Internally vs. Externally Installed Shaft Grounding Devices

There has been a strong industry trend to internally install onto the motor a shaft grounding device. This installation approach checks all the marketing boxes in terms of the perceived “set it and forget it” convenience factor. However, this article discusses why an internally installed shaft grounding device is less than ideal and also discusses a better, more reliable fix for electrically induced bearing damage. It also addresses the concept of product robustness in the context of industrial applications.

Why Not Just Install the Grounding System Inside the Motor?

Claims stating a shaft grounding device does not need to be maintained because its contact surfaces are friction free and therefore never wear are simply false. When two surfaces come in contact there will always be friction and wear. Surfaces with the lowest coefficients of friction will wear over time. That’s just basic physics. Our lab studies have shown fibre-based grounding devices do in fact wear down just like any other material. To address this problem, these grounding devices proclaim that if shaft-to-fibre contact is lost, then the flow of electrons can occur across that gap. Our lab studies have shown the fibre wear rate over a relatively short period of time will be too great for flow of electrons to occur rendering the grounding device ineffective. Furthermore, shaft voltages will begin to climb in a short period of time and well beyond what is known to cause dielectric breakover through the grease to the bearing.

A little math to illustrate just how much friction wear a fibre-based grounding device is subject to:

For a 3-3/8” shaft diameter running continuously at 1800rpm, each bristle will experience 506 feet/minute of shaft traveling along its surface. Extrapolated over the service life of the motor, each bristle will experience over 500,000 miles of travel. That is a lot of friction to not have significant wear to occur. Even installed inside the motor, the grounding device will be subjected to dust, grease, etc. causing additional wear and tear of the device thereby further accelerating how quickly wear rates occur and how fast shaft voltages climb.

Knowing fibre-based grounding devices have their own wear rates like any other grounding system, installing a grounding device inside a motor then becomes very problematic as maintaining that grounding device is almost non-feasible. Maintaining an internally installed grounding device for the service life of the motor and beyond is simply not an option and causes large downstream maintenance and downtime costs.

Why Not Just Install the Grounding System on the Outside of the Drive End of the Motor?

Fibre-based shaft grounding devices are often installed on the outside of the drive end of the motor, but their fundamental design makes them non-maintainable. In short, maintaining a long-term path of least resistance away from the bearing means the entire grounding device must be replaced. The fibres are not maintainable. If the ring is not a split ring design, then the motor needs to be uncoupled to remove the grounding device. If the ring is a split ring design, then the motor still needs to be taken offline to replace the device. Replacing these rings can be very cumbersome as they are often epoxied onto the motor’s housing in an effort to make shaft-to-frame continuity.

An Alternative to Drive End Internally and Externally Installed Grounding Devices:

Over the course of three decades, SGS^TM non-drive end shaft grounding systems have proven to be the only long-term fix for electrically induced bearing damage. Why is that?

1. As the only end-of-shaft application on the market, our patented brush running surface does not ride directly on the steel shaft, but rather on a special end-of-shaft rotor assembly. Because of this patented and proprietary design, the rotational travel our brushes are subjected to is far less distance than any grounding system with a running surface along the shaft’s diameter. Less wear equals increased system performance and long-term reliability. SGS^TM non-drive end shaft grounding systems are expected to run for up to 10 years at 1800rpm prior to needing brush maintenance. That run time is far greater than any other grounding system on the market.
2. Because of our patented non-drive end, end-of-shaft applications, SGS^TM shaft grounding systems possess a very unique feature. The brush can be safely checked or replaced without needing to shut down the motor. Just unscrew the brush barrel cap and the brush can be removed, checked and if needed, replaced in just a couple of minutes. Inexpensive replacement brushes are always available for same day shipping nationwide. DP&A Sales continues to support originally installed SGS^TM shaft grounding systems from 20+ years ago.
3. SGS^TM non-drive end shaft grounding systems completely and effectively address the negative effects of the environment on the brush’s running surface. The end of shaft application allows the brush and its running surface to be fully sealed from the environment while keeping its class leading maintainability. Dust, grease, dirt, etc. cannot affect the path of least resistance established by the grounding system. This patented design contains all the brush’s wearables, but in a manner that keeps the system’s wearables away from the running surface. That design feature allows SGS^TM shaft grounding systems to maintain a very low path of least resistance away from the motor’s bearings and for the entire wear life of the brush.
4. When comparing a fibre based or carbon-based shaft grounding device with an SGS^TM shaft grounding system, you will notice how much more robustly an SGS^TM grounding system is built. It’s like comparing a plastic Casio watch to a Swiss built stainless steel watch. There simply is not a comparison. If properly maintained via very infrequent brush changes as described above, this product robustness allows an SGS^TM grounding system to survive in just about any environment for decades while performing its duty of being the path of least resistance away from the bearing. Although SGS^TM shaft grounding systems can cost a bit more up front than its competitors, that cost differential is fully justified when considering the true downstream costs of a less robustly built and poorly performing shaft grounding device such as a fibre-based shaft grounding product or a small carbon-based device that rides directly on the steel shaft. A maintenance free shaft grounding product that can protect a motor for its service life is a “too good to be true” situation. When considering your shaft grounding options, be thinking in terms of product robustness relative to the environment, product maintainability and its basic design principals that allow the system to withstand a 24/7 industrial environment.

Patented SGS^TM CR Series non-drive end shaft grounding system installed on a TEFC motor. Note the brush barrel cap in the middle of the robust grounding system body. This cap can be removed allowing full access to the brush while the motor is running. All SGS^TM products are heavy duty and manufactured from corrosion resistance materials and add to the class leading longevity of the system.