2. Dynamic seals are used to seal moving parts.
Dynamic seals include shaft and rod seals and
compression packings. A slight leakage in these
seals is acceptable for seal lubrication.
SEALING MATERIALS.There have been many
different materials used in the development of sealing
devices. The material used for a particular application
depends on several factors: fluid compatibility;
resistance to heat, pressure, and wear resistance;
hardness; and type of motion.
The selection of the correct packings and gaskets
and their proper installations are important factors in
maintaining an efficient fluid power system. The types
of seals to be used in a particular piece of equipment
are specified by the equipment manufacturer.
Seals are made of materials that have been
carefully chosen or developed for specific
applications. These materials include polytetra-
flouroethylene (PTFE), commonly called TEFLONTM,
synthetic rubber, cork, leather, metal, and asbestos.
The most common types of materials are discussed in
the following paragraphs.
Cork.The physical properties of cork make 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 used under various
pressures and allows the gasket to be cut to any desired
thickness to fit any surface, while still forming an
excellent seal. Cork is generally recommended for use
where sustained temperatures do not exceed 275°F.
Synthetic Rubber.The materials used in synthetic
rubber seals are either neoprene or nitrile-butadiene base.
These seals are available in a wide range of density,
tensile strength, and elongations. Many factors
contribute to make synthetic rubber ideal for seals. The
elasticity of the material makes it easier in many
applications. Since synthetic rubber seals are virtually
impermeable in their compressed state, they require less
sealing load than many other types of gaskets. Synthetic
rubber seals are used in a variety of applications and
are capable of functioning in temperature ranges as
wide as -65°F to +300°F.
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 to make it useful as a sealing
material. The tanning processes are those normally used in
the leather industry. It is generally resistant to abrasion
regardless of whether the grain side or 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 leather is properly
impregnated, it is impermeable to most liquids and some
gases. Leather is capable of withstanding the effects of
temperatures ranging from -70°F to +220°F.
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. Copper is easily bent and requires
careful handling. In addition, copper becomes hard when
used over a long period of time and when subjected to
compression. It is advised that when a component is
disassembled, the copper sealing rings should be
In some fluid power actuating cylinders, metallic
piston rings are used as packing. These rings are similar
in design to the piston rings used in engines.
TYPES OF SEALS.Fluid power seals are
usually typed according to their shape or design. These
types include T-seals, O-rings, quad-rings, and U-cups,
and so on. Some of the most commonly used seals are
discussed in the following paragraphs.
T-Seals. The T-seal has an elastomeric
bidirectional sealing element resembling an inverted
letter T. This sealing element is always paired with two
special extrusion-resisting backup rings, one on each side
of the T. The basic T-seal configuration is shown in figure
3-44, view A. The backup rings are single turn, bias cut,
and are usually made of TEFLONTM, nylon, or a
combination of TEFLON and nylon. Nylon is widely
used for T-seal backup rings because it provides excellent
resistance to extrusion and has low friction
The special T-ring configuration adds stability to the
seal, eliminating spiraling and rolling. T-seals are used in
applications where large clearances could occur as a
result of expansion.
O-Rings. An O-ring is doughnut-shaped. O-rings
are usually molded from rubber compounds; however,
they can be molded or machined from plastic materials.
The O-ring is usually fitted into a rectangular groove that
is machined into the item to be sealed.
An O-ring sealing system is often one of the first
sealing systems considered when fluid closure is