Mechanics: The Science of Machinery
by A. Russell Bond · English · Gutenberg #49445 · 569,408 chars · public domain (US)
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Title: Mechanics: The Science of Machinery
Author: A. Russell Bond
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*** START OF THE PROJECT GUTENBERG EBOOK MECHANICS: THE SCIENCE OF MACHINERY ***
[Illustration:
Copyright, Ewing Galloway]
_The drop forge. A fourteen-ton hydraulic press is employed in forging
an immense open hearth steel plate._
POPULAR SCIENCE LIBRARY
EDITOR-IN-CHIEF
GARRETT P. SERVISS
AUTHORS
WILLIAM J. MILLER HIPPOLYTE GRUENER A. RUSSELL BOND
D. W. HERING LOOMIS HAVEMEYER ERNEST G. MARTIN
ARTHUR SELWYN-BROWN ROBERT CHENAULT GIVLER
ERNEST INGERSOLL WILFRED MASON BARTON
WILLIAM B. SCOTT ERNEST J. STREUBEL
NORMAN TAYLOR DAVID TODD
CHARLES FITZHUGH TALMAN
ROBIN BEACH
ARRANGED IN SIXTEEN VOLUMES
WITH A HISTORY OF SCIENCE, GLOSSARIES
AND A GENERAL INDEX
_ILLUSTRATED_
[Illustration: LOGO]
VOLUME FIVE
P. F. COLLIER & SON COMPANY
NEW YORK
Copyright 1922
BY P. F. COLLIER & SON COMPANY
MANUFACTURED IN U. S. A.
MECHANICS
THE SCIENCE OF MACHINERY
BY
A. RUSSELL BOND
FORMERLY MANAGING EDITOR, SCIENTIFIC AMERICAN
[Illustration]
P. F. COLLIER & SON COMPANY
NEW YORK
PREFACE
ALTHOUGH strictly speaking the term “Mechanics” applies to that branch
of Physics that deals with the actions of forces on material bodies,
originally the word had a broader meaning embracing all machinery
and mechanical inventions. To-day popular usage is restoring to the
term its original broad interpretation, and it is in this popular but
rather unorthodox sense that “Mechanics” has been chosen as the title
of this book; for although certain elementary principles of mechanics
are described and explained, the major portion of the book deals with
machines and their evolution to their present stage of perfection.
Machines are man’s creation, and yet in a sense the man of to-day is a
machine product; for modern civilization owes its material and in large
measure its esthetic development to machinery. The story of machinery,
from primitive man’s first attempts to augment his physical powers with
mechanical aids down to the present era of gigantic, steel-muscled
machinery and marvelously intricate mechanisms, is the story of human
progress. It is this story that we have endeavored to tell in the
following pages, but the subject is too large to be covered in a single
volume or even a dozen volumes. Under the circumstances we have been
obliged to confine ourselves to a mere outline, selecting certain
avenues of progress more marked than others and presenting brief sketch
maps of them. We have aimed in this way to give a bird’s-eye view of
the whole story of human progress in things material.
The book has not been written for the mechanical engineer, but for the
layman who would learn of the mechanical contrivances that contribute
to his material welfare; hence technical terms have been avoided, as
far as possible, and where unavoidable have been explained and defined.
A. RUSSELL BOND
CONTENTS
CHAPTER PAGE
I. TOOL-MAKING ANIMALS 9
II. THE ANATOMY OF A MACHINE 20
III. MACHINES FOR MAKING MACHINES 42
IV. COUNTING SECONDS 57
V. PUTTING RIVERS TO WORK 75
VI. LIQUID LEVERS AND GEARS 94
VII. AIR _vs._ WATER 109
VIII. AIR SPRINGS AND CUSHIONS 126
IX. POWER FROM HEAT 139
X. BURNING FUEL IN THE ENGINE CYLINDER 155
XI. WHEN COAL AND OIL ARE EXHAUSTED 171
XII. INVASION OF THE SEA 182
XIII. SLIDING, ROLLING, WALKING, AND CREEPING 203
XIV. INVASION OF THE SKY 219
XV. HELPING THE FARMER 239
XVI. MACHINING THE EARTH 251
XVII. FROM FIBER TO FABRIC 268
XVIII. THE MAKING OF PAPER 289
XIX. TYPESETTING AND PRINTING MACHINERY 300
XX. CAST IRON, WROUGHT IRON, AND STEEL 314
XXI. ANIMATED MACHINERY 326
XXII. HEAT VACUUMS 345
XXIII. ENGINES OF DESTRUCTION 359
XXIV. SUMMARY OF MECHANICAL PROGRESS 376
LIST OF ILLUSTRATIONS
THE DROP FORGE--FORGING A STEEL PLATE _Frontispiece_
FACING PAGE
RAISING WATER WITH A CHAIN OF POTS--A PRIMITIVE
PUMP USED IN EGYPT 32
HORSE-OPERATED CHAIN PUMP USED IN GREECE 32
MULTIPLE SPINDLE DRILL IN A MOTOR CAR FACTORY 33
BUCKET OF A GIANT DREDGE WITH ITS JAWS WIDE OPEN 64
GOLD DREDGE EATING ITS WAY THROUGH RIVER SANDS 65
SUCTION DREDGE EXCAVATING A CHANNEL AND BUILDING NEW LAND 65
AIR LOCK OF A PNEUMATIC CAISSON 96
SUBAQUEOUS TUNNEL SHOWING THE SHIELD IN THE BACKGROUND 97
WHARF PROTECTED BY A PNEUMATIC BREAKWATER 128
SUN-POWER PLANT IN EGYPT 129
CLOSE-UP OF ONE OF THE REFLECTORS OF A SUN-POWER PLANT 129
GIANT CAPRONI HYDROAEROPLANE 160
CONCRETE STEAMSHIP “FAITH” 161
“VIRGINIAN,” THE LARGEST LOCOMOTIVE IN THE WORLD 161
LOOM PROVIDED WITH A JACQUARD ATTACHMENT 224
BATTERY OF MULE SPINNING FRAMES 225
JACQUARD ATTACHMENT FOR LOOM 225
DIGGING A TRENCH WITH A MACHINE OF THE CHAIN TYPE 256
MULTIPLE PLOWING ON A WESTERN WHEAT FIELD 257
SAWING LOGS OF PULPWOOD INTO 16-INCH LENGTHS 288
HEATER IN A PULP MILL WHERE THE WOOD PULP IS MIXED WITH CLAY 288
MODERN FOURDRINIER PAPER-MAKING MACHINES 289
MOLTEN METAL FROM A BLAST FURNACE 320
BESSEMER CONVERTER BLOWING AIR THROUGH A MASS OF MOLTEN IRON 320
STEEL BEAM PASSING THROUGH THE FINISHING ROLLS 321
COMPOSITOR AT WORK ON A LINOTYPE MACHINE 352
THE OPTOPHONE, WHICH ENABLES THE BLIND TO READ PRINT 353
BLIND MAN READING WITH HIS EARS 353
CHAPTER I
TOOL-MAKING ANIMALS
WHEN we review the marvelous achievements of modern civilization we
are quite willing to agree with the ancient psalmist that man is
“little lower than the angels.” But at the other end of the scale our
complacency is liable to receive a rude shock; apparently the boundary
between man and beast is not so very easy to draw.
We used to be told that one important superiority of mankind lies in
the fact that he makes use of tools, while the beast never uses any
implement except those that nature has furnished him as part of his own
organism. But a gorilla will throw stones at his enemy; and he knows
how to brandish a club and use it with telling force. Some of the apes
are known to use sticks to knock down fruit which is out of the reach
of their hands, and they will crack nuts with a stone. Clearly these
animals are tool users. A very intelligent orang-utan in the Bronx
Zoölogical Garden, New York, after trying for days to wrench off a
bracket from the wall of his cage eventually used the horizontal bar of
his trapeze as a lever and with it pried the offending bracket from its
fastenings. Here was real invention and the discovery of the principle
of leverage. The great black arara cockatoo of New Guinea uses his beak
as a saw to weaken the shells of hard nuts, and to keep his bill from
slipping off the smooth shell he is ingenious enough to wrap a leaf
around the nut to hold it steady.
Even in the insect world we find creatures resourceful enough to make
use of tools. Prof. Franz Doflein of the University of Breslau tells
of an interesting study of certain ants, known as the _Oecophylla
smaragdina_, who build their nests in bushes by fastening leaves
together with fine threads. But the ants that build the nests cannot
spin these threads, because they possess no spinning glands. They must
depend upon their larvæ for this product. When a rent was made in one
of these nests, a band of the tiny creatures ranged themselves side
by side along the torn edge of the leaf and reached across the gap
until they could catch hold of the opposite edge with their mandibles.
Then they drew back step by step, with perfect teamwork, until the two
edges were brought together. In the meantime, other ants had rushed to
the nursery and each one had picked up a larva, not with the idea of
bearing it off to safety, but in order that the babies might spin the
thread which the adult ants were unable to do. The larvæ were carried
to the breach in the nest and moved back and forth across the rent.
They were pressed first against one side of the tear and then the other
and all the while were squeezed tightly, evidently with the purpose of
making them spin. Gradually a fine silky web was woven across the torn
leaf and eventually the rent was completely patched.
Unquestionably these little ants are tool-using animals, because they
make their larvæ serve as spinning spindles and also as weavers’
shuttles. However, this can hardly be cited as a point in common with
even the lowest type of man, for the ants merely use the tools they
find at their disposal. They certainly cannot be credited with having
produced or even improved the tool which they use, whereas even in
the most primitive of men we find that the tools used are not only
carefully selected for the work to be performed, but are actually,
shaped, be it ever so crudely, to suit the job.
Clearly we must shift the boundary between man and beast,
distinguishing the former as the creature who artificially improves his
tools. But even here it is not absolutely certain that the boundary
will stand. Wilhelm Boelsche, a well-known German writer on natural
history, calls attention to the “blacksmith woodpecker” which will
thrust hard pine nuts into cracks in the trunk of a tree, so that they
are held as if in a vise, enabling the bird to operate upon the seed
more easily. Furthermore, this woodpecker will actually make a hole in
the tree to receive the nut if there is not a hole or crack handy, so
that evidently this animal does produce or artificially improve the
tool that it uses.
There are a few such examples in nature, just enough to cast a bit of
uncertainty on the boundary we have set. But although the actual line
of demarcation may not be clear, there is no question but that the
lowest type of humanity now existent, or of which we have any record,
is or was a tool maker. Chipped stones evidently fashioned by man
for some useful purpose are found even in the remains of the Middle
Tertiary Epoch. The spirit of inquiry, of experiment, of invention,
and the ambition to dominate over other members of the animal kingdom
or over the obstacles imposed by nature, are to be found more or less
active among all peoples, no matter how lowly a position they may
occupy in the scale of civilization.
WAR AS A STIMULUS OF INVENTION
The most primitive implements were probably developed for the purpose
of war. From the very earliest times, down to the present day, war
has been a most potent stimulus of invention. The first tools ever
used were probably intended to enable the user to cope with dangerous
enemies. They marked the first stage in the conquest of brain over mere
brawn. The primitive weapons were used not only in fighting other men,
but in fighting off dangerous animals, and then in hunting animals for
food. No doubt the first implement ever used was a club, which gave
a real advantage over the unarmed, scratching, tearing, and biting
enemy. This was a lever which increased the reach of the fighter, and
also increased the power of his blow. The heavier the club, the more
dangerous the weapon, particularly when most of the weight was centered
at the outer end of the stick. But he was a real genius who first
fastened a rock to the end of his club.
THE ART OF BREAKING STONES
Then arose the art of breaking stones--breaking them skillfully, so as
to form a jagged cutting edge. When man began to fashion tools of stone
he left imperishable records of his craftsmanship which enable us to
trace his progress in invention. The first finished tool we find was
the fist hatchet--a stone roughly chipped to form a cutting edge and of
convenient shape for the grasp of the hand. This primitive tool very
slowly, through a period covering thousands of years, developed into
all manner of cutting implements, some with handles of wood and bone.
The ax head was followed by the spearhead and this finally by the
arrowhead, showing that man had at last found a mechanical substitute
for his muscles to hurl projectiles farther and with greater accuracy
than he could throw them by hand.
We marvel at the resourcefulness and skill of the primitive savage in
working so difficult a material as stone. It would baffle a modern
mechanic to be required to shape a piece of flint into an arrowhead
with no other tool than a piece of bone. He is so accustomed to using
tools which are harder than the material they are intended to shape
that he cannot conceive of making any impression upon a piece of flint
with a piece of bone, to say nothing of a stick of hard wood, and yet
such tools were used away back in the Stone Age. At first stones were
roughly shaped by hammering them together. Then the artisans became
more skilled. They discovered that certain stones could be chipped
more regularly and evenly, and the art of flaking off chips of flint
sprang up. Some specimens that belonged to ages long preceding that of
recorded history are beautifully done. The spearheads are symmetrically
shaped like a long narrow leaf, and the stone is evenly furrowed on
both sides with a keen edge all around.
Not only did primitive artisans shape the stone implements with hammer
blows, but they learned how to shape stone by pressure as well, using
a tool that was relatively soft. It is not a very difficult matter to
shape even so hard a substance as glass merely by pressure. If a piece
of glass is laid on a table with its edge slightly overhanging that of
the table, it is possible to chip off the overhanging edge by pressing
a nail or even a hard stick of wood against this edge. A small flake
of glass is thus removed, and, by continuing the process, arrowheads
of any shape may be formed. The tool is placed not against the upper
surface of the glass, but against the edge of the glass, so that only
the lower surface of it is split or flaked off. Then the glass is
turned over and a chip is taken off the opposite face.
In the Middle Stone Age we find the primitive craftsman equipped with
a very complete assortment of stone tools. He had hammers, chisels,
scrapers, drills, and polishing tools. He knew how to make useful
household implements, such as spoons and ladles, out of bone. He
polished his work and ornamented the implements with carvings of
animals. Ivory pins and needles show that he had begun to make himself
clothing from the skins of animals and that he sewed them together with
thongs or tendons.
In the Late Stone Age he had learned how to make vessels of fire-baked
clay. His axes were ground to a sharp edge, and he bored holes in the
ax head to receive the ax handle. The Swiss lake dwellers built houses
of wood and fitted them with all sorts of wooden furniture carved
with stone tools. Among the remains of these interesting settlements
may be found balls of clay which, from the fact that one of them was
discovered with a spool of flax still attached to it, were evidently
used as spinning “whorls” used for spinning flax into thread. Clothing
of skins was giving way to or being supplemented with clothing of woven
fabric.
DISCOVERY OF THE LEVER AND THE WEDGE
Prior to the Stone Age the club was undoubtedly used, in time of
peace--if there ever was a time of peace in those days--to batter down
trees, to beat through entanglements and to dislodge great stones.
Here the first idea of leverage was evidently employed. The club with
a rock tied to it, particularly if the rock was shaped with a sharp
edge, made a better implement for hewing trees. It is quite probable
that soon after this stage of development had been reached, some one
discovered the use of the wedge, particularly in splitting timber. Of
course, no one realized in those early days why it was that he could
pry up a greater weight with a lever than he could lift directly by
hand, or why he could split open a log by driving wedges into it. The
_art_ of mechanics was in existence long ages before _science_ of
mechanics began to be studied. But it was not until men began to look
into the why of things that rapid progress was made.
We can go on endlessly with our speculations on the evolution of tools
and machinery up to the time when historians began to record the
mechanical achievements of man. Unfortunately even after historians
began to write they were so filled with admiration for the destructive
work of man that they had no time to record his constructive work.
The warrior who spread havoc and terror received all the glory, and
his deeds were written on parchment, inscribed in clay and carved in
stone; but the humble artisan was not worthy of mention. Even when the
science of mechanics came to be studied, it was shrouded in a veil of
mystery, and it was beneath the dignity of the man of science to impart
his knowledge to the artisan. There was a lack of cooperation between
science and industry that has persisted to a certain extent even up
to the present time. Some of the most ingenious inventions of the
ancients were employed by a corrupt and crafty priesthood to produce
apparently miraculous effects and hoodwink the general public; and so,
in looking back to the early days of mechanics, we are obliged to draw
upon our imagination to trace its evolution, supplementing this by a
study of the tools of primitive people of more recent time. Practically
every form of hand tool we now use must have been known to the ancient
artisan.
INVENTION OF THE WHEEL
We are not going to attempt to write a history of the evolution of
machinery, but there is one invention whose origin is lost in the
remote prehistoric ages which deserves more than passing attention. It
is a pity that we have no clue as to who invented the wheel or how this
most important element that enters into the construction of nearly all
machinery was evolved. The invention called for a remarkable degree of
originality. There is nothing like a wheel in nature. Levers we have
in our own physical frame. But a wheel is something that is distinctly
a human creation. Whoever invented it must have been a real genius, a
James Watt or a Thomas Edison of his day. Certainly we owe more to the
invention of the wheel than we do even to so revolutionary a machine
as the steam engine, or the flying machine. How it was ever first
conceived is a mystery. Maybe this primeval genius got his idea from
seeing a stone rolling downhill, or he may have seen a tumbling weed
rolling along the ground before the wind. It may be that the forerunner
of the wheel was a roller shaped out of a log, for certainly primitive
civilization must have advanced enough to have known how to hew timber
before it would have been capable of fashioning a wheel. Some observant
man might have noticed that he could drag a heavy timber over a
rolling log much more easily than he could along the bare ground, and
gradually the roller evolved into a wheel.
