For electric motors, there’s no universal “one-size-fits-all” solution for anti-friction bearings. Every bearing type incorporates specific performance characteristics and so suits particular operating conditions. A mismatch between bearings and motor application can lead to significant problems.
Consider, for example:
- Using motors larger than 125 hp, faster than 1,200 rpm and having deep-groove ball bearings optimized for direct-connect duty with belts may lead to mechanical overload and premature failure.
- Putting a motor designed for heavy belting on a coupled load may not give enough radial loading for the bearing’s rolling elements to roll. They will skate or skid on the race, causing high temperatures and potentially rapid and complete lubrication failure.
- Underloading a roller bearing, even with a belted load, may result in premature failure from the same skidding phenomenon.
- Installing a motor with bearings selected for horizontal mounting in a vertical application adds an unplanned axial bearing loading and may cause loss of grease in the bearings.
- Not addressing stray currents passing across bearings (often encountered in large variable-speed electric motors) may prompt electrical erosion damage leading to premature bearing failure.
While there’s always the possibility that a replacement motor may perform differently from the original or, even worse, fail, selecting the bearings inside a motor consistent with application and conditions greatly improves the chances for success.
Bearings and motor design
Rolling bearings (ball and roller) 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. The bearings should enable high- and low-speed operation, minimize friction, exhibit low noise, promote long life and save power.
Ultimately, the application will drive bearing selection. Appropriate bearing types for electric motors include deep-groove ball bearings, angular-contact ball bearings, cylindrical roller bearings, tapered roller bearings, spherical roller bearings, toroidal roller bearings and spherical roller thrust bearings. Each has its own configurations and performance characteristics.
Electric motors typically incorporate a locating and non-locating bearing arrangement to support the rotor radially and locate the rotor axially relative to the stator. Locating bearings position the shaft and support axial loads while non-locating bearings permit shaft movement in the axial direction and compensate for overload conditions when thermal expansion of the shaft occurs.
For smaller motors used in horizontal equipment, the most common bearing arrangement consists of two deep-groove ball bearings mounted on a short shaft in a cross-locating bearing arrangement to control shaft movement. In most medium and large electric motors for horizontal machines, deep-groove ball bearings act as the locating bearing while the bearing in the non-locating position may be a ball bearing or cylindrical or toroidal roller bearing, depending upon the loads, speeds, temperature and environment of the application. The non-locating bearing accommodates any axial expansion due to heat or machine tolerances.
Vertical machines typically rely upon deep-groove or angular-contact ball bearings or spherical roller thrust bearings, depending upon the vertical load, shaft and rotor weights, speeds, temperature and operating environment.
The impact of load
The nature of the load is a primary factor in bearing selection. Bearings always require a given minimum load for proper rolling-element rotation and enhanced lubricant film formation in rolling contact areas. Otherwise, skidding can occur, resulting in higher operating temperatures and lubricant degradation.
The bearings, of course, must provide a sufficient load capacity to yield satisfactory service life in the application. All loads must be considered — not just the weights involved and the forces derived from the power transmitted but also coupled and belted loads from connection to the driven load.
With a coupled load, the motor shaft usually is connected via a flexible coupling to the driven load’s shaft. This type of load presents no axial or radial load to the motor bearings except for the weight of the motor’s rotors and shaft assembly. (Misalignment from mounting errors, however, can add radial load.) Ball bearings are typical choices for coupled loads.
The most common type of belted load is when a V-groove pulley is mounted on the motor shaft in an overhung arrangement and is connected to another pulley on the driven load by one or more belts held in tension. This can generate high radial loads on the motor shaft and the drive-end bearing (because it’s closest to the applied external loading). Depending upon the magnitude of radial load, either ball or roller bearings can be employed.
On smaller motors under normal conditions, a ball bearing may serve in applications with either coupled or belted loads.
In general, heavy loads are carried by roller bearings and lighter loads by ball bearings. Loads can be radial, axial or a combination of the two. Certain bearings, such as cylindrical roller types, usually are used for radial loads only; others, such as angular-contact ball bearings, mainly suit axial loads.
Cylindrical and toroidal roller bearings only can support pure radial loads with minimal axial loads. Other radial bearings (such as tapered and spherical roller bearings) can accommodate axial loads in addition to radial loads but also have minimum load considerations.
