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RELATIONSHIP OF FORCE,
PRESSURE,
AND HEAD
In dealing with fluids, forces are usually
considered
in relation to the areas over which they are
applied. As previously discussed, a force acting
over a unit area is a pressure, and pressure can
alternately be stated in pounds per square inch or
in terms of head, which is the vertical height of
the column of fluid whose weight would produce
that pressure.
In most of the applications of fluid power in
the
Navy, applied forces greatly outweigh all other forces,
and the fluid is entirely confined. Under these
circumstances it is customary to think of the forces
involved in terms of pressures. Since the term
head is encountered frequently in the study of
fluid power, it is necessary to understand what it
means and how it is related to pressure and force.
All five of the factors that control the actions
of
fluids can, of course, be expressed either as force,
or in terms of equivalent pressures or head. In
each situation, the different factors are referred to
in the same terms, since they can be added and subtracted
to study their relationship to each other.
At this point you need to review some terms
in
general use. Gravity head, when it is important enough
to be considered, is sometimes referred to
as head. The effect of atmospheric pressure is referred
to as atmospheric pressure. (Atmospheric pressure
is frequently and improperly referred to as
suction.) Inertia effect, because it is always directly
related to velocity, is usually called velocity
head; and friction, because it represents a
loss of pressure or head, is usually referred to as
friction head.
STATIC AND DYNAMIC FACTORS
Gravity, applied forces, and atmospheric
pressure
are static factors that apply equally to fluids at rest or in motion, while inertia and friction
are dynamic factors that apply only to fluids
in motion. The mathematical sum of gravity,
applied force, and atmospheric pressure is
the static pressure obtained at any one point in
a fluid at any given time. Static pressure exists in
addition to any dynamic factors that may also be
present at the same time.
Remember, Pascal’s law states that a pressure
set
up in a fluid acts equally in all directions and at
right angles to the containing surfaces. This covers
the situation only for fluids at rest or practically
at rest. It is true only for the factors making
up static head. Obviously, when velocity becomes
a factor it must have a direction, and as
previously explained, the force related to the velocity
must also have a direction, so that Pascal’s
law alone does not apply to the dynamic factors
of fluid power.
The dynamic factors of inertia and friction are
related
to the static factors. Velocity head and friction
head are obtained at the expense of static head.
However, a portion of the velocity head can always
be reconverted to static head. Force, which can
be produced by pressure or head when dealing with
fluids, is necessary to start a body moving if
it is at rest, and is present in some form when the
motion of the body is arrested; therefore, whenever
a fluid is given velocity, some part of its
original static head is used to impart this velocity,
which then exists as velocity head.
BERNOULLI’S PRINCIPLE
Consider the system illustrated in figure 2-18.
Chamber
A is under pressure and is connected by a
tube to chamber B, which is also under pressure. The
pressure in chamber A is static pressure of 100
psi. The pressure at some point (X) along the connecting
tube consists of a velocity pressure of
Figure 2-18.—Relation of static and dynamic factors— Bernoulli’s
principle.
10 psi exerted in a direction parallel to the line of
flow, plus the unused static pressure of 90 psi, which
still obeys Pascal’s law and operates equally in
all directions. As the fluid enters chamber B it
is slowed down, and its velocity is changed back to
pressure. The force required to absorb its inertia
equals the force required to start the fluid moving
originally, so that the static pressure inchamber B is equal to that in chamber
A. This situation (fig. 2-18)
disregards friction; therefore, it
would not be encountered in actual practice.
Force or head is also required to overcome
friction but, unlike inertia effect, this force
cannot be recovered again, although the energy
represented still exists somewhere as heat. Therefore,
in an actual system the pressure in chamber
B would be less than in chamber A by the
amount of pressure used in overcoming friction
along the way.
At all points in a system the static pressure is always
the original static pressure, less any velocity head
at the point in question and less the friction head
consumed in reaching that point. Since both the
velocity head and the friction head represent energy
that came from the original static head, and
since energy cannot be destroyed, the sum of the
static head, the velocity head, and the friction head
at any point in the system must add up to the
original static head. This is known as Bernoulli's
principle, which states: For the horizontal
flow of fluid through a tube, the sum of
the pressure and the kinetic energy per unit volume
of the fluid is constant. This principle governs
the relations of the static and dynamic factors
concerning fluids, while Pascal’s law states the
manner in which the static factors behave when
taken by themselves.
MINIMIZING FRICTION
Fluid power equipment is designed to reduce friction
to the lowest possible level. Volume and velocity
of flow are made the subject of careful study.
The proper fluid for the system is chosen. Clean,
smooth pipe of the best dimensions for the particular
conditions is used, and it is installed along
as direct a route as possible. Sharp bends and
sudden changes in cross-sectional areas are avoided.