We can speculate upon the evolution of vehicles and transportation,
once the wheel was invented. Of course, the first method of
transporting loads was to carry them in the arms. Possibly loads were
placed on skids and dragged along by one end. Away back in early times,
it was discovered that two persons could carry more than twice as much
as one, if the load were placed on a couple of poles. There was no
friction to contend with, and not only was the load cut in two, because
each man bore half of it, but the position of the load was such that
it could be borne more easily. After the wheel was discovered, some
one must have conceived of the idea of dispensing with an assistant by
placing a wheel between the poles of the stretcher, thus making a crude
wheelbarrow. It is more likely that two wheels were first used, making
a cart of the stretcher, because the crude workmen of those days could
hardly have produced anything but a very wobbly wheelbarrow. At any
rate, the wheel, or pair of wheels, robbed one man of his job. Only one
bearer was required where before two had been used. Labor costs were
immediately reduced 50 per cent.
DISPLACING MEN WITH MACHINES
In the very earliest days of invention machines began to displace men.
Had there been unions in those days, no doubt there would have been
strenuous opposition to the introduction of this substitute for an
honest worker. But among the ancients, even more than at the present
time, invention meant greater production rather than less work,
because the laborer of that time was not a hired man but a slave.
There was no object in cutting down labor when it cost practically
nothing. The only stimulus to invention was greater production.
The invention of the wheel meant the dawn of transportation, which
is the backbone of civilization, and from it resulted no end of
other inventions. It made it possible for communities to come into
closer touch with each other. It meant circulation--an interchange of
knowledge and of products. Food was transported from one locality to
another, enabling certain communities to dispense with agricultural
work and specialize in certain lines of manufacture; for they could
barter their products for food raised by other communities. There are
some tribes to-day which are most backward because they are separated
from other tribes by rivers, while other tribes similarly placed owe
their progress to the fact that they have developed sufficient skill to
build crude bridges and thus gain access to the outside world.
RAISING WATER
In Egypt the wheel had a wonderful effect on agriculture. In that dry
land water is, and always has been, most precious. No wonder the Nile
was venerated! It meant life--life to crops, and hence life to man.
How to raise water from this stream of life in time of drought was
the great problem of the Egyptian. As slave labor was cheap, it was
customary to haul up the precious water, a bucket at a time, and pour
it over the fields. Then some one discovered that this process could be
simplified by using a _shadoof_ or swape; in other words, a long pole
fulcrumed near one end, with a heavy rock for a counterbalance lashed
to the shorter arm, and a bucket tied by a long rope to the longer
arm of the lever. This primitive machine is still to be found in some
rural districts. With this contrivance, a heavier load could be lifted
than by hand, because, when raising the bucket, the weight of the rock
would assist in lifting the water. After the swape came all manner of
ingenious devices for lifting the water. There were seesaw arrangements
which would scoop up some of the water at each oscillation of the
seesaw, and in one ingenious contrivance there was a succession of
seesaws by which the water was raised to a considerable height, whence
it poured down into ditches that irrigated the fields.
Then some one invented a water wheel or a great wheel, fitted with
buckets, which was turned by human or ox power, and which poured a
steady stream of water into the irrigating ditches.
But the greatest invention was that of the engineer who actually made
the river turn the wheel. It was probably on the Nile that the _noria_,
as this machine was called, was first put into service.
The wheel was provided with paddles, so that the current made it
revolve, and the water spilled out of the buckets into a trough as
they were turned over by the wheel. We can imagine the triumph of the
ancient inventor who developed that machine. True, the river might
arise in its wrath now and then and wreck the machine, but in wrecking
the wheel it had to flood the land, which, after all, was exactly what
was aimed at. The anger of the river was short-lived; it soon quieted
down and went on placidly turning the wheel which robbed it of the
precious water. It was a great event in the history of engineering. The
Nile had been harnessed. One of the great powers of nature had been set
to work.
CHAPTER II
THE ANATOMY OF A MACHINE
EVERY animal is a complex machine, provided with its own motive power
and a brain for directing the operation of its own mechanical elements.
Not satisfied with the mechanism that nature has put into the human
machine, man has reached for other elements and devised mechanisms
of his own in order to supplement the human machine and increase its
efficiency. At first, as we have seen, these elements were hand tools
of the crudest sort; but they were gradually improved and then they
were combined into what we term machines. In developing these machines,
he naturally took his own system as a pattern and was guided to a
large extent by an examination of his own physical structure. We see
this very clearly in the names of the different parts of machinery,
which are taken from the names of similar parts in the human frame.
Almost every member of the body is used in mechanical terminology. For
instance, we have the “head” and the “foot,” the “arms” and the “legs,”
the “fingers” and the “ankles,” “elbows,” “shoulders,” “trunk,” “hips,”
and various parts of the face, such as the “eyes,” “ears,” “nose,”
“mouth,” “teeth,” “lips,” and even the “gums,” to indicate parts of
machinery which have some remote resemblance to these features.
Before we can understand machinery we must have some general knowledge
of the elements of which it is composed. Probably most of the readers
of this book already possess a fair knowledge of machine elements and
mechanical movements and they can well afford to skip this chapter.
However, for the benefit of the uninitiated, we must put a machine on
the operating table, dissect it, and explain its anatomical structure.
We cannot attempt a very detailed study, but will confine ourselves to
the most important elements.
Every machine is made up of movable parts and fixed parts, the latter
serving to guide or constrain the motion of the former; for no
combination of elements will constitute a machine unless the parts are
_constrained_ to move in certain predetermined directions.
THE LEVER
Among the moving elements the first to be considered is the lever,
which really forms a broad classification comprising many elements that
will hardly be recognized as levers at first blush. Levers in some form
are to be found in practically every machine. A wheel, a gear, and a
pulley are really levers in disguise, as will be explained presently.
Of course everyone knows that a simple lever consists of a rigid bar
that swings on a fulcrum. The fulcrum may be a knife edge, a shaft
passing through the bar or any element on which the bar can be swung
or oscillated. The purpose of the lever is to give a certain advantage
in the application of a force to a load. This may be a change of speed
and distance of travel, and hence of power, or merely a change of
direction.
[Illustration: FIG. 1.--THREE ORDERS OF SIMPLE LEVERS]
There are three types or _orders_ of levers produced by varying the
relative positions of the points where the fulcrum, the force or
effort, and the weight or load are applied. These are shown in Figure
1. In the lever of the _first order_ the fulcrum is placed between the
effort and the weight; in the lever of the _second order_ the weight
is applied between the fulcrum and the effort; and in the lever of the
_third order_ the effort is applied between the fulcrum and the weight.
In each case that part of the lever which extends from the fulcrum to
the point where the effort is applied is called the _effort arm_, and
that which extends from the fulcrum to the point where the weight is
supported is the _weight arm_. The weight that can be lifted with a
given effort depends upon the ratio of the effort arm to the weight
arm. If the two arms are of equal length, the effort is equal to the
weight, but twice the weight can be lifted with the same effort if the
effort arm is twice as long as the weight arm. You can lift a ton with
an effort of only 100 pounds if your effort arm is twenty times as long
as your weight arm but the end of your effort arm would have to move
twenty inches to raise the ton weight one inch. We are assuming in all
these cases that the lever itself has no weight and that there is no
friction at the fulcrum.
Of course levers are not used merely for the purpose of lifting weight,
but to overcome any resistance or merely to apply pressure upon an
object. In almost every household we may find examples of the three
orders of levers. A pair of shears, for instance, is composed of two
levers of the first order, swinging on a common fulcrum. The effort is
applied at the handles, and the weight or load is the material that is
cut by the blades or, speaking more technically, the handles are the
_effort arms_ and the blades are the _weight arms_. A material that
is too tough to be cut at the tip ends of the blades may be easily
cut if we move it in near the fulcrum or pin that hinges the blades
together; for by doing this we shorten the weight arms, because the
weight arm is measured not to the end of the blade, but to the point
where it is cutting into the material. To cut very tough material, such
as heavy tin or sheet steel, we use long-handled short-bladed shears.
The cutting pressure depends upon the ratio of the effort arm to the
weight arm. If the effort arms are twice as long as the weight arms,
the cutting pressure is twice as great as that applied at the handles.
A nutcracker consists of a pair of levers of the second order. The
fulcrum is at one end and the effort or pressure is applied at the
opposite end of the levers or handles, while the equivalent of the
weight (in this case the nut) is placed between the effort and the
fulcrum. Again the _effort arm_ is measured from the fulcrum or hinge
pin of the tool to the point where the hand pressure is applied, and
the weight arm is measured from the fulcrum to the nut. The effort
arm may be four or five times as long as the weight arm, so that the
pressure exerted on the nut is four or five times as great as that
exerted by the hand on the ends of the handles.
[Illustration: FIG. 2.--AN ANGULAR OR BELL-CRANK LEVER]
In the case of a pair of sugar tongs we have another tool something
like the nutcracker in construction, but here the weight, i.e., the
lump of sugar, is seized by the ends of the tongs while the hand
pressure is applied somewhere between the fulcrum and the weight. Hence
we have here a lever or pair of levers of the third order. The _effort
arm_ of a pair of tongs is always shorter than the _weight arm_ and
the pressure on the sugar lump is always less than that exerted on the
tongs by the hand. Evidently the most powerful tool of the three is the
nutcracker, because the effort arms extend over the full length of the
tool and are always longer than the weight arms.
A lever need not consist of a straight bar; the effort arm may form
an angle with the weight arm, forming what is known as an angular or
bell-crank lever (Figure 2). When a common claw hammer is used to pull
out a nail, the claws that slip under the head of the nail form the
weight arm and the hammer handle the effort arm. A horizontal pull on
the handle produces a vertical lift on the nail.
Sometimes two or more levers are interconnected, as in Figure 3, the
effort arm of one being linked to the weight arm of the other. This
serves to increase the lifting force at the weight and at the same time
keep the mechanism within compact limits. Such compounding can go on
indefinitely and is subject to all sorts of variations.
[Illustration: FIG. 3.--COMPOUND LEVERAGE]
One thing we must not forget, and it is a matter that is commonly
overlooked by perpetual motion cranks, namely, that while a pound of
pressure on the effort arm may be made to lift two, four, or a hundred
times as many pounds on the weight arm by varying the relative length
of these arms, it has to move two, four, or a hundred times as far as
the weight arm, so that the work done on one side of the fulcrum is
always exactly equal to that done on the other side.
CONTINUOUS REVOLVING LEVERAGE
[Illustration: FIG. 4.--PRIMITIVE GEAR WHEELS--TWO COACTING GROUPS OF
LEVERS]
If we take a number of levers radiating from a common fulcrum like the
spokes of a carriage wheel, we have a primitive gear wheel. Two such
groups of levers may be mounted on parallel shafts so that when one
is turned its spokes will successively engage the spokes of the other
group and make the latter turn (see Figure 4). Each spoke is first
an effort arm on one side of the wheel and then a weight arm as it
turns around to the other side of the wheel, and as the effort arms
and weight arms are of the same length there is no multiplication of
power. A pound on one side of the wheel cannot lift more than a pound
on the other. The driven wheel receives the same power as the driving
wheel except for such loss as may be due to friction at the bearings or
where the spokes contact. The only advantage of such a pair of gears
is that the direction of rotation of the driven wheel is the reverse
of that of the driving wheel. If the spokes of one wheel are longer
than those of the other, we have at once a variation in the rate of
rotation proportional to the relative diameters of the two wheels. In
Figure 5, for instance, the diameter of the driving wheel A is twice
the diameter of the driven gear B, and so, for each revolution of A, B
must make two revolutions, i.e., the driver must make two revolutions
for each revolution of the driven wheel. In other words, the speed of
revolution is doubled. However, if we make B the driver the speed of
the driven wheel A will be half of that of wheel B.
[Illustration: FIG. 5.--COACTING LEVERS OF UNEQUAL LENGTH]
In primitive machines spoke gears were seldom mounted on parallel
shafts because of the difficulty of keeping the spokes in alignment.
Instead, one shaft was mounted at right angles to the other so that
one set of spokes would cross the other (Figure 6), thus producing
the equivalent of a bevel gear. This was of advantage in changing the
plane of rotation. A later development was the _barrel_ or _lantern
gear_, which permitted transfer of power without changing the plane of
rotation. A cylindrical bundle of rods constituted one of the wheels
(as shown in Figure 7). Instead of being crudely formed of spokes, the
other wheel sometimes consisted of a disk with pins radiating from
its rim. Such gears in far more refined form are still used in modern
clocks and watches. A still further development for transmitting motion
to a plane at right angles to that of the driving shaft is shown in
Figure 8. Here we have a _crown_ gear in which the pins instead of
radiating from the periphery of the disk project from the side face of
the gear.
[Illustration: FIG. 6.--PRIMITIVE EQUIVALENT OF THE BEVEL GEAR]
[Illustration: FIG. 7.--PRIMITIVE LANTERN GEAR]
Turning back to our first spoked wheels, it is very evident that we
may put a rim over the spokes or even fill in between the spokes and
convert the wheels into solid disks that are in frictional engagement
with each other without getting away from the fact that we are dealing
with levers. Each wheel, then, consists of a continuous revolving
lever. Friction gears are used quite commonly in machinery when it is
desirable to have the wheels slip if subjected to excessive strain.
TOOTHED GEARS
By forming teeth on one gear to mesh between similar teeth on the
other, we convert the friction gears into a pair of spur gears (Figure
9). We need not go into the intricacies of the form of gear teeth.
They are designed to be in continuous rolling contact while they are
in mesh. The novice is apt to call all spur gears “cogwheels” and
gear-teeth “cogs.” Mechanics, however, recognize a difference between
cog wheels and spur wheels. In the former, the teeth, or cogs, are not
cast upon or cut out of the wheel body, but are separate pieces fitted
to the wheel. Such wheels are found in old water mills. They consist of
wooden wheels with iron or steel teeth mortised in the wooden rim of
the wheel. In general it is safer to speak of spur gears because there
are few cogwheels now in use.
[Illustration: FIG. 8.--CROWN AND LANTERN GEAR]
When a small gear engages a large one, the former is commonly known as
a _pinion_.
[Illustration: FIG. 9.--SPUR AND PINION GEAR]
[Illustration: FIG. 10.--BEVEL FRICTION GEARS]
If two friction wheels are to turn at right angles one to the other,
they must have conical bearing surfaces, as in Figure 10. The angle
between the shafts of the two gears and the relative size of the gears
may be changed as desired, provided each cone surface has its apex at
the intersection of the two shafts or axes. It is easy to understand
how such conical friction gears may be converted into toothed bevel
gears (Figure 11), by forming teeth on the conical surfaces, and it
will be evident that the teeth must taper toward the apex of the two
cones. Two bevel gears of equal diameter, and with shafts set at right
angles one to the other, are known as _miter_ gears.
[Illustration: FIG. 11.--TOOTHED BEVEL GEARS]
So far we have not shown any combination of gearing that will multiply
power. In Figure 5, the driver A is twice the diameter of the driven
wheel B, and the latter makes two revolutions for one of A, but the
speed at the periphery of the two wheels is the same. A pull of one
pound at the point _a_ produces a pressure of one pound at _b_, and
this in turn produces a lift of one pound at _c_ because the levers
in each wheel are perfectly balanced, that is, each lever has equal
effort and weight arms. The way to obtain an increase of power and of
peripheral speed is to fasten two wheels of unequal diameters together
on the same center and apply the effort to one of the wheels (as in
Figure 12) and the weight to the other wheel. This gives us what is
technically known as a _wheel and axle_. The dotted lines show that
we have here a lever of the first order which can be used to multiply
power in the same way that a bar lever does. If one wheel is twice the
diameter of the other then a pound of effort will lift two pounds of
weight.
[Illustration: FIG. 12.--WHEEL AND AXLE OR REVOLVING LEVER OF FIRST
ORDER]
[Illustration: FIG. 13.--REVOLVING LEVERS OF THE 2D AND 3D ORDER]
Figure 13 shows how the effort and weight can be shifted about in such
fashion as to give us a lever of the second and one of the third order.
The power may be enormously increased and the speed of the final wheel
greatly reduced by setting up a train of gears in which the effort
is received by the larger one of each couple and is delivered by the
smaller one. In Figure 14 the smaller wheels are half the diameter of
the larger ones. A pound of pressure at A will amount to 2 at B, 4 at
C, 8 at D, 16 at E, and 32 at F. On the other hand, point A will have
to move through 32 inches to make the point F move an inch.
