LIQUID LEVERS AND GEARS
MANY an inventor has strayed off into the delusive pursuit of perpetual
motion because he did not know that the pressure in a body of water
at any given point is equal in all directions, upward, downward, or
laterally.
A cubic foot of water weighs 62½ pounds. Take a hollow column with an
internal cross-sectional area of one square foot and if it be filled
with water to a depth of ten feet there will be a weight of 625 pounds
of water in the column and hence a pressure of 625 on the bottom of
the tube or 4.34 pounds on every square inch of the bottom. But the
water presses on the sides of the tube as well and the amount of
this pressure depends upon the depth or “head” of water and not upon
the quantity of water. At the bottom of the tube the pressure on the
side walls is 625 pounds per square foot or 4.34 pounds per square
inch; at a depth of one foot the pressure on the side walls is 62.5
pounds per square foot or .434 pound per square inch; at a depth of
two feet it will be .864 pound per square inch, etc. The pressure on
each square inch depends not upon the mass of the water but upon its
depth. If the column of water had a cross-sectional area of a mile or
a thousand miles, the pressure at a depth of one foot would always be
.434 pound per square inch. (Of course there are slight variations from
this figure due to salt or other substances dissolved in water or to
changes in density produced by variations of temperature, but we need
not consider such minute differences here.) That is why a dam which is
strong enough to hold back the waters of a pond will be just as able to
hold back the waters of the whole ocean if it be placed in a sheltered
bay where ocean waves cannot tear it to pieces. The ocean, despite its
enormous mass, can exert no more pressure per foot of depth than the
water in a cistern.
WHY A SHIP FLOATS
It is because the pressure of water at a given depth is exerted upward,
as well as laterally and downward, that a ship floats. It is the
upward pressure of the water that holds up the boat. When an object
is placed in a reservoir of water it sinks into zones of increasing
pressure until it finally reaches a depth at which the pressure on
the bottom of the object balances the weight of the body. If the body
is entirely submerged before reaching such a point, it will continue
to sink to the bottom of the reservoir because water will flow over
the top of the object and keep adding downward pressure to offset the
increasing upward pressure. The amount of water in the reservoir makes
no difference. A battleship will float just as high in a flooded dry
dock as it will in the open ocean. If the dry dock were so narrow as
to leave a clearance of but a few inches of water around the ship, the
latter would still float even though the ship weighed considerably more
than the water in the dock.
There is a big difference, then, between the weight of water and the
pressure it exerts. In Figure 38 we have an L-shaped receptacle with
the lower arm of the L terminating in a chamber A. The top wall B
of this chamber measures ten square inches. The tube C has a cross
sectional area of one square inch. If tube C is filled to a height of
twelve inches above wall B we shall have an upward pressure of 0.434
pound on every square inch of wall B, or a total of 4.34 pounds. If
by means of a plunger D we add a hundred pounds of pressure to the
column of water in tube C, we shall be adding a thousand pounds to the
pressure on the wall B. The side walls and bottom of the chamber A
will also be subjected to a pressure of 1,000 pounds per inch plus the
pressure due to the depth or head of water.
[Illustration: FIG. 38.--DIAGRAM ILLUSTRATING HYDROSTATIC PRESSURE]
[Illustration: THE AIR-LOCK OF A PNEUMATIC CAISSON]
[Illustration: SUBAQUEOUS TUNNEL, SHOWING THE SHIELD IN THE BACKGROUND]
[Illustration: FIG. 39.--PRINCIPLE OF THE HYDRAULIC PRESS]
Here, then, we have a convenient means of multiplying force or effort
and it is a means that is used very largely in certain classes of
machinery. Figure 39 is a diagrammatic representation of a hydraulic
press. It consists of a cylinder A in which is fitted a ram B. An
L-shaped tube C connects with the cylinder and is fitted with a plunger
D. The cylinder and tube are filled with water and then when the
plunger is depressed the ram B has to rise, If the area of the plunger
is one square inch and that of the ram thirty square inches, a 100
pounds pressure on the plunger will exert 3,000 pounds of lift on the
ram.
