Aluminum .Aluminum is limited to low-
pressure use, yet it has good flaring and bending
Plastic .Plastic tubing lines are made from a
variety of materials; nylon is the most suitable for use in
low-pressure hydraulic applications ONLY.
There are three important dimensions of any tubular
productoutside diameter (OD), inside diameter
(ID), and wall thickness. Sizes of pipe are listed by the
nominal (or approximate) ID and wall thickness. Sizes of
tubing are listed by the actual OD and the wall thickness.
The material, the inside diameter, and the wall
thickness are the three primary considerations in the
selection of lines for the circulatory system of a particular
fluid power system.
The manufacturers of tubing and pipe usually supply
charts, graphs, or tables which aid in the selection of
proper lines for fluid power systems. These tables and
charts use different methods for deriving the correct sizes
of pipe and tubing.
Line should normally be kept as short and free of
bends as possible. However, tubing should NOT be
assembled in a straight line, because a bend tends to
eliminate strain by absorbing vibration and compensates
for thermal expansion and contraction. Bends are
preferred to elbows, because bends cause less of a power
loss. A few of the incorrect and correct methods of
installing tubing are shown in figure 3-35.
FLEXIBLE HOSE.Hose is used in fluid power
systems where there is a need for flexibility, such as
connection to units that move while in operation or to
units attached to a hinged portion of the equipment. It
is also used in locations that are subjected to severe
vibration. Flexible hose is usually used to connect the
pump to the system. The vibration that is set up by the
operating pump would ultimately cause rigid tubing to
Flexible hose is designated by a dash number,
which is the ID of the hose expressed in 16ths of an
inch and is stenciled on the side of the hose. For
example, the inside of a -16 hose is 1 inch. For a few
hose styles, this is approximate and is not a true ID.
Rubber hose is designed for specific fluid,
temperature, and pressure ranges and is provided in
various specifications. Flexible hydraulic hose is
composed of three basic parts (fig. 3-36):
Inner Tube.The inner tube is a synthetic rubber
layer that is oil-resistant. It must be smooth, flexible,
and able to resist heat and corrosion.
Reinforcement Layers.The reinforcement layers
vary with the type of hose. These layers (or plies) are
constructed of natural or synthetic fibers, braided wire, or
a combination of these. The strength of this layer depends
upon the pressure requirement of the system.
Figure 3-36.Flexible rubber hose construction.
Figure 3-35.Correct and incorrect methods of installing tubing.