[Illustration: RAISING WATER WITH A CHAIN OF POTS
A primitive pump still used in Egypt]
[Illustration: A HORSE-OPERATED CHAIN-PUMP USED IN GREECE]
[Illustration: MULTIPLE SPINDLE DRILL IN MOTOR CAR FACTORY]
[Illustration: FIG. 14.--A TRAIN OF SPUR GEARS]
[Illustration: FIG. 15.--PULLEYS OF THE 1ST, 2D, AND 3D ORDERS]
[Illustration: FIG. 16.--TYPICAL ARRANGEMENT OF BLOCK AND TACKLE]
A pulley is merely a modification of the wheel. Figure 15 shows how it
may be arranged to correspond to the three orders of simple levers.
If the pulley axis is fixed, as in the first order, the effort and
weight arms are equal and hence balanced. In the second order the
wheel is bodily movable, hence one pound will raise two pounds of
weight because the power arm is twice as long as the weight arm, while
in the third order it takes two pounds of lift to raise one pound of
weight. There is no end of possible combinations of pulleys which will
multiply power in the same way that bar levers do when compounded. A
common arrangement of block and tackle is given in Figure 16. There is
a four-sheave pulley block above and a three-sheave block below, but
in order to trace the rope clearly the pulley wheels or sheaves are
represented as of different diameters. The arrangement consists of a
series of levers of the first order in the upper pulley block coupled
to a series of levers of the second order in the lower block. To find
the weight that a given power will lift, multiply the effort by the
number of strands of rope that are supporting the weight. In this case
there are seven such strands, not counting the strand E, to which the
effort or pull is applied. This means that a pull of a hundred pounds
at E will lift 700 pounds at W. Of course a pull of seven feet at E
will raise the weight only one foot.
[Illustration: FIG. 17.--INCLINED PLANE WITH EFFORT PARALLEL TO THE
INCLINED FACE]
THE INCLINED PLANE AND ITS FAMILY
The inclined plane constitutes a second broad classification of machine
elements. The wedge, the screw, the cam, and the eccentric, all belong
to the family of the inclined plane.
[Illustration: FIG. 18.--INCLINED PLANE WITH EFFORT PARALLEL TO THE
BASE]
A simple form of inclined plane is pictured in Figure 17, which shows
a weight W being rolled up an incline. The effort required to carry it
to the top of the incline depends, of course, upon the steepness of the
incline. The drawing shows a rise of 3 feet on a slope 5 feet long, and
the weight of the wheel is, say 20 pounds. To find the effort required,
the weight is multiplied by the rise (20 × 3 = 60) and divided by
the length of the slope (60/5 = 12) and we find that it takes only
12 pounds to roll the 20-pound wheel to the top of the incline. This
holds true when the pull is parallel to the inclined face. If the
pull is parallel to the base of the incline, as in Figure 18, we must
divide by the length of the base instead of the length of the incline
(60/4 = 15) and we find that it takes 15 pounds of effort to pull the
weight up the incline. If the pull is exerted at an angle both to the
base and the inclined face, we have a problem that is slightly more
complicated and we need not go into it here because it involves a bit
of trigonometry. In all cases, however, it may be noted that the amount
of rope that is taken in, in hauling the weight up the incline, bears a
definite relation to the amount of effort required to raise the weight.
In Figure 17, 5 feet of rope must be pulled in, in order to raise the
weight 3 feet, so that ⅗ of 20 or 12 pounds is all that is required
to pull up the weight, while in Figure 18, 4 feet of rope is hauled
in for a lift of 3 feet, so that ¾ of 20 or 15 pounds is required to
pull up the weight. In this respect the inclined plane is exactly like
the lever or the pulley, for the _effort_ multiplied by the _distance_
through which it is exerted is always exactly equal to the _weight_
multiplied by the _distance_ through which it moves. Thus in Figure
17, the effort 12 pounds multiplied by the distance 5 = the weight 20
pounds times the distance 3, and in Figure 18, effort 15 x distance 4
= weight 20 x distance 3. Of course, we are ignoring the weight of the
rope and the friction which, in actual practice, are important factors
to be reckoned with.
[Illustration: FIG. 19.--ENDLESS SCREW OR WORM GEAR]
So far we have considered a fixed inclined plane, but when the inclined
plane is moved between the weight and a fixed base it is known as a
wedge, and in this case, too, the effort required to move the wedge
multiplied by the distance the wedge moves is equal to the weight
multiplied by the distance it is lifted.
The commonest form of inclined plane is the screw which is merely an
inclined plane bent around a cylinder. A screw engaging a toothed
wheel, as in Figure 19, gives a combination known as an “endless
screw,” or, more commonly, as a worm gear. The screw or worm is always
the driver, and as it must make a complete turn to move the gear
through a space of one tooth, the power of this combination is very
great. It is practically impossible to turn the worm by using the gear
wheel as a driver because the friction developed at the point where
the worm and gear contact is very great. For this reason worm gearing
is used in the steering gear of automobiles. The shaft of the steering
wheel is fitted with a worm which meshes with a worm gear on the parts
connected with the wheels. It is very easy to turn the wheels by
operating the steering wheel, but if the wheels strike a rut or a stone
they are not deflected from their course, because the worm makes it
impossible for them to turn the steering wheel.
[Illustration: FIG. 20.--HELICAL OR SPIRAL GEARS]
The spiral gear shown in Figure 20 is a cross between a worm gear and
spur gear. The teeth are spirals set at an angle of 45 degrees to the
axis of the wheel. In this case either gear can be used to drive the
other, and the advantage of such a pair is that power is transmitted
from one shaft to another in a different plane and at right angles to
the first.
[Illustration: FIG. 21.--DRUM CAM]
[Illustration: FIG. 22.--PROFILE OR DISK CAM]
Cams are usually irregular revolving inclined planes. Figure 21, shows
a cylinder or drum cam. A groove is cut in the cylindrical wall of the
cam and an arm or lever is provided with a roller which rolls in the
groove. When the cam is revolved the lever is constrained to follow all
the twists and turns of the groove. A different form of cam is shown in
Figure 22. It is formed with an irregular periphery against which the
roller is pressed by a spring. As the cam wheel revolves, the roller
and the arm to which it is attached must move in and out over all the
hills and valleys of the periphery. The cam is one of the most useful
elements in modern machinery, for it provides a very simple means of
producing the most complicated and irregular motions.
[Illustration: FIG. 23.--ECCENTRIC BY WHICH ROTARY MOTION IS CONVERTED
INTO RECTILINEAR MOTION]
We cannot attempt to describe all the different types of cams, but
reference should be made to the eccentric, which is a form of cam
commonly used to operate the valves of a steam engine. The cam in this
case is a perfectly circular disk, but the shaft that turns it does not
lie at the center of the disk, consequently an object bearing against
the periphery must move toward and away from the center as the disk
revolves. Instead of using a spring-pressed roller to bear against one
side of the disk, the whole disk is encircled with a ring of steel
known as an eccentric strap. This strap is bolted to a valve rod and
as the eccentric revolves the strap makes the valve rod move back and
forth. (See Figure 23.)
A description of all the various combinations of gearing, link motions,
ratchets, escapements, clutches, and miscellaneous movements would
easily fill the rest of this book, and we must therefore content
ourselves with this very brief survey of a few of the more important
elements employed in the construction of modern machinery.
CHAPTER III
MACHINES FOR MAKING MACHINES
WHILE we may glory in the wonderful mechanical progress of to-day, we
must not overlook the marvelous skill of the ancient artisan nor forget
that it is to his inventive genius that we are indebted for practically
every hand tool we possess. Only a few special tools owe their origin
to the modern inventor. All the rest date back beyond the twilight of
history. We have merely improved upon these tools by slight changes of
design or the employment of better materials in their construction.
As users of these tools we cannot begin to compare with the skilled
workman of ancient days. Our progress is shown not in the _development_
of skill, but in the _loss_ of it. We have taken the tool out of the
human hand and put it into an inanimate machine. It is only very
recently that the tool was delivered to the machine and that act marked
the dawn of the present remarkable mechanical era.
Machines for making machines date back to the time of the early
Egyptians. They had their pole lathes and bow drills, but these
machines only partially relieved the workman of his labors, and the
quality of the work still depended upon a degree of skill that was
acquired only through years of patient apprenticeship.
The pole lathe, by the way, consisted merely of a pair of centers
between which the work was mounted, a pole attached to the ceiling
and a strap or rope passed around the work and fastened at one end to
a pole and at the other to a pedal resting against the floor. (See
Figure 24.) When the pedal was depressed, the strap was pulled down
and the work was revolved. On releasing the pedal, the spring of
the pole pulled the strap up and reversed the rotation of the work.
Thus by alternately depressing and releasing the pedal, the work was
intermittently revolved against a chisel which was rested on a block
and guided by the workman. Small work could be turned out on such a
lathe with considerable precision, but when it came to large parts,
particularly parts of steel, the workman was easily tired by the effort
of operating the pedal and was apt to be irregular in the guiding of
the tool.
[Illustration: FIG. 24.--PRIMITIVE POLE LATHE]
Up to the middle of the eighteenth century practically no advance had
been made over the ancient lathe of the Egyptians, and when, 150 years
ago, the steam engine was invented the task of building the engine
seemed almost insuperable.
James Watt was a maker of mathematical instruments, a man of great
skill and precision as a craftsman, but he dealt with parts of small
dimensions. When he conceived of his steam engine, he mentally pictured
the various parts as turned out with all the accuracy and finish that
was possible in the diminutive members of a scientific instrument. To
him it seemed perfectly feasible to turn a cylinder which would be
practically perfect in contour, and to fit it with a piston around
which no steam could leak. With the lathe then in existence such a fit
was easily possible on small work. But when he undertook to have the
cylinder of his engine bored, he discovered that there was no machine
that could begin to do the work properly. In fact, when Smeaton, who
was a prominent engineer of that time, investigated Watt’s steam
engine, he declared that it was such a complicated piece of work that
neither tools nor workmen existed that could build it. In Watt’s first
engine, the cylinder was only six inches in diameter and two feet long,
and a special type of boring machine was devised to bore the forged
cylinders. But the boring was so irregular that when the piston was
inserted and the steam was turned on, nothing would stop the flow of
steam that leaked around the piston. In vain did James Watt use cork,
oiled rags, tow, paper, and even old hats to stop the leakage. However,
the boring machine was improved and later a cylinder, eighteen inches
in diameter, was bored with such accuracy that the large diameter
exceeded the small diameter in the worst place by only ⅜ of an inch.
This Watt considered a very good bit of turning. To-day cylinders of
that size that vary from true by half the thickness of the paper that
this is printed on would be thrown out as defective.
It was in 1769 that Watt invented the steam engine, but that great
event did not mark the dawn of the present era of machinery. For a
quarter of a century thereafter there was little progress in the
development of machine tools. A boring machine was built that did
fair work. There were a few sawmills in which wind power was employed
to drive the saw. But lathes were still driven by foot power and the
cutting tool was still held and guided by hand.
MAUDSLEY’S “GO-CART”
The real father of the present era was a very clever British mechanical
engineer, Henry Maudsley, who undertook to eliminate the uncertainties
of the human hand by clamping the cutting tool of the lathe in a rest
and arranging the rest to slide along the length of the lathe or
transversely toward or away from the center. These two motions made it
possible to accomplish all that the workman could accomplish by hand
and at the same time the tool was held so firmly that accuracy and
precision of turning was assured. Furthermore, he provided this slide
rest with a nut that engaged a screw driven through suitable gearing
by the lathe spindle. Then, as the work revolved, the slide rest
was compelled to move along the bed of the lathe at a uniform rate.
By varying the gearing, the speed of the slide rest and the tool it
carried could be varied at will, thus making it possible to cut screw
threads of any pitch desired with a degree of accuracy unattainable by
hand.
Remarkable as was this improvement, it met with the usual opposition
that every real advance in machinery received in those days. People
referred to the slide rest as Maudsley’s “go-cart,” but it proved such
an important element of the lathe and so very valuable that before
long it was universally adopted. From that time on the skill of the
workman began to lose its importance. The man began to give way to the
machine. Precision was possible in large as well as small work. The
human element was also dispensed with in the driving of the lathe. The
foot pedal was superseded by the steam engine, and the machine came to
be known as the engine lathe.
There are many ways of working metals now in common use. Metals may be
cast in a molten state, or they may be pressed and molded into shape in
a cold state, or they may be hammered either cold or hot, but in nearly
all cases in which metal is removed in order to form a piece of work,
the chisel is used as a cutting instrument. This is perfectly apparent
in lathes and planers, but not quite so apparent in sawing, drilling,
filing, and grinding. A drill is merely a spiral chisel which revolves
upon its own center. A saw is a gang of tiny chisels, and a file
consists of still smaller chisels which are broader than those of the
saw. In grinding we have rough surfaces in which particles of emery or
carborundum act as tiny chisels. The shears, the punch, and the cutting
torch are practically the only exceptions to the rule that metals
are always cut by chisels, and even the shears may be conceived as
consisting of a pair of broad coacting chisels, while it takes little
imagination to see a form of chisel in the punch. The cutting torch is,
of course, in no sense a chisel.
In the cutting of metals the work may move against a fixed tool or the
tool may move against a fixed piece of work. In a lathe, it is the work
that revolves or rotates against the tool. In the drill and the milling
machine the tool revolves against the work.
In the planer, the tool is fixed and the work slides against it. The
shaper reverses the operation; the work is fixed and the tool moves in
a rectilinear direction.
Up to the nineteenth century practically the only machines for
cutting metals were the lathe and a crude form of boring machine.
The machinists of that day had not reached the stage where they were
able to produce anything but round work on a machine. The planer had
not been born. It was for this reason that Watt had a great deal of
difficulty in getting rectilinear motion for the piston of his engine.
He had to invent a complicated system of links and levers in order to
obtain a practically parallel motion to guide his piston in and out
of the cylinder. When the planer was invented and it was possible to
produce straight surfaces with a considerable degree of accuracy, all
of Watt’s ingenious parallel motions, went into the discard and the
cross-head and guides took their place.
It was not until long after the planer had been invented that Eli
Whitney, the American genius of cotton-gin fame, conceived the milling
machine. He reversed the operation of the lathe by placing the cutting
tool on the revolving spindle and sliding the work against it. Milling
cutters consist of wheels formed with a number of cutting edges or
chisels which are arranged either on the periphery of the wheel or on
the face of the wheel.
Following the milling machine, came the grinder, in which a revolving
wheel of an abrasive material served to wear away the surface of a
piece of work, and with this form of machine steels of great hardness
could be finished with accuracy and a high polish.
THE INTERCHANGEABLE SYSTEM
The most notable advance in machine work came early in the nineteenth
century, when what was known as the “American System” of manufacture,
or the interchangeable system, was introduced. As long as mechanics
were obliged to perform their operations largely by hand, it was
impossible to attain great accuracy. Each workman put his own
individuality into the work. As a consequence, no two pieces were of
exactly the same size or shape. This was true even with the early
power-driven machine tools. The parts might be very close to the same
size, but careful measurements showed that they varied by a minute
fraction of an inch. Hence, when a machine was assembled the unyielding
metal parts had to be filed and trimmed and hammered to fit them
together. If any accident occurred to a machine, the damaged part
could not be replaced by another taken from stock. The entire machine
had to go back to the shop where an experienced mechanic would make a
new part to replace the damaged one. In those days a machine was not
manufactured but was built as an individual mechanism, just as a house
or a boat is built to-day.
With the advent of accurate machine tools came the idea of
standardizing the parts so that hundreds and thousands of pieces could
be made of exactly the same dimensions, and in assembling a machine
the parts could be picked at random from the stock and put together
without the use of special tools and without requiring any special
fitting. This was of special importance in the tools of warfare,
because armies need quantity production, i. e., rifles, cannon, etc.
More machines of the same kind were required for an army than for any
other organization or line of work. At the close of the Napoleonic
War the British Government had 200,000 parts of muskets either partly
finished or waiting repairs. Their muskets were made after the old
system. Each one was built separately with its parts individually
fitted together, so that whenever any part was injured, the musket had
to be laid aside and sent back to the workshop for repairs.
Long before that time, the idea of making standard guns had been hit
upon in France. Thomas Jefferson, while Minister to France, in 1785,
wrote of the French system which was then being developed by a mechanic
named Le Blanc. He was building a musket in which the parts were
of standard pattern, and which could be assembled by taking pieces
haphazard as they came to hand and putting them together without
special fitting. Thomas Jefferson called the attention of the American
Government to this system and showed that it was possible to produce
muskets cheaper by that method of manufacture. However, our Government
at that time failed to avail itself of the opportunity of utilizing
this system of manufacture.
Later on, the idea was taken up in this country by Eli Whitney and by
Simeon North. When Whitney attempted to introduce the system, he was
laughed at by French and English ordnance officials, and even our own
Government officials were skeptical, particularly when they found that
it required so much preparation in the way of machinery and designing
of parts before a single musket was completed. It seemed like a waste
of money to invest in so much preparation. But Whitney was soon able
to silence all his critics by taking to Washington ten pieces of each
part of a musket and then selecting at hazard from each pile of pieces
the requisite parts and putting together ten muskets. It was not long
before the foreign governments saw the importance of this method of
manufacture. Great Britain later adopted it in the making of her own
rifles and called the process the “American System.”
