Viscosity of Hydraulic
Oil
By Martin
Cuthbert MEng (Hons), Webtec Products Ltd.
The internal resistance
to flow of a liquid is measured by a fluidís viscosity. More precisely
absolute viscosity (
) is defined in terms
of the force between two parallel layers of fluid for a certain slip velocity
between them. This is represented by Newtonís equation (
).
Very often a hydraulic
fluid will be selected on the basis of its viscosity and the operating temperature
of the system. A fluid will flow more easily the less viscous it is, since less
energy is required to overcome the internal frictional forces. Any saving in
energy must be balanced against an increase in leakage due to the lower fluid
viscosity.
There are two measures
of viscosity: absolute (also known as dynamic) and kinematic. The S.I. unit
for absolute viscosity is N s m-2 or Pa.s. The non-S.I. unit is the
poise (P) equivalent to 0.1 N s m-2 (not to be confused with
the poiseuille (Pl), used in France, and equal to 10 poise) though the
centipoise (cP) is more commonly used. In the hydraulics industry kinematic
viscosity is more frequently used, where:
The S.I. unit for
kinematic viscosity (
) is mm2s-1
which corresponds to the older but still commonly used unit the centistoke (cSt).
Past measures of
viscosity using arbitrary scales like Redwood No 1 seconds, Saybolt Universal
Seconds (SUS), or degrees Engler should no longer be used. These units have
been superseded by the empirical measures previously mentioned; conversion tables
do exist but are only true at a fixed temperature.
Effect of temperature
on viscosity
The temperature and
viscosity of hydraulic oil are inversely related; as temperature increases, viscosity
decreases. In order to define the kinematic viscosity of oil, its viscosity is
quoted at a set temperature (40ƒC for the ISO standard) and the oil is given a
value according to the viscosity index (V.I.). For example an oil quoted as conforming
to ISO 22 will have a viscosity of 22 mm2s-1/ cSt at 40ƒC.
Viscosity Index
The viscosity index
is a single number representation of the viscosity temperature characteristics
of a fluid. The greater the value of the V.I. the smaller the change in viscosity
for a given change in temperature, and vice-versa. Oils with a V.I. of 80 or more
are said to have a high V.I. value. Oils with a V.I. between 80 and 40 are said
to have a medium value and those below 40 a low value. Typically mineral oils
used by the fluid power industry have a high V.I. of about 100. If temperature
and kinematic viscosity are plotted to give a linear relationship (using logarithmic
scales) then the V.I. is a measure of the gradient of the line. As the V.I. is
increased the gradient is reduced. A typical temperature-viscosity curve for ISO
oils can be seen below.
Figure:
Effect
of Temperature on Kinematic Viscosity
Effect of pressure
on viscosity
Contrary to popular
belief, varying pressure can lead to significant variations in viscosity. In a
closed flow circuit at a fixed temperature, a change in pressure of 40 MPa (400
bar) can lead to a change of up to 8% in viscosity. However there are problems
in calculating this variation.
Density and specific
volume
The density of mineral
oils is typically around 870 kg m-3 (in comparison synthetic oils
usually have a density of around 1200 kg m-3). The specific gravity,
the ratio of the density of the fluid to the density of water, is a dimensionless
quantity typically 0.87 for mineral oils.
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Webtec Products Ltd (March 2004)