HYDRAULIC LEVERAGE
However, we must remember that in mechanics, as in all walks of life,
we cannot get “something for nothing.” If we multiply the pressure or
force, we must pay for it in some way, otherwise we should be getting
more work out of the press than we put in it, which is what the
perpetual motion crank is ever trying to do. As the cross-sectional
area of the plunger D is only 1/30th of that of the ram, the plunger
must descend thirty inches to raise the ram one inch. We need not
consider the difference in the head of water because it would not
amount to more than a few ounces at most, nor need we consider
frictional losses. The case is parallel to that of the lever. In fact,
we may consider the hydraulic press as a fluid lever with the water in
tube C as the effort arm and that in cylinder A as the weight arm. The
two arms are here so proportioned that the power arm must move thirty
times as far as the weight arm. The work put into the press is exactly
balanced by that we get out of it. An effort of 100 pounds exerted
through a distance of thirty inches is exactly balanced by the moving
of 3,000 pounds through a distance of one inch.
It is a decided disadvantage to have to move the plunger so far and
in actual commercial practice hand-operated hydraulic presses are not
worked in that way. A pump is used to force water into the cylinder
so that a great many short strokes may be taken in place of one long
one, and the pump handle provides an added leverage, enabling a man
with little effort to exert an enormous lift. The water enters the ram
cylinder through a valve, and the pressure is maintained on the ram
until relieved by the opening of an outlet port.
MAUDSLEY’S LEATHER COLLAR
All this seems very simple and one would suppose that the inventor
of the hydraulic press must have been exceptionally free from the
troubles and trials that beset most inventors. However, there is a vast
difference between a laboratory apparatus and a commercial machine.
When, towards the close of the eighteenth century, Joseph Bramah,
the eminent British tool builder, invented the hydraulic press, he
experienced all sorts of difficulty in holding the water in the ram
cylinder. Of course, the ram has to slide freely into and out of the
cylinder, but how could he prevent the water from leaking out past the
ram? He resorted to all the plumbing expedients of the day. He used a
stuffing box and gland, but when this was packed tight enough to hold
the water in, it gripped the ram so tightly that the latter would not
move down into the cylinder on the return stroke. Bramah had in his
employ a very clever young mechanic named Henry Maudsley, who later
became famous as an inventor and designer of machine tools. We read
of him in Chapter III. Maudsley attacked the baffling problem of the
hydraulic press and provided a solution that survives to this day. In
place of the stuffing box which is a means of jamming a mass of cotton
waste about the collar, he provided a cupped leather collar. When the
pressure was applied it expanded the collar and made it bear tightly
against the ram, but on relieving the hydraulic pressure the pressure
of the cup leather was also reduced automatically.
There are many machines analogous to the hydraulic press in principle.
They do not use water in every case for the fluid lever. Where the
fluid is used over and over again oil is frequently employed. The
compactness of this form of lever makes it most useful wherever an
operation calls for the overcoming of a very heavy load or resistance
through a relatively short distance. For instance, there are machines
for bending pipe, for curving railroad rails, for punching holes in
metal, for pulling wheels off their shafts, for jacking up heavy
weights, for baling cotton, paper, and other materials, all of which
operate on the same principle as the hydraulic press. Water pressure is
supplied sometimes by a hand pump, sometimes by a power-driven pump,
and sometimes it is taken from a reservoir in which compressed air
imparts the requisite pressure to the water.