But it was not only in the field of rifles that interchangeable
manufacture made itself felt. The New England clock industry provides
an interesting illustration. At first the clocks were made of wood,
but early in the nineteenth century, a clock maker, Chauncey Jerome
by name, designed a brass clock in which the parts were made on the
interchangeable system. Instead of building each clock as a separate
piece of work, clocks were turned out by the thousands, and at an
extremely low price. Soon he had flooded this country with his clocks
and began to look around for other markets. Machinery had been used by
other clock makers in producing wooden clocks, and movements which had
cost $50 each in 1840 had been reduced to $5. But Chauncey Jerome’s
clock was made of brass and by means of the interchangeable system of
manufacture he could produce it for less than 50 cents. The clock was
such a success in this country that Jerome decided to try it abroad.
Consequently he made arrangements with an agent in England and shipped
over a large consignment. The British Government was astonished at the
low price of the clock and was convinced that it had been undervalued.
At that time, they had a simple and ingenious method of punishing a
consigner who undervalued the goods he wished to introduce into the
country. This consisted in promptly appropriating the property at the
price given in the invoice. In due course of time, much to Jerome’s
astonishment, he received a letter from the British Government stating
that his clocks had been confiscated, and with the letter came a check
paying for them at the invoice price. Jerome was not in the least
dejected by the rebuke; on the contrary, he was rather elated, for, as
far as he could figure it out, he had a spot-cash buyer for his goods
and no selling expenses. He did not mind at all letting the British
Government have the clocks at the invoice price. So he decided to try
again with a larger shipment. To his great delight this shipment met
the same fate as the first, and in due course another good British
check arrived. Thus encouraged, Jerome sent over a third and still
larger shipment, but by that time Johnny Bull began to suspect that the
Yankee clock maker was getting the best of the bargain and, finally
convinced that clocks really could be produced with profit at the
low invoice price, he permitted them to enter his country. With this
striking example, the fame of the Yankee system of manufacture spread
over the world.
In order to have two parts alike, they must be placed under a machine
in exactly the same way. In other words, they must be set in “jigs” or
frames which are fitted into the machine in such a way that the tools
will approach the work from exactly the same angle or penetrate the
work to exactly the same depth in ten, or a hundred, or a thousand,
or a million pieces, as the case may be. Making jigs and dies consumes
a great deal of time in preparation work, but once the preparation
stage has been passed, articles are produced with wonderful rapidity
and very little waste of time. Formerly it was necessary to determine
the location of each hole in a casting separately and spend precious
time in adjusting the work to the proper position under the tool. If
the hole was to be threaded, it had to pass through several separate
operations.
THE TURRET LATHE
After the slide rest invented by Maudsley, the next great improvement
on the lathe was a turret head or a sort of turntable which carried
a number of tools. The tools are arranged to come automatically into
play one after the other. One tool, for instance, may cut a groove in
the work, another finish the face of the work, another bore a hole in
the piece and another tap the hole. In many cases, several of these
operations are performed simultaneously. The head of the lathe is
provided with a hollow spindle so that the work is automatically fed to
the tools through this spindle, and as soon as one piece is finished,
it is automatically cut off and the jaws of the clutch which holds
the work, or stock as it is called, open automatically so that a new
length may slide forward and be operated upon by the tools. The machine
requires no attention once the tools have been set up to the proper
angle, except that it must be kept supplied with bars of stock as they
are consumed, and with a copious flow of lubricant on the tools. One
operator can therefore take charge of a number of automatic lathes.
All he does is to feed them; they do the rest.
The modern drill has also gone through a great many developments in
order to speed up the work that it performs. When a casting is to have
forty or fifty holes drilled and tapped in it, instead of following the
old method of drilling each hole separately, a lot of separate drilling
spindles are used, each fitted with a drill, and these are brought
simultaneously into play. As many as fifty or sixty holes may be
drilled at a single operation, and after the holes have been drilled,
the drilling spindles move to one side to make way for the taps, which
thread such of the holes as are to receive screws. By first setting the
spindles in the proper position and then using jigs to locate the work
properly under them, the assurance is had that every one of the scores
of holes drilled will be accurately spaced apart and the spacing in
every casting will be identical.
The multiple tool system is also used in milling machines in which a
number of milling cutters either of the face or the end type come into
play simultaneously upon a piece of work set in a suitable jig, and cut
the piece with absolute precision, so that all castings will have faces
accurately spaced apart and cut to exactly the same level.
It is by such methods as these that we are able to produce such large
quantities of machinery at remarkably low cost. One of the most
notable examples of such work was the development of the Liberty
engine during the World War. This engine did not differ in principle
from others built in Europe or in this country, but its design was
carefully adapted to permit of interchangeable manufacture. No careful
finish was used except where indispensable. Special jigs, tools, and
fixtures were prepared. Ingenious wrinkles of American manufacture were
introduced. All this consumed time, and great was the irritation of the
general public. Under ordinary conditions, it would have taken years to
have developed the Liberty engine to the manufacturing stage, but under
the urgent stress of war, the whole work of design and preparation
for manufacture was crowded into a few short months, and then Liberty
engines began to be manufactured on a stupendous scale.
CUTTING WITH RED-HOT TOOLS
One of the most remarkable advances in machine tools was due to the
studies of Fred W. Taylor. He entered a large steel plant in 1880
and was immediately struck with the enormous waste of effort on the
part of the men in the plant. There was at that time considerable
dissatisfaction among the workmen, and when Taylor endeavored to speed
up work he was faced by the incontrovertible argument that he had no
idea how much work a certain machine ought to turn out. There was
nothing for him to do but either back down or study machine tools and
discover their maximum capacity. This led him to investigate the matter
of cutting speeds. For years he spent all of his spare time studying
this subject, timing machines and experimenting with different types
of cutting tools. He estimated that in the twenty-six years of his
investigation he converted 800,000 pounds of steel into chips. What
he wished to discover was the best depth of cut, the best speed of
cutting, and the best speed at which the tool should be fed into the
work. He soon discovered that, contrary to prevailing opinion, the
round-nosed tool was better than the diamond-pointed tool, that the
coarse slow-cutting speed was better than a fine cut at high speed. He
discovered that the best method of lubricating the tools was to keep
them bathed in a heavy stream of water, supersaturated with carbonate
of soda, so as to prevent the metal from rusting. The best tool steel
of that day was known as a self-hardening steel. Manufacturers of the
cutting steels had warned Taylor that he must not use water on these
tools. Taylor, however, was not satisfied to take the word of others,
but proceeded to investigate the matter himself, and discovered that
he could safely increase the cutting speed of his tools 33 per cent by
the use of a heavy stream of water for lubricating purposes. This led
Taylor and his associate, Maunsel White, to investigate the different
kinds of tool steels, and eventually they evolved a chrome tungsten
tool which could do from two to four times the work of other tools.
Later vanadium was added to the alloy, further improving the tool.
At the Exposition in Paris, in 1900, foreign manufacturers were
astonished to find enormous lathes operating at high speed with the
cutting tools taking such heavy cuts and feeding so fast that the nose
of the tool was actually heated to a dull red heat, and yet it kept its
cutting edge perfectly. This was a revelation to tool makers abroad,
and it led immediately to the adoption of American high-speed cutting
tools.
The development of the automobile, which began to take on serious
proportions at about that time, is responsible above all other machines
for improvements in American machine tools, and for the extension of
the American system of interchangeable manufacture. When automobiles
came to be made on the interchangeable system and in enormous
quantities so that the cost was reduced to within the limits of the
average man’s pocketbook, they began to make mechanics of men who
before that had never used a tool; and this new and widespread interest
in machinery stimulated the production of better and more efficient
tools. Hence the progress of machine tools in the past few years has
been simply phenomenal.
CHAPTER IV
COUNTING SECONDS
YOU CAN measure civilization by its timepieces. The higher the
civilization of a community the more it appreciates the value of time
and the more minutely does it measure the passage of time. The savage
divides his day into but two periods: the period of light and that
of darkness; the early Romans divided their day into eight watches,
four watches of daylight and four of night, but the higher and more
complex civilization became, the smaller became the subdivisions of
time. People began to feel the need of carrying the time with them,
and about 1500 A. D. watches came into use, but it was not until 1665
that watches began to be equipped with minute hands, and it was almost
exactly a century later that they were equipped with a second hand.
To-day time means so much to us that we will fight our way into a
subway express, instead of riding more comfortably in a local train,
merely for the sake of saving five minutes. The tiny second hands of
our watches divide the day into eighty-six thousand four hundred parts,
and in some operations we measure time intervals down to the thousandth
part of a second. Only among the most highly civilized nations are
timepieces carried by the common people.
It used to be that time was made for slaves, but now time has made
slaves of us. Shift the hands of the clock and the whole nation is
forced to change its habits.
Time-measuring mechanism is given early prominence in this book because
clocks were among the earliest machines invented, and they furnish an
example of the wonderful ingenuity of inventors before the dawn of
the modern era of machinery. Naturally this chapter must be largely
historical.
The first thought of measuring time came from the ancient astronomers
and astrologers, who, in watching the motions of heavenly bodies, the
sun by day and the moon by day and night, found it necessary to keep a
record of these motions and sought about for some mechanical means of
doing so. The studies of the old astrologers were closely associated
with religion, and as a consequence the most advanced intellects were
centered upon astronomical matters and incidentally upon horology.
Fortunately the design and construction of mechanisms for measuring
time were not considered beneath the dignity of the scientists of those
early days. Mathematicians felt free to record their investigations in
this branch of mechanics, and as a consequence of the early cooperation
of science and mechanics in this field much real progress was made, and
the development of timepieces was more rapid than that of any other
machine.
TELLING TIME WITH A LEAKY BUCKET
The ancient Egyptians early felt the need of a better clock than the
sundial, because it operated only on cloudless days and was absolutely
worthless to tell off the hours of darkness. Realizing that time is a
measure of motion, they sought for some slowly moving body whose motion
could be used to measure time, and naturally they turned to water. The
earliest form of clock consisted merely of a leaking bucket. Either the
bucket was filled with water which was allowed to escape through a very
tiny orifice, or else the heavily loaded bucket was placed in water
and the water was allowed to leak into the bucket until it sank. The
period it took for a bucket to run dry or for a bucket to fill and sink
indicated a lapse of an hour or some other standard of time.
The idea of subdividing this period was a later development. As
the water leaked out of a bucket, the water level descended, but
unfortunately not at a uniform rate. The weight of water in a full
bucket made the drops come faster than when the bucket was nearly
empty. Consequently the time graduations on the side of the bucket had
to be set farther apart at the top of the bucket than at the bottom.
Various ingenious schemes were devised for maintaining a uniform
discharge. In one type of water clock or clepsydra a conical bucket was
used so that there would be a constant relation between the head of
water and the volume in the bucket and the time graduations could be
spaced uniformly.
THE REMARKABLE WATER CLOCK OF CTESIBIUS
The most remarkable clepsydræ were those invented by an old Alexandrian
mathematician, Ctesibius, who lived about 250 years before Christ.
Ctesibius introduced the siphon principle into his clocks, and also
employed gear wheels and even a cord and pulley. Furthermore, he was
the first man to employ jeweled bearings in a timepiece. The use of
jewels in timepieces was reinvented in 1704 A. D. However, Ctesibius
used his jewels in a very different way from that in which they are
used now, as will be described below.
[Illustration: FIG. 25.--THE JEWELED WATER CLOCK BUILT BY CTESIBIUS
ABOUT 250 B. C.]
Figure 25 illustrates the most interesting clock he built and it was
arranged to run year in and year out. The clock had a cylindrical face
mounted on a hollow pedestal in which the mechanism was concealed.
The column was divided off into twenty-four hours and a pointer that
rose vertically marked off the lapse of time. But here he was faced
with a serious complication. Hours in those days varied with the time
of the year. A day from sunrise to sunset consisted of twelve hours.
In summer, when the days were long, the hours were long, and in the
short days of winter the hours were correspondingly shortened. To be
sure, the variations in the length of the day are not so great in Egypt
as they are in our latitude, because it is nearer to the equator;
nevertheless there is a difference which the precise old mathematician
had to take into account. In order to provide for variations in the
hours, Ctesibius ran the lines spirally around the column and arranged
his cylindrical clockface to turn slightly each day, so that in the
winter months the clock hand or pointer moved over that part of the
face where the daylight hour lines were closer together and the hours
of night were farther apart, while in summer, the reverse would obtain.
At the bottom of the column were two little cherubs. The cherub on the
left was a sad little fellow who was constantly weeping. Tears trickled
from his eyes and dropped into a basin. The tears passed into the
hollow pedestal of the clock and gradually filled a cylinder formed in
the base of the clock. A piston in this cylinder supported the other
cherub. As the water gradually filled the cylinder this cherub was
slowly raised and a wand he held in his hand pointed off the hours on
the clockface. When the twenty-fourth hour was reached, a siphon came
into play, which suddenly emptied the cylinder, permitting the pointer
to drop. The siphon discharged its water into a small water wheel,
which, by means of the system of gears, turned the column slightly
to bring the hour lines in proper positions for measuring the time
intervals of the next day. The column made one complete turn in 365
days. The jeweled bearings, referred to above, were placed in the eyes
of the weeping boy, so that the holes that pierced them would not be
enlarged by the constant wear of the water and thereby increase the
rate of flow.
TIMING ANCIENT ORATORS
We have dwelt at considerable length upon this old clock of the
pre-Christian era to show the ingenuity of inventors of that day, and
also the careful study that was made of time by ancient mathematicians
and astronomers. Of course water clocks were used before the time of
Ctesibius. In fact, we read of them in the comedies of Aristophanes,
written 400 B. C. Water clocks were used to limit the long speeches
of orators at court, and in one place we find Demosthenes accusing
a man of “talking in my water,” while at another time, when he was
interrupted, he called to the officer to stop the water, showing that
he valued every moment of time allotted to him for his speech.
In 807 Charlemagne was presented with a clock by the King of Persia.
This consisted of an elaborate mechanism in which were all manner of
wheels, and the clock would actually strike the hours. The driving
power, however, was water.
But there were serious disadvantages in the use of water for the
measurement of time. No great accuracy was ever obtainable with it,
owing to the fact that its volume varied considerably with the
temperature, and also with the dryness or moisture in the surrounding
atmosphere. The idea of using a weight instead of water is claimed to
have originated as far back as 990 A. D. The next important advance in
the motive power of clocks was in 1500, when Peter Hele of Nuremberg
invented the mainspring.
That was long before the pendulum made its appearance. The clock
mechanism was slowed down and kept under control by what was known as
a balance lever. (See Figure 26.) This was a horizontal lever mounted
to oscillate in a horizontal plane. The lever was fitted with sliding
weights, so that it could be carefully adjusted. The last wheel of the
train of gears was provided with escapement teeth, somewhat similar to
those used on our clocks and watches, but which would alternately move
the lever this way and that. The inertia of the lever with the heavy
weights on it was sufficient to prevent the mechanism from racing, and
by this means, the motion was governed and slowed down, so that it
measured time with a fair degree of accuracy.
DISCOVERY OF THE PENDULUM
[Illustration: FIG. 26.--THE BALANCE LEVER, PREDECESSOR OF THE PENDULUM]
The pendulum had an interesting origin. Galileo, while a student in
Pisa in 1581, was attending a service in the cathedral one day when
his attention was drawn to the swinging of a large hanging lamp. One
of the attendants had drawn the lamp toward him, so that he could
reach it more readily to light it. When he let go the lamp began to
swing slowly back and forth, and the observant young student noticed
that although the oscillations gradually slowed down the period of
oscillation was constant. He had no watch with which to measure the
length of the period, but being a medical student he knew that he had a
fair timepiece in his own heartbeats, and so by counting his pulse he
proved that it took the lamp just as long to complete each oscillation
when it swept through a long arc as when it died down to but a few
inches. He made note of this peculiar action and began experimenting
with pendulums of different lengths. Then it occurred to him that if he
could time the pendulum with his pulse he could time his pulse with a
pendulum. So he devised a pendulum whose length could be adjusted until
its oscillation would coincide with the throb of a patient’s pulse, and
then the length of the pendulum would give him the rate of the pulse
beat. The invention was seized upon by the medical profession of that
day, and the _pulsilogia_, as it was called, became an indispensable
instrument for physicians.
[Illustration: BUCKET OF A GIANT DREDGE WITH ITS JAWS SPREAD WIDE OPEN]
[Illustration: GOLD DREDGER EATING ITS WAY THROUGH RIVER SANDS]
[Illustration: A SUCTION DREDGE EXCAVATING A CHANNEL AND BUILDING NEW
LAND]
The idea of using the pendulum to control the action of the clock also
occurred to Galileo, and in later years, after he had lost his sight,
he passed the idea down to his son, Vincent. However, it is generally
conceded that the credit for introducing the pendulum and doing
away with the balance lever belongs to Christian Huygens, the Dutch
mathematician, whose first pendulum clock dates back to 1659.
About the same time Dr. Robert Hooke invented the balance spring
which made it unnecessary to use the pendulum in portable timepieces.
Prior to that watches were fitted with balance levers and they gave
a great deal of trouble, because the time varied with the position
in which they were carried. But the balance wheel overcame all these
difficulties and made it possible for a person to carry an accurate
timepiece in his vest pocket, although the early watches were very
bulky mechanisms.
