How Shaft Grounding Systems Prevent Corrosion and Bearing Damage

Shaft grounding systems are widely used in electric motors, generators, wind turbines, gearboxes and other rotating equipment to control unwanted shaft voltage and electrical current. Although shaft grounding is sometimes described as a method of preventing corrosion, its primary purpose is to provide a controlled electrical path for stray shaft currents and protect bearings and other rotating components from electrical erosion.

When shaft currents are allowed to pass through rolling bearings, microscopic electrical discharges can occur between the rolling elements and raceways. Over time, these discharges can create pitting, fluting, surface damage and premature bearing failure. A properly designed shaft grounding system provides a lower-resistance path from the rotating shaft to ground, reducing the likelihood of current passing through the bearings.

What Causes Shaft Current?

Modern motors and generators can develop shaft voltage for several reasons. Variable-frequency drives (VFDs), high-frequency switching, electromagnetic effects and electrostatic charges can all contribute to unwanted voltage on a rotating shaft.

Fluke notes that PWM-driven motors can experience shaft-to-frame voltage that may result in bearing discharge currents. When these currents pass through the bearing, they can produce pitting and scarring of the bearing surfaces.

The basic current path can be represented as:

Motor or generator → shaft → bearing → machine frame → ground

If the bearing becomes part of this electrical circuit, repeated microscopic discharges may occur at the contact points between the rolling elements and raceways.

This is particularly important in high-speed motors, inverter-driven motors, generators and wind turbine drivetrains.

How Does a Shaft Grounding System Work?

A shaft grounding system creates an intentional electrical path between the rotating shaft and the machine ground.

A typical system consists of:

• Carbon grounding brush

• Carbon fiber grounding contact

• Brush holder

• Shaft contact surface

• Electrical connection to ground

The grounding brush remains in sliding contact with the rotating shaft. Instead of allowing unwanted current to travel through a bearing, the current can flow from the shaft through the grounding brush and into the designated grounding connection.

In other words:

Shaft → Grounding Brush → Brush Holder → Ground → Electrical Grounding System

Schunk describes shaft grounding systems as a way of safely diverting parasitic shaft currents away from bearings and gear components. Its industrial solutions include carbon brushes, carbon-fiber grounding systems and dedicated brush holders.

Why Can Shaft Current Damage Bearings?

Bearing damage caused by electrical current is often associated with electrical discharge machining (EDM)-type effects.

When the voltage across the bearing becomes high enough to overcome the insulating properties of the lubricant film, a small electrical discharge can occur. The localized energy can produce microscopic melting or surface damage.

Repeated discharges may eventually produce:

• Pitting

• Fluting or washboard-like patterns

• Raceway surface roughness

• Increased bearing noise

• Increased friction and temperature

• Premature bearing failure

Fluke identifies shaft voltage and bearing current as important factors in motor bearing damage, noting that arcing across bearing surfaces can create pitting and fluting.

This is why shaft grounding is not simply an electrical accessory. In suitable applications, it can be an important part of the machine's bearing-protection strategy.

Carbon Brushes for Shaft Grounding

Carbon is particularly suitable for shaft grounding because it can provide electrical conductivity while maintaining a controlled sliding contact with the rotating shaft.

A grounding carbon brush normally carries much less current than a conventional motor or generator power brush. Its primary function is to provide a stable electrical discharge path rather than transmit large operating currents.

Schunk notes that grounding carbon brushes are designed to transfer electrical charge across the sliding interface and have been used in industrial and wind-power applications for decades.

The brush material must nevertheless be selected according to the application. Important factors include:

• Shaft material

• Shaft diameter

• Rotational speed

• Expected shaft voltage

• Electrical resistance

• Temperature

• Humidity

• Dust and contamination

• Vibration

• Required service life

• Available installation space

Therefore, a grounding brush should not be selected only by its external dimensions.

The Role of the Carbon Brush Holder

The brush holder is another important component of the grounding system.

Its purpose is to maintain the grounding brush in the correct position while allowing the brush to follow the rotating shaft. Stable contact is essential because intermittent contact can increase electrical resistance and reduce the effectiveness of the grounding path.

