As we’ve mentioned before, electric motors account for a majority of power usage in the US industrial sector and when they invariably fail, bearings are usually the main culprit.
Protecting the bearings on electric motors would, logically, help prolong the operation life of a motor. One solution would be using grounding brushes to prevent shaft currents or contamination from damaging the bearings. Another solution would be to use coated bearings. Writing at Design News, Daniel Juchniewicz, an applications engineer for SKF USA, suggests coated bearings could be the antidote for many bearing related problems.
Among other safeguards, a variety of specialty coatings uniquely formulated for bearings offers added lines of defense against adverse operating conditions. Most bearing types can be coated. Coated bearings can be specified into new applications or serve as “drop-in” replacements within design envelopes — eliminating any requirements for drastic design changes or reconfigurations.
Juchniewicz provides three areas where specialty coatings would benefit bearings:
- Delivering wear resistance
- Insulating against electric currents
- Turning to wind turbine reliability
Delivering wear resistance
Bearings can be prone to wear for any number of reasons, whether due to poor lubrication conditions (associated with high temperatures and/or low lubricant viscosities), sudden variations in load or speed, standstill vibration, and/or particle or liquid contaminants. All can dramatically shorten bearing (and equipment) service life over time.
As an effective solution for premature wear, a patented wear-resistant carbon coating can be applied (by the manufacturer) to the bearing rolling elements and/or raceways. The coating (only a few microns thick) covers the contact surfaces using a specialized process ideally suited for bearings. The resilience of the bearing steel is not only retained, but its surface hardness is also doubled compared with standard grades.
Juchniewicz said that for a coating to properly function in a bearing, it has to meet three standards: withstand rolling contact fatigue, create little friction, and prevent adhesive wear. He states that this type of specialty wear resistant coating meets all three standards.
Insulating against electric currents
Variable frequency drives (VFDs) provide many benefits when properly installed to manage motor functions, however they also create a potential hazard: shaft currents. Currents generated by a VFD pass along the motor’s shaft and, if correctly installed, go to ground. However, if there are no other options, the current may use the bearings as its path to ground. The resulting damage to the bearings from electric arc aren’t the only hazard to the health of the bearing.
Stray currents can damage the contact surfaces of a bearing’s rolling elements and raceways (electrical erosion) and rapidly degrade bearing grease. An additional risk develops from high-frequency currents due to inherent stray capacitance — and the risk of damage increases if the application employs a frequency converter.
Once electric arc bearing damage has begun, excessive vibration, increased heat, increased noise levels, and reduced effectiveness of the lubricant will combine to shorten a bearing’s service life. The extent of damage to bearings will depend on the amount of energy and its duration, but the effects usually will be the same: pitting damage to the rolling elements and raceways, rapid degradation of the lubricant, and premature bearing failure.
Designers can “insulate a bearing from the outside” by specifying a very thin aluminum oxide coating to form a barrier against electric arcing and subsequent erosion of the bearing rolling surfaces. The coating is applied (by the manufacturer) on a bearing’s outer ring or inner ring using a spraying technique and then sealed with a resin to protect against the conductive effects of water and moisture. Standard insulating coatings can withstand voltages of up to 1,000 V DC, or coatings can be developed for voltages as high as 3,000 V DC.
Turning to wind turbine reliability
One of the more challenging applications for bearings, Juchniewicz writes, are wind turbine systems. Varying temperatures, speeds, and loads continually test the bearings while the turbine is in service. In addition, the bearings will also have to withstand contaminants, moisture, and chemicals, which pose direct threats to the life-cycle of the bearings and effect turbine operation and maintenance costs.
In response, a newly developed black oxidation surface treatment has been introduced in the marketplace to create a robust layer of protection against these demanding conditions. The coating can be specified for all types of critical bearings in wind turbine systems.
The surface treatment is applied (by the manufacturer) to a bearing’s rings and/or rolling elements. The process creates a thin, dark black surface layer delivering a significant performance upgrade for the broad range of bearing types and sizes in wind turbines (up to 2.2 m in diameter and up to 1,000 kg per individual bearing component).
Because of the multitude of conditions in which bearings operate, your mileage may vary when using specialty coated bearings. However, regardless of your bearing application, consulting with experts is a very important first step.
These examples underscore that an array of specialty coating options exists for designers seeking to maximize bearing service life and performance in tough conditions. Our recommended best practice for designers at the outset: Consult with a qualified and experienced bearing manufacturer early in the design stage to help identify the inevitable application challenges, as well as the available solution products for application success.