Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/544388
40
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July - August 2015
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www.machinerylubrication.com
with lithium, calcium, polyurea and silica thickeners, are short-fi -
bered. The greases formulated with these thickeners typically have
better channeling characteristics and are more easily pumped.
Long-fi bered thickeners, such as those with sodium, aluminum
and barium, tend to have worse channeling characteristics. The
longer thickener fi ber can also be sheared through the churning
process, which can cause a change in consistency. In addition, since
these greases often fl ow back into the channel that has been cut by
a bearing, they can result in an increase in heat and exacerbate the
shearing process.
NLGI Grade
The base oil viscosity and the amount of thickener concentration
greatly infl uence the NLGI grade of the fi nished lubricating grease.
The NLGI number is a measure of the grease's consistency. The
higher the NLGI number, the thicker the overall consistency. The
scale ranges from 000 (fl uid like) to 6 (solid block). When it comes
to high-speed greases for rolling-element bearings, the NLGI grade
tends to go up while the base oil viscosity goes down. This balance
is to ensure there isn't excess oil bleed from the thickener. Based
upon the bearing's speed factor as well as the temperature in which
the bearing operates, you can draw solid conclusions about the
appropriate NLGI grade of the grease.
Bearing Type
Rolling elements in bearings come in a variety of shapes. The
shape of the element makes an impact on the required viscosity,
NLGI grade and regreasing interval. This all has to do with the
amount of surface area in contact with the grease between the
element and the race. The more surface area, the more the oil will
be wrung out of the thickener. Also, the bearings that have more
contact (spherical, cylindrical, needle, tapered roller, etc.) tend to
be more heavily loaded than a standard ball bearing. This added
load lends itself to an increased separation rate as well as the need
for higher viscosity base oils.
Dropping Point
Perhaps one of the most notable considerations when selecting
a high-speed grease is the temperature at which the bearing will
operate. To ensure the selected grease will perform at elevated
temperatures, you should check the dropping point of the grease
(ASTM D566 and D2265). These test results can be found on most
all grease technical data sheets. The test uses a small cup with a
hole in the bottom in which the grease is applied to the inside walls.
A thermometer is then inserted but does not touch the grease. This
apparatus is then heated until a single drop of oil separates and
drips out of the bottom of the cup. The temperature at which this
occurs is the dropping point of the grease.
A high dropping point is important for bearings operating at
elevated temperatures. However, just because a grease has a high
dropping point doesn't mean the base oil can withstand elevated
temperatures. The dropping point does not equate to the
maximum usable temperature. There should be a buffer between
the temperature at which the bearing operates and the dropping
point of the grease.
Incompatibility Issues
When changing grease types, it is important to remove as much
of the old grease as possible to minimize any incompatibility issues
with the new grease. If feasible, disassemble the equipment and
clean out as much of the grease as possible.
Although the majority of applications will be properly lubri-
cated with a general-purpose grease, for those instances when the
NDm value is excessively high, it is essential to ensure the lubri-
cant is able to protect the equipment. Even if you are diligent and
select a grease based upon all the previously mentioned proper-
ties, the only way to truly know if the grease will perform in the
desired manner is to conduct a fi eld trial. Monitor the bearing
temperature and look for any signs of grease or oil leaking out
from the seals or purge vents.
Finally, be sure to do your homework and calculate the NDm
values of your bearings in order to select the appropriate lubricant.
With proper attention and lubricant selection, your high-speed
equipment will enjoy a longer service life.
About the Author
Wes Cash is a senior technical consultant with Noria Corporation.
He holds a Machine Lubrication Technician (MLT) Level II certifi cation
and a Machine Lubricant Analyst (MLA) Level III certifi cation through the
International Council for Machinery Lubrication (ICML). Contact Wes at
wcash@noria.com.
IN THE TRENCHES
BEARING TYPE RELATIVE LIFE OF GREASE
Deep-groove, single-row
ball bearing
1
Angular contact, single-row
ball bearing
0.625
Self-aligning ball
bearing
0.77-0.625
Thrust ball
bearing
0.2-0.17
Cylindrical, single-row roller
bearing
0.625-0.43
Needle roller
bearing
0.3
Tapered roller
bearing
0.25
Spherical roller bearing 0.14-0.08
TYPICAL MAXIMUM OPERATING GREASE TEMPERATURE
• If dropping point (DP)<300° F, subtract 75° F from the DP
• If 300° F