One way to head off arcing problems is to insulate the bearings from the shaft currents. Specialized ceramic coatings can be applied on the outside or inside diameter of the bearing to prevent currents from flowing through the bearings. Hybrid bearing designs, which substitute ceramic balls or rollers for the metal rolling elements within a bearing, offer another solution (Figure 2). They effectively insulate bearings “from the inside.”
Moisture. This can’t always be controlled but it can be managed. When motors are running, humidity usually isn’t harmful. However, when they’re turned off and cool, condensation builds up. Condensation can’t be stopped but using grease fortified with rust inhibitors in bearing assemblies and frequently rotating the shafts of idle motors whenever condensation is suspected can guard against the harmful effects. Good seals can help keep humidity from invading the cavity. Avoiding direct water spray on seals during washdowns also is important.
Shaft misalignment. A common root cause of premature bearing failure, such misalignment between the shafts of the motor and the driven equipment introduces excessive vibration and internal bearing loads and will shorten the working life of an electric motor.
Couplings typically are flexible to accommodate misalignment. However, don’t take their flexibility for granted. For ideal shaft alignment, first secure the driven equipment and then install the coupling. Only after the coupling is attached to the equipment should the motor be moved into proper alignment and secured.
Improper lubrication. Effective bearing lubrication demands the proper type and quantity of lubricant, replenishment interval and application method.
A new electric motor should arrive with its bearings properly lubricated for the dimensions of the bearing envelope (Figure 3).
No general rule governs correct lubrication intervals. Instead, base the intervals on bearing size and type, speed of operation, the general operating environment and the type of electric motor. (Vertical motors require lubrication twice as often as horizontals.) Bearings sealed or shielded for life normally shouldn’t be relubricated.
Before lubricating a bearing, determine the grease currently in use and select either the same type of grease or a compatible product — not all greases are compatible. (Compatibility charts are available from lubricant manufacturers.) Always take into account the motor manufacturer’s recommendation.
Avoid over-lubrication. Adding more lubricant than specified can result in increased friction and temperature and, thus, reduced grease life — potentially harming a bearing and adversely affecting motor performance. Rolling elements require more energy to rotate if there’s too much grease. This places a greater burden on the motor. Over-lubricating also can cause undesirable heat buildup as the rolling elements attempt to push the extra grease out of the way. Heat buildup leads to friction, wear and reduced grease life.
Don’t lose your bearing
Proper selection, installation and maintenance of bearings can contribute to optimized performance and service life of electric motors. It can be tough to sort through all the interrelated factors, but you’d don’t have to do this on your own. A knowledgeable bearing manufacturer can help you find the best solutions to meet specific application demands.
Daniel R. Snyder, P.E., is director, applications engineering, for the industrial division of SKF USA, Kulpsville, Pa. E-mail him at email@example.com.