CHAPTER 5
FLUID LINES AND FITTINGS
The control and application of fluid power
would be impossible without suitable means of
transferring the fluid between the reservoir, the
power source, and the points of application. Fluid
lines are used to transfer the fluid, and fittings
are used to connect the lines to the power source
and the points of application.
This chapter is devoted to fluid lines and
fittings. After studying this chapter, you should
have the knowledge to identify the
monly used lines and fittings, and
explain the procedure for fabricating,
labeling the lines.
TYPES OF LINES
The three types of lines used in
systems are pipe (rigid), tubing
and hose (flexible). A number of
considered when the type of line is
most com-
be able to
testing, and
fluid power
(semirigid),
factors are
selected for
a particular fluid system. These factors include
the type of fluid, the required system pressure,
and the location of the system. For example,
heavy pipe might be used for a large stationary
fluid power system, but comparatively lightweight
tubing must be used in aircraft and missile
systems because weight and space are critical
factors. Flexible hose is required in installations
where units must be free to move relative to each
other.
PIPES AND TUBING
There are three important dimensions of any
tubular product—outside diameter (OD), inside
diameter (ID), and wall thickness. Sizes of pipe
are listed by the nominal (or approximate) ID and
the wall thickness. Sizes of tubing are listed by
the actual OD and the wall thickness.
SELECTION OF PIPES AND TUBING
The material, ID, and wall thickness are
the three primary considerations in the selec-
tion of lines for a particular fluid power
system.
The ID of a line is important, since it
determines how much fluid can pass through the
line in a given time period (rate of flow)
without loss of power due to excessive friction
and heat. The velocity of a given flow is less
through a large opening than through a small
opening. If the ID of the line is too small for the
amount of flow, excessive turbulence and friction
heat cause unnecessary power loss and overheated
fluid.
Sizing of Pipes and Tubing
Pipes are available in three different weights:
standard (STD), or Schedule 40; extra strong
(XS), or Schedule 80; and double extra strong
(XXS). The schedule numbers range from 10
to 160 and cover 10 distinct sets of wall
thickness. (See table 5-1.) Schedule 160 wall
thickness is slightly thinner than the double extra
strong.
As mentioned earlier, the size of pipes is
determined by the nominal (approximate) ID. For
example, the ID for a 1/4-inch Schedule 40 pipe
is 0.364 inch, and the ID for a 1/2-inch Schedule
40 pipe is 0.622 inch.
It is important to note that the IDs of all pipes
of the same nominal size are not equal. This is
because the OD remains constant and the wall
thickness increases as the schedule number
increases. For example, a nominal size 1-inch
Schedule 40 pipe has a 1.049 ID. The same size
Schedule 80 pipe has a 0.957 ID, while Schedule
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