How to Prevent Electrical Erosion in Bearings


Written by:
Daniel R. Snyder, SKF USA Inc.

Pumps and Systems, February 2007

Bearings perform vital roles in enabling optimized operation and service life of pumps used throughout industry. In centrifugal pump applications, for example, crucial functions abound.

The bearings support hydraulic loads imposed on the impeller, the mass of the impeller and shaft, and induced loads due to couplings and drive systems. They help keep the shaft end-play and axial deflections within acceptable limits for the impeller and seal assembly. They also act to reduce friction, which, if uncontrolled, can result in loss of power, excessive heat generation, increased noise and/or wear, and early seal failure.

Within the electric motors driving pumps, bearings particularly serve to support and locate the rotor, keep the air gap small and consistent, and transfer loads from the shaft to the motor frame.

The "perfect" operating environment for bearings in virtually any application would be contaminant-free and have low humidity, low non-fluctuating temperature, good alignment, and no vibration. Loads would be perfectly balanced. Theoretically, bearings could last indefinitely in a perfect world.

In practice, however, premature bearing failure can occur. In the case of electric motors, an estimated 50 percent of all failures are attributed to bearings, although the bearings themselves are not usually the root cause.  There are other forces at work, and if bearings exhibit signs of damage, they can often point the way to the true cause and fix. The damage may indicate alignment, mounting, temperature, lubrication, loading, and/or sealing problems.

Many problems can be traced back to poor installation or maintenance practices. But when pumps are run by variable speed electric motors using frequency converters, the motors can tend to generate stray electric currents.  And stray currents in electric motors pose an increasingly common problem for bearings.

When a stray current in a motor uses a bearing as its path to ground, the resulting damage is referred to as electric arc bearing damage. The most common causes of electric arc bearing damage include asymmetry in the motor's magnetic circuit, unshielded power cables, and fast-switching variable frequency drives (VFDs).

Electric arcing will develop if there is a difference in potential between the shaft and the bearing housing. (Even a difference of a few volts in potential can produce the effect.) The motor bearings are not the only components that can be impacted: a stray current can also damage bearings in the machinery directly coupled to a motor, too.

Once electric arc bearing damage has begun, excessive vibrations, increased heat, increased noise levels, and the reduced effectiveness of the lubricant will combine to shorten a bearing's service life.

The extent of damage to bearings depends upon the amount of energy and its duration. However, the effect is usually the same: pitting damage to the rollers and raceways and eventual bearing race spalling, rapid degradation of the lubricant, and premature bearing failure. Unscheduled equipment downtime and unanticipated maintenance costs will likely follow.

The damage occurs when an electric current passes through the contact zone of a bearing's rolling elements and raceway and the energy of the electric discharge across the contact's lubricant film generates heat, which causes localized melting of the surface. The effect on a bearing is almost like a series of small "lightning strikes" which melt and retemper internal bearing surfaces. Some surface material flakes away and spalls out, potentially shortening the bearing's service life.

The good news is that there are signals telegraphing this problem and ways to "insulate" against such interference or recurrence. Among telltale warning signs, "micro-cratering" on a bearing is perhaps the most commonly experienced effect of electric arc damage. This is characterized by molten pit marks (invisible to the eye). A dull gray surface of the rolling element will send a visual warning sign of cratering to telegraph that bearing deterioration is present.

Another indicator of bearing damage from electric arcing will be characteristic "fluting" (or "washboarding") patterns in the raceways of bearings. Fluting is caused by the dynamic effect of the rolling elements continually moving over the micro-"craters" and etching a rhythmic pattern into the running surfaces of a bearing's races. Noise and vibration from the bearing increases and, eventually, the deterioration will lead to complete bearing failure.

Fluting damage

Characteristic "fluting" patterns (or "washboarding") on bearing raceways can indicate bearing damage from electric arcing.

The overriding message: If and when electric arc bearing damage is suspected, bearings should be replaced and proper insulation should be provided to prevent electric currents from passing through. Replacement solutions have advanced with the engineering of hybrid ball bearings in motors using ball bearings. As an alternative, most bearing types (including roller bearings) can be specially coated to form a superior insulating barrier against stray currents and the damage they can inflict.

 

Both the hybrid bearings and the coated bearings have proven to be highly effective solutions in controlling arcing damage.

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