Lubricating Electric Motor Bearing

Electric motors come in a range of sizes and are employed in applications as diverse as irrigation pumps, industrial cooling fans and household appliances. Yet, no matter their size or application, the bearings used in electric motors all depend on proper lubrication for their long-term reliability and performance.

Proper lubrication of motor bearings involves using the right lubricant for a particular application to accommodate such factors as high temperatures or speeds. It means avoiding common maintenance errors, such as overlubrication, under-lubrication, and contamination during the relubrication process. It also means selecting lubricants with characteristics that satisfy the demands of the customer's application, such as stringent noise level requirements.

The Role Of Lubrication

During operation, a bearing lubricant's function is to form a thin film of lubricating fluid between a bearing's rolling and sliding surfaces, minimizing wear and friction. The lubricant also protects bearing rings and rolling elements from corrosion, and provides a barrier against solid and liquid contamination.

There are two types of lubricants used for bearings: grease and oil. In essence, grease is made up of a base oil and a thickener, which retains the oil. Base oil types include mineral, other natural and synthetic oils. The thickener (in most cases lithium, calcium, sodium or polyurea) forms a lattice which carries the oil between its fibers. A variety of additives, such as antioxidants, rust inhibitors, and anti-wear compounds, may be included in the mixture.

Bearings in large vertical shaft motors and some specialty motors are lubricated with oil, but approximately 95 percent of all motor bearings are grease-lubricated. The most popular grease is rust-inhibiting, polyurea-based thickener that is used as the standard factory fill for most sealed or shielded motor bearings.

The Hazards Of Over- And Underlubrication

Bearings supplied from the factory are pre-lubricated with the proper quantity of grease(an amount sufficient to lubricate the bearing properly while not impeding the bearing's rolling elements. Electric motor users often add grease to bearings before installing them in the mistaken belief that more grease is always better than less. Such overgreasing, however, can lead to a condition called churning. As bearing balls or rollers struggle to push excess grease out of the way as they rotate, the running friction increases and the temperature within the bearing rises. Eventually, the heat breaks down the grease, which then loses its ability to lubricate effectively, and premature bearing failure occurs. Another result of too much grease is that the churning consumes more energy and places a greater burden on the motor.

Underlubrication can be as damaging to bearings as overlubrication. The consequences of underlubrication are excessive heat from metal-to-metal contact between a bearing's rolling elements and the raceways, leading to wear and premature bearing failure. During normal operation, some grease can gradually evaporate, bleed out of a bearing, or exhaust its service life. For this reason, it is important to relubricate within the time frame outlined in the accompanying relubrication intervals chart.

Frequency And Quantity Of Relubrication

The life expectancy of a grease depends on several factors, including the type of grease, the bearing's size, the application's speed, and operating temperature. Depending on these variables, the relubricating interval can range from a few hundred operating hours to 10,000 hours or more. Applications with high temperatures require more frequent lubrication. Vertical motors require lubrication approximately twice as often as horizontal motors. Recommendations on lubrication intervals are usually provided with motor documentation.

The correct grease quantity depends on bearing size, with larger bearings obviously requiring more grease. Follow the recommendations on quantity provided by the motor or bearing supplier. If these are not available, the formula below can be used to determine the correct amount of grease needed:

Gq = 0.114 x D x B
where
Gq = grease quantity in ounces
D = bearing outside diameter in inches
B = total bearing width in inches

Application And Cleanliness
When lubricating electric motor bearings, be sure to follow the correct application procedures. Most electric motors are equipped with a grease fitting and a drain plug. Pump new grease into the bearing through the fitting while allowing the old grease to exit through the open drain. After injecting the recommended amount of grease, run the motor with the drain open until the bearing has a chance to purge all excess grease. When grease stops exiting the drain, plug the drain securely. The bearing is now properly lubricated. Avoid purging excess grease through the seals, since contaminated grease is likely to collect in the bearing cavity and cause a buildup of heat.

When relubricating electric motor bearings, cleanliness is of paramount importance. If not applied correctly, grease can collect ambient dust and other contaminants, which can cause bearing noise and eventually damage bearings by leaving dents in rotating elements and raceways. In one recent example, an electric motor manufacturer found that an abrasive residue left on shaft journals was entering nearby bearings as they were being installed, contaminating the lubricant and causing a noise problem during operation. The company improved its installation procedures, and the noise problem was eliminated.

