|
| |
Back
Stratopower Pump | Up
Content Moved | Next
Tube cutting and deburring |
[ Back ] [ Home ] [ Up ] [ Next ]
CHAPTER 5
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
Table 5-1.—Wall Thickness Schedule Designations for Pipe
160 pipe has a 0.815 ID. In each case the OD is 1.315
(table 5-1) and the wall thicknesses are 0.133
 0.179 
and 0.250 
respectively. Note that the
difference between the OD and ID includes
two wall thicknesses and must be divided by
2 to obtain the wall thickness.
Tubing differs from pipe in its size classification. Tubing
is designated by its actual OD. (See
table 5-2.) Thus, 5/8-inch tubing has an OD of
5/8 inch. As indicated in the table, tubing is available
in a variety of wall thicknesses. The diameter
of tubing is often measured and indicated
in 16ths. Thus, No. 6 tubing is 6/16 or 3/8
inch, No. 8 tubing is 8/16 or 1/2 inch, and so
forth.
The wall thickness, material used, and ID determine
the bursting pressure of a line or fitting. The
greater the wall thickness in relation to the ID
and the stronger the metal, the higher the bursting
pressure. However, the greater the ID for a
given wall thickness, the lower the bursting pressure,
because force is the product of area and pressure.
Materials
The pipe and tubing used in fluid power systems
are commonly made from steel, copper, brass,
aluminum, and stainless steel. Each of these metals
has its own distinct advantages or disadvantages
in certain applications.
Steel pipe and tubing are relatively inexpensive and
are used in many hydraulic and pneumatic systems.
Steel is used because of its strength, suitability
for bending and flanging, and adaptability
to high pressures and temperatures. Its
chief disadvantage is its comparatively low resistance
to corrosion.
Copper pipe and tubing are sometimes used for
fluid power lines. Copper has high resistance to
corrosion and is easily drawn or bent. However, it
is unsatisfactory for high temperatures and has a
tendency to harden and break due to stress and vibration.
Aluminum has many of the characteristics and qualities
required for fluid power lines. It has high resistance
to corrosion and is easily drawn or bent. In
addition, it has the outstanding characteristic of
light weight. Since weight elimination is a vital factor
in the design of aircraft, aluminum alloy tubing
is used in the majority of aircraft fluid power
systems.
Stainless-steel tubing is used in certain areas of
many aircraft fluid power systems. As a general rule,
exposed lines and lines subject to abrasion or
intense heat are made of stainless steel. An
improperly piped system can lead to serious
power loss and possible harmful fluid
Table 5-2.—Tubing Size Designation
contamination. Therefore in maintenance and PREPARATION
OF PIPES repair of fluid power
system lines, the basic design AND
TUBING requirements must be
kept in mind. Two primary requirements
are as follows:
1. The lines must have the correct ID to provide
the required volume and velocity of flow with
the least amount of turbulence during all demands
on the system.
2. The lines must be made of the proper material
and have the wall thickness to provide sufficient
strength to both contain the fluid at the required
pressure and withstand the surges of pressure
that may develop in the system. Fluid
power systems are designed as compactly as
possible, to keep the connecting lines short. Every
section of line should be anchored securely in
one or more places so that neither the weight of
the line nor the effects of vibration are carried on
the joints. The aim is to minimize stress throughout
the system.
Lines 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
also compensates for thermal expansion and contraction. Bends are preferred to elbows, because
bends cause less of a power loss. A few of
the correct and incorrect methods of installing tubing
are illustrated in figure 5-1.
Bends are described by their radius measurements. The
ideal bend radius is 2 1/2 to 3 times the
ID, as shown in figure 5-2. For example, if the
ID of a line is 2 inches, the radius of the bend should
be between 5 and 6 inches.
While friction increases markedly for sharper curves
than this, it also tends to increase up to a
certain point for gentler curves. The increases in
friction in a bend with a radius of more than 3
pipe diameters result from increased turbulence near
the outside edges of the flow. Particles of fluid
must travel a longer distance in making the change
in direction. When the radius of the bend is
less than 2 1/2 pipe diameters, the increased pressure
loss is due to the abrupt change in the direction
of flow, especially for particles near the inside
edge of the flow.
During your career in the Navy, you may be required
to fabricate new tubing to replace damaged
or failed lines. Fabrication of tubing consists
of four basic operations: cutting, deburring,
bending, and joint preparation.
[ Back ] [ Home ] [ Up ] [ Next ]
This information is now available on CD in Adobe PDF Printable Format
|
