Jay S. Carlson, MRC Bearing Services
Ball bearings in electric motors 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 contribute to 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 result 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 voltage level when arcing occurs will depend 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 Damage Results: 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.
Warning Signs: 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 warning sign 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.
Should electric arc bearing damage be suspected, bearings should be replaced and proper insulation should be provided to prevent electric currents from passing through: Hybrid ball bearings (which substitute ceramic balls for steel rolling elements) offer an advanced and practical solution.
Hybrids incorporate 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 both in ac and dc motors. Hybrid bearings further possess a higher speed capability and can sustain longer service life than all-steel bearings in most applications for a variety of reasons.
Among the key characteristics of hybrid bearings compared with all-steel counterparts:
- Lower density: Silicon nitride balls are 40% less dense than similarly sized steel balls, reducing 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% 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% of similar steel rolling elements. This means less sensitivity to temperature gradients for more accurate preload control.
All these characteristics can deliver key potential benefits to users:
- 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% 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. 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 minimize the potential for false brinelling.
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.