THE FIRST SHIP’S CHRONOMETER
The importance of having accurate timepieces aboard ship was felt as
far back as the time of Columbus. When ships began to go out beyond the
sight of land, it was highly important that they be equipped with some
means of locating their position at sea. By noting the elevation of the
sun at noon with a sextant it was possible to determine the latitude of
the ship, but there was no means of determining its longitude, except
by dead reckoning, that is, using a log to measure the speed of a ship,
and estimating its position by calculating the number of miles it
had traveled since leaving port. If ships could be provided with an
accurate timepiece which would keep the same time as that of some fixed
observatory, it would be possible definitely to locate the position of
the ship east or west by noting how fast or slow the sun was at noon on
the particular day on which the observation was taken. This use of the
chronometer is common in these days, but up to 150 years ago there was
no timepiece sufficiently accurate to permit a navigator to tell with
any certainty just where he was.
It was John Harrison, the son of a Yorkshire carpenter, who was the
first to build a chronometer worthy of the name. A prize of 10,000
pounds was offered by the British Parliament for anyone who could
invent and sell a chronometer which would enable a ship to take a
voyage from England to any of the West Indian islands and back and keep
track of the longitude within one degree. If this could be defined
within two-thirds of a degree, the prize would be 15,000 pounds.
Harrison made a bid for this prize, and after years of effort and
patient labor, he succeeded in being granted a trial. His son, William,
was sent on a ship to Jamaica with the now celebrated chronometer,
which was mounted on a large cushion. The instrument was constantly
attended by the young man, its position being adjusted from time to
time to suit the “lie” of the ship. When the ship was eighteen days
out, the vessel was estimated by dead reckoning to be 13° 50′ west of
Portsmouth, but the chronometer indicated the position as 15° 19′.
The timepiece was immediately condemned as worthless, but William
Harrison had not lost faith in the instrument, and insisted that if
the ship continued on the same course, a certain island, if properly
marked on the chart, would be seen the following day. True to the
prediction, the next morning at seven o’clock the island appeared.
By means of his chronometer William Harrison was able to predict the
appearance of the other islands, and at the end of the voyage, which
occupied sixty-one days, the chronometer was only nine seconds slow.
When he returned to Portsmouth, after an absence of five months, the
error of the chronometer was only one minute and five seconds, giving
an error in distance of only eighteen miles, whereas thirty miles was
the margin of error allowed by the prize conditions. Such accuracy
seemed so incredible that the chronometer had to be tested on a second
voyage, during which it was kept under lock and key and when William
Harrison had to wind the instrument he was obliged to do so in the
presence of two witnesses, lest he move the hand of the chronometer
surreptitiously. At the end of the second voyage there was no further
doubt that Harrison was fully entitled to the prize. Chronometers soon
came to be used extensively, until now they are one of the most perfect
of machines made by man, and operate with an accuracy that is almost
incredible. Usually a ship is provided with several chronometers, so
that one may be used as a check upon another. They are mounted in ball
sockets and gimbal joints, so that they are not affected by the roll of
the ship, but always lie in a horizontal position.
MARVELOUS PRECISION OF MODERN WATCHES
While we may well marvel at the precision of the chronometer, it is
equally marvelous, if not more so, that we may equip ourselves for
a few dollars with a timepiece which is so wonderfully accurate as
to vary little more than a second per day. If one took the pains to
regulate his watch carefully, any of the better makes could be adjusted
to such accuracy.
There is nothing very mysterious about the mechanism of a watch. It
consists merely of a train of gears which slow down the motions of
the mainspring to a convenient speed; and these gears moreover keep
the proper relation between the hour and minute and second hands. But
when we reflect that a small watch possesses a tiny second hand which
travels something like ten miles in a year, and that if carefully
regulated it will not vary from that of another watch in the whole
journey by more than six or eight inches at the most, we certainly have
a reason to marvel. There are 86,400 seconds in a day, and a watch is
usually arranged to make five beats per second or 432,000 per day.
The interval between beats must be adjusted with such minuteness that
one beat must not differ from another by 1/86000 part of a second,
else the watch will register more than a second fast or slow at the
end of a day. And yet watches capable of such precision are being
turned out daily by the thousands. Of course, such perfection would be
absolutely impossible without the use of extremely accurate machine
tools. It would have been impossible as long as we had to depend upon a
watchmaker to make a watch by hand.
THE PACEMAKER OF A WATCH
If we look at the works of a clock the most conspicuous feature is
the rapidly oscillating balance wheel which, by the way, is the most
important part of the watch, for it governs the release of the power
stored up in the spring.
It controls the escapement which brings the whole mechanism of the
clock to a standstill five times each second--in fact it is the
pacemaker of the watch, for it gives the watch a step-by-step movement
and fixes the rate at which the steps are taken.
[Illustration: FIG. 27.--ESCAPEMENT OF A WATCH]
The last wheel of the watch train is what is known as an escape wheel.
It is formed with teeth of an odd shape, such as shown at A in Figure
27. These teeth are engaged by a pair of pallets B and C, carried by
a three-armed lever D. The pallets are usually bits of sapphire or
similar hard stone to prevent wear. The third of the lever is slotted
at its extremity to engage a sapphire pin E, carried by a disk F,
which is mounted on the staff of the balance wheel. The escape wheel
A revolves in the direction of the arrow, being impelled by the
mainspring acting through the train of gears. One of the teeth of this
wheel engages the pallet B, causing the lever D to swing on its axis
and push the sapphire pin E toward the left, thereby giving the disk
F an impulse in the same direction. Here a delicate coil spring, known
as the hairspring, comes into play. Without the hairspring the parts
would stand still, the escape wheel being blocked by the pallet B. The
hairspring is attached at one end to the shaft or staff of the disk
F and the other to the frame of the watch. It tries to hold the disk
F in a fixed position, but is disturbed by the action of the escape
wheel and is constantly oscillating the disk in its effort to bring
it back to its normal position. When the disk swings over to the left
the pallet B is clear of the teeth of the escape wheel. This releases
the escape wheel and it springs forward in the direction of the arrow,
but before it can move through an interval of one tooth it is arrested
by the second pallet C, which has been projected into its path by
the swing of the lever D. The lever swings back until the pallet C
clears the escape wheel and the pallet B engages the next tooth. And
so the action continues, the lever swinging back and forth and at each
complete oscillation releasing one tooth of the escape wheel.
The hairspring takes up the shock of this intermittent motion and a
balance wheel carried by the staff to which F is fastened steadies
the oscillatory motion of the lever D. A watch is full of microscopic
parts. In a small timepiece there are machine-made screws so small
that without the aid of a magnifying glass one cannot see the screw
threads cut upon them. But the most marvelous part of the whole watch
is the delicate hairspring and the means of adjusting its tension
and compensating for its expansion and contraction with changes of
temperature.
INANIMATE MATTER IN CONTINUAL MOTION
When working with minute intervals of time many factors must be
considered which are not even thought of in machines of grosser
proportions. It never occurs to the man in the street that not only the
animate world but the inanimate as well is in ceaseless and variable
motion. If our eyes were capable of taking in minute microscopic
details, we should see that everything is expanding or contracting,
swelling or shriveling, twisting and warping in response to the
atmospheric changes. Our steel bridges and skyscrapers are in constant
motion; solid concrete dams must be provided with expansion joints; the
Washington Monument goes through a diurnal gyration in response to the
sun’s rays. Of course all this motion is almost immeasurably small. A
bar of steel a mile long will expand ⅖ of an inch for every increase
of a degree Fahrenheit in temperature. The expansion of a hairspring,
which may be nine or ten inches long, is infinitesimally small and yet
this must be considered by the watchmaker. We must remember that the
escapement mechanism divides the day into 432,000 parts, each of which
contains some minute error, for absolute perfection is impossible,
and if we add up all these 432,000 errors they must not foot up to
more than a second per day. If the hairspring expands ever so slightly
its power is weakened, but this loss of power is compensated by an
ingenious form of balance wheel. The rim is in two parts, half of it
being attached to one spoke of the wheel and the other half to the
other, as shown in Figure 28. Each half rim is formed of two strips
of metal, an inner strip of steel, and an outer strip of brass fused
together. Brass expands and contracts almost twice as much as steel,
and hence when there is a rise of temperature the rim sections tend
to curl in, bringing their center of gravity nearer the center of the
wheel and making less of a load for the weakened hairspring to move,
while on the other hand, when the spring is contracted by cold, the
rims spread out slightly, giving it a greater load to oscillate. The
weight of the balance wheel is thus automatically adjusted against
variations in power of the spring.
[Illustration: FIG. 28.--THE SELF-CORRECTING BALANCE WHEEL OF A WATCH]
Aside from this automatic variable adjustment, the balance wheel must
have a primary permanent adjustment. The rim of the wheel is loaded
by means of small screws. Screws placed near the free ends of the
rim sections will have a greater inward or outward play as the rim
contracts and expands, and by their locations in different positions
on the rims the balance wheel may be adjusted with great accuracy to
compensate for temperature variation in the hairspring.
THE PENDULUM ESCAPEMENT
[Illustration: FIG. 29.--PENDULUM ESCAPEMENT]
In the case of clocks such delicacy of adjustment is hardly necessary.
Pendulum clocks, if they are to run accurately, must have their
pendulums automatically adjustable for variations in temperature,
because the longer the pendulum the more slowly it oscillates. Many
years ago a grid type of pendulum was invented in which various alloys
were used, which reacted one against the other and preserved the center
of gravity always at the same distance from the center of oscillation.
The action of a pendulum movement is similar to that of a balance-wheel
movement described above. The pendulum operates a pallet lever similar
to that shown in Figure 29. The teeth of an escape wheel, which are
commonly somewhat different from those of the watch movement, strike
the pallets of the pallet lever, and the escape wheel is intermittently
stopped and permitted to proceed. The pressure of the escapement teeth
against the pallets is just enough to keep the pendulum swinging and
the speed of the clock is regulated by lengthening or shortening the
pendulum.
We have added little to clock or watch movements in recent years. About
the only conspicuous modern invention is the torsion pendulum. The
pendulum in this case is a heavy horizontal disk suspended by a wire,
and the disk rotates first in one direction and then in the other,
twisting and untwisting the wire. The advantage of this pendulum is
that the oscillations are very slow, and hence it is possible to keep
the clock going for a year at a single winding.
The electric regulation of clocks is another important improvement. A
clock need not be a very perfect timepiece, but if provided with an
electric regulator its hands are brought up to the correct time every
hour in response to an impulse sent from an accurate master clock which
in turn has its time corrected daily from the National Observatory in
Washington.
CHAPTER V
PUTTING RIVERS TO WORK
FLOWING water exerts a strange fascination upon mankind, even to the
present day. Tourists travel hundreds of miles to view the glorious
spectacle of a riotous tumbling cataract. Is it strange, then, that in
the olden times, when the world was peopled with gods and genii and
strange spirits, the ancients looked upon ceaselessly flowing rivers as
the symbol of life? It was most natural for them to covet the endless
power of a river and eventually, despite their superstitions, to try to
utilize some of its energy.
It may be that sailboats antedate the first water wheel, but it seems
much more probable that flowing water was the first inanimate power
harnessed by man. Windmills were certainly a later development. They
possessed the advantage that they could be located anywhere while the
water mill had of necessity to be built along the bank of a stream.
However, the power of the wind is so unreliable and fluctuates so
widely that it was little used, except in flat countries, where there
was little if any available water power.
Water power predominated until the steam engine was introduced, when
it had to give way to an even more reliable power and one which could
be located at any place to which fuel could be transported. Now,
however, we are going back to our first power, seeking it out in the
most inaccessible mountainous regions, because we have discovered the
means of taking the power it yields and transmitting it hundreds of
miles, over hills and plains to the point where we can put it to useful
service. Hydroelectric power has very aptly been termed “white coal.”
The first prime motor was the current wheel, that is, a wheel fitted
with paddles, which was journaled over a stream with the paddles
projecting into the water. This was a very inefficient machine; it
converted very little of the energy of a stream into useful mechanical
power. The idea of damming the stream and letting the waters flow
over the dam through a raceway upon a water wheel was a much later
development.
WATER WHEELS
Three types of water wheel which were in universal use before the
advent of the steam engine were the undershot wheel, the overshot
wheel, and the breast wheel (Figure 30). In the undershot wheel the
water stored back of a dam is let out near the bottom of the dam and
strikes the under side of the wheel, so that the top of the wheel turns
toward the dam. In the overshot wheel, the water flows over the wheel
striking the paddles or buckets on top and on the forward side, so that
the wheel turns forward. In the breast wheel, the water strikes the
paddles half way up the wheel on the rear side and drives the wheel in
the same direction as that of the undershot wheel.
When we speak of water power we are apt to think of the water as
actually furnishing the energy. As a matter of fact, it is not water
but gravity that drives the wheel, the water being merely the medium
that gravity acts upon. By having the water drop from a great height,
its velocity is greatly increased and the power it imparts to the wheel
is much higher. In mountainous regions it is easy to obtain a high head
of water and thus generate a great deal of power from a relatively
small stream. However, the ancient type of wheel with its paddles or
buckets has now practically passed out of existence, being superseded
by the Pelton wheel for high heads and the turbine for low heads of
water.
[Illustration: FIG. 30.--UNDERSHOT, OVERSHOT, AND BREAST WHEELS]
INVENTION OF THE PELTON WHEEL
[Illustration: FIG. 31.--SECTIONAL VIEW OF A PAIR OF PELTON WHEEL
BUCKETS SHOWING HOW THE WATER JET IS DIVIDED AND FOLLOWS THE CONTOUR OF
THE BUCKETS]
In California, a number of years ago, they made use of what was known
as the hurdy-gurdy wheel. This consisted of an ordinary wheel with
bucket-shaped paddles against which was directed a stream of water
at high velocity through a nozzle. There was a carpenter, named L.
A. Pelton, who used to make a business of building and repairing
such wheels and the flumes that carried the water to them. Although
uneducated, he was possessed of considerable native ingenuity and was a
very observant man. One day, when he was called in to repair a wheel,
he noticed that one of the buckets which had been misplaced received
the water from the nozzle without any splashing. The water struck the
edge of the bucket with practically no shock, whereas the other buckets
produced a great deal of splashing. Pelton had enough knowledge of
the principles of mechanics to realize that a splash means a waste of
energy, and that here was a bucket which, although out of plumb and
apparently defective, was really more efficient than any of the others
in the wheel. It occurred to him then that instead of having the jet of
water strike the middle of the buckets it ought to strike the edge, so
that all its power would be absorbed without any wasteful splashing. He
might have displaced the jet laterally so as to accomplish this result,
but he realized that that would have produced a considerable side
thrust on the wheel, which, of course, would have been objectionable,
and so he hit upon the plan of using double buckets and letting the
stream of water strike the pair of buckets along their dividing line.
(See Figure 31.) This would split the stream in two and let each half
strike the slanting face of the bucket, and follow the surface around
in the same way that it did on the single misplaced bucket, but the
reaction or side thrust on one bucket would be counteracted by that
on the other. This idea proved successful and out of it has grown
the Pelton wheel which is now universally used in all power plants
employing high heads of water.
A 4,000-FOOT HEAD OF WATER
A notable illustration of such a plant is the great installation at
Big Creek, Cal. Big Creek, despite its name, used to be a small stream
flowing down the mountains into a canyon. One would hardly suppose that
it was capable of yielding much power, but it had its source high up in
the Sierras and was fed mainly by melting snows. In the springtime, it
swelled to a good-sized torrent. By building three dams near the top of
the mountain, a lake was formed in which the water of the melting snow
was impounded, so that a steady stream of water could be supplied the
year round for power purposes. But even so, the stream hardly amounts
to very much if we consider only the quantity of water that passes
through it. The particular advantage of this installation is the fact
that in a distance of six miles from the dam the creek falls 4,000 feet.
An inhabitant of the Eastern States who is unused to mountain
heights may gain some conception of the meaning of this elevation by
gazing up to the pinnacle of the Woolworth Tower, which rises 795
feet above street level, then mentally multiplying its altitude by
five. Evidently even a small stream of water dropping from such an
elevation would develop an enormous amount of power. In fact, it was
considered inexpedient to use the entire fall at a single drop and so
it was divided into two stages. The water is carried through a tunnel
three-quarters of a mile long and then through a flow pipe along the
face of the mountain to a point where it may drop 2,000 feet to the
first power plant. After passing through this plant the water is
discharged into the creek and is then diverted into a second tunnel
four miles long and a series of steel conduits to a point from which it
may drop 2,000 feet more to the second power plant. In each power house
there are two electric generators, each fitted with a pair of Pelton
wheels. These wheels are a little less than eight feet in diameter and
each one develops 23,000 horsepower.
