Procedures For Protecting Electric Motor Bearings
Bearing contamination — the entry of foreign particles or liquids into the bearing cavity — is 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 underestimate the real problem.
Some contaminants do not leave physical evidence, such as dents, in the bearing raceway. Instead, they degrade the lubricant, reducing its viscosity, and ultimately cause 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 downtime. Maintenance professionals should be knowledgeable about the effects of contamination and well-trained in the procedures to prevent it.
There are five basic ways to maintain bearings and reduce the number of failures caused by contamination.
Identify signs of contamination
Many electric motors operate in harsh conditions full of solid or liquid contaminants. Solid contaminants include dirt, lint, dust residue from shaft polishing, or fragments of coal, brick, or cement.
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 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 form at the fatigue site. The site then spalls, and metal flakes away from the raceway (Fig. 1).
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 turned on, it ran noisily. An inspection of the bearing components revealed static corrosion which resulted from rainwater mixing with the lubricant. This corrosive mixture etched the raceways at the rolling element locations.
Static corrosion can also occur because of excess humidity in the plant. Increasing the frequency of relubrication to purge contaminants and running idle motors periodically help prevent static corrosion.
Water and other liquid contaminants can also damage bearings when they are in operation. Under dynamic conditions, these contaminants degrade the lubricant and reduce its viscosity. The viscosity may drop below the minimum level required for the application, increasing operating temperatures and metal-to-metal contact between rolling surfaces.
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 feel or see evidence of solid contamination.
Close analysis of a dismounted bearing can also be informative. Grooves or dents in the raceway surface indicates the presence of solid contamination. Etching at ball-spaced intervals along the raceway indicates the probability of liquid contamination.
Lubricate to purge contaminants
Although some bearings in specialty motors are oil lubricated, the majority of electric motor bearings use grease. Electric motors usually operate between 140 and 160 F, an ideal temperature range for grease lubrication. Grease provides a thin film of lubricating fluid between a bearing’s rolling and sliding surfaces, minimizing wear and friction. Grease also acts as a barrier against solid and liquid contaminants.
Frequent lubrication is an excellent means of preventing contamination. The process of greasing purges used grease, and any contaminants it may contain, from the bearing and its housing (Fig. 2). If contamination is suspected, shorten the lubrication interval.
For example, if lubrication is currently done monthly, increase the frequency to twice a month, but cut the quantity of grease in half. The goal is to reestablish the protective 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 drain plug. Before lubricating, clean the fitting thoroughly to avoid introducing contaminants to the bearing along with fresh grease. Then pump in new grease while allowing the old to exit through the open drain.
Most electric motor bearings are protected by seals or shields (Fig. 3).
Seals provide a superior barrier against small contaminants because, unlike shields, they actually make contact with the bearing inner ring. But this frictional contact results in increased bearing operating temperatures. As electric motor speeds increase and bearing sizes get larger, seals become a less viable option.
Shields are used in the vast majority of electric motor applications, especially high-speed situations. When replacing a bearing, check that the new unit contains the same seal or shield type used in the original. If there are any questions about sealing options, contact the bearing manufacturer.
Follow installation guidelines
Technicians can inadvertently introduce contamination into a motor bearing during installation if they fail to follow accepted procedures. The consequences of poor practices are easily demonstrated.
Lightly sprinkle some cigarette ash into an open bearing and spin it by hand. Bearing noise increases dramatically. Even after a short time in service, under no load, examination of the bearing raceway reveals 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 of dirt, dust, and moisture. Make sure that shafts, housings, and other motor components are clean and dry. Do not remove new bearings from their packaging until immediately before mounting.
When using an induction heater, remember that it magnetizes bearings as it heats them (Fig. 4). Always use a heater that has a built-in demagnetization cycle. Otherwise, the magnetized bearing may attract metal particles during installation.
Contamination can cause bearings to run hot or emit an altered vibration signal. Basic monitoring of temperature and vibration provide early signs of a problem.
In the first few weeks of a motor’s operation, establish a temperature benchmark. Since most electric motor bearings are lubricated with grease containing a polyurea thickener that begins to degrade at about 200 F, 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 the motor housing. Drilling small dimples at these locations ensures that readings will be taken at the same locations every time.
Critical motors should be monitored at least weekly, others monthly. If a high vibration level is detected, sophisticated analysis should be used to determine the problem’s source.
— Edited by Joseph L. Foszcz, Senior Editor, 847-390-2699, firstname.lastname@example.org
The author is available to answer questions about procedures for protecting bearings. He can be reached by fax at 610-265-8047.
Four previous articles presented information relative to protecting and extending bearing life.
“Solving Bearing Overheating Problems” (PE, April 1994, p 93, File 2060/5560) outlines techniques for pinpointing the true source of overheating.
“Preventing Bearing Failures” (PE, 6/5/95, p 58, File 2060) explains how to avoid bearing failure due to faulty mounting and poor lubrication.
“Establishing An Integrated Condition Monitoring Program” (PE, 12/11/95, p 64, File 2001/5540) discusses the elements required to have a successful program.
“Extending Bearing Life” (PE, August 1997, p 62, File 2060/5540) spells out the steps to follow in handling, installing, and maintaining bearings.
Texts of recent articles are posted on www.planteng.com. Copies of articles are available by calling 847-390-2692.
Contamination can cause up to 30% of motor bearing failures.
Contamination can be solid or liquid.
Lubricate bearings often in harsh environments.
How to protect bearings
– Determine the type of contamination and its source
– Lubricate as required to maintain oil film and purge contaminants
– Always use seals or shields to protect electric motor bearings
– Always work on bearings in clean surroundings using the proper tools
– Monitor bearings for heat and vibration to detect early signs of failure