Efficient Plant March 2020 - 18

feature | lubrication strategies

Gp (g) = 0.005 x D x B
(Note: D and B in mm)
Gp (oz.) = 0.114 x D x B
(Note: D and B in inches)
Gp = grease replenishment amount
D = bearing outside diameter
B = bearing width
Fig. 1: Pay attention to units when using these formulas to calculate the amount of
grease required for replenishment.

consistency-that creates film thickness, but rather the viscosity of the base oil contained with the
grease. For motors operating in the 1,200- to 3,600rpm range, the ideal viscosity is in the range of 90
to 120 cSt at 40 C. This viscosity will typically create
sufficient separation of moving parts, without creating
churning from excess fluid friction.
The most common motor-grease-selection mistake
is to use multipurpose grease. Multipurpose greases
are formulated with base oils in the ISO VG 220 to
460 cSt range, far too high for motor bearings at typical operating speeds. To compound the issue, multipurpose greases often contain EP additives which, in
some circumstances, can volatilize and corrode the
copper windings of a motor. Multipurpose greases
are designed for slower-speed bearings or bearings
with higher loads such as pillow-block bearings used
on conveyors. They should never be used for electric-motor lubrication.
By contrast, well-formulated electric-motor grease
typically contains a base oil with a viscosity around
100 cSt at 40 C, a shear- and temperature-stable thickener such as polyurea, and mild antiwear additives for
mixed-film lubrication.

LUBRICANT QUANTITY
The amount of grease to apply to each motor bearing depends on bearing size. To calculate the right
amount, we simply need to know the outside diameter of the bearing (D) and the bearing's width, or
thickness (B). Multiplying D and B, together with a
constant, as shown in Fig. 1, will yield a good estimate

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for the correct quantity of grease. When using this
approach, pay attention to the units. To obtain grease
quantity in ounces, measure the bearing dimensions
in inches and multiply by 0.114. For grams, determine
D and B in millimeters, then multiply by 0.005.
Since most grease guns dispense in "shots," rather
than ounces or grams, it is often necessary to convert
from the calculated quantities in Fig. 1 to shots from
each grease gun. This is easily done by dispensing
ten shots of grease onto a scale and dividing by ten to
calculate the amount of grease dispensed by one shot
of the grease gun. Be sure to do this for each grease
gun since different styles and models of grease guns
dispense different amounts.

RE-LUBRICATION FREQUENCY
Unlike calculating grease volume, determining the
optimum re-grease interval is slightly more complicated. There are several methods to do this but perhaps the easiest is to use the method first proposed by
Lloyd "Tex" Leugner in his book, Practical Handbook
of Machinery Lubrication (Maintenance Technology
International Inc). The Leugner equation uses shaft
diameter, speed of rotation, and bearing type to estimate the number of hours between re-greasing.

F = re-grease interval (hr.)
n = rotational speed (rpm)
d = shaft diameter (mm)
K = bearing factor:
ball bearing = 10
cylindrical roller = 5
spherical roller = 1
tapered roller = 1

As the formula suggests, as speed increases, the
numbers of hours between re-grease decreases since
the stress placed on the lubricant will be higher. In
addition to speed, size and bearing operating conditions should also be considered. Depending on the
operating environment, one or more of the correction
factors shown in Fig. 2 on p. 19 should be applied to
the Leugner equation.

MARCH 2020

2/18/20 3:33 PM

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Efficient Plant March 2020

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