Valves, gauges, and other components are
designed to interrupt flow as little as possible. Careful
thought is given to the size and shape of the
openings. The systems are designed so they can be kept clean inside and variations from normal
operation can easily be detected and remedied.
OPERATION OF HYDRAULIC COMPONENTS
To transmit and control power through pressurized
fluids, an arrangement of inter-connected components
is required. Such an arrangement is
commonly referred to as a system. The
number and arrangement of the components vary
from system to system, depending on the particular
application. In many applications, one main
system supplies power to several subsystems, which
are sometimes referred to as circuits. The complete
system may be a small compact unit; more
often, however, the components are located at
widely separated points for convenient control and
operation of the system.
The basic components of a fluid power system are
essentially the same, regardless of whether the system
uses a hydraulic or a pneumatic medium. There
are five basic components used in a system.
These basic components are as follows:
1. Reservoir or receiver
2. Pump or compressor
3. Lines (pipe, tubing, or flexible
hose)
4. Directional control valve
5. Actuating device
Several applications of fluid power require only
a simple system; that is, a system which uses only
a few components in addition to the five basic
components. A few of these applications are presented
in the following paragraphs. We will explain
the operation of these systems briefly at this
time so you will know the purpose of each component
and can better understand how hydraulics
is used in the operation of these systems.
More complex fluid power systems are described
in chapter 12.
HYDRAULIC JACK
The hydraulic jack is perhaps one of the simplest
forms of a fluid power system. By moving
the handle of a small device, an individual can
lift a load weighing several tons. A small initial
force exerted on the handle is transmitted by
a fluid to a much larger area. To understand this
better, study figure 2-19. The small input piston
has an area of 5 square inches and is directly
connected to a large cylinder with an output
piston having an area of 250 square inches. The
top of this piston forms a lift platform. If
a force of 25 pounds is applied to the input piston,
it produces a pressure of 5 psi in the fluid, that
is, of course, if a sufficient amount of resistant
force is acting against the top of the output
piston. Disregarding friction loss, this pressure
acting on the 250 square inch area of the output
piston will support a resistance force of 1,250
pounds. In other words, this pressure could overcome
a force of slightly under 1,250 pounds. An
input force of 25 pounds has been transformed into
a working force of more than half a ton; however,
for this to be true, the distance traveled by
the input piston must be 50 times greater than the
distance traveled by the output piston. Thus, for
every inch that the input piston moves, the output
piston will move only one-fiftieth of an i
n c h.
This would be ideal if the output piston needed to
move only a short distance. However, in most instances,
the output piston would have to be capable
of moving a greater distance to serve a practical
application. The device shown in figure 2-19
is not capable of moving the output piston farther
than that shown; therefore, some other means
must be used to raise the output piston to a
greater height.
Figure 2-19.—Hydraulic jack.
The output piston can be raised higher and maintained
at this height if additional components are
installed as shown in figure 2-20. In this illustration
the jack is designed so that it can be raised,
lowered, or held at a constant height. These
results are attained by introducing a number of
valves and also a reserve supply of fluid to be used
in the system.
Notice that this system contains the five basic components—the
reservoir; cylinder 1, which serves
as a pump; valve 3, which serves as a directional
control valve; cylinder 2, which serves as
the actuating device; and lines to transmit the fluid
to and from the different components. In addition,
this system contains two valves, 1 and 2,
whose functions are explained in the following discussion.
As the input piston is raised (fig. 2-20, view A),
valve 1 is closed by the back pressure from the
weight of the output piston. At the same time, valve
2 is opened by the head of the fluid in the reservoir.
This forces fluid into cylinder 1. When the
input piston is lowered (fig. 2-20, view B), a pressure
is developed in cylinder 1. When this pressure
exceeds the head in the reservoir, it closes valve
2. When it exceeds the back pressure from the
output piston, it opens valve 1, forcing fluid into
the pipeline. The pressure from cylinder 1 is
Figure 2-20.—Hydraulic jack; (A) up stroke; (B) downstroke.
thus transmitted into cylinder 2, where it acts to raise
the output piston with its attached lift platform.
When the input piston is again raised, the
pressure in cylinder 1 drops below that in cylinder
2, causing valve 1 to close. This prevents the
return of fluid and holds the output piston with
its attached lift platform at its new level. During
this stroke, valve 2 opens again allowing a
new supply of fluid into cylinder 1 for the next power
(downward) stroke of the input piston. Thus,
by repeated strokes of the input piston, the lift
platform can be progressively raised. To lower the
lift platform, valve 3 is opened, and the fluid from
cylinder 2 is returned to the reservoir.
