002

LOADED4X4.COM.AU
RICK THE OIL GUY
the form of soluble additives
or lubricating solids may be
added to the lubricant. Have you
ever wondered why differential
and gear oil often have a funky
sulphur smell? Contrary to
popular belief, it’s not so you can
smell oil leaking from your Land
Rover. Diffs and gearboxes are
two great examples of places
where a boundary layer condition
can easily exist, and that sulphur
smell indicates the presence of
sulphur/phosphorus extreme
pressure additives in the oil.
These additives react under
pressure and heat to form a solid
lubricating layer between gear
teeth. A hypoid differential has an
extreme sliding/wiping action of
the crown wheel and pinion teeth,
and it’s possible to have localised
temperature spikes of over 300+
degrees. The extreme pressure
additives present in the diff’s oil
react to this pressure and heat by
adding a new layer of solid film
lubrication that ensures all the
parts keep sliding against each
other nicely. The plating action of
molybdenum disulphide (moly)
in constant velocity (CV) joints
is another example of solid film
lubrication at work, to prevent
metal to metal contact.
Grease is the word
Did you know that grease
is a mix of oil and a specialist
soap? The soap acts as a type
of thickener that absorbs the oil
and releases it when put under
pressure. The oil is squeezed out
under load when it’s needed and
then reabsorbed into the soap
when the load is reduced. How’s
that for clever!
Grease is used where oil can’t
easily be contained within the
bearing or assembly that requires
lubrication. The downside
of grease compared to
oil is that its use will
increase the heat of
an operating assembly
due to increased drag and lower
conduction with a slightly lower
load carrying capacity - generally
- compared to a high viscosity
extreme pressure oil.
There are a variety of
different soaps used in grease,
all with differing characteristics,
and much the same as oils,
there are a variety of grease
types available, each with its
unique qualities that are suited to
specific uses.
So that’s the basics on how
oil and grease go about easing
the pain of the various parts in
our 4X4s that are in contact with
each other.
In issue 003 we’re going
to take a look at engine oils;
what the various numbers on
the container mean and why
manufacturers specify certain
grades and types of oil for
their vehicles.
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