|
Application
Notes:
|
|
viscosity index (V.I.)
is the empirical, unitless number indicating the effect
of temperature change on the kinematic
viscosity of an oil. Liquids change viscosity
with temperature, becoming less viscous when heated; the higher the V.I. of
an oil, the lower its tendency to change viscosity with temperature. The V.I.
of an oil — with known viscosity at 40°C and at 100°C — is determined by
comparing the oil with two standard oils having an arbitrary V.I. of 0 and
100, respectively, and both having the same viscosity at 100°C as the test
oil. The following formula is used, in accordance with test method
ASTM-D-2270:
V.I. = L-U/L-H x 100
where:
L = viscosity at 40°C of the 0-V.I. oil;
H = viscosity at 40°C of the 100-V.I. oil;
U = viscosity at 40°C of the test oil.
There is an alternative calculation, also in
ASTM-D-2270, for oils with V.I.’s above
100. The V.I. of paraffinic oils is inherently
high, but is low in naphthenic oils, and even
lower in aromatic oils (often below 0). The V.I.
of any petroleum oil can be increased by adding
a viscosity index improver.
High-V.I. lubricants are needed wherever relatively constant viscosity is
required at widely varying temperatures. In an automobile, for example, an
engine oil must flow freely enough to permit cold starting, but must be
viscous enough after warm-up to provide full lubrication (see
multi-grade oil). Similarly, in an
aircraft hydraulic system, which may be exposed to temperatures above 38°C
at ground level and temperatures below -54°C at high altitudes, consistent
hydraulic fluid performance requires a
high viscosity index.
Also see V.I. Calculator,
Viscosity Classification
.
REAL Services
700 Portage Trail
Cuyahoga Falls, OH
44221.3057
voice: 330.630.3700
fax: 330.630.3733
© 1995-2005 REAL Services® U.S.A.
- Analytical Almanac
All Rights Reserved
|
|
|