CHAPTER 7
SEALING DEVICES AND MATERIALS
Recall from chapter 1 that Pascal’s theorem,
from which the fundamental law for the science
of hydraulics evolved, was proposed in the
seventeenth century. One stipulation to make the
law effective for practical applications was a
piston that would “fit” the opening in the vessel
“exactly.” However, it was not until the late
eighteenth century that Joseph Brahmah invented
an effective piston seal, the cup packing. This led
to Brahmah's development of the hydraulic press.
The packing was probably the most important
invention in the development of hydraulics as a
leading method of transmitting power. The
development of machines to cut and shape closely
fitted parts was also very important in the
development of hydraulics. However, regardless
of how precise the machining process is, some type
of packing is usually required to make the piston,
and many other parts of hydraulic components,
“fit exactly.” This also applies to the components
of pneumatic systems.
Through years of research and experiments,
many different materials and designs have been
created in attempts to develop suitable packing
devices. Suitable materials must be durable, must
provide effective sealing, and must be compatible
with the fluid used in the system.
The packing materials are commonly referred
to as seals or sealing devices. The seals used in
fluid power systems and components are divided
into two general classes-static seals and dynamic
seals.
The static seal is usually referred to as a gasket.
The function of a gasket is to provide a material
that can flow into the surface irregularities of
mating areas that require sealing. To do this, the
gasket material must be under pressure. This
requires that the joint be tightly bolted or
otherwise held together.
The dynamic seal, commonly referred to as
a packing, is used to provide a seal between two
parts that move in relation to each other.
These two classifications of seals—gaskets
and packing—apply in most cases; however,
deviations are found in some technical publi-
cations. Certain types of seals (for example, the
O-ring, which is discussed later) may be used
either as a gasket or a packing.
Many of the seals in fluid power systems
prevent external leakage. These seals serve two
purposes—to seal the fluid in the system and to
keep foreign matter out of the system. Other seals
simply prevent internal leakage within a system.
NOTE: Although leakage of any kind results
in a loss of efficiency, some leakage, especially
internal leakage, is desired in hydraulic systems
to provide lubrication of moving parts. This also
applies to some pneumatic systems in which drops
of oil are introduced into the flow of air in the
system.
The first part of this chapter deals primarily
with the different types of materials used in the
construction of seals. The next section is devoted
to the different shapes and designs of seals and
their application as gaskets and/or packings in
fluid power systems. Also included in this chapter
are sections concerning the functions of wipers
and backup washers in fluid power systems and
the selection, storage, and handling of sealing
devices.
SEAL MATERIALS
As mentioned previously, many different
materials have been used in the development of
sealing devices. The material used for a particular
application depends on several factors: fluid
compatibility, resistance to heat, pressure, wear
resistance, hardness, and type of motion.
The selection of the correct packings and
gaskets and their proper installation are important
factors in maintaining an efficient fluid power
system. The types of seals to be used in a
particular piece of equipment is specified by the
equipment manufacturer.
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