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CHAPTER 12
In the preceding chapters, you learned about hydraulic
and pneumatic fluids and components of
fluid power systems. While having a knowledge of
system components is essential, it is difficult to
understand the interrelationship of these components
by simply watching the system operate.
The knowledge of system interrelation is
required to effectively troubleshoot and maintain
a fluid power system. Diagrams pro-vided in
applicable technical publications or drawings
are a valuable aid in understanding the operation
of the system and in diagnosing the causes
of malfunctions.
This chapter explains the different types of diagrams
used to illustrate fluid power circuits, including
some of the symbols that depict fluid power
components. Included in this chapter are
descriptions and illustrations denoting the differences
between open-center and closed-center fluid
power systems. The last part of the chapter describes
and illustrates some applications of basic fluid
power systems.
DIAGRAMS
As mentioned earlier in this chapter, to troubleshoot
fluid power systems intelligently, a mechanic
or technician must be familiar with the system
on which he or she is working. The mechanic
must know the function of each component
in the system and have a mental picture
of its location in relation to other components.
This can best be done by studying the
diagrams of the system.
A diagram may be defined as a graphic representation
of an assembly or system that indicates
the various parts and expresses the methods
or principles of operations. The ability to
read diagrams is a basic requirement for understanding
the operation of fluid power systems.
Understanding the diagrams of a system requires
having a knowledge of the symbols used in
the schematic diagrams.
SYMBOLS
The Navy uses two military standards that list
mechanical symbols that must be used in preparing
drawings that will contain symbolic representation.
These standards are as follows: 1. Military
Standard, Mechanical Symbols (Other
than Aeronautical, Aerospacecraft, and Spacecraft
Use), Part 1, MIL-STD-17B-1.
2. Military
Standard, Mechanical Symbols for Aeronautical,
Aerospacecraft, and Spacecraft Use,
Part 2, MIL-STD-17B-2.
Some of the symbols frequently used in fluid power
systems have been selected from these two
standards and are shown in Appendixes II and
III. Appendix II contains symbols from MIL-STD-17B-1.
Appendix III contains symbols from
MIL-STD-17B-2.
While the symbols shown in the appendixes are
not all encompassing, they do provide a basis for
an individual working with fluid power systems
to build upon. Some rules applicable to graphical
symbols for fluid diagrams are as follows:
1. Symbols show connections, flow paths, and
the function of the component represented only.
They do not indicate conditions occurring during
transition from one flow path to another; nor
do they indicate component construction or values,
such as pressure or flow rate.
2. Symbols do not indicate the location of ports,
direction of shifting of spools, or position of
control elements on actual components.
3. Symbols may be rotated or reversed without
altering their meaning except in cases of lines
to reservoirs and vented manifolds.
4. Symbols may be drawn in any size.
5. Each symbol is drawn to show the normal or
neutral condition of each component unless multiple
circuit diagrams are furnished showing various
phases of circuit operation.
For more detailed information concerning the symbols
used in fluid power diagrams, consult the above-mentioned
military standards. Additional
information
concerning symbols and the reading of
diagrams is contained in BIueprint
Reading and Sketching, NAVEDTRA
10077-F1.
TYPES OF DIAGRAMS
There are many types of diagrams. Those that are
most pertinent to fluid power systems are discussed
in this text.
Pictorial Diagrams
Pictorial diagrams (fig. 12-1) show the general
location and actual appearance of each component,
all interconnecting piping, and the general
piping arrangement. This type of diagram is
sometimes referred to as an installation diagram.
Diagrams of this type are invaluable to maintenance
personnel in identifying and locating components
of a system.
Cutaway Diagrams
Cutaway diagrams (fig. 12-2) show the internal working
parts of all fluid power components in a
system. This includes controls and actuating mechanisms
and all interconnecting piping. Cutaway
diagrams do not normally use symbols.
Figure 12-1.—Hydraulic system pictorial diagram.
Figure 12-2.—Cutaway diagram—pneumatic.
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