CHAPTER 10
HYDROSTATIC AND HYDRAULIC MACHINES
CHAPTER LEARNING OBJECTIVES
Upon completion of this chapter, you will be able to do the following:
l Explain the difference between hydrostatic and hydraulic liquids.
l Discuss the uses of hydrostatic machines.
l Discuss the uses of hydraulic machines.
In this chapter we will discuss briefly the pressure
of liquids: (1) hydrostatic (liquids at rest) and (2)
hydraulic (liquids in motion). We will discuss the
operation of hydrostatic and hydraulic machines and
give applications for both types.
HYDROSTATIC PRESSURE
You know that liquids exert pressure. The pressure
exerted by seawater, or by any liquid at rest, is known
as hydrostatic pressure.
If you are billeted on a submarine, you are more
conscious of the hydrostatic pressure of seawater. When
submerged, your submarine is squeezed from all sides
by this pressure. A deep-sea diving submarine must be
able to withstand the terrific force of water at great
depths. Therefore, the air pressure within it must be
equal to the hydrostatic pressure surrounding it.
PRINCIPLES OF HYDROSTATIC
PRESSURE
In chapter 9 you found out that all fluids exert
pressure in all directions. That’s simple enough. How
great is the pressure? Try a little experiment. Place a pile
of blocks in front of you on the table. Stick the tip of
your finger under the first block from the top. Not much
pressure on your finger, is there? Stick it between the
third and fourth blocks. The pressure on your finger has
increased. Now slide your finger under the bottom block
in the pile. There you will find the pressure is greatest.
The pressure increases as you go lower in the pile. You
might say that pressure increases with depth. The same
is true in liquids. The deeper you go, the greater the
pressure becomes. However, depth isn’t the whole story.
Suppose the blocks in the preceding paragraph were
made of lead. The pressure at any level in the pile would
be considerably greater. Or suppose they were blocks of
balsa wood-then the pressure at each level wouldn’t
be as great. Pressure, then, depends not only on the
depth, but also on the weight of the material. Since you
are dealing with pressure—force per unit of area, you
will also be dealing with weight per unit of volume-or
density.
When you talk about the density of a substance, you
are talking about its weight per cubic foot or per cubic
inch. For example, the density of water is 62.5 pounds
per cubic foot; the density of lead is 710 pounds per
cubic foot. However, to say that lead is heavier than
water isn’t a true statement. For instance, a 22-caliber
bullet is the same density as a pail of water, but the pail
of water is much heavier. It is true, however, that a cubic
foot of lead is much heavier than a cubic foot of water.
Pressure depends on two principles-depth and
density. You can easily find the pressure at any depth in
any liquid by using the following formula:
P = H x D
in which
P = pressure, in lb per sq in. or lb per sq ft
H = depth of the point, measured in feet or inches
and
D = density in lb per cu in. or lb per cu ft
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