square inches. With a resistant force on piston 2, a
downward force of 20 pounds acting on piston 1 creates
10 psi (20÷2) in the fluid. Although this force is much
smaller than the applied forces in figure 10-1, the
pressure is the same because the force is concentrated
on a relatively small area.
This pressure of 10 psi acts on all parts of the fluid
container, including the bottom of the output piston 2;
therefore, the upward force on the output piston 2 is 10
pounds for each of its 20 square inches of area, or 200
pounds (10 x 20). In this case, the original force has
been multiplied tenfold while using the same pressure
in the fluid as before. In any system with these
dimensions, the ratio of output force to input force is
always 10 to 1 regardless of the applied force; for
example, if the applied force of the input piston 1 is
50 pounds, the pressure in the system is increased to 25
psi. This will support a resistant force of 500 pounds on
the output piston 2.
The system works the same in reverse. Consider
piston 2 as the input and piston 1 as the output; then the
output force will always be one-tenth the input force.
Sometimes such results are desired.
Therefore, the first basic rule for two pistons used
in a fluid power system is the force acting on each is
directly proportional to its area and the magnitude of
each force is the product of the pressure and its area, is
Volume and Distance Factors
In the systems shown in views A and B of figure
10-1, the pistons have areas of 10 square inches. Since
the areas of the input and output pistons are equal, a
force of 100 pounds on the input piston will support a
resistant force of 100 pounds on the output piston. At
this point, the pressure of the fluid is 10 psi. A slight
force, in excess of 100 pounds, on the input piston will
increase the pressure of the fluid, which will, in turn,
overcome the resistance force. Assume that the input
piston is forced downward 1 inch. This displaces 10
cubic inches of fluid. Since liquid is practically
incompressible, this volume must go some place. In the
case of a gas, it will compress momentarily but will
eventually expand to its original volume at 10 psi. This
is provided, of course, that the 100 pounds of force is
still acting on the input piston. Thus this volume of fluid
moves the output piston. Since the area of the output
piston is likewise 10 square inches, it moves 1 inch
upward to accommodate the 10 cubic inches of fluid.
The pistons are of equal areas; therefore, they will move
equal distances, though in opposite directions.
Applying this reasoning to the system in figure 10-2,
it is obvious that if the input piston 1 is pushed down 1
inch, only 2 cubic inches of fluid is displaced. The
output piston 2 will have to move only one-tenth of an
inch to accommodate these 2 cubic inches of fluid,
because its area is 10 times that of the input piston 1.
This leads to the second basic rule for two pistons in the
same fluid power system, which is the distances moved
are inversely proportional to their areas.
While the terms and principles mentioned above are
not all that apply to the physics of fluids, they are
sufficient to allow further discussion in this training
manual. It is recommended that Fluid Power,
NAVEDTRA 12964 (latest edition), be studied for a
more detailed and knowledgeable coverage of the
physics of fluids and basic hydraulic/pneumatic
Since fluids are capable of transmitting force and at
the same time flow easily, the force applied to the fluid
atone point is transmitted to any point the fluid reaches.
Hydraulic and pneumatic systems are assemblies of
units capable of doing this. They contain a unit for
generating force (pumps), suitable tubing and hoses for
containing and transmitting the fluid under pressure, and
units in which the energy in the fluid is converted to
mechanical work (cylinders and fluid motors). In
addition, all operative systems contain valves and
restrictors to control and direct the flow of fluid and limit
the maximum pressure in the system.
Because of the similarities of hydraulic and
pneumatic systems (that is, from a training point of
view), only the components of hydraulic systems are
covered in this section. Remember that most of the
information is also applicable to pneumatic systems and
The heart of any hydraulic system is its pumps; it is
the pump that generates the force required by the
actuating mechanisms. The pump causes a flow of fluid;
thus, the amount of pressure created in a system is not
controlled by the pump but by the workload imposed on
the system and the pressure-regulating valves.
Basically, pumps may be classified into two groups
based on performance: (1) fixed delivery when running