HYDRAULIC
FLUID
Hydraulic system
liquids are used
primarily to transmit
and distribute forces
to various units
to be actuated. Liquids are
able to do
this because they
are almost incompressible. Pascal’s Law
states that pressure
applied to any
part of a
confined liquid is
transmitted with undiminished
intensity to every
other part. Thus if
a number of
passages exist in
a system pressure
can be distributed
through all of
them by means
of the liquid.
Manufactures of
hydraulic devices usually
specify the type
of liquid best
suited for use
with their equipment, in
view of the
working conditions, the service
required, temperature
expected inside and
outside the systems, pressure the
liquid must withstand, the possibilities
of corrosion and
other conditions that
must be considered. Some of
the properties and
characteristics that must
be considered when
selecting a satisfactory
liquid for a
particular system are
discussed below.
Viscosity
One of
the most important
properties of any
hydraulic fluid is
its viscosity. Viscosity is
internal resistance to
flow. A satisfactory liquid
for a given
hydraulic system must
have enough body
to give a
good seal at
pumps, valves and pistons; but
it must not
be so that
it offers resistance
to flow, leading to
power loss and
higher operating temperature. A fluid
that is too
thin will also
lead to rapid
wear of moving
parts, or of parts
which have heavy
loads
Chemical Stability
Chemical stability
is another property
which is exceedingly
important in selecting
a hydraulic liquid. It
is the liquid
ability to resist
oxidation and deterioration
for long periods. All
liquids tend to
undergo unfavorable chemical
change under severe
operating conditions.
Liquids may
break down if
exposed to air, water, salt, or other
impurities especially if
they are in
constant motion or
subject to heat. It
should be noted
that the temperature
of the liquid
in the reservoir
of an operating
hydraulic system does
not always represent
a true state
of operating conditions. Localized hot
spots occur on
bearings, gear teeth, or at
the point where
liquid under pressure
is forced through
a small orifice. Continuous passage
of a liquid
through these points
may produce local
temperatures high enough
to carbonize or
sludge the liquid, yet
the liquid in
the reservoir may
not indicate an
excessively high temperature.
Some metals
such as zinc, lead, brass and
copper have an
undesirable chemical reaction
on certain liquids. These chemical
process result in
the formation of
sludge, gum and carbon
or other deposits
which clog openings, cause valves
and pistons to
stick or leak, and
give poor lubrication
to moving parts. It
may cause changes
in the physical
and chemical properties
of the liquid. The
liquid may become
darker in colour, higher in
viscosity and acids
are formed.
Flash
Point
Flash point is
the temperature at
which a liquid
gives off vapour
in sufficient quantity
to ignite momentarily
or flash when
a flame applied. A high flash
point is desirable
for hydraulic liquids
because it indicates
good resistance to
combustion and a
low degree of
evaporation at normal
temperatures.
Fire
Point
Fire point is
the temperature at
which a substance
gives off vapour
in sufficient quantity
to ignite and
continue to burn
when exposed to a spark
or flame. Like flash point
a high fire point
is required for
the hydraulic liquids.
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