Rolling element bearings in electric motors generally serve to support and locate the rotor, keep the air gap small and consistent, and transfer loads from the shaft to the motor frame. Applications abound in consumer appliances.

Among them, split-phase, capacitor-start, and permanent split-capacitor motors typically drive clothes washer drums; shaded-pole and permanent split-capacitor motors power dishwasher water pumps, range hoods, air-conditioning systems, and fans; and single-phase induction motors drive tumble-dryer drums.

All these motor applications for appliances rely on various types of ball bearing arrangements to deliver machine efficiency, low noise and vibration levels, and optimized service life without lubrication.

But premature bearing failure can arise from any number of root causes, ultimately impacting on appliance performance, warranty, and remedial service and recurrence. A particular threat arises from stray electric currents passing across bearings and causing significant electrical erosion damage in their wake.

The problem

Close-up shot of fluting patterns in bearing raceways, a warning sign of bearing damage from electric arcing.

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) for variable-speed motors.

Once electric arc bearing damage has begun, excessive vibrations, increased heat, increased noise levels, and the reduced effectiveness of the motor bearing’s lubricant will all contribute to shortening the bearing’s service life.

The extent of damage to bearings will depend on the amount of energy and its duration. However, the effect on bearings usually will be the same -- pitting damage to the rollers and raceways, rapid degradation of the lubricant, and premature motor bearing (and appliance) failure.

Electric arcing will occur if there is a difference in potential between the motor shaft and the bearing housing. (Even a difference of a few volts in potential can produce the effect.). The voltage level when arcing occurs depends on ball size, operating speed, current frequency, and bearing geometry.

The damage

Enlarge this picture Table 1. Comparison of properties of standard bearing steel and silicon nitride used in hybrid bearings.

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 analogous to a series of small “lightning strikes,” which melt and retemper internal bearing surfaces. The result is that some surface material flakes away and spalls out. This creates noise in the bearing and has the potential to shorten service life.

“Cratering” is perhaps the most commonly experienced effect of electric arc damage. This effect is characterized by molten pit marks that are generally too small to be visible to the eye. However, a dull gray surface of the rolling element can serve as a visual warning sign of cratering to indicate that bearing deterioration is present.

Another telltale and noticeable warning sign of bearing damage from electric arcing will present itself as characteristic “fluting” patterns in the raceways of bearings. Fluting is caused by the dynamic effect of the rolling elements continually moving over the microscopic 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.

A solution

In the quest to “insulate” against the arc damage problem, recent advances in technology and materials have been shown to make a difference. One solution is to use hybrid ball bearings, which substitute ceramic balls for steel rolling elements.

Hybrid ball bearings have 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.

Compared to conventional all steel ball bearings, hybrid bearings exhibit some key advantages:

• Lower density. Silicon nitride balls are 40 percent less dense than similarly sized steel balls, which results in a reduction of centrifugal force and friction. This permits higher speeds, less weight, lower inertia, and more rapid starts and stops. In short, the bearings can run faster and cooler, thereby saving energy.
  
• 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 load distribution.

From the perspective of appliance performance and reliability in particular, hybrid bearings can afford the following advantages:

• Extended service life. The properties of ceramics combine to hold the promise of service life up to 10 times that of a standard steel bearing.
  
• Reduced 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 vibration. In appliances exposed to static vibration, there is an inherent risk of false brinelling, a wearing away of the surfaces within the ball and raceway contacts that  can eventually lead to spalling and premature failure. Lighter-weight ceramic balls keep the potential for false brinelling to a minimum.
  
  Experienced product and service partners can serve as reliable resources to help keep appliance designers and manufacturers current on new materials developments and bearing designs that can help OEMs obtain the best possible performance and service life for bearings.