Angular-contact ball bearings can support moderate axial loads at relatively high speeds. For moderate and heavy axial loads acting in one direction, specify spherical roller thrust bearings. A bearing’s ability to carry an axial load will be determined by the angle of contact or load action internal to the bearing (the greater the angle the more suitable the bearing for axial loads).
Single-row angular-contact ball bearings and tapered roller bearings only can accommodate axial loads acting in one direction. In applications with axial loads of alternating direction, these bearings must be combined with a second bearing capable of supporting axial loading.
A combined load results when radial and axial loads act simultaneously. The most common bearing solutions in these cases are single- and double-row angular-contact and single-row tapered ball bearings (although deep-groove ball bearings may also be appropriate, depending upon the ratio of axial to radial loading).
Other primary factors for bearing selection include:
Connection type. The nature of the device placed between the drive and the driven unit (coupling, belt or gear drive) influences the loads on motor bearings and resulting performance.
Good alignment is important in both flexible and rigid coupling drives because misalignment can induce additional forces into the bearing system and reduce service life. Proper alignment is particularly crucial with a rigid coupling where typically three bearings are on a shaft (two in the motor and a third in the coupled device). When rigid couplings are aligned very accurately, the drive-end bearing might become relatively unloaded. In these cases, a deep-groove ball bearing would be recommended at the drive end.
A belt or gear drive frequently will load the motor bearings more heavily than a coupling drive; cylindrical roller bearings most often are used at the drive end where significant loads are encountered. In applications with heavy loads and the possibility of misalignment or shaft deflection, spherical and toroidal roller bearings offer solutions.
Speed. The rpm influences operating temperature and, in turn, bearing and lubricant life. So, cage, lubricant, running accuracy and clearance of the bearings, the resonance frequency of the system, and the balancing of the rotating components are significant factors in bearing selection.
For high-speed applications, ball bearings generally prove more appropriate than roller bearings. In very-high-speed applications, precision or hybrid bearings may offer benefits.
Temperature. The permissible bearing operating temperature in a motor application will limit the speed at which rolling bearings can perform. Bearing types with low friction and corresponding low heat generation inside the bearing will work well in high speed operation. The highest speeds can be achieved with deep-groove ball bearings when loads are purely radial and with angular-contact ball bearings for combined loads. This holds especially true for such bearings with ceramic rolling elements.
Type of lubrication. Under normal speed and temperature conditions, the bearings in electric motors usually are lubricated with grease. Grease allows for simpler more-cost-effective housing and sealing designs, offers better adhesion of lubricant to critical surfaces, and provides more reliable protection against contaminants than oil.
The life expectancy of grease depends upon several factors, including the type of bearing, the type of grease, the orientation and speed of the motor, and the operating temperature of the bearings.
Small ball bearings in standard electric motors usually are fitted with seals or shields and lubricated for life — they aren’t intended to be relubricated but replaced at normal motor repair intervals. Severe-duty motors, regardless of size, often are supplied with open bearings and provisions for regreasing. (If the grease life is shorter than the expected bearing life, the bearings obviously will need to be relubricated while the grease is still performing as intended.)
Sometimes, rotational speeds or operating temperatures make it impractical or impossible to use grease because the grease life or relubrication times are too short. These cases demand oil lubrication. In general, only large electric motors are oil-lubricated, in part due to the need for more sophisticated seals and the potential risk of leakage from the systems.
Tips for longer life
When motors fail, bearings may be the culprit but possible non-bearing causes also abound. These encompass windings, wiring, grease or seal failures, which, in turn, may result in bearing failures (although bearings are not the root cause). Improper motor use and inadequate maintenance can add to potential problems and premature bearing failure.
Here are some potential problems along with pointers on how to avoid or address them:
Electric arcing. In variable-speed motors, stray currents from arcing may cause bearing damage. Although arcing typically tends to be isolated and localized, the effect on a bearing is almost like a series of small lightning strikes that melt and retemper internal bearing surfaces. The result is that some surface material flakes away and spalls out to create noise in the bearing and potentially shortened service life.
Users can be sure that bearing damage from electric arcing has occurred when they notice characteristic “fluting” patterns (Figure 1).
Fluting is caused by the dynamic effect of the rolling elements continually moving over the microcraters 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.
Even if a bearing isn’t affected by these discharges, its lubrication could be. Grease composition can degrade rapidly due to the high localized temperatures generated by current discharges.
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 firstname.lastname@example.org.