What Is Fluting? — Bearing Glossary

Fluting is an electrical discharge damage pattern on bearing raceways — parallel grooves spaced at regular intervals, resembling the flutes of a wind instrument, caused by stray electric current passing through the bearing.

Fluting occurs primarily in variable frequency drive (VFD) motors, welding equipment, and electrical machinery with leakage current. For a full overview of electrical and mechanical bearing damage, see bearing failure modes guide and browse spherical roller bearings for high-power VFD drive applications. High-frequency current from the inverter induces shaft voltage — when voltage exceeds approximately 0.3–0.5 V, electrical arcing jumps across the thin lubricant film at the contact point, eroding the steel surface. The arc discharge frequency approximates the VFD carrier frequency (1–16 kHz) multiplied by the number of rolling elements, creating a regular erosion cycle that leaves evenly spaced parallel grooves.

Sources and Conditions for Fluting

Three primary current sources cause fluting: (1) Circulating current — shaft voltage induced by asymmetric magnetic flux, current flowing through the bearing to complete the circuit between shaft and housing. (2) VFD common-mode current (EDM current) — common-mode voltage from the inverter drives current through the bearing when no adequate ground path exists. (3) Leakage current from welding machines or inadequately isolated power sources near the bearing.

Visual inspection after disassembly makes fluting easy to identify: the raceway shows parallel wavy grooves, perpendicular to the rolling direction, evenly spaced 0.5–3 mm apart. The surface appears grey or black from the oxide layer. The damage texture shows small irregular craters (cratering) rather than the deep pitting of spalling from fatigue — distinguishing electrical from mechanical damage under magnification is straightforward.

The key question is not whether damage has occurred, but how to prevent recurrence. Replacing a standard bearing without addressing the electrical path results in identical failure within the same timeframe.

Practical Example: Bearing 6316 C3 in a VFD Motor

At a textile plant in Dong Nai, a 55 kW motor operating on a 15 kHz VFD caused fluting in a 6316 C3 bearing (d=80, D=170, B=39 mm, C=72 kN, C₀=53 kN) after only 4,000 hours — against a 30,000-hour design life. The bearing produced high-frequency noise; shaft voltage measurement showed 2.8 V (exceeding the 0.5 V safety threshold by a factor of 5.6). Solution: fit an SKF INSOCOAT insulated outer ring bearing at the non-drive end plus a shaft grounding ring at the drive end.

The INSOCOAT bearing costs 3–5 times more than a standard 6316 C3. Against the cost of an unplanned motor failure — rewinding, downtime, lost production — it represents a small fraction of avoided losses. Prevention here is straightforwardly more economical than repair.

Shaft Voltage Measurement and Monitoring

Measuring shaft voltage is the most direct method for assessing fluting risk. Use a digital multimeter with test leads contacting the shaft end and the motor housing while running at operating speed. Interpret readings as follows:

Measure at operating speed — shaft voltage varies with VFD carrier frequency. Per ISO 15243:2017, electrical erosion is the second most common failure mode in VFD motors after lubrication errors.