The water is directed into the buckets of the Pelton wheel in a stream
six inches in diameter, and it issues from the nozzle with a velocity
of 300 feet per second or about 210 miles per hour. A jet of water is
almost like a solid bar of wood. In fact, it is impossible to chop
through it with an ax. The water would swing the ax out of one’s hand
before it got part way through the jet. Traveling at such a high speed
the friction is so great that it would tear the skin off one’s hands,
if it did not actually tear the hand off the arm, and yet it strikes
the buckets of the wheels with no shock at all, for the first part of
the bucket it touches is nearly parallel to the jet, and as the water
sweeps around the curved face of the bucket it loses practically all of
its pressure and velocity and falls into the tail race. The electric
power generated by the two plants is stepped up to 150,000 volts and
sent out over transmission lines to points of service. The street cars
of Los Angeles are connected by a 240-mile electric harness to the
hydraulic horses of Big Creek.
Powerful as this stream is, a still higher head is used in Switzerland,
at Lake Fully, where there is a drop of over a mile in a distance of
2.8 miles. The water is carried by a short tunnel through the mountain,
and then makes a drop of over 5,000 feet to the power plant, where it
strikes the Pelton wheels at a velocity of 400 miles per hour, or about
seven times the speed of a fast express train.
HARNESSING THE MISSISSIPPI
In contrast to such high heads, we have the low-head power plants
which are employed where a large volume of water is available. The
most notable installation of this type, and the largest in the world,
is that at Keokuk, Iowa, where a dam has been thrown across the
Mississippi River. For many years it was thought impossible to make
any use of the vast volume of water that flows through this great
river. But above Keokuk there used to be a rapid extending back about
twelve miles. By building a dam across the river just below the rapid
it was possible to obtain a working head of about thirty-two feet, and
with the enormous volume of water available this provided sufficient
energy to make the development worth while. In marked contrast to
the installation at Big Creek, it is volume rather than velocity
that is employed, and hence turbines rather than Pelton wheels are
used. More water goes through a single turbine than is used in the
whole of the city of New York with all its elaborate aqueduct system.
Enormous turbines are used, fifteen feet in diameter, and when the
installation is complete there will be thirty units, each yielding
10,000 horsepower, or a total of 300,000 horsepower. A turbine, it may
be explained, differs from the ordinary water wheel in the fact that
the water runs through the wheel instead of around it (Figure 32). The
water may enter at the center and then flow out at the periphery, or
it may enter at the periphery and then be discharged from the center
of the wheel, or it may run axially through the wheel. In a Pelton
wheel there is a single jet which strikes but one pair of buckets at a
time, but in a turbine there are many jets distributed all around the
circumference of the wheel. The water is divided into a series of jets
by being forced through a stationary set of curved vanes. The blades of
the rotor or revolving part of the turbine are oppositely curved. If
the rotor were immovable the jets would have to change their direction
in passing through the rotor, but as the rotor is free to turn, the
jets react against these blades and set the wheel to revolving. The
turbine may be designed to run either on a horizontal axis or on a
vertical one.
[Illustration: FIG. 32.--TURBINE WHEELS; INFLOW TYPE SHOWN ON THE LEFT
AND OUTFLOW TYPE ON THE RIGHT]
The turbines used at the Keokuk plant are of the inflow type. The rotor
is mounted on a vertical shaft in a scroll-shaped concrete chamber,
something like a snail shell. Water pouring into this chamber is thus
given a swirling motion in the direction of rotation of the wheel. As
it flows into the wheel it passes first through a ring of fixed vanes,
which divide it into the jets.
The highest velocity of a wheel is naturally at the periphery and the
advantage of an inflowing turbine such as this is that the water is
traveling at its highest velocity when it strikes the periphery of the
rotor. As it loses its velocity it flows in toward the slower-moving
portions of the rotor. Finally it reaches the center, after giving up
practically all its energy, and falls into the tail pool through a
draft tube at the center of the rotor.
The scroll chambers at Keokuk are thirty-nine feet in diameter and the
draft tubes are eighteen feet in diameter. Water enters the scroll
chambers with a velocity of fourteen feet per second and comes out of
the draft tubes into the tail pool with its velocity cut down to but
four feet per second. Compare this with the velocity of the water jets
at Big Creek!
The current generated at Keokuk goes to St. Louis and surrounding towns
and serves a population of 1,120,000.
Now that we have learned how to transmit electrical power without
serious loss over enormous distances, it is only a question of time
before all the water power in the world is harnessed and put to the
service of man. The power costs nothing after once the plant has been
built; the only expense is that of maintaining the machinery and
keeping it in repair. It is estimated that there is some 200,000,000
horsepower available in this country, but this includes all flowing
water, much of which it would be impracticable, if not almost
impossible, to utilize. However, there is about 60,000,000 horsepower
commercially available, according to the figures of the U. S.
Geological Survey, of which we have developed so far only 6,000,000
horsepower.
The ancients used flowing streams not so much for power purposes as
to lift water to a higher level so that it would flow into their
irrigating ditches. Nowadays, electricity, steam, or air is used for
elevating water, but we have a very ingenious machine which makes
the stream lift a part of itself. This machine is very different in
principle from the old Egyptian noria. It depends upon the kinetic
energy of water in motion. You cannot push a nail into a piece of wood
with a hammer but you can easily drive it in by striking it with the
hammer. As the hammer is swung it acquires what we term kinetic energy
or energy of motion.
SETTING KITCHEN FAUCETS TO WORK
It is not generally realized that water in motion also acquires kinetic
energy. Whenever a faucet is turned off very quickly, there is a
hammering sound which is due to the fact that the moving water in the
water pipe is brought to an abrupt stop. This puts a severe strain on
the piping. A great deal of trouble was experienced from this source in
the early days of plumbing. At a hospital in Bristol, England, there
was a lead pipe leading from a cistern in one of the upper stories
to the kitchen. Every time the faucets were turned off abruptly the
momentum of the water caused the lead pipe to expand, and every now and
then the pipe was burst. In order to relieve the situation, a plumber
connected a pipe to the faucet and carried it up the side of the
building to the level of the cistern. His idea was that whenever the
water was turned off suddenly it would have a vent leading up to the
level of the water reservoir. Much to his surprise, the water issued
from the pipe in a jet of considerable height. To prevent the escape
of the water, he extended the pipe considerably, and still a jet of
water would issue from it. Eventually the relief pipe was carried up
twice the height of the cistern and even then the water would squirt
out occasionally when the faucets in the kitchen were turned off very
suddenly. Then the idea was conceived of placing a reservoir on one of
the upper floors of the hospital and letting the jet of water fill this
reservoir. Every time the faucet was operated in the kitchen a certain
amount of water flowed into the new cistern, and in this way it was
kept supplied with enough water to furnish that which was required for
the upper floors of the hospital.
[Illustration: FIG. 33.--SECTIONAL VIEW OF A WATER RAM]
It is on this principle that the hydraulic ram operates. Water from a
stream is made to flow down a pipe, and as it gains velocity a check
valve suddenly stops the flow which produces enough pressure to force
open a valve in an air chamber and let some of the water enter the
chamber. As soon as the pressure is relieved the check valve opens
and the valve into the chamber closes automatically until a moment
later the stream of water has gained sufficient velocity to repeat the
performance. Thus an intermittent jet of water is forced into the air
chamber and thence through a pipe to a reservoir. The height to which
the water will rise depends entirely upon the velocity of the water
flowing through the system. The air chamber is necessary to cushion
the action of the hydraulic ram and provide a fairly steady pressure
upon the water that flows up through the vent pipe. The check valve is
entirely automatic. It is held open against the pressure of the water
by a spring or a weight, but when the water is in motion is dragged
shut, only to spring open again when the pressure is reduced by the
escape of the water into the air chamber.
[Illustration: FIG. 34.--THE GYRATING WATER METER]
There is a very ingenious water-driven motor which is employed merely
to record the amount of water flowing through it. This is the Thomson
water meter which is illustrated in Figure 34. It consists of a
circular chamber with inwardly dished or conical top and bottom walls.
In the chamber is a flat disk with a ball and socket bearing. At one
side there is a vertical diaphragm in the chamber which passes through
a slot in the disk. This prevents the disk from revolving, but it
is free to oscillate. It has a motion similar to the gyrations of a
top when it is beginning to lose speed and die down, except that the
disk does not revolve. When the disk is in contact with the bottom
wall of the chamber on one side it contacts with the top wall on the
other so that the chamber is virtually divided into two compartments
by the disk, but by gyrating the disk these compartments are made to
revolve. Water enters at one side and discharges at the other side of
the vertical diaphragm. Now, if the disk is in the position shown in
Figure 34, the water, on entering, bears upon the upper face of the
inclined disk and wedges its way between the disk and the upper wall
of the chamber, making the disk oscillate on its ball center. As the
edge of the disk rises across the face of inlet port the water entering
the chamber bears against the under side of the disk, continuing the
gyratory motion. The water cut off on the upper side of the disk is
carried around to the outlet and discharges, while a fresh supply flows
in on the other side of the vertical partition and at the next half
turn the water in the lower compartment discharges at the outlet side
of the partition, while the compartment is filling on the other side of
the partition. A measured amount of water flows through the chamber at
each gyratory oscillation of the disk. A train of gearing is driven by
the gyrating disk which operates a set of dial pointers and a measure
of the amount of water passing through the meter is indicated.
DIGGING WITH WATER JETS
We have referred to the enormous velocity of the jets used to drive
Pelton wheels. Where high heads of water are obtainable water jets are
used very effectively for excavating purposes, particularly in mining
plants for washing down gold-bearing gravel banks. If water is not
found near such banks expensive canals, flumes, and pipe lines are
constructed and even tunnels are bored to bring the water to the point
where it can be utilized. Some of the giant nozzles spout streams from
2 to 8 inches in diameter with a pressure of from 50 to 200 pounds per
square inch. The powerful streams tear into the gravel banks, washing
them away into sluices in which riffle boxes are placed to catch the
precious metal. The back pressure of these nozzles is very heavy and
the larger ones have to be provided with strong anchorages. Water in
motion resists any change of direction and long levers have to be
provided to permit the miners to guide the nozzles.
The hydraulic jet is also used for general excavating wherever water
power is available. Sometimes it is employed under water when clearing
a channel to level down piles of stones that are too large to be picked
up by a suction dredge. Hollow iron piles are driven into a sandy
bottom by means of hydraulic jets. No hammer is needed. Water is pumped
into the pile and on issuing from the bottom of the pile it carries
sand with it, making a hole into which the pile sinks. Wooden piles
are driven in the same way by loosely attaching a water pipe to them
so that the pipe may be withdrawn when the pile has been driven far
enough. The pile is grooved at the lower end so that the pipe outlet
may be centered at the bottom of the pile.
COMPRESSING AIR WITH WATER
[Illustration: FIG. 35.--HOW THE VENA CONTRACTA RAISES WATER]
A very ingenious apparatus for compressing air was invented in the
earliest years of the iron age to furnish a continuous blast of air for
the Catalan forges. This compressor, known as a “trompe,” can hardly
be termed a machine because it contains no moving parts except water,
which is the motive power, and the air which it traps and compresses.
To understand its operation we must look into a peculiar property of
water flowing out of a reservoir into a pipe or nozzle. There is a
converging motion that tends to contract the jet of water just after it
leaves the pipe. This is known as the _vena contracta_. It produces
a partial vacuum in the pipe. If air ports are opened into the pipe
at this point, air will be sucked in to fill the vacuum and will be
carried out of the pipe by the friction of the water. In Figure 35 a
pipe is shown running from the _vena contracta_ to a water tank below.
The rise of water in this pipe indicates the degree of vacuum produced
by the jet.
[Illustration: FIG. 36.--THE “TROMPE” BY WHICH COMPRESSED AIR WAS
FURNISHED FOR CATALAN FORGES]
This principle is used in the hydraulic-air compressor or “trompe” as
it is called. Water flows out of one reservoir through a pipe into
another reservoir lower down. (See Figure 36.) Air enters the pipe
through ports at the point where the vein of water contracts and is
carried down into the second reservoir. This reservoir is sealed so
that the air is trapped in it. The water passes out through a pipe
which is carried high enough to keep a certain pressure of air in the
reservoir and prevent it from blowing out.
[Illustration: FIG. 37.--DIAGRAMMATIC SECTION OF A LARGE HYDRAULIC AIR
COMPRESSOR IN MICHIGAN]
There is a compressor of this type, constructed on an enormous scale,
in the northern part of Michigan. A sketch of the compressor is given
in Figure 37. The air reservoir in this case is a huge underground
rock-walled chamber nearly 350 feet below the surface, 8 feet wide, 26
feet high, and about 280 feet long. There are three intake pipes, 5
feet in diameter, each filled with an annular funnel-shaped head, which
sucks air into the water and carries it down into the chamber. At the
bottom of each intake pipe there is a concrete block with a conical
top projecting up into the pipe. The column of water flowing down the
pipe is spread out into an annular stream by the conical block and the
bubbles of air escape into the chamber. The water outlet of the chamber
is an inclined shaft which leads up about 270 feet to the surface of
the ground where it discharges into the tail race. The water is forced
up this inclined shaft by the pressure of the air trapped in the
chamber. The mouth of the shaft is, of course, below the level of the
water in the chamber so that there is no chance for the air to escape
unless the pressure becomes excessively high, when it will force the
water level below the mouth of the shaft and blow out. The discharge
sometimes forms a geyser 700 feet high. The air will continue to blow
until the pressure is reduced enough for the water level to rise and
cut off access of the air to the mouth of the shaft. Under normal
conditions there is a fall of 343 feet from the water level at the top
of the intake pipe to the water level in the chamber, and a vertical
rise of 271 feet from the water level in the chamber to the tail-water
level. The difference, or 72 feet, represents the working head. With
all three intakes operating, a total of 5,000 horsepower is developed.
Each intake delivers 11,930 cubic feet of air per minute at a pressure
of 128 pounds. Air enters the intake heads through tubes ⅜ inch in
diameter and there are 1,800 of them to each head. The air is employed
to operate machinery and tools in an adjacent mine.
One advantage of this type of air compressor is that it cools the air
while compressing it. This was hardly an advantage in the Catalan
forges, but when the air is used to drive machinery it is important
that it be precooled. When air is compressed by mechanical means
a great deal of heat is generated and the machines must be water
jacketed to extract this heat, but in the hydraulic compressor the
air bubbles are compressed as they pass down with the water to the
reservoir and the water absorbs the heat, delivering cool compressed
air at the bottom of the intake pipe.
CHAPTER VI
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.
CHAPTER VII
AIR _VS._ WATER
THE STEAM engine had its origin in a kitchen; the modern clock was
first conceived in a cathedral; our laws of motion were discovered
under an apple tree, but, strangest of all, pneumatic engineering had
its beginning in a _barber shop_. In each case it was an inquisitive
boy who played the leading rôle.
We do not hear much about the youthful father of pneumatic engineering,
possibly because he lived so very long ago, but his story is fully
as interesting as that of Watt or Galileo or Newton. Young Ctesibius
dwelt in the city of Alexandria, Egypt, 250 years before the birth of
Christ. His father kept a barber shop, and the young lad used to watch
his father practice his tonsorial art on the Greek and Egyptian dandies
of the time. No doubt Ctesibius’s father expected to make a first-rate
barber of his son, but history does not tell us much about his early
life.
The barbers of those days had their mirrors as do the barbers of the
present time. But a mirror in those days was a treasured possession.
It consisted of a brightly polished plate of metal in which a person
could see himself “darkly” or it might have been a plate of glass with
a black backing. These mirrors had to be carefully preserved from
injury and from moisture, and so, instead of having them mounted on the
wall as in a modern barber shop, they were stowed carefully away and
brought out only after the tonsorial artist had completed his operation
and was ready to exhibit his finished product to the customer.
THE BOY WHO DISCOVERED COMPRESSED AIR
Now in the tonsorial parlors of Ctesibius’s father the precious mirror
was suspended above the head of the customer and when the barber had
finished the shave or haircut he pulled down the mirror and let the
customer survey his remodeled countenance. After the customer viewed
himself to his heart’s content he merely released the mirror and it
was automatically drawn back into place by a counterweight that slid
in a case fastened to the wall. Young Ctesibius noticed that every
time the mirror was released a curious whistling sound came from
the case in which the counterweight moved. No doubt this phenomenon
had been recurring day after day for years, but it excited no more
curiosity than would a squeaky door, until, one day, the barber’s son
happened to notice it. Immediately he was all curiosity. No one knew
anything about air in those days and the boy could get no satisfactory
explanation of the phenomenon, so he started an investigation of his
own. He found that the counterweight fitted very closely in its case
and when it dropped it forced a stream of “wind” out of a crevice which
produced a whistling sound. That set Master Ctesibius to thinking. He
had discovered a method of harnessing the wind. No more barber shop for
him; he was launched upon a career of scientific research and invention.
The first use he made of his discovery was to build a pipe organ which
was driven by a water wheel and hence was known as a water organ. But
there were other and more lucrative fields for the newly discovered
power. Being invisible, compressed air proved a most valuable medium
for performing seemingly miraculous tricks with which the corrupt
priesthood of that time hoodwinked the public. Naturally such
air-controlled apparatus was kept secret, and we have only a meager
record of a few of the ingenious devices used in the temples. However,
Ctesibius made many scientific discoveries and became the most famous
mathematician and scientist of his age. We have read in Chapter IV
of his famous water clock adjusted to record hours of varying length
according to the season.
