Keeping current - ball bearings for electric motors

by Jay S. Carlson, MRC Bearing Services

The “perfect” operating environment for ball bearings in variable-speed electric motor applications 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 and maintenance staffs must grapple with a variety of root causes. A particular threat arises from stray electric currents passing across bearings and causing significant electrical erosion damage in their wake, as well as potential premature bearing failure, unscheduled machine downtime, and unanticipated maintenance costs. The good news is that there are warning signs for this problem and ways to “insulate” against further interference or recurrence.

The problem
Ball bearings in electric motors serve to support and locate the rotor, keep the air gap small and consistent, and transfer loads from the shaft to the motor frame. When a stray current in a machine 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).

Once electric arc bearing damage has begun, excessive vibrations, increased heat, increased noise levels, and the reduced effectiveness of the lubricant will shorten a bearing’s service life. The extent of damage to bearings will depend on the amount of energy and its duration.

However, the effect usually will be the same: pitting damage to the rollers and raceways, rapid degradation of the lubricant, and premature bearing failure.

Why arcing occurs
Electric arcing will happen 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.) And it is not only the motor bearings that can be impacted: A stray current can damage bearings in the machinery directly coupled to a motor, too.

The voltage level when arcing occurs depends on ball size, cage type, and seal design. For two bearings the same size, arcing occurs at a higher voltage level for the open variant than for the sealed variant. If the bearing is equipped with pressed steel shields, the risk of arcing will be higher, because the “insulating part” is only the air gap between the electrically conducting shield and the bearing inner ring.

How the damage result
When an electric current passes through the contact zone of a bearing’s rolling elements and raceway, the energy of the electric discharge generates heat, causing 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. The outcome is that some surface material flakes away and spalls out to create noise in the bearing and potentially shortened service life.

“Cratering” is perhaps the most commonly experienced effect of electric arc damage. This type 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 telltale and noticeable warning sign of bearing damage from electric arcing will present itself as 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.

And even if a bearing is not directly influenced by electric discharge, the bearing’s lubricant may become a target and begin to degrade with dire consequences.

This is because localized high temperatures can cause additives in lubricants to char or burn the base oil and, in turn, additives will be consumed more quickly and the lubricant will turn black and hard. The ensuing rapid breakdown can drastically shorten grease life and lead to secondary bearing damage due to poor lubrication.

The clear message: Should electric arc bearing damage be suspected, bearings should be replaced and proper insulation should be provided to prevent electric currents from passing through.

The solutions
In the quest to “insulate” against the problem, recent advances in technology and materials have blazed new ground. Solutions now include hybrid ball bearings (which substitute ceramic balls for steel rolling elements) and coatings for insulation.

Hybrid ball bearings
Hybrid ball bearings feature rings made from bearing steel and rolling elements manufactured from bearing grade silicon nitride. Because silicon nitride has high resistivity, hybrid bearings provide ideal insulation from electric currents in both AC and DC motors. In addition, hybrid bearings possess a higher speed capability and can sustain longer service life than all-steel bearings in most applications for a variety of reasons.

Some key characteristics of hybrid bearings compared with all-steel counterparts are:

• Lower density. Silicon nitride balls are 40 percent less dense than similarly sized steel balls, which reduces centrifugal force and friction. This means higher speeds, less weight, lower inertia and more rapid starts and stops. In short, the bearings can run faster and cooler.

• Higher hardness. Ceramic balls are harder than both steel and most potential particle contaminants. This means the bearings can eliminate contaminant particles either by crushing them or pressing them into the (softer) steel rings, where they can be rendered harmless.

• Lower friction. Silicon nitride’s low coefficient of friction enhances wear resistance to enable the bearing to run cooler even under poor lubrication conditions. This means better lubrication, less noise, and lower operating temperatures.

• Higher modulus of elasticity. Ceramic rolling elements have a 50 percent higher modulus of elasticity than steel. This means increased bearing stiffness and reduced deflection under load to promote reliable performance.

• Lower coefficient of thermal expansion. Ceramic rolling elements have a thermal expansion of only 29 percent of similar steel rolling elements. This means less sensitivity to temperature gradients for more accurate preload control.

From an MRO perspective these characteristics allow users to realize:

• Lower maintenance and energy costs. Maintenance costs can quickly multiply if a bearing must be changed frequently and an extension in the service life of a bearing without increasing maintenance costs can contribute to reductions in the overall operating cost of equipment. Less friction adds up to lower energy costs.

• Extended service life. Most bearings are designed into applications based on loading conditions and do not take into account factors such as lubrication, contamination, and maintenance. Without proper attention to these external factors, a steel bearing rarely reaches its optimized design and service life.

The properties of ceramics combine to hold the promise of service life up to 10 times that of a standard steel bearing.

• Extended grease life. In environments imposing high demands on the bearing lubricant, standard bearings experience surface wear due to insufficient lubricant film and bearings can fail if the initial grease charge is not replenished within an acceptable timeframe. Hybrid bearings run cooler and can operate with thinner lubricant films, so there is less aging of the grease and relubrication intervals can be longer for increased service life compared with standard bearings in the same operating conditions.

• Lower operating temperatures. The heat generated in bearings is attributed to viscous friction between the balls and raceways. The source of the loading is both external and internal, and little can be done to reduce the external loads. However, since ceramic balls have only 40 percent of the density of steel balls, less centrifugal load is generated by the balls and the internal friction is lower. This translates to cooler running for the same operating conditions (or, if applicable, a higher rotational speed while maintaining the same temperature).

• Reduced wear from contamination. In contaminated environments solid particles create dents in the rolling surfaces and raised edges around those dents, causing noise and premature wear as steel balls roll over those surfaces. As noted, the harder ceramic ball material puts contaminants in their place.

• Reduced wear from vibration. In equipment exposed to static vibration there is an inherent risk of false brinelling (wearing away of the surfaces within the ball and raceway contacts), which can eventually lead to spalling and premature failure. Lighter-weight ceramic balls keep the potential for false brinelling to a minimum.

Coatings for insulation
Another solution has arisen with the development of very thin aluminum oxide coatings to help form a barrier for steel ball bearings against electric arcing. Coatings can be applied (using a plasma spraying technique) either on a bearing’s outer or inner ring. The standard layer thickness recommended for these coatings has been shown to prevent most current passage problems.

Viable coatings for such “insulation from the outside” have been performance tested to 1,000 volts DC with a minimum ohmic resistance of 50 M. Coated bearings further can be treated with special sealants to provide high thermal stability and resistance to heat, chemicals, and moisture.

Whether selecting hybrid bearings (“insulation from the inside”) or specialized coatings, insulated bearings generally will be mounted on the non-drive end of converter-driven induction motors.

One more pointer: Experienced product and service partners can serve as reliable resources to help keep users “current” about these and other remedial solutions for electric arc bearing damage. The best possible performance and service life for bearings will follow.

Jay S. Carlson is marketing manager for MRC Bearing Services, 1510 Gehman Road, Kulpsville, PA 19443. Phone: 716-661-2727; Fax: 716-661-2740. Web site: www.mrcbearingservices.com.

This article appeared in the October/November 2006 issue of MRO Today magazine. Copyright 2006.

Back to top

Back to Uptime archives