Additional information on the various types
of viscometers and their operation can be found
in the Physical Measurements Training Manual,
The viscosity index (V.I.) of an oil is a number
that indicates the effect of temperature changes
on the viscosity of the oil. A low V.I. signifies
a relatively large change of viscosity with changes
of temperature. In other words, the oil becomes
extremely thin at high temperatures and extremely
thick at low temperatures. On the other hand, a
high V.I. signifies relatively little change in
viscosity over a wide temperature range.
An ideal oil for most purposes is one
that maintains a constant viscosity throughout
temperature changes. The importance of the V.I.
can be shown easily by considering automotive
lubricants. An oil having a high V.I. resists
excessive thickening when the engine is cold and,
consequently, promotes rapid starting and prompt
circulation; it resists excessive thinning when the
motor is hot and thus provides full lubrication and
prevents excessive oil consumption.
Another example of the importance of the V.I.
is the need for a high V.I. hydraulic oil for military
aircraft, since hydraulic control systems may be
exposed to temperatures ranging from below
65°F at high altitudes to over 100°F on the
ground. For the proper operation of the hydraulic
control system, the hydraulic fluid must have a
sufficiently high V.I. to perform its functions at
the extremes of the expected temperature range.
Liquids with a high viscosity have a greater
resistance to heat than low viscosity liquids which
have been derived from the same source. The
average hydraulic liquid has a relatively low
viscosity. Fortunately, there is a wide choice of
liquids available for use in the viscosity range
required of hydraulic liquids.
The V.I. of an oil may be determined if its
viscosity at any two temperatures is known.
Tables, based on a large number of tests, are
issued by the American Society for Testing
and Materials (ASTM). These tables permit
calculation of the V.I. from known viscosities.
If motion takes place between surfaces in
contact, friction tends to oppose the motion.
When pressure forces the liquid of a hydraulic
system between the surfaces of moving parts, the
liquid spreads out into a thin film which enables
the parts to move more freely. Different liquids,
including oils, vary greatly not only in their
lubricating ability but also in film strength. Film
strength is the capability of a liquid to resist being
wiped or squeezed out from between the surfaces
when spread out in an extremely thin layer. A
liquid will no longer lubricate if the film breaks
down, since the motion of part against part wipes
the metal clean of liquid.
Lubricating power varies with temperature
changes; therefore, the climatic and working
conditions must enter into the determination of
the lubricating qualities of a liquid. Unlike
viscosity, which is a physical property, the
lubricating power and film strength of a liquid
is directly related to its chemical nature.
Lubricating qualities and film strength can be
improved by the addition of certain chemical
Chemical stability is another property which
is exceedingly important in the selection of a
hydraulic liquid. It is defined as the liquids ability
to resist oxidation and deterioration for long
periods. All liquids tend to undergo unfavorable
changes under severe operating conditions. This
is the case, for example, when a system operates
for a considerable period of time at high
Excessive temperatures, especially extremely
high temperatures, have a great effect on the life
of a liquid. The temperature of the liquid in the
reservoir of an operating hydraulic system does
not always indicate the operating conditions
throughout the system. Localized hot spots occur
on bearings, gear teeth, or at other points where
the liquid under pressure is forced through small
orifices. Continuous passage of the liquid through
these points may produce local temperatures high
enough to carbonize the liquid or turn it into
sludge, yet the liquid in the reservoir may not
indicate an excessively high temperature.
Liquids may break down if exposed to air,
water, salt, or other impurities, especially if they
are in constant motion or subjected to heat. Some
metals, such as zinc, lead, brass, and copper, have
undesirable chemical reactions with certain
These chemical reactions result in the forma-
tion of sludge, gums, carbon, or other deposits
which clog openings, cause valves and pistons to
stick or leak, and give poor lubrication to moving