While Ctesibius is credited with being the first man to make scientific
use of compressed air, the use of free air or wind as a power far
antedates him. Sailing vessels were probably in use long before
windmills were invented. As pointed out elsewhere, windmills possessed
an advantage over water wheels in that they could be placed anywhere,
instead of being confined to the banks of streams. They were widely
used for pumping water, grinding grain, and sawing wood until the steam
engine arrived and displaced them.
Although wind power is but little used to-day, air is much more widely
used in modern machinery than is generally realized. A catalogue of the
various devices in which it plays a prominent part is likely to prove
astonishing. For instance, we employ it to bore through rock, to stop
the speeding express train, and even to quell ocean billows. Being
elastic and compressible, it makes an ideal spring for the storage of
power and a cushion for absorbing shocks. It is also used to fight
back water and permit men to work in the depths of the sea or in
water-bearing sand and earth.
Despite his extensive use of air Ctesibius knew nothing of the
atmospheric pressure. It was not until the seventeenth century that
atmospheric pressure was discovered and measured. It was then learned
that there is no such thing as suction.
RAISING WATER WITH ATMOSPHERIC PRESSURE
[Illustration: FIG. 40.--SECTIONAL VIEW OF A SUCTION PUMP]
A suction pump should be called an atmospheric pump. The operation
of such a pump is illustrated in Figure 40. A piston A slides in a
cylinder B. There is an opening in the bottom of the cylinder which
is normally closed by a clack valve C that opens upwardly. There is
a similar valve D in the piston A. When the piston is pushed down it
compresses the air in the cylinder, closing the valve C while valve
D opens, permitting the air to escape. Then on the upward stroke the
valve D closes and valve C opens permitting air to flow into the
cylinder. In this way the air in the pipe and cylinder are exhausted
after a few strokes. As the air is rarefied and the pressure is reduced
above the column of water in the pipe the greater pressure of the water
outside the pipe forces the column to rise until it eventually flows
through the valve in the piston and out of the spout of the pump.
The maximum height to which water can be raised by a suction pump
is thirty-four feet but in actual practice the lift seldom exceeds
twenty-six to twenty-eight feet because the fit between the cylinder
and piston is not perfect.
[Illustration: FIG. 41.--CONVENTIONALIZED SECTIONAL VIEW OF A FORCE
PUMP]
When water is raised to a greater height a force pump is used. Such a
pump is shown in Figure 41. The pump cylinder A has two valves B and C.
The valve B opens upwardly and valve C opens downwardly. The piston D
is a solid member containing no valve. When the piston rises the valve
C closes and valve B opens, letting water into the cylinder. On the
reverse stroke the valve B closes and the descending piston forces the
water out through valve C into pipe E. The height to which the water
may be lifted depends upon the power applied to the piston and not upon
atmospheric pressure. In a fire pump two force pumps are used which
alternately deliver water into an air chamber. The air is compressed
by the water that enters the chamber and exerts a steady pressure on
the water, forcing it out through the fire hose in a stream that is
practically steady.
It is a decided disadvantage to install a force pump in a deep well
because of the mechanical difficulty of operating the piston at a great
depth. Often the suction and force pump principles are combined. The
force pump piston and cylinder are located twenty-six and twenty-eight
feet above the level of the water in the well with a pipe leading down
into the water. The water is forced up into the cylinder by exhausting
the air in the cylinder and pipe and then is forced out of the cylinder
and up a pipe to the top of the well. This also has its disadvantages.
In deep wells the pump piston must still be located and operated at
considerable depth.
THE AIR LIFT
A much more simple pump for deep wells is the air lift in which there
are no valves and no piston. The pump consists of two pipes that are
let down into the water. (See Figure 42.) The larger pipe is open at
the top and bottom, and the smaller pipe discharges air into the bottom
of the larger one. This air being directed upward has a lifting effect,
and mingling with the water in the pipe produces a sort of froth which
is so much lighter than solid water that the atmospheric pressure on
the water in the well is sufficient to raise the froth. The water is
thoroughly aerated and to a large extent purified by this system of
pumping, so that the air lift commends itself particularly for city
waterworks. Sometimes a series of air lifts are used which lift water
by successive stages to a greater height than would be possible with a
single direct lift.
[Illustration: FIG. 42.--PUMPING WATER WITH COMPRESSED AIR]
EXCAVATING THROUGH QUICKSAND
[Illustration: FIG. 43.--SECTIONAL VIEW OF A PNEUMATIC CAISSON]
Centuries ago it was realized that it would be possible for men to
descend into the open sea if they were protected by a bell-shaped
chamber, for the air trapped in the chamber would furnish them with
oxygen requisite for breathing purposes, and would prevent the water
from drowning them out. This same idea of a diving bell is used on
land when sinking a shaft through quicksand or water-bearing strata. A
large box or caisson is used. (See Figure 43.) This box may be either
cylindrical or rectangular, and it is open at the top and bottom. The
lower edges of the box are shod with steel and form cutting edges that
will sink into the soil that is being excavated. At a height of about
seven feet above the bottom of the box there is a transverse diaphragm
known as the deck, and the space below is known as the working
chamber. This deck is very strongly constructed, as it has to support
the weight of the concrete shaft that is built above it. Laborers,
commonly known as “sand hogs,” enter the working chamber and dig out a
shallow pit in the floor of clay or sand. This pit is then extended to
the cutting edges of the caisson. The caisson thus undermined settles
down into the excavation and another pit is started. In this way, step
by step, the caisson is sunk into the ground. In order to overcome the
friction of the caisson against the sides of the excavation and to
insure its sinking, it is heavily weighted. When water-bearing sand is
reached, compressed air is admitted into the working chamber to force
the water out. When the air pressure is greater than the water pressure
it drives the water out of the sand in the working chamber, so that the
men can work in perfectly dry ground, even though the surrounding sand
may be so saturated with water as to form a quicksand.
Of course the farther the excavation proceeds below the water level the
greater the air pressure required, and the caisson would be blown up
out of the ground by this air pressure or would float on the water were
it not for the weights with which it is loaded. When an open shaft is
to be dug, pig iron is loaded on the caisson to force it down, but in
most work the object is merely to sink a concrete column down to rock
and so the caisson is filled with concrete above the deck. Sections
are added to the caisson as it sinks into the ground, and these
sections are filled with concrete. This method of building the column
facilitates the work of laying the concrete, and at the same time
provides the weight necessary to overcome the buoyancy of the caisson
and the skin friction on the side walls of the excavation.
After the caisson has been carried down to rock and a good seat has
been blasted out of solid rock, the working chamber is completely
filled with concrete and the concrete shaft is thus anchored to the
rock.
THE AIR LOCK
Access to the working chamber is had through a central shafting. In
order to hold the air pressure in the working chamber and yet provide
for the entrance of men and materials, an air lock is fitted to the
upper end of the shafting. This air lock, as shown in Figure 43,
consists of a chamber formed with a trapdoor at the top and the bottom.
Both doors open downwardly. To enter the caisson, the bottom door of
the chamber must first be closed and means are provided for doing this
from outside the air lock. The compressed air in the chamber is then
let out through a valve, and when the pressure drops almost to normal
the upper trapdoor falls open of its own weight, giving access to the
chamber. After entering, the upper door is closed while compressed air
is admitted into the chamber; the air pressure then serves to hold the
upper trapdoor closed. The bottom door, in the meantime, has been kept
closed by the air pressure below, which is greater than that above the
door. But after sufficient air has been introduced into the chamber to
equalize the pressure on both sides of the lower trapdoor, it falls
open of its own weight. The occupants of the chamber can then proceed
down a ladder to the working chamber. When leaving the caisson, the
action of the air lock is reversed. The bottom door is pushed up and
held closed for a moment while air is released from the chamber, when
the greater pressure beneath will hold the door closed, and after the
pressure within the air lock has been lowered practically to normal,
the upper door drops of its own weight, permitting the occupants of the
chamber to climb out. The same process must be undergone by buckets
loaded with sand or earth from the excavation and by empty buckets
returning to the working chamber.
More time is required for a man to pass through an air lock than for a
load of sand or any inanimate load. In some of the larger caissons a
separate small air lock is provided just for the use of the workmen.
In the air lock the pressure must be built up slowly so that it will
permeate a man’s whole system. When we realize that the pressure
that men have to support in caisson work may amount to from fifty
to one hundred tons on the whole body, it is difficult at first to
understand why the body is not flattened out like a pancake. It is only
by permitting the system to absorb the pressure so that there is as
much internal pressure as that outside that a man is able to enter a
compressed air chamber without harmful results.
THE CAISSON DISEASE
Compression is rather annoying to a man who has not experienced
the sensation, but the principal danger comes in decompression,
particularly after a person has been in the working chamber for a
long time. When breathing compressed air much larger volumes of
oxygen are taken into the lungs at each breath than in the ordinary
atmosphere, and one feels decidedly exhilarated by this unusual supply
of stimulating oxygen. But with the oxygen large volumes of nitrogen
are taken into the system as well. The nitrogen permeates the blood
while the oxygen is consumed and exhaled in the form of carbon dioxide.
The longer a man is exposed to the pressure the more nitrogen does he
absorb. When the pressure is released suddenly the nitrogen begins
to froth, just as a bottle of soda water does when the stopper is
removed. The nitrogen bubbles in the small veins stop the circulation
and produce the dreaded “caisson disease” which makes itself felt in
the form of severe cramps and excruciating pains. Not infrequently this
disease ends in death. By reducing the pressure gradually the nitrogen
is enabled to pass off completely without bubbling or frothing, just
as it is possible to let out the gas from a soda bottle without
frothing by permitting it to escape through a pin hole in the stopper.
The surest cure for a victim of the caisson disease is to put him
immediately into what is known as a “hospital tank” and build up the
pressure in the tank equal to that which he has just been subjected to
in the caisson, after which the air in the tank is let out so slowly
that there is plenty of time for the nitrogen to pass off without
forming bubbles.
A pressure of forty-five pounds per square inch above that of the
atmosphere is considered a severe pressure for excavation work.
But work has been carried on in pressures up to fifty-two pounds,
corresponding to a depth of 120 feet below water level.
Now that the cause of the caisson disease is understood formulas have
been worked out to insure proper decompression. On one occasion a diver
descended 306 feet into the ocean where the water pressure was 133
pounds on every square inch of his body. This meant that the air which
was pumped down to him had to be compressed to the same pressure. The
diver actually remained at the bottom only a very short time, but it
took two hours and three-quarters to bring him to the surface. He came
up half the distance very quickly and then had to rest on the bottom
rung of a Jacob’s ladder. The rungs on this ladder were ten feet apart
and he was instructed to rest on each rung a certain specified time.
When he was ten feet below the surface he had to wait three-quarters of
an hour for the last trace of nitrogen absorbed by his blood to pass
off.
BORING TUNNELS THROUGH RIVER BEDS
It is comparatively simple to sink a vertical shaft into water-bearing
soil, but a horizontal shaft involves serious difficulties. The action
of a diving bell is easily illustrated by inverting a tumbler and
pressing it down into a basin of water. The air trapped in the tumbler
will keep the upper part of the glass dry, and by inserting a tube in
the tumbler it is possible to fill the tumbler so full of compressed
air as to drive out all the water. This is virtually what is done in
the caisson; but when excavating a horizontal bore, the caisson must be
turned on its side. Turn the tumbler on its side and it is impossible
to keep the water out of it, no matter how much air we may blow into
it. The reason for this is that the pressure on the open end of the
tumbler is not uniform. At the bottom, where the water is deeper, it
will be greater than at the top. If air is pumped in to equalize the
water pressure at the upper edge of the glass, it will not prevent
water from flowing in at the bottom; and if it be equal to that at the
bottom, the water pressure at the top cannot hold the air in and keep
it from pouring out.
[Illustration: FIG. 44.--SECTIONAL VIEW OF A TUNNEL SHIELD]
Fortunately most of the soil through which a subaqueous tunnel is
driven is not very fluid. It is either sticky, as in clay, or sluggish
enough to prevent the water from flowing in rapidly. If there is enough
cover of silt or earth above the tunnel bore, it will help to hold
the air in the tunnel. When the bore comes very close to the surface
of the bed of the stream that is being tunneled, loads of clay are
dumped along the line of the tunnel to provide the requisite cover. In
tunnel boring a shield is used which is the equivalent of the caisson
in vertical boring. The shield is a cylindrical box with a diaphragm
across it corresponding to the deck of the caisson. (See Figure 44.)
In front of the diaphragm there is a small working chamber which is
protected above by an extension of the shield known as an apron. In
the diaphragm there are a number of doors at different levels, which
may be closed in case of danger. If work is proceeding near the top of
the shield, the upper doors are opened and the pressure is regulated
to equal the water pressure at that level. If the work is carried on
near the bottom of the shield, the upper doors are closed and only the
lower doors are open, and the pressure is increased to equal the water
pressure at that point. Sometimes the material is of such a nature
that the men can safely pass out of the doors into the working chamber
outside, but more often it is possible to work only within a limited
area immediately in front of the doors.
When the material is very soft it is often unnecessary to do any actual
excavation by hand in front of the diaphragm. The shield is merely
pushed forward through the mud or silt and the doors are opened to let
the material flow in through them. Workmen dig out this mud and it
is hauled out of the tunnel. Whenever bowlders are encountered it is
necessary for the men to work outside of the diaphragm to chip away the
rocks with compressed-air drills, or else bore them and blast them with
small charges of dynamite.
In quicksand the material is so fluid that it is unsafe to open the
ordinary doors of the caisson, and they are then provided with shutters
which are raised one at a time, to permit of operating on a very small
section of the head of the tunnel. After enough material has been
excavated from in front of the shield, the latter is pushed forward and
the excavating is renewed.
Unlike caisson work, the weight of the shield is of no assistance in
making it penetrate the soil, nor is it possible to move the entire
lining of the tunnel with the shield. The tunnel is lined with rings
of cast iron which are bolted together, the rings themselves being
made up of heavily ribbed curved plates. The shield is formed with a
“tail” which fits over the end of the tunnel line like a cap. When the
shield is to be pushed forward, a set of hydraulic jacks are fitted
between the end of the tunnel lining and the diaphragm of the shield,
and by means of these the shield is given a shove forward far enough
for a new section of the cast-iron lining to be added within the tail.
The tunnel-lining rings are usually sixteen inches wide so that it is
customary to move the shield ahead sixteen inches at a time or just far
enough for a new ring of lining to be installed. Of course, the tunnel
is fitted with air locks by which men can enter the working section
without permitting the compressed air therein to escape, and these
locks are just like those used in sinking a caisson, except that they
are horizontal instead of vertical.
QUELLING OCEAN BILLOWS WITH AIR
Air is also used in another and very novel way to battle against water.
In this case it is not quiet water pressure, but the tremendous power
of ocean storms that is combatted.
The influence of a shoal upon ocean waves has often been observed.
A sand barrier even when submerged to a depth of twenty or thirty
feet will break the waves of a heavy storm and leave an area of
comparatively quiet water behind it. The reason for this is that the
water in the waves does not travel with the waves, but undergoes
local oscillatory motion. This motion is in the form of circular or
elliptical currents which travel in a vertical plane. When a sand bar
is encountered it interferes with these local currents and breaks up
the waves.
Knowing this to be the case, it occurred to Mr. Philip Brasher that
some other means of disturbing the rhythmic movement of the water
might be found, and he determined to try the experiment of using
compressed air for this purpose. Accordingly he laid a perforated pipe
under water and connected it with an air compressor. Then when a storm
arose air was pumped into the pipe and it rose in bubbles through
the water. The effect of this upward movement of air was instantly
observable. It upset the rhythmic water currents and the waves which
struck this wall of air bubbles curled and broke as if they had
encountered a shoal of sand.
Several exposed piers on the Pacific coast have had a pneumatic
breakwater built around them so as to protect them or ships lying
alongside from being pounded by the waves. There is no expense attached
to the breakwater except in time of storm when the air pumps must be
kept going. One important advantage of the breakwater is that it does
not block navigation. A ship can sail right over the wall of bubbles
and find refuge behind it.
CHAPTER VIII
AIR SPRINGS AND CUSHIONS
UNLIKE water, air is a highly compressible, elastic fluid, and as such
furnishes an excellent medium for the storage of energy. It acts just
like a clock spring into which energy may be introduced and stored
by winding or compressing the spring. The energy remains locked up
in the spring and when the spring is released, it gives back just
as much energy as was put into it, except for slight frictional and
heat losses. Air is a much better spring than steel or any other
metal because it never loses its elasticity from fatigue and because
it has an enormous capacity for the storage of energy. It possesses
one serious drawback, however. Much of the energy that is expended
in compressing it is converted into heat. If the air were to be used
immediately and without transmitting it to a distance, there would be
no advantage in extracting the heat, but heat cannot be stored in air
for long and it would gradually escape from the storage reservoir or
air receiver and from the pipes leading the air to the machines that it
was to operate. As the heat escaped it would lower the pressure of the
air and hence much of the energy would be lost.
