm.


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