Figure 10-14.-Controlling fluid pressure.
hydraulically by remote control, although
emergency they may operate manually.
Hydraulics are used in many other ways aboard
submarines. They are used to raise and lower the
periscope. The submarines are steered and the bow and
stern planes are controlled by hydraulic systems. The
windlass and capstan system, used in mooring the
submarine, is hydraulically operated. You will find
many more applications of hydraulics aboard the
Controlling Fluid Pressure
In some hydraulic systems, oil is kept under
pressure in a container known as an accumulator. As
shown in figure 11-14, the accumulator is a large
cylinder; oil is pumped into it from the top. A free piston
divides the cylinder into two parts. Compressed air is
forced into the cylinder below the piston at a pressure
of 600 psi. Oil is then forced into it on top of the piston.
As the pressure above it increases, the piston is forced
down, squeezing the air into a smaller space. Air is
elastic; you can compress it under pressure, and it will
expand as soon as the pressure is reduced. When oil
pressure is reduced, large quantities of oil under
working pressure are instantly available to operate
hydraulic rams or motors any place on the submarine.
The Navy uses many devices whose operation
depends on the hydrostatic principle. You should
remember three points about the operation of these
Pressure in a liquid is exerted equally in all
Hydrostatic pressure refers to pressure at any depth
in a liquid that is not flowing.
Pressure depends upon both depth and density.
The formula for finding pressure is
P= H x D
The working principle of all hydraulic mechanisms
is simple enough. Whenever you find an application that
seems hard to understand, keep these points in mind:
Hydraulics is the term applied to the behavior of
enclosed liquids. Machines that operate liquids
under pressure are called hydraulic machines.
Liquids are incompressible. They cannot be
squeezed into spaces smaller than they
A force applied on any area of a confined liquid
transmits equally to every part of that liquid.
In hydraulic cylinders, the relation between the
force exerted by the large piston to the force
applied on the smaller piston is the same as the
relationship between the area of the larger
piston and the area of the smaller piston.
Some of the advantages of hydraulic machines are:
We use tubing to transmit forces, and tubing can
readily transmit forces around corners.
Tubing requires little space.
Few moving parts are required.