What is Bearing Fluting

Bearing fluting is a common issue that arises in electric motors, causing premature bearing failure and costly repairs. This article explores the causes, identification, and prevention of bearing fluting to help maintain the longevity and performance of electric motors.

What is Bearing Fluting

Bearing fluting is a type of electrical bearing damage caused by shaft voltages and currents induced by variable frequency drives (VFDs). This phenomenon results in a distinctive washboard or fluting pattern on the bearing raceways, leading to premature bearing failure, increased noise, and vibration.

Causes of Bearing Fluting

• Induced shaft voltages and currents from VFDs: The high-frequency switching of power semiconductors in VFDs can create common-mode voltages, leading to parasitic capacitance between the stator windings, rotor, and motor shaft. This capacitively coupled voltage causes shaft currents to flow through the motor bearings to ground.
• Frequent electrical discharges melt and pit bearing surfaces: When the induced shaft voltage exceeds the dielectric strength of the bearing lubricant film, it breaks down, allowing the current to discharge through the bearing. This process, similar to electrical discharge machining (EDM), causes localized melting and pitting of the bearing surfaces.
• Shaft voltage builds up and discharges through motor bearings: As the shaft voltage accumulates, it seeks the path of least resistance to ground, which is often through the motor bearings. The repeated discharge of current through the bearings leads to progressive damage and the formation of fluting patterns on the raceways.

Identifying Bearing Fluting

• Audible bearing noise: Fluted bearings often produce distinct audible noise during operation, which can be an early indicator of bearing damage. The noise is caused by the irregular surface of the fluted raceways, leading to increased vibration and noise levels.
• Fluting pattern in bearing raceways: Upon inspection, bearing fluting is characterized by evenly spaced, parallel grooves or ridges on the bearing raceways. This washboard-like pattern is a clear sign of electrical bearing damage caused by shaft currents.

Preventing Bearing Fluting

• Proper VFD cable shielding and grounding: Ensure that VFD power cables are properly shielded and grounded to minimize the generation of common-mode voltages and reduce the risk of shaft currents.
• Grounding motor shaft: Install shaft grounding devices, such as carbon brushes or grounding rings, to provide a low-impedance path for shaft currents to flow to ground, bypassing the motor bearings.
• Using conductive grease: Apply conductive bearing grease containing metal particles or carbon nanotubes to provide a controlled path for shaft currents to discharge, reducing the risk of bearing damage.
• Installing insulated ceramic bearings: Replace standard steel bearings with insulated ceramic bearings, which have lower electrical conductivity and can help prevent the flow of shaft currents through the bearings.
• Fitting shaft with Faraday shield: Install a Faraday shield around the motor shaft to block capacitive coupling between the stator and rotor, reducing the generation of shaft voltages and currents.
• Adding VFD output filters: Implement VFD output filters, such as dV/dt filters or common-mode chokes, to reduce the magnitude and rise time of voltage pulses, minimizing the risk of shaft currents and bearing damage.
• Creating low-impedance grounding paths: Establish low-impedance grounding connections between the motor frame, bearing housings, and VFD to provide alternative paths for shaft currents to flow, reducing the likelihood of bearing damage.

FAQs

What is spalling in bearings?

Spalling is a form of bearing damage characterized by the flaking or chipping of bearing surfaces, often caused by excessive loads, inadequate lubrication, or material fatigue. Unlike fluting, which has a distinct washboard pattern, spalling appears as random pitting or flaking of the bearing raceways or rolling elements.

What is electrical pitting?

Electrical pitting is a type of bearing damage caused by electric current passing through the bearing, resulting in localized melting and the formation of small craters or pits on the bearing surfaces. This damage is similar to the pitting caused by electrical discharge machining (EDM) and can lead to premature bearing failure if left unchecked.

Conclusion

Bearing fluting is a prevalent issue in VFD-driven motor systems, causing premature bearing failure and increased operational costs. By understanding the causes, identification, and prevention strategies for bearing fluting, maintenance professionals can proactively address this problem and extend the life of motor bearings.

Implementing proper shielding, grounding, and bearing protection techniques is essential for mitigating the effects of shaft voltages and currents, ensuring the reliability and efficiency of VFD-driven motor systems.