Efficient Plant October 2022 - 20

feature | lubrication solutions
Fig. 1: Viscosity Index formula
500
VI = L - U x 100
L - H
50
L40 C
U40 C
H40 C
VI = 0
VI = 100
5
20 30
40
50 60
70
Temperature (C)
Viscosity Index can be calculated by comparing the viscosity of an
unknown oil at 40 C, with viscosity at 40 C of two reference oils L and H.
as outlined in ASTM D341. As a result, two oils that have nominally
the same ISO viscosity grade, i.e., identical viscosities at 40 C, can
have very diff erent viscosities at temperatures above and below 40 C
(Fig. 2. p. 21).
Th is concept is particularly important for equipment that needs
to start at very low ambient operating temperatures, such as engines,
transmissions, gearboxes, and hydraulic systems that operate outside
in colder climates where startup may occur at -40 C (-40 F) or
colder. In these instances, choosing an oil that has a very high VI
is of paramount importance to have suffi cient pumpability to avoid
starving the equipment of lubricant at startup.
An oil's VI is largely defi ned by the type of base oil selected. Most
conventional Group I and II mineral-based industrial oils have VIs
in the 95 to 105 range. By contrast, a PAO synthetic will have a VI in
the 140 to 160 range. Other types of fl uids will have VIs consistent
with the rheological properties of the base oil (Fig. 2).
An oil's VI can be artifi cially increased with VI improver additives.
VI improvers are typically made from long-chain organic polymers
such as polymethacrylates (PMA) or polyisobutylene (PIB). Th ey
work because their structure and molecular shape changes with
temperature. At very low temperatures, the polymers are coiled
tightly into a " ball. " As such, their contribution to an oil's measured
kinematic viscosity is low. As temperature increases, the polymer
chain uncoils, causing internal resistance to dynamic fl ow and shear.
A simple visual analogy is to think about dropping uncooked,
dried pasta (the VI polymer) into a large pan of cold water (the
base oil). A small, uncooked ball of spaghetti in a large pan of water
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80 90
100
has little impact on the resistance to stirring with a wooden spoon.
However, as the water is heated, the pasta will begin to swell or
uncoil, creating greater resistance to stirring.
While the analogy with cooking pasta fails when temperature
decreases since, unlike pasta, the VI polymer contracts when the
temperature drops, it's useful to explain two other issues with oils
that are heavily fortifi ed with VI improvers.
One is the impact of permanent shear thinning. VI polymers work
because their large molecular shape creates resistance to fl ow and
shear. However, in applications such as engines, gearboxes, and
hydraulics, the lubricant is exposed to signifi cant shearing action
from rotating and reciprocating contacts. In some instances, this can
shear the VI polymer, reducing the average molecular weight (size)
of the polymer. Th e impact will be a lower overall viscosity, particularly
at elevated temperatures.
Referring to the pasta example, permanent shear thinning is
like chopping long spaghetti strands into small one-inch lengths.
Because the spaghetti is now much smaller, it off ers much lower
resistance to stirring with a spoon, analogous to the viscosity drop in
a lubricant that has undergone permanent shear thinning.
Another issue with oils that have been heavily fortifi ed with VI
improvers is the far more nuanced problem of temporary shear
thinning. Temporary shear thinning occurs under higher RPMs and
is caused by the VI polymers aligning themselves in the direction of
the applied shearing force.
Again, the example of stirring pasta helps. While cooked spaghetti
may provide resistance when the water is stirred slowly, if stirred at
high speed in a constant (radial) direction, the spaghetti strands will
align in the direction of the stirring, eff ectively lowering the resistance.
Th is is analogous to a temporary reduction in viscosity under
applied shear stress. Once the applied shear force (stirring) stops,
the large pasta strands re-orient randomly. Th is same phenomenon
holds true for VI polymers under high radial stress.
It's for this reason that SAE engine-oil specifi cations include measurement
of kinematic and high-temperature, high-shear (HTHS)
viscosity. Th rough the use of HTHS fl uids and the impact of dynamic
viscosity shear thinning, highway truck fl eets experienced a 0.5%
to 1.5% gain in fuel savings since the lower eff ective viscosity results
in a thinner oil fi lm and less fl uid " drag. "
While this may be good for trucks, the same cannot be said for
other applications. In hydraulic systems operating in colder climates,
as an example, it's common practice to use HVI (high viscosity
index) and VHVI (very-high viscosity index) fl uids. Th ese fl uids
have VIs oſt en as high as 300, making them usable at much lower
start-up and operating temperatures.
However, a common problem with VHIVI fl uids that are poorly
OCTOBER 2022
Log10 kinematic viscosity
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Efficient Plant October 2022

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