A properly designed holder should provide:

• Accurate brush positioning

• Stable contact pressure

• Controlled brush movement

• Mechanical stability

• Reliable electrical connection

• Convenient inspection and replacement

Schunk specifically notes that matching brush holders help maintain the grounding brush in the correct position and continuous contact with the shaft.

Shaft Grounding in Wind Turbines

Wind turbines are an important application for shaft grounding systems.

The generator and drivetrain operate continuously under changing loads, rotational speeds and environmental conditions. Electrical currents associated with generator operation and power electronics can potentially reach rotating shafts and bearings.

For this reason, specialized grounding brushes and grounding systems are used in wind turbine generators to divert unwanted shaft currents.

Schunk's wind-turbine solutions specifically identify high-frequency and parasitic shaft currents as potential causes of damage to bearings and gear components, and describe grounding brushes as a means of diverting these currents away from the shaft-to-bearing path.

For offshore wind turbines, additional factors such as humidity, salt exposure, vibration and limited maintenance access can make material selection and brush-holder design particularly important.

Does Shaft Grounding Prevent Corrosion?

Strictly speaking, shaft grounding should not be described as a universal anti-corrosion system.

Its primary purpose is electrical protection, particularly against bearing currents and electrical erosion.

However, reducing electrical discharge can help prevent a form of surface damage that may sometimes be confused with conventional corrosion. Electrical erosion can create microscopic craters and damaged metal surfaces, while moisture, contaminants and other environmental conditions may contribute to conventional chemical corrosion.

Therefore, a more technically accurate description is:

Shaft grounding systems reduce the risk of electrical erosion and bearing damage by providing a controlled path for unwanted shaft currents.

For equipment operating in corrosive environments, the grounding system itself should also use materials and protective designs appropriate to the environment.

How to Select a Shaft Grounding Brush

Before selecting a carbon grounding brush, engineers should evaluate the complete application rather than selecting a generic brush based only on size.

Key parameters include:

Shaft diameter: Determines the contact geometry and holder arrangement.

Rotational speed: Affects friction, wear and contact stability.

Shaft surface: Surface condition, roughness, coatings and contamination can influence contact performance.

Electrical conditions: Shaft voltage, frequency and expected current determine the grounding requirements.

Environment: Temperature, humidity, dust, oil and corrosive gases can affect brush life.

Installation: Available space determines the appropriate holder and mounting configuration.

Maintenance requirements: Applications with limited access may require longer-life carbon or carbon-fiber grounding solutions.

Carbon-fiber grounding systems are another option for industrial applications. Schunk describes systems using thousands of coated, unidirectional carbon fibers contacting the rotating shaft, with designs available for different environmental conditions.

Vocarbon Shaft Grounding Solutions

Vocarbon focuses on carbon-based electrical contact components for industrial applications, including carbon brushes, carbon brush holders and shaft grounding components.

For shaft grounding applications, the carbon material, brush geometry, holder design and contact conditions should be evaluated together. Depending on the motor, generator or rotating equipment, a grounding solution may be developed around the shaft diameter, operating speed, installation space and environmental conditions.

Typical applications include:

• Industrial electric motors

• VFD-driven motors

• Generators

• Wind turbine generators

• Gearboxes

• Hydropower equipment

• Large rotating machinery

• Bearing-current protection systems

A properly engineered shaft grounding system does not simply “ground the shaft.” It creates a controlled electrical path that helps keep unwanted current away from sensitive bearings and rotating components.

Conclusion

Shaft grounding systems are an important technology for protecting rotating electrical equipment from unwanted shaft voltage and current.

By maintaining a reliable electrical contact between the rotating shaft and ground, a shaft grounding brush can provide an alternative current path and reduce the possibility of current flowing through bearings. This can help minimize electrical erosion, pitting, fluting and premature bearing damage.

For motors, generators and wind turbines, the effectiveness of the system depends on more than the carbon brush itself. Brush material, shaft condition, contact pressure, holder design, electrical characteristics, rotational speed and operating environment all need to be considered.

For this reason, selecting the correct carbon grounding brush and shaft grounding system should be treated as an application-specific engineering decision rather than simply a replacement-parts purchase.

Vocarbon can provide carbon brush and grounding component solutions for industrial rotating equipment, including customized designs based on application requirements, dimensions and operating conditions.