Outer Cover.The outer cover protects the
reinforcement layers. A special rubber is most commonly
used for the outer layer because it resists abrasion and
exposure to weather, oil, and dirt.
Flexible hose is provided in four-pressure ranges.
Low pressure is used in a low-pressure system and for the
exhaust lines of high-pressure systems. Medium-
pressure hose is used in systems with pressures up to
1,200 psi; high-pressure hose is used with pressures up to
3,000 psi; and extra-high-pressure hose is used in
systems with pressures up to 5,000 psi. High- and extra-
high-pressure hoses normally come as complete
assemblies with factory installed end fittings. Medium-
and low-pressure hose are available in bulk and are
usually fabricated locally.
Flexible hose must NOT be twisted on installation,
since this reduces the life of the hose considerably and
may cause fittings to loosen as well. You can determine
whether or not a hose is twisted by looking at the lay line
that runs along the length of the hose. This lay line should
not tend to spiral around the hose (fig. 3-37).
Hose should be installed so that it will be subjected to
a minimum of flexing during operation. Support clamps
are not necessary with short installations, but with hoses
of considerable length (48 inches for example), clamps
should be placed not more than 24 inches apart. Closer
supports are desirable and, in some cases, needed.
Hose must NEVER be stretched tight between two
fittings. About 5 to 8 percent of the total length must be
allowed as slack to provide freedom of movement under
pressure. When flexible hose is under pressure, it
contracts in length and expands in diameter. Examples of
correct and incorrect installations of flexible hose are
shown in figure 3-37.
Connectors and Fittings
There are many types of connectors and fittings
required for a fluid power system. The type of
connector or fitting depends upon the type of
circulatory system (pipe, tubing, or flexible hose), the
fluid medium, and the maximum operating pressure of
the system. Some of the most common connectors and
fittings are described in the following paragraphs.
connectors (fig. 3-38) are used in low-pressure pipe
systems. The connectors are made with standard pipe
threads cut on the inside surface of the connector. The
end of the pipe is threaded on the outside for
connecting with the connector. Standard pipe threads
are tapered slightly to ensure a tight connection.
Figure 3-37.Correct and incorrect installation of
Figure 3-38.Threaded pipe connectors.