Feature for Plant EngineeringBearing contaminationthe entry of foreign particles or liquids into the bearing cavityis a leading cause of failure in electric motors. According to failure analysis statistics, about 15% of all motor bearing failures can be directly attributed to contamination. But this figure may understate the real problem. Some contaminants leave behind little physical evidence, such as dents in the bearing raceway. Instead, they degrade the bearing lubricant, reducing its viscosity, and ultimately result in bearing failure. These failures are often classified as lubricant-related, although their root cause was contamination. Consequently, the actual number of motor bearing failures attributable to contamination may be as high as 30%.

     Contamination can dramatically shorten bearing life and lead to costly shutdowns and downtime. Maintenance professionals should be knowledgeable about the effects of contamination, and well-trained in the procedures that can prevent it. Below are five basic recommendations for motor bearing maintenance:
1. Know how to identify signs of contamination.

Contaminants can be either solid or liquid. Solid contaminants can range from dirt and lint, to dust residue left over from polishing the shaft, to fragments of coal, brick, cement and other materials present in the plant environment. Many electric motors operate in harsh conditions full of contaminants. The basic design of electric motors tends to aggravate the problem. Virtually all electric motors contain a cooling fan that draws air, as well as potential contaminants, across the motor’s two bearing positions.

     Solid contaminants create small dents or fatigue sites in the bearing raceway. This damage is accompanied by an increase in noise as the bearing operates. Gradually, a series of cracks will form at the fatigue site. Then the site will suffer spallingthe flaking away of metal from the raceway.

     Liquid contaminants have a different but equally serious impact on motor bearings. In a recent case, a fan located on a roof was left idle and exposed to heavy rainfall. When the fan was finally turned on, it ran noisily. An inspection of the bearing components revealed static corrosion, which resulted when rainwater mixed with the bearing’s lubricant. This corrosive mixture etched the raceways at rolling element locations. Static corrosion of this type can also occur because of excess humidity in the plant environment. More frequent relubrication to purge the contaminants and running an idle motor periodically can help prevent static corrosion.

     Water and other liquid contaminants can also damage bearings when they are in operation. Under dynamic conditions, these contaminants will degrade the lubricant and reduce its viscosity. Viscosity is a measurement of the thickness, or ability to flow, of a lubricating oil. The viscosity may drop below the minimum level required for the application at hand, and increased operating temperatures and metal-to-metal contact between rolling contact surfaces may result.

     Visual inspection of the bearing lubricant can often reveal evidence of contamination. For example, technicians can take a small amount of bearing grease, rub it between their fingers, and both feel and see evidence of solid contamination. If water droplets appear instead, then the grease has probably suffered liquid contamination. Close analysis of the bearing itself after dismounting can also be informative. Grooves or dents in the raceway surface indicate the presence of solid contamination. Etching at ball-spaced intervals along the raceway, on the other hand, points to the probability of liquid contamination.
2. Lubricate topurge contaminants.

Although some bearings in specialty motors are oil-lubricated, the majority of all electric motor bearings are lubricated with grease. Electric motors commonly have an operating temperature of 140-160Fan ideal temperature range for grease lubrication. A bearing grease consists of a base oil and a thickener, which forms a lattice that carries the oil between its fibers. During operation, bearing grease provides a thin film of lubricating fluid between a bearing’s rolling and sliding surfaces, minimizing wear and friction. It also acts as a barrier against solid and liquid contaminants.

     Frequent relubrication is an excellent means of preventing contamination. The process of regreasing purges used grease, and any contaminants it may contain, from the bearing and bearing housing. If you suspect a contamination problem, shorten the relubrication interval. For example, if you currently relubricate once a month, increase the frequency to twice a month, but cut the quantity of grease in half. The goal is to reestablish the protective grease barrier rather than to replenish the grease entirely. Too much grease can cause a condition called churning, which results in excessive heat.

     Most electric motors are equipped with a grease fitting and a drain plug. Before relubricating, clean the grease fitting thoroughly in order to avoid introducing contaminants to the bearing along with the new grease. Then pump in new grease while allowing the old grease to exit through the open drain.
3. Seal motorbearings properly
.
Most electric motor bearings are protected by either seals or shields. Seals provide a superior barrier against small contaminants because, unlike shields, they actually make contact with the bearing inner ring. But this frictional contact also results in increased bearing operating temperatures. As electric motor speeds increase and bearing sizes get larger, seals become a less viable option.

     Shields are employed in the vast majority of electric motor applications, especially in high-speed situations. When replacing an electric motor bearing, make sure the new bearing contains the same seal or shield type used in the original bearing. If you have questions about sealing options, contact the bearing manufacturer.
4. Follow guidelines for clean installation.

Technicians can inadvertently introduce contamination into a motor bearing during installation if they fail to follow accepted procedures. The consequences of poor practices are vividly illustrated in a seminar demonstration conducted by the SKF Bearing Maintenance Institute. A technician lightly sprinkles cigarette ash over an open bearing, which is spun by hand. The effects of contamination are immediately apparent: the bearing noise increases dramatically. Even after a short time in service, under no load, examination of the bearing raceway would reveal small dents caused by the cigarette ash.

     To prevent contamination during installation, ensure that the work area is clean. Mount bearings in an area that is free from dirt, dust and moisture. Make sure that shafts, housings and other motor components are clean and dry. Finally, do not remove new bearings from their packaging until immediately before mounting.

     When using an induction heater, remember that the heater magnetizes bearings as it heats them. Always use a heater that has a built-in demagnetization cycle. Otherwise, the magnetized bearing may attract metal particles during installation.
5. Perform basic monitoring of motor bearings.

Contamination can cause bearings to run hot or emit an altered vibration signal. Basic monitoring of temperature and vibration can provide early signs of a problem. In the first few weeks of a motor’s operation, establish a temperature benchmark by taking a series of measurements using a hand-held thermometer. Ideally, these readings should fall between 140 and 160F. Since most electric motor bearings are lubricated with grease containing a polyurea thickener that begins to degrade at about 200F, sharp temperature increases above the benchmark may indicate impending failure and call for a shutdown.

     Motor vibration should also be trended. Take vibration readings at three or four locations on a motor’s housing. Drill a small dimple in these locations so that measurements are taken at the same places each time and can be accurately compared. Critical motors should be monitored once a week or more, other motors perhaps once a month. If a high vibration level is detected, more sophisticated analysis should be used to determine the problem’s source.

     Following the recommendations outlined above will reduce contamination-related motor bearing failures, and contribute to longer bearing life and improved electric motor reliability.

     Adapted from an article originally published in the March 1998 issue of Plant Engineering magazine. Used with permission.”