Bearing failures are the root cause for the great majority of electric motor downtime, repair and replacement costs. Bearing and motor manufacturers are aware of the situation. Motor repair shops can attribute much of their business to bearing failures. And motor users see bearing failure as the fundamental cause of virtually every electric motor repair expense. Studies conducted by the Electrical Apparatus Service Association also demonstrate that bearing failures are by far the most common cause of motor failures.
Knowing that shaft bearings are the Achilles’ heel of industrial electric motors is not a new idea in maintenance departments, but what is new is recognizing that something can be done to prevent most motor bearing failures.
Factors affecting bearing failure:
Electric motors actually present a relatively easy duty for shaft bearings. The motor rotor is lightweight, yet because of its large shaft diameter, the bearings are large. For example, the bearings supporting the 140 lb. Rotor for a typical 40 hp. 1800 rpm industrial motor are so large that they have an L-10 minimum design fatigue life of 3000 years, or 10 percent of the bearings are statistically expected to fail from fatigue after 3000 years of operation. Plant operating experience, however, strongly contradicts such optimistic estimates of motor bearing life. In actual industrial environments, bearing failure is rarely caused by fatigue; it is caused by less-than-ideal lubrication. Because of contaminated lubrication, bearings fail well before they serve their theoretical fatigue life. There are many reasons for less than-ideal bearing lubrication. Lubricants can leak out; chemical attacks or thermal conditions can decompose or break down lubricants; lubricants can become contaminated with non-lubricants such as water, dust, or rust from the bearings themselves. These lubrication problems can be eliminated. Motor bearings can last virtually forever by simply providing an ideal contamination-free, well-lubricated bearing environment. Conventional wisdom teaches that such an ideal motor bearing environment can be provided by using a dry-running lip seal or using sealed (lubricated-for-life) bearings.
Indeed, for many light-duty applications, such bearing protection techniques are often sufficient to allow bearings to last as long as the equipment itself. However, these bearing protection methods have not significantly reduced the rate of bearing failure in severe-duty industrial motors.
Bearings in industrial applications continue to fail because of inadequate lubrication caused by lubricant loss, contamination, and decomposition and break-down. Lip seals invariably wear out well before the bearing fails, and sealed bearings inherently foreshorten the life of a bearing to the service life of the contained grease (usually only about 3,000 to 5,000 hours for most industrial services).
Maintenance professionals may find the following suggestions on how to forestall motor hearing failure obvious, but some new techniques and technologies are available.
Lubricate Bearing at Correct Intervals:
Despite years of warnings from bearing manufacturers, over lubrication continues to plague many motor bearings. Too much grease can cause overheating of the bearings. The lubrication instructions supplied by the motor manufacturer will specify the quantity and frequency of lubrication. Generally, two-pole motors should be greased twice a year, four-pole and slower motors only once a year.
use the best available grease:
The most commonly used bearing grease is polyurea-based, a low-cost, low-performance, highly compatible lubricant. However, it does not handle water well, a serious drawback for many industrial applications. It reacts readily with water and loses its ability to lubricate bearings.
Industrial motor bearings should be lubricated with a synthetic-based aluminum complex grease. A high-quality grease pays for its additional cost in reduced motor downtime and repair costs.
Keep Out Moisture:
Unless the motor is being hosed down or it operates in a humid environment, reasonably shielded motor bearings may not become seriously contaminated with moisture while the motor is running. However, when the rotor is shut down, moisture and condensation can collect on the surface of the bearing components. Eventually, this water breaks through the oil and grease barrier, contacts the metal parts of the bearing, and produces tiny particles of iron oxide. These rust particles make an excellent grinding compound when mixed with the grease. resulting in premature failure of the bearing because of surface degradation.
Preventing water contamination is a major challenge to bearing housing design. Close shaft-to-endbell clearances cannot stop the movement of humid air. Contact seals will quit contacting, resulting in large gaps that allow movement of air and water vapor across the bearing.
Vapor-blocking bearing isolators, such as the one illustrated, are among the more successful devices presently available to prevent water vapor from entering a stationary bearing. When the motor shaft is rotating, the isolator opens, eliminating the possibility of friction and wear. However, when the shaft is stationary, the isolator closes, preventing movement of air or water across its face. With no wear from rotating friction, the seal may last indefinitely, and surely as long as the fatigue-failure life of the bearing.
Keep Out Dirt:
Lip seals, contact seals, and frequent grease replacement help minimize the amount of dirt and other air-borne abrasives that can contaminate bearing lubricant. These solutions, however, have some drawbacks. Lip seals have a short service life, and frequent grease displacement is expensive and messy.
One successful approach to keeping air-borne dirt and liquids out of an operating bearing is to install a labyrinth-type non-contact seal over the bearing housing. These bearing isolators, readily available from suppliers, combine a tortuous labyrinth path with impingement and centrifugal forces to trap and remove air-borne dirt and liquid; virtually no contamination can reach the bearing. Because the bearing isolator is a non-contact device, it will generally be the longest-lasting component of the motor.
Although not intended as such, a bearing isolator could serve as an emergency sleeve bearing if the primary bearing fails, possibly preventing damage to the motor’s stator and rotor. In emergency situations, the bearing isolator can allow continued operation for a short time and still prevent the need to rewind the motor when the bearing is replaced. Bearing isolators constructed of bronze or other non-sparking materials also can prevent hazardous sparks that could otherwise occur when the bearing’s rolling elements fail.
