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Rolling element bearings
used in electric motors are at risk for various modes of
failure if an incorrect maintenance or lubrication strategy is
implemented. These include incorrect lubricant selection,
contamination, loss of lubricant and overgreasing. This
article discusses several effective strategies to minimize the
likelihood of these failure modes.
Most electric motors are designed with grease-lubricated,
antifriction, rolling element bearings. Grease is the
lifeblood of these bearings because it provides an oil film
that prevents the harsh metal-to-metal contact between the
rotating element and races. Bearing troubles account for 50 to
65 percent of all electric motor failures, and poor
lubrication practices account for most of these bearing
troubles. Proper maintenance procedures, planning and the use
of the correct lubricant can increase productivity by reducing
these bearing troubles and motor failures.
Failures Get to know the failures.
By knowing the failure modes, focus can be placed on reducing
or even eliminating them.
Incorrect Lubricant - It is important to use the
correct grease for specific applications. Regreasing with the
wrong grease can lead to premature bearing failure. Most oil
suppliers have grease that is specifically designed for
electric motors, which is different from their multipurpose
extreme purpose (EP) grease.
Grease Incompatibility - Greases are made with
different thickeners, such as lithium, calcium or polyurea.
Unfortunately, not all greases are compatible with each other,
even those with the same thickener type. Therefore it is
important to use the same grease or compatible substitute
throughout the life of the bearing.
Motor Casing Full of Grease - If the grease cavity
is overfilled, and high pressure from the grease gun is
applied, the excess grease can find its way between the shaft
and the inner bearing cap and press into the inside of the
motor. This allows the grease to cover the end windings of the
insulation system and can cause both winding insulation and
bearing failures.
Lubricant Starvation - There are several possible
causes of lubrication starvation. The first is insufficient
grease being added during installation. The second is
inappropriate, elongated relubrication intervals. The third
involves the possibility that the oil has separated from the
thickener base due to excessive heat.
Overpressurization of the Bearing Housing - Anytime
there is an overpressurization of the bearing housing,
stresses are placed on parts that were not designed to handle
the pressure. Keep in mind that the standard manual grease gun
can produce pressures up to 15,000 psi.
Overheating Due to Excess Grease -Too much volume
will cause the rotating bearing elements to churn the grease,
trying to push it out of the way. This results in parasitic
energy losses and high operating temperatures, and increases
the risk of oil separation and bearing failure.
Getting Started To
begin, a plan must first be in place. The following
suggestions are the bare minimum that need to be discussed and
implemented to get the program started.
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Make an equipment list that includes all the assets
needed in the program.
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Verify the type of bearings and their seals that are
installed in both the inboard and outboard ends of motors.
This will determine if the bearings are regreasable. A
policy should also be determined for the regreasing of
shielded bearings, commonly found in motors. (Some experts
recommend not greasing double-shielded bearings.)
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Choose a grease type that will be adequate for the
program. Remember that once a grease type and manufacturer
are chosen, it's best to not deviate from this choice. If
this grease is different from a grease previously used in
the bearings, the previous grease will need to be cleaned
out or flushed from the bearings and housings.
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Make all necessary modifications to the electric motors.
This includes adding fittings and making them accessible.
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Establish a set of procedures for maintaining the motors.
Develop a PM System
There are many choices to make when deciding on a
preventive maintenance (PM) system. In some plants it may be
beneficial to use only a spreadsheet, while others have the
need for complete dedicated systems. The end goal is the same.
Each motor needs to be tracked as an asset, accomplished by
noting the attention each motor receives. Some factors to
include in the PM system are: date of installation,
horsepower, frame size, rpm, bearing type and environmental
conditions. Setting up a system like this may take some time,
but once completed it will be a great tool.
Determining Lube Type
When searching for a lube type and manufacturer or
supplier, there are several things to consider. The following
is a list of qualities of a good electric motor grease:
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Good channeling characteristics
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NLGI Grades 2 to 3
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Base oil viscosity of an ISO VG 100 to 150 or more
specifically 90 to 120 cSt at 40°C
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High dropping point, 400°F minimum
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Low oil bleed characteristics, per D1742 or D6184
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Excellent resistance to high-temperature oxidation
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Good low-temperature torque characteristics
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Good antiwear performance (but not EP)
Polyurea grease is popular with many bearing and motor
manufacturers. A significant proportion of equipment
manufacturers also specify some type of polyurea grease in
their electric-powered machinery. A polyurea-based grease is
an excellent grease for electric motors; however, this
thickener is incompatible with most other thickeners. Some
manufacturers don't recommend mixing one brand of polyurea
with another. Instruct the motor rebuild shop on what grease
to use, and make sure the grease type is specified on new
motor purchase orders.
Determine Regrease Time Cycle
There are several methods for determining a
regrease time cycle. It is important to realize that no single
method will provide a magical answer to a plant's problems.
The multiple calculators, tables and charts available can
provide a good starting point. They can be used to determine
how to set the cycles. The real fine tuning, however, must be
done by trial and error. The factors that most calculators
have in common are load, operation time, bearing type,
temperature, environment and speed. This is where the database
that was built will be beneficial.
Grease Volume Control
Grease volume control has been a long-standing
problem for industry, and simply following OEM recommendations
may not be enough to solve this problem. There is a simple
equation that takes a logical approach to determining the
volume of grease to be added. The formula is:
G = 0.114 x D x B
Where G = the amount of grease in ounces, D = the bore
diameter in inches and B = the bearing width in inches.
Once the volume is found, it must be converted into shots,
or pumps of the grease gun. There is one way to obtain the
value used to convert the number; for this the user will need
the grease gun to be used and a postal scale. After finding
the output per full stroke of the handle, label the gun so
that it is now "calibrated". The average value is
approximately 18 shots per ounce for most manual guns but
grease gun output can vary by a factor of 10, so be sure to
calibrate each gun. The Use of
Feedback Instruments Feedback from the lubrication
points is needed to verify that the proper frequency and
volume has been chosen. Ultrasonic instrumentation might be
the best tool available to dial in the optimum relubrication
frequency. A conservative approach is to use a frequency
generation method as a starting point, and continuously refine
that value based on feedback from the ultrasonic equipment.
Likewise with volume, ultrasonics can be used to hone in on
the correct amount of grease by using the hybrid method.
Ultrasonics will be fully discussed in a later issue of
ML because it is somewhat of an art form and warrants a
separate article on this subject.
Procedure The intent
of a good maintenance program is to extend the service life of
a motor. In most cases, improper lubrication procedures can
have a negative impact on the program. A basic set of
procedures should include some variation of the following:
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Ensure the grease gun contains the appropriate lubricant.
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Clean the areas around the relief and fill fittings.
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Remove the grease relief valve or drain plug.
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Grease the bearing with a calculated amount of grease.
Slowly add grease to minimize excessive pressure buildup in
the grease cavity.
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Watch for grease coming out the relief port. If excessive
amounts of grease are pumped into the motor and the old,
used grease is not being purged, stop and check for hardened
grease blocking the relief passage.
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If regreasing is performed with the motor out of service,
run the motor until bearing temperature increases to
operating temperature to allow for thermal expansion of the
grease. Ensure the relief valve or drain plug is left out
during this process.
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Allow the motor to run at this temperature for a short
time to expel any excess grease before installing the bottom
grease relief valves.
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After excessive grease has been purged, reinstall the
drain plug and clean excessive grease from the relief port
area.
This article was written to inform the reader of some of
the thought processes that go into the creation of a
lubrication program. Remember to take your time and do it
right the first time. The rewards are worth the effort. |
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