BLASTING WITH WATER
A novel use of water pressure has been developed in England. In certain
mines it is dangerous to use dynamite for blasting purposes owing
to the presence of explosive gases, and successful experiments have
been made with hydraulic cartridges. This consists of a cylinder of
steel fitted with a series of little plungers arranged in a row in the
cylindrical wall of the cartridge. As in powder blasting, a series
of holes are drilled in the face of the rock and the cartridges with
the plungers retracted are fitted into the holes. Then the cartridges
are connected to a high-pressure water supply. The water forces the
plungers out, exerting enough pressure to burst the rock. Not only is
this system perfectly safe, but it is economical, because the gallery
does not have to be cleared of workmen before every blast. There is
more certainty in the use of water cartridges, and the danger, common
where dynamite is used, of having the rock drill or pick strike and
explode a stick of dynamite which failed to go off with the rest of the
charge in a previous blast is avoided.
Hydraulic pressure is also used in jacks for lifting heavy weights. The
principle of the hydraulic jack is the same as that of the hydraulic
press.
RAISING A BRIDGE SPAN WITH WATER
An interesting illustration of the use of these liquid levers was
afforded in the construction of the Quebec Bridge. This huge bridge, it
will be recalled, consists of two cantilevers which stretch out from
opposite shores of the St. Lawrence River and support between them a
center span 640 feet long and weighing 5,400 tons. The span was built
on barges, towed down to position between the cantilever arms and then
lifted up 150 feet to the floor level of the bridge. Eight 1,000-ton
hydraulic lifting jacks were used, two at each corner of the span.
They were ideally suited to this kind of work. The bridge span had to
be lifted with utmost care and the motion had to be simultaneous on
all four corners. If one corner were raised faster than another the
span would be twisted and subjected to serious strains. In the first
attempt the fastening gave way at one corner and the span crumpled up
and plunged down into the river. But a year later at the second attempt
the span was hoisted successfully to position. The jacks had a lift
of two feet, and, counting the time required to secure the huge plate
chains by which the span was suspended, move the jacks down and give
them a fresh hold, it took fourteen minutes to complete each two-foot
lift. The work was done only during the daylight hours and on the third
day the span was finally brought into position and made fast to the
cantilever arms by means of twelve-inch pins driven home at each corner.
Hydraulic power is used very largely in the operation of cranes. As
the plunger or ram has a very limited range of motion some means of
multiplying distance of travel is required. One simple scheme is to use
a set of pulley wheels or sheaves attached to the cylinder and another
set to the ram and pass the hoisting chain around them. If we refer
back to Figure 16, on page 34, we shall see that seven feet of rope
must be pulled in at E in order to raise the lower pulley block one
foot. It is very evident that the process can be reversed. Power might
be applied to spread the two-pulley blocks apart when a movement of one
foot would produce a travel of seven feet at E. That is what is done
on the hydraulic crane. The ram is lifted, spreading the pulley blocks
apart and thus multiplying the motion of the lifting cable to any
extent, depending upon the number of sheaves, so that a travel of but a
few feet will result in lifting a load forty or fifty feet.
HYDRAULIC ELEVATORS
The same principle is used in hydraulic elevators. The hydraulic
cylinder lies horizontally on the basement floor and by means of pulley
gearing a short motion of the plunger is sufficient to raise the car
several stories.
In a more modern type of elevator the plunger acts directly on the car.
The plunger is long enough to reach to the top story of the building
which means that it must sink far into the ground in order to let the
car down to the first story or basement. A steel pipe is sunk into
the ground and serves as the cylinder and in this the long plunger
operates. In one of New York’s tall buildings the cars are lifted to
a height of 282 feet. The plungers are 6½ inches in diameter and they
travel at a speed of over 400 feet per minute. The car which, when
loaded, weighs 1,617 pounds, is supported on the top of the plunger.
However, counterweights are provided which balance the weight of the
car, so that practically the only weight lifted by the plunger is that
of the passengers. The advantage of this type of elevator is that it
reduces the danger of accident. The motion of the car is steady and
easily controlled. The car cannot move down above a given speed and
the only possible danger is that the plunger might break. However, a
shaft of steel 6½ inches in diameter, even if hollow, is hardly likely
to give way even under the most extraordinary loads to which it might
be subjected in elevator service.
