BASIC DIAGRAMS AND SYSTEMS
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.
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.
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
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
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.