Significance of Viscosity Index: Lubricating
oils are subjected to wide ranges of tempera-
tures in service. At high temperatures, the
viscosity of an oil may drop to a point where the
lubricating film is broken, resulting in metal-to-
metal contact and severe wear. At the other
extreme, the oil may become too viscous for
proper circulation, or may set up such high
viscous forces that proper operation of ma-
chinery is difficult. Consequently, many
applications require an oil with a high viscosity
index.
oils are subjected to wide ranges of tempera-
tures in service. At high temperatures, the
viscosity of an oil may drop to a point where the
lubricating film is broken, resulting in metal-to-
metal contact and severe wear. At the other
extreme, the oil may become too viscous for
proper circulation, or may set up such high
viscous forces that proper operation of ma-
chinery is difficult. Consequently, many
applications require an oil with a high viscosity
index.
In an automobile, for example, the crankcase oil
must not be so thick at low starting temperatures
as to impose excessive drag on the engine and
to make cranking difficult. During the warm-up
period, the oil must flow freely to provide full
lubrication to all engine parts. After the oil has
reached operating temperature, it must not thin
out to the point where consumption is high or
where the lubricating film can no longer carry its
load.
must not be so thick at low starting temperatures
as to impose excessive drag on the engine and
to make cranking difficult. During the warm-up
period, the oil must flow freely to provide full
lubrication to all engine parts. After the oil has
reached operating temperature, it must not thin
out to the point where consumption is high or
where the lubricating film can no longer carry its
load.
Similarly, fluid in an aircraft hydraulic system
may be exposed to temperatures of 100° F or
more on the ground, and to temperatures well
below zero at high altitudes. For proper opera-
tion under these varying conditions, the viscosity
of the hydraulic fluid should remain relatively
constant, which requires a high viscosity index.
more on the ground, and to temperatures well
below zero at high altitudes. For proper opera-
tion under these varying conditions, the viscosity
of the hydraulic fluid should remain relatively
constant, which requires a high viscosity index.
As suggested by the relationship between
naphthenic and paraffinic oils, the viscosity
index of an oil can sometimes be taken as an
indication of the type of base stock. A straight
mineral oil with a high V.I., 80 or above, is
probably paraffinic, while a V.I. below about 40
usually indicates a naphthenlc base stock.
naphthenic and paraffinic oils, the viscosity
index of an oil can sometimes be taken as an
indication of the type of base stock. A straight
mineral oil with a high V.I., 80 or above, is
probably paraffinic, while a V.I. below about 40
usually indicates a naphthenlc base stock.
In general, however, this relationship between
V.I. and type of base stock holds only for
straight mineral oils. The refining techniques
and the additives that are available today make
it possible to produce naphthenic oils with many
of the characteristics, Including V.I., of paraffinic
oils. V.I., then, should be considered an
Indication of hydrocarbon composition only in
the light of additional information.
V.I. and type of base stock holds only for
straight mineral oils. The refining techniques
and the additives that are available today make
it possible to produce naphthenic oils with many
of the characteristics, Including V.I., of paraffinic
oils. V.I., then, should be considered an
Indication of hydrocarbon composition only in
the light of additional information.
WATER WASHOUT
ASTM D 1264
Lubricating greases are often used in applica-
tions that Involve operations under wet
conditions where water may enter the mecha-
nism and mix with the grease. Therefore, the
ability of a grease to resist washout becomes an
important property in the maintenance of a sat-
isfactory lubricating film, and tests for evaluating
the effect of water on grease properties are of
considerable interest.
tions that Involve operations under wet
conditions where water may enter the mecha-
nism and mix with the grease. Therefore, the
ability of a grease to resist washout becomes an
important property in the maintenance of a sat-
isfactory lubricating film, and tests for evaluating
the effect of water on grease properties are of
considerable interest.
Greases can be resistant to water in several
ways. Some greases completely reject the ad-
mixture of water or may retain it only as
occluded droplets with little change in structure.
Unless these greases are adequately inhibited
against rusting they may be unsuitable for lubri-
ways. Some greases completely reject the ad-
mixture of water or may retain it only as
occluded droplets with little change in structure.
Unless these greases are adequately inhibited
against rusting they may be unsuitable for lubri-
cation under wet conditions since the "free"
water could contact the metal surface and cause
rusting.
water could contact the metal surface and cause
rusting.
Yet other greases that absorb water may be
satisfactory under wet conditions. These types
of grease absorb relatively large amounts of
water by forming emulsions of water in oil. This
absorption has little effect on the grease struc-
ture and leaves no "free" water to wet and rust
the metal. Therefore, the grease continues to
supply the proper lubrication while also acting as
a rust preventive.
satisfactory under wet conditions. These types
of grease absorb relatively large amounts of
water by forming emulsions of water in oil. This
absorption has little effect on the grease struc-
ture and leaves no "free" water to wet and rust
the metal. Therefore, the grease continues to
supply the proper lubrication while also acting as
a rust preventive.
Other water-absorbing greases form thin fluid
emulsions so that the grease structure is de-
emulsions so that the grease structure is de-
A-21 (FIST 2-4 11/90)