MACHINERY LUBRICATION- INDIA SEPTEMBER-OCTOBER 2019 | Page 25

C M Sharma | VAS Tribology Solutions
WHAT IF..?
You have abnormal
foaming of
oil in a gearbox
Too many anti-foam additives can lead
to a significant deterioration of the
air-release capability. It is always a good
practice to consult your lubricant
supplier first for advice.
Symptoms
Foam is a collection of small bubbles of
air that accumulate on or near the
surface of the fluid. In severe cases, the
foam can leak out of the machine
through breathers, sight glasses and
dipsticks.
Lubricating oils are not completely free
of air. Whether during operation or
storage in barrels, oils are constantly in
an exchange process with their
air-containing environment. Even if the
oil is free of air bubbles, it will have a
proportion of dissolved air. This
depends primarily on the gas solubility,
but pressure and temperature also have
an effect. Some mineral oils can have air
content approaching 9 to 11 percent
volume at atmospheric pressure and
room temperature. As long as the air
remains dissolved in the oil, this
generally is not a problem. However,
free air bubbles, which usually are
caused by constantly immersing
machine parts or through oil returning
to the reservoir, can lead to serious
disruptions in equipment operation,
including impaired cooling effect,
increased
oxidation
tendency,
shortened oil life, reduced carrying
capacity of the lubricant film, oil spills,
decreased oil pump capacity, lack of
lubrication, cavitation and micro
dieseling. Air release cannot be
improved by additives. However, the
foaming behaviour of lubricating oils
can be improved by anti-foam additives,
which reduce the surface tension of the
oil, i.e., by the well-proportioned
addition of silicon - containing
compounds or oil - soluble polyglycols.
Foam is an efficient thermal insulator,
so the temperature of the oil can
become difficult to control.
Foam is an object formed by trapping
pockets of gas in a liquid or solid. A
bath sponge and the head on a glass of
beer are examples of foams. In most
foams, the volume of gas is large, with
thin films of liquid or solid separating
the regions of gas.
Foam and Air Release
Oil returning to a reservoir has enough
time to separate air in the form of air
bubbles. The main influences on the
speed at which these air bubbles
separate from the oil and rise include
the size of the bubbles, the oil’s viscosity
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