Efficient Plant June 2020 - 18

feature | lubrication strategies

Know Your

Kappa
Values

If your bearings require grease
lubrication, reliable long-term
performance is determined by
the Kappa value.
Mark Barnes, PhD CMRP
Des-Case Corp.

SELECTING THE CORRECT lubricant
for rotating or reciprocating assets is the
most fundamental aspect of any reliability-focused lubrication program. Unfortunately, for grease-lubricated assets, it's quite
common to select the incorrect lubricant.
Unlike bearings lubricated with oils,
where most OEMs provide a good starting
point, many grease-lubricated bearings do
not come with a lubricant recommendation, and for good reason. In an oil-filled
gearbox, input/output speeds and load are
all known. That's not necessarily the case
when rolling-element bearings are used in a
slow-turning conveyor or a high-speed fan.
Since lubricant selection is always based on
choosing the correct viscosity and additive
package for the required load and speed,
selecting the right grease for a 60-rpm conveyor pulley bearing is very different from
that for a 3,600-rpm direct-drive fan.

KAPPA IS KEY
To select the proper grease for element
bearings, we need to understand how a

EP2006flube.indd 18

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lubricant separates moving surfaces under
operating loads and speeds. The term for
this is film thickness, which is a measure of
how far apart the metal surfaces are under
operating loads and speeds. Too close
together and wear will occur. Too far apart
and fluid friction will create churning, causing operating temperatures to rise.
To determine optimum oil-film thickness,
lubrication engineers often use the Kappa
value of the grease/bearing at operating
speed and temperature. The concept of
Kappa applies equally to oil- and greaselubricated bearings but is most useful when
calculating the appropriate grease. Simply
put, Kappa is the ratio of the actual viscosity
of the base oil, within the grease at operating temperature, to the minimum viscosity
required to support a dynamic load within
a bearing to provide full film lubrication
(Figure 1).
At Kappa values less than 1.0, asperities
on rolling-element surfaces and raceways
will contact each other since the lubricant
viscosity is too low to create complete

separation, an effect known as boundary
lubrication. As base-oil viscosity increases,
so too does the Kappa value for the same
bearing under identical operating conditions. Once Kappa exceeds 1.0, complete
bearing separation is achieved. Kappa
values in the 1.5 to 2.0 range indicate
"full-film" elasto-hydrodynamic (EHD)
separation.
For higher-speed oil-lubricated bearings,
operating under constant speed, Kappa
values in the 1.5 to 2.0 range are recommended. For grease-lubricated bearings,
operating at slower (or variable) speeds,
Kappa values in the 2.0 to 4.0 range may
be more appropriate. Where shock loading
may occur, even higher Kappa values are
sometimes used.
The Kappa value of a grease/bearing
combination can be determined by calculating the minimum viscosity at operating
conditions, using the nomograph shown in
Figure 2, p. 20, and comparing the minimum viscosity to the actual viscosity of the
base oil at operating temperatures. When

JUNE 2020

5/28/20 3:46 PM

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Efficient Plant June 2020

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