Lubezine Magazine Vol. 9 April - June 2014 | Page 24

TECHNOLOGY

FEATURE

gear and engine numbers cover the same
range of viscosities; for example, an SAE 30
engine oil has approximately the same viscosity as a SAE 85W gear oil. This is because
the formulation of engine oils is very different to that of gear oils in the automotive
industry. An engine oil is far more stressed
than a gear oil because it must cope with
combustion by-products and blow-by gases
which severely degrade the oil. As a result
engine oils contain a much wider variety of
additives than gear oils. Although not ideal,
an engine oil will function in a gearbox
while a gear oil will destroy an engine.

Comparative Viscosity Classifications

Monogrades and multigrades
Engine (and gear type) oils come in a variety
of grades as the table 2 shows such as SAE 30
or SAE 80W. These are known as monograde
oils, but multigrade oils are also available
with SAE gradings like 15W40 or 80W90 etc.
All multigrade oils have the viscosity
properties of a low temperature ‘W’ oil and a
high temperature oil without the ‘W’ suffix.
An SAE 30 and an SAE 15W40 both have
roughly the same viscosity (kinematic) at
40 0C; they both approximate an ISO 100
oil. What then is the difference between a
monograde and a multigrade oil in viscosity
terms?
Remember that if temperature is
increased, viscosity will decrease. The viscosity is high at low temperatures and low
at high temperatures. However, not all oils
behave in the same manner. Some oils ‘thin’
out less than others when the temperature
is increased. This is the difference between
a monograde and a multigrade oil.
On a very cold winter morning the temperature could be -50C but when the engine
reaches full operating temperature it might
be 1000C. Ideally, what is required is a fairly
low viscosity oil which will flow readily
at low temperatures without thinning out
too much when operating temperature is
reached. Multigrade oils are formulated to
do this.
A typical monograde oil such as SAE 40
would thin out as temperature increases
while as a multigrade oil such as 20W50
would thin out less with increase in temperature .
This introduces the concept of the Viscosity Index (VI) which is a measure of an oil’s
‘multigradedness’.
The higher the VI, the more ‘multigraded’
the oil. In the above example, SAE 40 (mono22

Chart 1

grade) has a low VI while SAE 20W50 has a
high VI.
The advantage of using a multigrade oil
is that it has greater viscosity stability over
a wider range of temperatures. The oil
behaves like an S QH