LIFT LOCKS FOR CANALS
There is another type of elevator that completely dwarfs anything used
in office buildings. On some canals where it is necessary to make a
sudden change of level of considerable extent, instead of using a
flight of locks, a hydraulic elevator is employed to lift or lower not
only the vessel but the water it is floated in. There are two huge
tanks which ply between the upper and lower levels of the canal. These
are so connected that as one rises the other descends. The tanks are
big enough to take in the largest vessels that are likely to use the
canal. They are fitted with water-tight gates at each end, and after a
vessel has entered one of them, say at the lower level, the gate behind
it is closed. Then hydraulic mechanism is operated to lift the tank,
and when the top is reached the other gate is opened and the vessel
sails out into the upper level. It seems like a tremendous undertaking
to lift a heavy ship, but the two tanks balance each other and the
only work that has to be done is to overcome the friction and inertia.
No matter how heavy a vessel may be, it will not disturb the balance
between the two tanks. Nor is it necessary to have a ship in each tank,
for the same depth of water is maintained in the two tanks, and when
a ship enters the weight of the tank is not increased, for the ship
displaces its own weight of water.
HYDRAULIC VARIABLE SPEED GEAR
Water is frequently used to take the place of toothed gearing for the
transmission of power, particularly where it is desirable to vary
speed. One of the disadvantages of toothed gearing is that a change of
speed can only be made by shifting gears. The speed cannot be varied
gradually but is changed by abrupt steps. The guns of a battleship must
be kept trained on the target, while the ship rolls in the waves under
them, so as to be ready for a broadside at any instant. A telescope is
secured to the barrel of the gun and a man known as a “pointer” tries
to keep the cross-hairs of a telescope on the target by constantly
elevating or depressing the gun. The speed of the elevating machinery
must be constantly changing; now running at high speed, the next
instant barely moving, and the next moment reversing.
To produce these variations a hydraulic variable speed gear is used.
The construction is somewhat complicated, but the general principle of
operation is simple. In a common pump water is lifted by operating a
plunger in and out of a cylinder. It will be readily understood that
the operation may be reversed, and if power is applied to the water to
force it through the pump, it will make the plungers move in and out of
the cylinder. In the hydraulic variable speed gear the driving shaft is
made to operate a set of little pumps which are connected to a second
series of pumps that act upon the driven shaft. Instead of water oil
is used, and the oil pumped by the driving pumps is forced into the
driven pumps. The latter pumps are so connected to the driven shaft as
to cause it to rotate. But by a simple mechanical expedient the stroke
of the driving pumps can be varied at will from maximum to zero, and
the motion of the pumps can also be reversed. The variable amount of
oil pumped into the driven pumps varies the speed of the driven shaft,
for if it takes two strokes of a driving pump to fill the cylinder of a
driven pump the driven shaft will travel at only half the speed of the
driving shaft.
In turbine-driven vessels some sort of gearing is required between the
propeller and the turbine. In order to operate at its best efficiency
a turbine must run at very high speed, but this speed is entirely
too high for the propeller. There is a limit to the rate at which a
propeller may be driven. If this limit is exceeded, the propeller
merely bores a hole in the water, forming a vacuum which produces a
drag on the ship. It is customary to interpose some form of gearing
between the propeller and the turbine shaft, but because of the high
speed and the vast amount of power to be transmitted it is a difficult
matter to design gearing that will be reliable. Furthermore, it is
desirable to vary the speed of the vessel and even to reverse it
without slowing down the turbine engine. In some cases a specially
designed system of toothed gearing has been employed, in another the
power of the turbine is converted into electricity and then reconverted
into mechanical power by means of a motor on the propeller shaft. This
provides a very efficient transmission, because the electricity can
be very conveniently controlled to accelerate, retard, or reverse the
speed of the propeller motor. The same thing can also be done by using
a hydraulic transmission gear. This, although quite different from
the variable-speed gear, yet operates on the same general principle.