To reduce the risk of contamination, keep grease in its original container until used, and never leave the container uncovered or open. Wash all lubrication tools, including grease guns, with a cleaning solvent and wipe them dry before using. Also, carefully clean the bearing's grease fitting and the area around it before relubricating.

Grease Compatibility

During relubrication, technicians may inadvertently mix two lubricants that are incompatible. The polyurea grease used as factory fill in most electric motor bearings is generally not compatible with the typical greases used in other plant applications, particularly lithium-based varieties. If two incompatible lubricants are mixed, there is
often a deterioration in lubricating capability. The resulting mixture will tend to have a softer consistency and a lower operating temperature, leading to leakage and potential bearing failure. As a general rule, never mix greases with different thickeners, or a grease containing a mineral oil with one containing a synthetic oil. Whenever possible, relubricate a bearing with the same grease used originally.

The potential for grease compatibility problems has risen in recent years due to the increasing use of specialty greases. In search of optimal motor performance, electric motor users sometimes replace the factory-supplied grease with a grease specially designed for use in extremely high or low temperatures or at high speeds. Once a bearing is lubricated, however, it is often difficult to remove all traces of the original grease. Grease residue may mix with the specialty grease, leading to a breakdown in lubricating capability. In addition, the ultrasonic cleaning processes sometimes used to remove the factory fill can damage bearing raceways. Contact the bearing manufacturer for recommendations on the use of specialty greases.

Meeting Noise Requirements

A grease's noise characteristics should also be considered when selecting a motor lubricant. Different greases(or even different batches of the same grease(exhibit markedly different noise properties. A faint "clicking" in a lubricant may resonate with other motor components, such as the end belt or housing, and be greatly magnified. Such a noise may not in itself be a symptom of bearing problems. But the noise may be unacceptable in certain plant environments and in household applications like washing machines and dryers.

Because of the subjective nature of noise and the difficulty of predicting how grease noise will interact with the vibrations of other motor components, there is no internationally accepted standard for lubricant noise in electric motors. Nevertheless, many bearing suppliers test the noise properties of various lubricants. SKF, for example, conducts a "Be Quiet" test using a specially designed test rig. The test is able to determine whether a lubricant batch has "noisy" or "quiet" characteristics. Lubricant batches that pass such a test are likely to perform well in electric motor applications.

The cost of neglecting the lubrication fundamentals outlined above is measured in more frequent lubricant failures and longer periods of downtime. Following correct practices, on the other hand, can reduce bearing-related problems and prolong the life expectancy of electric motors.

Inadequate bearing lubrication can lead to an increase in temperature and cause discoloration and wear on the bearing raceways. (Photo courtesy of SKF.)

Small indentations in bearing raceways and rolling elements can be caused by contaminants in the lubricant. (Photo courtesy of SKF.)

Copy to accompany relubricating interval chart
(see chart provided for correct positioning of copy):

Relubricating interval, tf operating hours

n rev/min

Scale a: radial ball bearings
Scale b: cylindrical roller bearings, needle roller bearings
Scale c: spherical roller bearings, taper roller bearings, thrust ball bearings;
full complement cylindrical roller bearings (0.2 tf); crossed cylindrical roller bearings
with cage (0.3 tf); cylindrical roller thrust bearings, needle roller thrust bearings,
spherical roller thrust bearings (0.5 tf)

Example

A deep groove ball bearing with a bore diameter (d) of 100 mm rotates at 1,000 r/min. The operating temperature varies between 60 and 70C (140-160F). What is the correct relubricating interval?
Follow the line from 1,000 on the diagram's X-axis across to the intersection of the curve for d = 100 mm. Then follow a line from this intersection horizontally or parallel to the X-axis across to the value of approximately 12,000 in the "scale a" column (radial ball bearings). The relubricating interval is therefore 12,000 operating hours.

Testing shows the effects of lubrication on a bearing's operating temperature. In Graph A, a roller bearing fitted with a grease escape valve is relubricated at 24, 48 and 72 hours. In each instance, there's an increase in temperature immediately after relubrication. As the bearing runs, it channels excess grease away from rolling elements. Grease exits through the escape valve, and the temperature drops. Graph B shows the relubrication of a bearing without an escape valve. Churning results when excess grease cannot be evacuated from the bearing. After relubrication, the bearing temperature remains excessively high.