Efficient Plant June 2020 - 20

feature | lubrication strategies
was suspected to be the root cause.
MULTIPURPOSE
Fig. 2: Determine minimum
GREASE SCOURGE
By comparing the size of the bearoperating
viscosity
One of the primary reasons
ings, which had an OD of 250 mm,
Minimum viscosity (cSt)
grease-selection mistakes are so
with the linear speed of the crane,
prevalent is the over-reliance on
which varied from zero at idle to a top
multipurpose grease. Multispeed of 5 m/sec., the rotational speed
purpose (EP 2) grease is, by far,
of the bearings could be calculated to
the most commonly used grease,
range between 0 and 382 rpm.
and for good reason. Formulated
After talking with the bearing
with shear stable thickeners, base
manufacturer, it became apparent that
Rotational
speed
(rpm)
oils in the range of 220 to 320 cSt,
the standard sealed-for-life bearings
and anti-wear or mild EP addiused were typically designed for
tives, multipurpose grease is well
electric motors running in the 1,800
suited for "average" applications
to 3,600 rpm range and were prefilled
such as pillow-block bearings on
with a low-viscosity-base-oil grease
conveying systems, pins and bushconsistent with higher rpm applicaings, or slides and ways.
tions. By calculating the minimum
Make no mistake, multipurpose
viscosity requirements at linear speeds
does not mean "all-purpose."
between 1 and 5 m/sec. (equivalent to
When used to lubricate electric
rotational speeds between 76 and 382
Use
this
nomograph
to
determine
the
minimum
operating
motors, multipurpose greases
rpm) from Fig. 2, it was determined
viscosity based on the mean pitch diameter (dm) of the bearing
typically have two to three times
that the bearings were functioning at
and the rotational speed. Source: SKF Bearings
higher viscosity than recommenda Kappa range between 0.5 and 1.8.
ed for motors operating in the
The bearings only operated with a
1,800- to 3,600-rpm range. This equates to a Kappa value in the 5.0
Kappa greater than 1 at linear speeds greater than 3 m/sec., which
to 7.0 range. When the base-oil viscosity is this high, it can cause a
represented just a small fraction of the actual operating time. By
motor to run hot and increase energy consumption. These condiswitching to same-sized sealed bearings, pre-lubricated with a grease
tions deteriorate the motor-winding insulation.
formulated with a higher viscosity base oil, and including the use of
Aside from electric motors, the second most common misappliEP additives to assist during stop-start motions, a five-fold increase
cation of multipurpose grease is in couplings. Greased couplings
in bearing life was achieved.
require specialized greases matched to the type of coupling and
HIGH-SPEED FAN BEARINGS
formulated with base oils and thickeners that have a similar specific
A papermill was experiencing high operating temperatures and
gravity to prevent separation under high centrifugal force. Using a
excessively high failure rates on direct-drive fans used to vent the
multipurpose grease in a high-speed coupling can result in separaMachine Room. To try to correct the problem, greasing intervals
tion of the base oil from the thickener at higher rpms, causing oil to
were increased, and a high-quality synthetic paper-machine grease
leak out of the coupling, resulting in lack of lubrication.
was used, with little gain.
For rotating and reciprocating assets, selecting a lubricant with the
Suspecting a lubrication issue, the grease specification was recorrect viscosity is the most important factor for machine reliability
viewed. Comparing the viscosity of the paper-machine grease, at an
and longevity. When it comes to greased bearings, there is no such
operating temperature of 210 F, with the minimum viscosity for a
thing as "one size fits all." Use multipurpose grease judiciously and
6313 bearing rotating at 3,600 rpm, calculated from Figure 2, it was
always know your Kappa. EP
determined that the fan was operating at a Kappa value in excess of
7.0. An ideal Kappa for a high-speed fan is 1.5 to 2.5. By switching
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp.,
to an electric-motor grease with a base-oil viscosity closer to 110 cSt
Goodlettsville, TN (descase.com). He has 21 years of experience in
(after first checking grease compatibility data), operating temperalubrication management, oil analysis, and contamination control and
tures decreased by 35 F and bearing life increased fivefold.
has published more than 150 technical articles and white papers.

EP2006flube.indd 20

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JUNE 2020

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Efficient Plant June 2020

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