Often the selection of seals is limited to seals
covered by military specifications. However, there
are occasions when nonstandard or proprietary
seals reflecting the advancing state of the art may
be approved. Thus, it is important to follow the
manufacturer’s instructions when you replace
seals. If the proper seal is not available, you
should give careful consideration in the selection
of a suitable substitute. Consult the Naval Ships’
Technical Manual, military standards, military
standardization handbooks, and other applicable
technical manuals if you have any doubts in
selecting the proper seal.
Seals are made of materials that have
been carefully chosen or developed for spe-
cific applications.
These materials include
tetrafluoroethylene (TFE), commonly called
Teflon; synthetic rubber (elastomers); cork;
leather; metal; and asbestos. Some of the most
common materials used to make seals for fluid
power systems are discussed in the following
paragraphs.
CORK
Cork has several of the required properties,
which makes it ideally suited as a sealing material
in certain applications. The compressibility of
cork seals makes them well suited for confined
applications in which little or no spread of the
material is allowed. The compressibility of cork
also makes a good seal that can be cut to any
desired thickness and shape to fit any surface and
still provide an excellent seal.
One of the undesirable characteristics of cork
is its tendency to crumble. If cork is used as
packing or in areas where there is a high fluid
pressure and/or high flow velocity, small particles
will be cast off into the system. Cork use in fluid
power systems is therefore limited. It is sometimes
used as gasket materials for inspection plates of
hydraulic reservoirs.
Cork is generally recommended for use where
sustained temperatures do not exceed 2750F.
CORK AND RUBBER
Cork and rubber seals are made by combining
synthetic rubber and cork. This combination has
the properties of both of the two materials.
This means that seals can be made with the
compressibility of cork, but with a resistance to
fluid comparable to the synthetic rubber on which
they are based. Cork and rubber composition is
sometimes used to make gaskets for applications
similar to those described for cork gaskets.
LEATHER
Leather is a closely knit material that is
generally tough, pliable, and relatively resistant
to abrasion, wear, stress, and the effects of
temperature changes. Because it is porous, it is
able to absorb lubricating fluids. This porosity
makes it necessary to impregnate leather for most
uses. In general, leather must be tanned and
treated in order to make it useful as a gasket
material. The tanning processes are those
normally used in the leather industry.
Leather is generally resistant to abrasion
regardless of whether the grain side or the flesh
side is exposed to abrasive action. Leather remains
flexible at low temperatures and can be forced
with comparative ease into contact with metal
flanges. When properly impregnated, it is
impermeable to most liquids and some gases,
and capable of withstanding the effects of
temperatures ranging from –700F to +2200F.
Leather has four basic limitations. First, the
size of the typical hide limits the size of the seals
that can be made from leather. A second
limitation is the number of seals that are
acceptable. Another limitation is that under heavy
mechanical pressures leather tends to extrude.
Finally, many of the properties (such as
impermeability, tensile strength, high- and
low-temperature resistance, pliability, and
compatibility with environment) depend upon the
type of leather and impregnation. Leathers not
tanned and impregnated for specific conditions
and properties will become brittle, dry, and
completely degreased by exposure to particular
chemicals. Leather is never used with steam
pressure of any type, nor with acid or alkali
solutions.
Leather may be used as packing. When
molded into V’s and U’s, and cups, and other
shapes, it can be applied as dynamic packing,
while in its flat form it can be used as straight
compression packing.
METAL
One of the most common metal seals used in
Navy equipment is copper. Flat copper rings are
sometimes used as gaskets under adjusting screws
to provide a fluid seal. Molded copper rings are
sometimes used as packing with speed gears
operating under high pressures. Either type is
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