Electrical bearing damage: pitting, fluting and shaft currents
Electrical discharge across the lubricating film can damage rolling contacts. Fine craters or washboard-like fluting warrant an electrical investigation, particularly in converter-driven equipment. A photograph alone does not establish the current path or prove that an insulated replacement is sufficient.
Discharge through a contact can produce local melting and craters; developed damage may appear as groove-like fluting.
Checks & easily confused patterns
Evidence to investigate
Preserve magnified surface evidence. Qualified personnel should investigate the electrical system and possible current path with appropriate methods.
Do not infer from appearance alone
A striped track or a vibration spectrum alone does not establish electrical damage. Compare mechanical wear, false brinelling and corrosion evidence.
Technical basis: NSK Bearing Doctor, individual damage-type references. Original BearingScope illustrations and investigation prompts, reviewed 4 October 2026. Colour, shape and depth are illustrative; source snapshots are retained in the internal audit.
Compare where each measure acts. These are reference concepts, not a wiring diagram or an automatic recommendation for your motor.
Insulating interface at the housing side
Outer-ring insulation
Shaft
Steel rolling contacts
Coated outer ring
Housing
A coated ring changes the electrical interface, not the rolling-element material. Check the exact coating and assembly specification.
Reference example SKF VL0241 / VL0246
SKF 17344 EN, pp. 4–7
Insulating interface at the shaft side
Inner-ring insulation
Shaft
Coated inner ring
Steel rolling contacts
Housing
Coating location matters. SKF describes higher impedance from the smaller inner-ring coating area; this is not a universal motor retrofit approval.
Reference example SKF VL2071 / VL2076
SKF 17344 EN, pp. 5–7
Insulation within the rolling contact
Hybrid ceramic bearing
Shaft
Steel inner ring
Ceramic rolling elements
Steel outer ring
Ceramic balls and steel rings form a hybrid bearing. A ceramic coating on a steel ring is a different construction. Neither label alone specifies clearance, seals or fit.
Reference example NSK hybrid ceramic-ball bearings
NSK, Hybrid Bearings with Ceramic Balls
A separate route around bearing contacts
Shaft grounding
Shaft
Grounding contact
Frame / earth
System review
A grounding contact diverts shaft current. Review the motor, coupling and driven equipment together: insulating motor bearings can leave a path through the coupled machine.
Reference example Grounding brush or ring
ABB, Bearing currents and how to beat them
Highlighted boxes locate the protection concept. The sequence does not predict current direction, quantify leakage or cover every possible bypass path.
Read the complete suffix
6216 HDY2 C3 is NSK’s published decoding example: HDY2 identifies the insulated design; C3 identifies internal clearance. C3 by itself is not an insulation code.
SKF uses different suffixes for inner- and outer-ring coatings. Do not translate an NSK suffix into an SKF or FAG equivalent by matching the base number.
At high frequency, the coating’s capacitance also matters. A DC test rating does not specify the whole installed drive’s behaviour. Humidity, cleanliness, contact surfaces and mounting conditions affect verification.
Use the exact bearing’s current documentation and test conditions. No voltage limit, coating grade or replacement approval is inferred here from the suffix alone.
Fluting is an inspection finding, not a vibration frequency
NSK describes electrical erosion as arcing damage at lubricated contacts, with melted craters and sometimes corrugated tracks. A vibration peak alone cannot identify that mechanism. Record magnified surface findings and investigate the electrical system as well as mechanical alternatives.
Shaft-voltage and current measurements require qualified personnel and suitable equipment. This page gives no live-probing instructions. Follow the equipment’s electrical and mechanical isolation procedure before physical inspection.
What was damaged?
Keep motor position, bearing marking and as-found inspection evidence together. Do not replace an observation with a presumed cause.
What was measured?
Shaft-to-frame voltage, bearing current and insulation resistance are different quantities. Record which one the qualified test procedure measures.
What is the current path?
Include both motor bearing positions, the coupling and the driven machine. A motor-only record can miss the path being investigated.
ABB notes that there is no single acceptable shaft-voltage rule for every motor-drive installation. This worksheet records evidence; it never returns a pass/fail verdict.
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Reference basis and limits
SKF, INSOCOAT, publication 17344 EN, May 2017, pp. 4–7: coating positions and electrical test basis.
NSK, Ceramic-Coated Insulated Ball Bearings HDY2: construction and designation example.
NSK, Hybrid Bearings with Ceramic Balls: hybrid construction.
NSK, Electrical Corrosion: damage interpretation.
ABB, Bearing currents and how to beat them: system-level paths and measurement limitations.
Reviewed 4 October 2026. These summaries do not reproduce manufacturer drawings or supply an OEM installation procedure. Illustrations are original conceptual interface lists, not photographs of damage. Product availability and exact ratings must be checked for the complete designation.
Observation is not diagnosis
What the evidence can — and cannot — tell you
Observation or context
Interpretation to consider
Verification needed
Craters or fluted tracks
Possible electrical discharge
Preserve magnified images and compare with mechanical wear patterns.
Variable-frequency drive installation
Electrical system context matters
Record drive changes, earthing, cable arrangement and bearing insulation specification.
Repeat damage after bearing replacement
The cause may still be present
Review both bearing positions and the complete current path.
A practical investigation sequence
01
Identify and record
Isolate electrical and mechanical energy before physical inspection. Shaft-voltage/current tests require qualified personnel and suitable instruments.
02
Check the context
Document the exact motor and bearing design, including any insulated ring or hybrid designation. Do not infer insulation from appearance.
03
Verify before acting
Compare the observed damage with mechanical false brinelling, corrosion and lubricant distress. Keep competing explanations open until tested.
04
Keep the decision traceable
Evaluate earthing, electrical insulation and bearing options at system level. Any change must retain the required fits, loads, speed capability and thermal behaviour.
Capture the information a model search cannot guess
Keep this checklist with your inspection or comparison. Checking a box only records that you have collected the item; it does not certify the bearing.
Original BearingScope workflow, written for catalogue research and comparison. Manufacturer material is used to check principles, not copied as a product endorsement or substituted for model-specific instructions. Reviewed 1 October 2026.
NSK Troubleshooting — Electrical corrosion — Electrical pitting and current damage
Source addresses and collection status are retained in the internal audit. No unpublished dimensions, torque settings, service intervals or interchange approvals are inferred.
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