The turbine drives a centrifugal pump and the water thus pumped is fed
to a horizontal water turbine on the propeller shaft. By an ingenious
arrangement of turbine wheels the speed of the propeller shaft may be
varied at will.
Hydraulic machinery is notable for its reliability. Sometimes water
or oil is used as a convenient means of transmitting pressure from
one part of a machine to another. If we take a tube filled with water
and fit a plunger in each end, then, when one plunger is depressed or
pushed in, the other will be expressed or pushed out. The pipe may be
twisted around in any direction or be tied up in a double bowknot, and
yet pressure applied to one cylinder will immediately be felt by the
other. This method of transmission may do away with a vast number of
gears or levers.
Water is comparatively incompressible and hence not a very adaptable
means of transmitting power, but a certain amount of flexibility is
secured in some systems by the use of what is termed an “accumulator.”
This consists of a large cylinder fitted with a plunger. The plunger
is heavily weighted and it maintains a constant pressure upon the
column of water in the cylinder. If, for instance, a hydraulic crane
is being used and suddenly a larger quantity of water is required than
would normally be delivered by a pump, it is automatically supplied
from the cylinder of the accumulator, and at a constant pressure. The
accumulator is arranged to control the throttle valve of the steam
pumping engine so that when the plunger of the accumulator reaches
a predetermined height the stream is cut off and the engine stops
pumping.
TRANSMITTING POWER WITH SOUND WAVES
During the war a new use of water for power transmission was discovered
and it is now being developed for the operation of mining machinery.
The inventor of the new transmission is George Constantinesco, a
Rumanian engineer. His first application of the invention was to a
mechanism for synchronizing the firing of a machine gun with the
rotation of an airplane propeller, so that it was possible to fire
through the propeller without danger of striking its blades. There were
several existing methods of gearing the propeller to the machine gun,
but Constantinesco’s system proved so much more reliable that it was
adopted and widely used by the British in their battle planes.
Instead of gears and levers, the Rumanian engineer used a column of
water in a heavy steel tube to conduct impulses from the propeller to
the machine gun. The pulsations produced by the propeller were too
rapid to cause an actual displacement of the whole column of water in
the tube, but a wave of pressure traveled through the water column at
an enormously high velocity.
We are wont to think of liquids as incompressible, but they are
actually slightly compressible and highly elastic. This is demonstrated
by the submarine telephone or the submarine bell. Water, we know, is an
excellent medium for the transmission of sound, but sound, we know, is
produced by a succession of pressure waves. If water were absolutely
incompressible, it could not convey sound from one point to another.
When sound travels through a speaking tube the whole column of air
does not move back and forth simultaneously, but it is divided into a
series of waves. Each particle of air has a local oscillatory motion
which it communicates to its neighbor, producing alternate compression
and rarefication, and it is this wave action that travels through the
column. The same is true of water, except that the rate of travel of
the pressure wave in water is much higher than in air, namely 4,800
feet per second. It is this wave transmission that Constantinesco
employed and which he calls “sonic” wave transmission.
To-day sonic wave transmission is used to operate rock drills. A wave
generator is used, which acts somewhat on the principle of a pump. A
pair of plungers are reciprocated at a rate of forty strokes per second
by means of an electric or gasoline motor, and they produce a train
of pressure waves in a column of water. The water is contained in a
specially designed flexible steel piping which runs to the rock drill.
The pressure waves operate a plunger in the drill and the plunger
carries the drill steel. The latter pounds the rock under the wave
impulses at the rate of forty strokes per second.
Sonic wave transmission is analogous to alternating current
transmission of electricity. There are direct equivalents in the wave
transmission, of volts, amperes, frequency, angle of phase, induction,
inductance, capacity, resistance, condensers, transformers, and
single-phase or polyphase systems.