bearing life

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xtend Motor Life with Improved Bearing Care

earing failures are the root cause for the great majority of electric motor downtime, repair and

placement costs. Bearing and motor manufacturers are aware of the situation. Motor repair shops cantribute much of their business to bearing failures. And motor users see bearing failure as the fundamen

ause of virtually every electric motor repair expense. Studies conducted by the Electrical Apparatus Serv

ssociation also demonstrate that bearing failures are by far the most common cause of motor failures.

nowing that shaft bearings are the Achilles heel of industrial electric motors is not a new idea in

aintenance departments, but what is new is recognizing that something can be done to prevent most m

earing failures.

act ors Af fect ing Bear ing Li f e

ectric motors actually present a relatively easy duty for shaft bearings. The motor rotor is lightweight,

ecause of its large shaft diameter, the bearings are large. For example, the bearings supporting the 140

otor for a typical 40 hp. 1800 rpm industrial motor are so large that they have an L-10 minimum design

tigue life of 3000 years, or 10 percent of the bearings are statistically expected to fail from fatigue afte

000 years of operation. Plant operating experience, however, strongly contradicts such optimistic estima

motor bearing life. In actual industrial environments, bearing failure is rarely caused by fatigue; it is

aused by less-than-ideal lubrication. Because of contaminated lubrication, bearings fail well before they

rve their theoretical fatigue life. There are many reasons for less than-ideal bearing lubrication. Lubric

an leak out; chemical attacks or thermal conditions can decompose or break down lubricants; lubricants

ecome contaminated with non-lubricants such as water, dust, or rust from the bearings themselves. The

brication problems can be eliminated. Motor bearings can last virtually forever by simply providing an id

ontamination-free, well-lubricated bearing environment. Conventional wisdom teaches that such an idea

otor bearing environment can be provided by using a dry-running lip seal or using sealed (lubricated-for

e) bearings.

deed, for many light-duty applications, such bearing protection techniques are often sufficient to allow

earings to last as long as the equipment itself. However, these bearing protection methods have not

gnificantly reduced the rate of bearing failure in severe-duty industrial motors.

earings in industrial applications continue to fail because of inadequate lubrication caused by lubricant l

ontamination, and decomposition and break-down. Lip seals invariably wear out well before the bearingils, and sealed bearings inherently foreshorten the life of a bearing to the service life of the contained

ease (usually only about 3,000 to 5,000 hours for most industrial services).

aintenance professionals may find the following suggestions on how to forestall motor hearing failure

bvious, but some new techniques and technologies are available.

ubricat e Bear ing at Corr ect Int ervals

espite years of warnings from bearing manufacturers, over lubrication continues to plague many motor

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earings. Too much grease can cause overheating of the bearings. The lubrication instructions supplied by

e motor manufacturer will specify the quantity and frequency of lubrication. Generally, two-pole moto

ould be greased twice a year, four-pole and slower motors only once a year.

se t he Best Avai lable Grease

he most commonly used bearing grease is polyurea-based, a low-cost, low-performance, highly compatib

bricant. However, it does not handle water well, a serious drawback for many industrial applications. It

acts readily with water and loses its ability to lubricate bearings.

dustrial motor bearings should be lubricated with a synthetic-based aluminum complex grease. A high-

uality grease pays for its additional cost in reduced motor downtime and repair costs.

eep Out Moist ure

nless the motor is being hosed down or it operates in a humid environment, reasonably shielded motor

earings may not become seriously contaminated with moisture while the motor is running. However, wh

e rotor is shut down, moisture and condensation can collect on the surface of the bearing components.

ventually, this water breaks through the oil and grease barrier, contacts the metal parts of the bearing, oduces tiny particles of iron oxide. These rust particles make an excellent grinding compound when mix

th the grease. resulting in premature failure of the bearing because of surface degradation.

eventing water contamination is a major challenge to bearing housing design. Close shaft-to-endbell

earances cannot stop the movement of humid air. Contact seals will quit contacting, resulting in large g

at allow movement of air and water vapor across the bearing.

apor-blocking bearing isolators, such as the one illustrated, are among the more successful devices

esently available to prevent water vapor from entering a stationary bearing. When the motor shaft is

tating, the isolator opens, eliminating the possibility of friction and wear. However, when the shaft isationary, the isolator closes, preventing movement of air or water across its face. With no wear from

tating friction, the seal may last indefinitely, and surely as long as the fatigue-failure life of the bearing

eep Out Di r t

p seals, contact seals, and frequent grease replacement help minimize the amount of dirt and other air

orne abrasives that can contaminate bearing lubricant. These solutions, however, have some drawbacks

als have a short service life, and frequent grease displacement is expensive and messy.

ne successful approach to keeping air-borne dirt and liquids out of an operating bearing is to install abyrinth-type non-contact seal over the bearing housing. These bearing isolators, readily available from

ppliers, combine a tortuous labyrinth path with impingement and centrifugal forces to trap and remove

orne dirt and liquid; virtually no contamination can reach the bearing. Because the bearing isolator is a

ontact device, it will generally be the longest-lasting component of the motor.

though not intended as such, a bearing isolator could serve as an emergency sleeve bearing if the prima

earing fails, possibly preventing damage to the motors stator and rotor. In emergency situations, the

earing isolator can allow continued operation for a short time and still prevent the need to rewind the

otor when the bearing is replaced. Bearing isolators constructed of bronze or other non-sparking materi

so can prevent hazardous sparks that could otherwise occur when the bearings rolling elements fail.



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t her Suggest ions

mproved bearing protection and lubrication will reduce downtime and the maintenance costs of electric

otors, but other important motor design features contribute to long service life, including over-sized hig

uality bearings, high-tech winding insulation, superior fan design, high-performance paint (such as epoxy

nd a strong, rigid cast iron frame.

hese features, usually standard or readily available, are found in most industrial-grade severe-duty elecotors. High-performance bearing protection systems. however, are not universally accepted as essentia

ng motor life. Specifying permanent bearing protection for new motors, or retrofitting isolators onto

xisting equipment, usually requires initiative on the part of the users maintenance or engineering staff.

ermanent, absolute bearing protection has a greater effect on motor life than any other decisions made

ecifying, equipping, and caring for electric motors. Keeping bearings lubricated with the right amount o

ean, uncontaminated, high-quality lubricant allows bearings in most industrial motors to outlast all othe

otor components.

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formation supplied by InproSeal, Rock Island, IL.

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