How to make bearings last in electric motors

Daniel R. Snyder, P.E.
Director, Applications
Engineering
SKF Industrial Div.
SKF USA Inc.
Kulpsville, Pa.

Rolling-element bearings in electric motors support and locate the rotor, maintain a small and consistent air gap between the rotor and stator, and transfer loads from the shaft to the motor frame. The correct bearings for an application let a motor run efficiently across its design speed range, minimize friction and power loss, produce little noise, and have a long service life.

On the other hand, bearings can be quickly ruined when a motor is used improperly. For example, the deep-groove ball bearings optimized for in-line couplers can overload if motors fitted with them drive a belt pulley. Likewise, motors containing roller bearings for heavy belt loads may prematurely fail when run with an in-line coupler because a minimum load is not maintained.

Electric motors typically incorporate a locating and nonlocating bearing arrangement to support the rotor and locate it relative to the stator. Locating bearings position the shaft and support radial and axial loads, while nonlocating bearings handle radial loads and let shafts move axially to prevent overloading from thermal expansion.

The most common setup for smaller motors in horizontal machines consists of a pair of deep-groove ball bearings mounted on a short shaft in a cross-locating arrangement. Medium and large electric motors typically use deep-groove ball bearings for locating. The nonlocating bearing may be a ball bearing, cylindrical-roller bearing, or toroidal-roller bearing, depending on the loads, speeds, and operating environment. Motors for vertical machines typically incorporate deep-groove ball bearings, angular-contact ball bearings, or spherical-roller thrust bearings.

LOADS AND SPEEDS
Regardless of type, bearings need a minimum load so rolling elements rotate and form a lubricant film rather than skid. Skidding raises operating temperatures and degrades lubricants. A general rule of thumb for roller bearings places a minimum load equal to about 0.02 times the dynamic radial-load rating. For ball bearings, that number is 0.01. Maintaining (at least) minimum loads is especially important when bearings see high accelerations and speeds that are roughly 75% of recommended ratings.

In general, power output governs shaft size and bearing bore diameter. Of course, load magnitude and direction also determines bearing size and type. Designers sizing motor bearings should consider additional forces such as magnetic pull from unsymmetrical air gaps, out-of-balance forces, pitch errors in gears, and thrust loads.

The calculation of loads on a single bearing models the bearing shaft as a beam resting on rigid, moment-free supports. Assuming the resulting load is constant in magnitude and direction, the equivalent dynamic bearing load comes from the general ABMA and ISO equation:

P = XFr + YFa

where P = equivalent dynamic radial-bearing load (lb), Fr = actual radial-bearing load (lb), Fa = actual axial-bearing load (lb), X = radial-load factor for the bearing, and Y = axial-load factor for the bearing. X and Y load factors can be found in bearing catalogs.

The type of connector or coupling between the drive and driven unit also determines loads on motor bearings. A belt or gear drive, for example, applies greater radial loads than a coupling drive. Here, cylindrical roller bearings at the drive end handle the higher loads. Spherical-roller and toroidal bearings are a good choice in applications with a combination of heavy loads, misalignment, and shaft deflection.

In coupling drives, proper alignment is important because misalignment may introduce additional forces and vibration that shorten bearing and motor life. Rigid coupled machines typically use three bearings on a shaft: two in the motor and a third in the coupled device. Accurately aligned rigid couplings tend to un-load the drive-end bearing. In these cases a deep-groove ball bearing for the drive end is a good choice.

Flexible couplings help accommodate misalignment, but only to a point. For best alignment — regardless of coupler type — first secure the driven equipment, then install the coupling. Only after the coupling is attached to the equipment should the motor be aligned and secured.

Of course, loads affect bearing life. The general ABMA and ISO formulas for ball and roller bearings respectively are:

where C = dynamic radial capacity (lb). These life equations can be further tailored for specific reliability targets and environmental conditions.

Operating speed influences operating temperature and, in turn, bearing and lubricant life. High-speed applications tend to favor ball bearings over roller bearings, while extremely high-speed operations may call for precision or hybrid bearings with ceramic rolling elements.

In any case, bearing operating-temperature limits cap motor speed. The highest speeds are possible with hybrid deep-groove ball bearings (pure radial loads) and with angular-contact ball bearings (combined loads).

LUBRICATION OPTIONS
Bearings in electric motors run at more modest speeds and temperatures generally are lubricated with grease. Grease simplifies housing and sealing designs, better adheres to critical surfaces, and protects against contaminants. How long that grease lasts depends on several factors, including grease type, bearing design, motor speed and orientation, and operating temperature. Small ball bearings in standard-duty electric motors are typically fitted with seals or shields and lubricated for life. Bearings are replaced at normal intervals. Severe-duty motors, regardless of size, often come with open bearings and provisions for regreasing.

Before adding grease, however, be sure to check which grease is already in use and select either the same or a compatible type. Lubricant manufacturers can provide compatibility information. Most large electric motors contain a grease fitting and drain plug. A common practice is to pump new grease into the bearing through the appropriate fitting and let the old grease exit the drain plug. Don't add more grease than what is needed for proper lubrication. Excessive amounts raise friction and running temperature, shortening both grease and bearing life.

Unfortunately, there is no general rule governing grease relubri-cation intervals. Intervals are instead based on bearing size, type, operating speed and environment, and motor design. Vertically mounted motors, for example, need relubrication twice as often as those run in a horizontal orientation.

Consider oil lubrication when extremely high rotational speeds or operating temperatures make it impractical or impossible to use grease because of too-frequent relubrication intervals. In general, only large electric motors are oil lubricated, necessitating more sophisticated seals. Newer technologies, including circulating oil and mist, have advantages over conventional oil-sump systems in certain applications.

KNOW THE ENEMY
Water and humidity are enemies of all bearing lubricants. Heat from running motors tends to evaporate moisture, but condensation can build up when motors are powered down. Condensation is impossible to prevent, but its harmful effects can be negated by the use of lubricants fortified with rust inhibitors. Users should frequently rotate shafts of idle motors whenever condensation is suspected. Good seals can help keep humidity out of the bearings as well. And avoid direct water spray on seals during wash downs.

A less-obvious problem affecting motor bearings is electric arcing. Electric arcing tends to be an isolated and localized event, similar to a series of small lightning strikes that melt and retemper internal bearing surfaces. The result: some surface material flakes away and spalls out, leaving behind microcraters. Characteristic "fluting" patterns on bearing surfaces indicate damage from electric arcing. Fluting is caused by rolling elements touching the microcraters and etching a pattern into the races over time. This and wear debris raise bearing noise and vibration, leading to deterioration of contact surfaces and eventual bearing failure. Arcing may not always directly hurt bearings but instead damages the lubricant. Highly localized temperatures associated with arcing char or burn the oil and break down additives.

One fix for electric arcing is specialized ceramic coatings. The coatings apply to the outside or inside diameter of a bearing to electrically insulate it from the shaft. The use of hybrid bearings is another way to stop arcing. Such bearings substitute ceramic balls or rollers for metal rolling elements to effectively insulate bearings from the inside.

After a new motor is installed, it's a good idea to take a baseline vibration measurement, then repeat the measurements at regular intervals. In this way, users can track motor and bearing health over time and make repairs before failures happen.

MAKE CONTACT
SKF Industrial Div., SKF USA Inc., www.skfusa.com