Page 306 - The Tribology Handbook

Mineral oils c2

The distillates and residues are used to a minor extent 50 000
as such, but generally they are treated or refined both
before and after vacuum distillation to fit them for the 20 000
10 000
more stringent requirement!;. The principal processes
listed in Table 2.4 are selected to suit the type of crude oil
and the properties required.
Elimination of aromatics increases the VI of an oil. A 2000
lightly refined naphthenic oil may be LVI but MVI if 1000
highly refined. Similarly a lightly refined mixed-base
oil may be MVI but HVI if highly refined. Elimination of 500
aromatics also reduces nitrogen, oxygen and sulphur
contents.
The distillates and residues may be used alone or blended
together. Additionally, minor amounts of fatty oils or of
special oil-soluble chemicals (additives) are blended in
to form additive engine oils, cutting oils, gear oils, hydraulic
oils, turbine oils, and so on, with superior properties to
plain oils, as discussed below. The tolerance in blend
viscosity for commercial branded oils is typically *4y0
but official standards usually have wider limits, e.g. & 10%
for IS0 3448.

PHYSICAL PROPERTIES 10.0
8.0'
Viscosity-.Temperature 6.0
Figure 2.4 illustrates the variation of viscosity with 5.0
temperature for a series of oils with kinematic viscosity 4.0
index of 95 (dynamic viscosity index 93). Figure 2.2 shows
the difference between 150 Grade IS0 3448 oils with 3.0
-20 -
KVIs of 0 and 95. Temperature, "C
Vi scosity--P ressu re Figure 2.2 150 grade IS0 3448 oils of 0 and 95 KW
The viscosity of oils increases significantly under pres-
sure. Naphthtenic oils are more affected than paraffinic
but, very roughly, both double their viscosity for every
35 MN/m2 increase of pressure. Figure 2.3 gives an
impression of the variation in viscosity of an SAE 20LY
IS0 3448 or medium machine oil, HVI type, with both
temperature and pressure.
In elastohydrodynamic (e hl) formulae it is usually
assumed that the viscosity increases exponentially with
pressure. Though in fact considerable deviations from an
exponential increase may occur at high pressures, the
assumption is valid up to pressures which control ehl
behaviour, i.e. about 35 MN/m2. Typical pressure viscosity
coefficients are given in Table 2.5, together with other
physical properties~
Pour point
De-waxed paraffinic oils still contain 1% or so of waxy
hydrocarbons, whereas naphthenic oils only have traces
of them. At about O"C, according to the degree of de-
waxing, the waxes in paraffinic oils crystallise out of
solution and at about -IOo@ the crystals grow to the
extent that the remaining oil can no longer flow. This
temperature, or close to it, when determined under
specified conditions is known as the pour point. Naph-
thenic oils, in contrast, simply become so viscous with
decreasing temperature that they fail to flow, although no
wax crystal structure develops. Paraffinic oils are therefore Figure 2.3 Variation of viscosity with temperature
said to have 'wax' pour points while naphthenic oils are and pressure of an SAE 2OW (HVI) oil (courtesy.
said to have 'viscosity' pour points. Institution of Mechanical Engineers)
C2.3