The
viscosity index, commonly designated VI, is an arbitrary
numbering scale that
indicates the changes in oil viscosity with changes in
temperature. Viscosity index can be classified as follows:
low VI - below 35; medium VI - 35 to 80; high VI - 80 to
110; very high VI - above 110. A high viscosity
index indicates small oil viscosity changes with
temperature. A low viscosity index indicates high viscosity
changes with temperature. Therefore, a fluid that has a high
viscosity index can be expected to undergo
very little change in viscosity with temperature extremes
and is considered to have a stable viscosity.
A fluid with a low viscosity index can be expected to
undergo a significant change in viscosity as the
temperature fluctuates. For a given temperature range, say
-18 to 370°C
( 0 - 100°F),
the viscosity of one
oil may change considerably more than another. An oil with a
VI of 95 to 100 would change less than one
with a VI of 80. Knowing the viscosity index of an oil is
crucial when selecting a lubricant for an application, and
is especially critical in extremely hot or cold climates.
Failure to use an oil with the proper
viscosity index when temperature extremes are expected may
result in poor lubrication and equipment
failure. Typically, paraffinic oils are rated at 38°C
( 100°F)
and naphthenic oils are rated at -18°C
(0°F).
Proper selection of petroleum stocks and additives can
produce oils with a very good VI.