HEAT OF COMPRESSION
In Chapter V we described a hydraulic system of compressing air and
noted that one of the advantages of this system is that it delivers
air cooled to the temperature of the water. This, of course, does not
mean that there is no loss due to heat. The air bubbles as they are
compressed are cooled by the water that compresses them; in other
words, the heat of compression passes off into the water, making the
water warmer than it would otherwise be. Heat is not produced without
expenditure of energy and a certain proportion of the water power is
thus wasted.
In most cases air is compressed by steam or electrically driven
compressors, and in such machines the heat due to compression is a
serious matter. The air cylinders are water-jacketed to carry off the
heat. But air is a poor conductor; it acquires heat faster than it can
give it off to the water surrounding the cylinders. In compound air
compressors the air compressed in one cylinder is cooled before being
passed on to the next cylinder, where it is further compressed. The
heat loss in compressing air in a single stage up to 100 pounds gauge
pressure is about 30 per cent. Air that enters a compressor at the
normal pressure of the atmosphere and with an initial temperature of
60 degrees F., if not cooled will become heated to 415 pounds gauge
pressure. The higher the initial heat of the air, the greater the rise
of temperature. If a volume of air be subjected to 294 pounds gauge
pressure, it will occupy about one-tenth of its former volume. If the
air was introduced into the compressor at zero, it would acquire a
temperature of about 650 degrees; if introduced at 60 degrees, it would
show an increase of about 800 degrees; and, if started at 100 degrees,
it would show an increase of 900 degrees in passing through the
compressor. This shows the advantage of compressing the air in stages
and cooling the air between stages.
Sometimes the heat developed is sufficient to produce a disastrous
explosion. Air is noncombustible, but the oils used to lubricate the
compressor are vaporized by the heat and when mixed with air form a
powerful explosive. Care has to be taken that none but high-grade
oil with a high flash point be used in the compressor and that the
temperature of compression does not rise to near the flash point lest
the vaporized oil be ignited.
That fire can be produced by sudden compression of air has long been
known in the Philippine Islands. The natives use a small air tube with
a close-fitting plunger. Combustible matter is placed in the bottom of
the tube, and on striking the plunger a sharp blow this is ignited.
When compressed air is used in an air motor it expands and in so doing
absorbs heat. The more rapidly it expands, the more heat it absorbs.
This heat it extracts from the motor and from the atmosphere into which
it escapes, and it is a common occurrence to find a thick coating of
frost around the exhaust port. This is due to condensation of moisture
in the atmosphere or in the compressed air itself, which, because
of the rapid extraction of heat, is converted into snow. On cold
days enough frost may be produced in the exhaust pipe to clog it and
interrupt the operation of the motor, and frost sometimes clogs the air
lines leading to the motor.
[Illustration: WHARF ON THE PACIFIC COAST PROTECTED BY A PNEUMATIC
BREAKWATER
Air issuing from the submerged perforated pipes breaks up the waves]
[Illustration: GENERAL VIEW OF A SUN POWER PLANT IN EGYPT]
[Illustration: CLOSE-UP OF ONE OF THE REFLECTORS SHOWING THE BOILER]
The fact that compressed air on expanding is cooled, makes it an ideal
power for use in mining machinery. Steam is inconvenient because of the
difficulty of transmitting it to the machines without loss of heat,
because it would heat the machines so that they cannot be handled
readily and because the exhaust steam would fill the mine with an
impenetrable fog; electricity is dangerous in mines that are apt to
contain explosive gases, because the wires are liable to be broken and
cause sparks by short circuiting, and because sparks are likely to form
between the brushes and commutators of the motors; but compressed air
has none of these objections, and, furthermore, the discharge from the
machines furnishes the operators with ample supplies of fresh cool air
which drives out disagreeable and dangerous gases.
AIR-DRIVEN HAMMERS
The ordinary rock drill is really a pneumatic hammer. The tool, which
is chisel-shaped, is used as a hammer to pound a hole in the rock. The
tool is driven up and down or in and out by compressed air bearing
alternately on opposite sides of a piston. The elasticity of the
compressed air acts as a cushion to relieve the machine from shock. The
drill is mounted upon a tripod in such a way that it may be operated in
any direction.
The pneumatic riveter is another form of compressed-air hammer.
The machine is held in the hands, and the tool, which is rapidly
reciprocated by air pressure, pounds the red-hot shank of a rivet.
While this is being done an assistant holds a hammer against the head
of the rivet. However, there is a type of riveter which has a U-shaped
frame. One arm of the frame carries the reciprocating tool while the
other reaches around and bears against the head of the rivet.
All sorts of tools have been built in which a small air-driven motor
furnishes the motive power. There are hand drills in which the tool is
revolved by a set of pistons, shears for shearing sheep which operate
somewhat on the principle of hair clippers, and pneumatic chisels used
for chipping stone. Pneumatic motors are widely used in operating
cranes and air hoists. They consist of a simple cylinder and piston,
and are used for short, direct lifts of all sorts.
STOPPING TRAINS WITH AIR
Air has played a most important part in transportation. When the air
brake was first introduced its purpose was to prevent collisions and
provide greater safety of operation. It was not generally realized that
efficient air brakes are not only a safety precaution, but a means of
increasing schedule speeds. The more quickly a train can stop, the
better speed it may make, particularly on a schedule that calls for
frequent stops.
In the first air brake invented by George Westinghouse, the locomotive
was provided with a reservoir in which air was stored and compressed
by means of a steam-operated air pump. The cars were each provided
with a cylinder and piston connected to the air reservoir through a
valve conveniently located in the engineer’s cab. The piston of the
air cylinder was connected with the air brakes so that whenever the
engineer wished to stop his train he merely turned on the compressed
air and all the brakes in the train were operated. This was a great
saving over the previous system of providing brakemen to operate hand
brakes.
Unfortunately, the problem of stopping a train was not as simple as
all this. On a long train the cylinders near the engine were the first
to receive pressure sufficient to operate the brakes, and as a result,
the forward part of the train was retarded more than the rear part at
the start of the brake operation, and the train came to a stop with
a series of jolts. Another serious disadvantage was the fact that
an enormous reservoir was required to furnish all the air necessary
for a long train. But the most serious drawback was the fact that
occasionally a train broke apart and then the engineer was powerless
to control the brakes of the detached portion of the train. If the
accident occurred on a down grade, there would result a collision
between the cars running by their own momentum and gravity and those
attached to the engine. This led to the present system of reversing the
process which consists in keeping air pressure constantly in the train
pipe and setting the brakes by relieving the pressure in the train pipe.
Each car is provided with its own cylinder which furnishes the
necessary air for the operation of its brakes. There is, of course, a
main cylinder on the locomotive in which air is pumped at high pressure
by a steam-operated pump. When the pressure of the main reservoir
falls below a predetermined amount, the air pump starts operating
automatically and continues until the requisite pressure is restored.
Air from the reservoir is fed to the train pipe at a certain pressure
and feeds the local reservoirs on the cars of the train. At each car
it passes through a very ingenious triple valve, which consists of a
cylinder with a double piston, one operated by pressure in the train
pipe, and the other by pressure in the local cylinder. The piston
operated by the pressure of the train pipe is larger than the other and
consequently the valve piston is normally pressed back, uncovering a
small port through which air from the train pipe feeds into the local
reservoir. In this way the air in each local reservoir is maintained
at the same pressure as that of the train pipe. Whenever air is let
out of the train pipe, the piston is pushed out by the excess pressure
on the local reservoir side and uncovers another port which permits
air to flow from the local reservoir to the brake cylinder operating
the brakes. The advantage of this system is that each car is always
supplied with sufficient power to act on the brakes immediately without
having to draw its supply of air from the main reservoir. In case the
train breaks apart, the air in the train pipe escapes and the brakes
are set automatically.
There have been a number of improvements in air brakes aimed to make
the operation of the brakes more uniform and to insure immediate
action, even on a very long train. Modern express trains weighing 920
tons and traveling at a velocity of 60 miles per hour, can be brought
to a standstill in 860 feet, or practically their own length. Automatic
arrangements are provided to insure the application of the brakes
at graduated rate, except, of course, when the emergency brakes are
applied. As the speed of the train slows down the pressure is gradually
relieved, otherwise the train would stop with a severe jolt. The rate
of applying the brakes so as to provide a smooth retardation depends in
large measure upon the weight of the train.
On the New York subways, it is interesting to note, every passenger
who boards a train has his weight recorded by the brake mechanism, and
allowance is automatically made for the inertia that his mass adds to
the train when it is in motion. The weight operates through a system of
levers to control the amount of pressure that is applied to the brakes
so that when the engineer operates the brake lever, he does not have to
consider whether the cars are light or jammed full of passengers; the
brake mechanism itself takes care of this.
PROPELLING CARS WITH AIR
If air can be used to stop a train, why cannot air be used to propel
it? This question occurred to many inventors and they answered it by
building air-propelled cars and locomotives. Pneumatic cars were tried
out on street railways, but they have had to give way to electric
cars which do not need to carry their power around with them, but can
draw it from a central power plant through a trolley wire. In only a
few situations, such as in mines where electric sparking is feared,
are air-propelled cars and locomotives still used to any considerable
extent.
However, air plays another and highly important part in transportation
and here it is employed not to deliver energy stored in it, but to
serve as a cushion. Without the soft, flexible grip of the air-filled
rubber tubes with which automobile wheels are shod, high-speed
motoring would be practically impossible. There is nothing that can
compare with air for absorbing shocks and unevennesses in the road.
Whenever an obstruction is encountered, not only is the shock absorbed
by elasticity of the air, but the impact is immediately distributed
uniformly over the whole tire, so that the strain on the tire is not
localized and the life of the tire is correspondingly increased. There
have been many attempts to introduce substitutes for pneumatic tires,
such as combinations of metallic springs and straps, but these have
failed, chiefly for the reason that they cannot distribute the shock
as the pneumatic tire does. Consequently they are not only liable
to damage, but they do not absorb the obstructions as readily as the
pneumatic tire does and the vehicle which they carry is subjected to
heavy stresses and strains.
Air as a cushion is used to prevent the rebound of the springs of an
automobile and is also widely employed in machinery to absorb the
momentum of moving parts or to slow the action of a spring. In a door
check, for instance, a powerful spring is provided which would slam the
door shut were it not for the cushioning action of air. A plunger is
connected to the door and slides in a cylinder on the door frame. The
air compressed by the plunger can escape only very slowly through a
small port.
AIR CUSHIONS FOR ELEVATORS
Air cushions are used in elevators to prevent too rapid a fall of
the car in case of accident. The lower part of the elevator shaft is
completely inclosed and fitted with steel doors to form an air pocket.
The car fits the shaft closely enough to compress the air under it when
it is moving downward. At normal speeds, this air escapes quite readily
around the sides of the car and through cracks in the doors, but when
the speed is excessive the air cannot escape fast enough and sufficient
pressure is built up to retard the car so that it will strike the
bottom with a moderate impact not at all dangerous to the passengers.
In the Woolworth Building, New York, the highest elevator shafts are
680 feet high and the air pockets are 137 feet deep--i. e., they reach
up to the tenth story. If the car broke away from its supports at the
top of the shaft it would be traveling at the rate of 132 miles an hour
when it struck the air pocket, but before it reached the bottom the
air would bring it practically to a stop. Imagine a heavy automobile
traveling at 132 miles per hour and brought down to a standstill in
half a city block! The passengers would be hurled out of their seats,
but in an elevator the passengers are standing and can brace themselves
against the pressure produced by their own momentum. The velocity
acquired in falling 543 feet to the air pocket must be overcome in 137
feet or ¼ of the distance. This means that the weight of each passenger
is multiplied by four. A man who weighed 150 pounds would find that
his pressure on the floor of the car had mounted to 600 pounds. One of
the inventors of the pneumatic cushion for elevators was killed when
testing out a car, not because of the impact of the car, but because
he was foolish enough to sit in a chair. The chair gave way under his
suddenly acquired weight and the poor man was fatally stabbed by one of
the splinters.
Air jets are used for a variety of purposes ranging from tamping
railway tracks to painting buildings. When a track is depressed under
the tamping action of trains passing over it the ties must be raised
and new ballast introduced under them so as to bring them up to level.
In order to save hand labor and insure the perfect filling of all
cavities a pneumatic tamper is sometimes used. The ballast, consisting
of sand or gravel up to ¾ inch in diameter, is fed out of a hopper and
meets a blast of air that hurls it into place like shot from an air
rifle. This furnishes a very solid and compact road bed.
It is in much the same way that weather-stained stone walls are
cleansed by means of a sand blast. The sand particles projected by the
jet of air act like myriads of tiny bullets which chip away the face
of the stone.
LAYING CEMENT WITH AN AIRGUN
It is after this same fashion that the cement gun is used to project
cement against wire reenforcement to form walls of buildings. Carl
E. Akeley was led to the invention of the cement gun by his efforts
to find an expeditious and economical method of mounting specimens
of large animals for the Field Museum in Chicago. He constructed a
pneumatic device for spraying cement and water upon a canvas-covered
framework, thus building up a body upon which the skin of the animal
could be mounted. This machine was improved and tried out successfully,
on a large scale, in constructing buildings.
The machine as now constructed consists of a hopper into which a
proper mix of sand and cement is introduced. Compressed air blows this
mixture out of a nozzle. Here it meets a jet of water also propelled
by compressed air. The water, sand, and cement combine and strike the
wall or surface to be coated with such an impact as to make a compact
fine-grained coating known to the trade as “gunite.”
Large surfaces can be painted much more readily and more evenly with
compressed air than with a brush. A widely spreading air jet is used
which draws the paint out of a receptacle, breaking it up into minute
droplets that are projected as a mist against the surface, covering it
with a uniform coating.
AIR AS A TENSION SPRING
So far we have been considering air as a compression spring. It may
also be employed in a manner somewhat analogous to a tension spring.
When air is exhausted from a receptacle we have a partial vacuum which
will produce work, but in the opposite direction from that produced by
compressed air. Of course there is no such thing as suction, and it is
the pressure of the atmosphere which is employed to do the work.
We are apt to overlook the fact that a vacuum cleaner depends upon
atmospheric pressure to drag the dust and dirt out of a carpet.
A vacuum cleaner would not work on the moon, where there is no
atmospheric pressure. The apparatus consists of an air pump, usually
driven by an electric motor, which exhausts, or partially exhausts,
the air from a foot piece that is dragged along the carpet. As the air
under atmospheric pressure rushes in through the carpet to take the
place of the air that has been pumped out, it carries with it all loose
particles of dirt. The dirt is filtered out of the air or a mechanical
trap is provided to catch and retain the dirt while the air is allowed
to escape. Large-scale vacuum cleaners are used for cleaning city
streets. The dirt is loosened by revolving brushes and is then drawn up
by atmospheric pressure into a tank.
Another use of atmospheric pressure is to be found in the pneumatic
tubes which convey mail from one post office to another. Air is
exhausted from the tube and when a carrier is inserted which fits the
tube closely the atmospheric pressure behind it forces the carrier
along to its destination. The same system is used to carry money to and
from the cashier’s office in department stores.
In later developments of the pneumatic mail carrier compressed air
was used back of the carrier, but the principle of operation was not
altered. In either case propulsion is effected by the difference of
air pressure on the opposite sides of the car.
In 1869 Alfred E. Beach built an experimental subway line under
Broadway, New York, near Warren Street. The line was only 200 feet
long. A cylindrical car large enough to hold eighteen people, fitted
the bore snugly and was propelled in one direction by compressing the
air in the tunnel and in the other direction by exhausting the air.
CHAPTER IX
POWER FROM HEAT
THE TERM “water power” is a misnomer. There is no power inherent in
water. It is gravity that makes water fall in cataracts or flow down
a river bed, and hence it is the force of gravity that is responsible
for the turning of our turbines and Pelton wheels. Water is merely the
medium through which the force of gravity acts.
In the steam engine, water, again, serves as a medium and it is the
energy of heat that is actually the working force. A closed vessel is
partly filled with water and heat is applied to it. The water grows
hotter and hotter until at a temperature of 212 degrees it begins to
vaporize. The temperature of the water remains stationary until the
space above the water is completely filled with steam at a pressure
equal to that of normal air pressure. The steam gauge then registers
zero. Continued application of heat raises the pressure of the steam
and also raises the temperature of the water. When the steam gauge
shows a pressure of 25 pounds per square inch, the water temperature
is nearly 267 degrees; at 100 pounds it is over 337 degrees; at 200
pounds it is 387 degrees; at 500 pounds it is about 467 degrees. As
steam is drawn off from the boiler a proportionate amount of water
turns immediately into steam to take its place. If the boiler should
burst, the entire mass of water would instantly flash into steam
because its temperature is far above the boiling point at normal air
pressure. That is why the explosion of a steam boiler is so violent. If
the boiler were entirely filled with steam the effect of an explosion
would not begin to be so destructive as if the boiler were half full
of water. It is really the explosion of water that does such serious
damage.
When the steam pressure in a boiler corresponds to the temperature of
the water it is said to be _saturated_. The water in a boiler boils
violently when the steam is drawn off rapidly and tiny droplets of
water are carried off in the steam, producing what is known as _wet
steam_. Steam that carries no water particles in suspension is called
_dry steam_. When the steam is heated above the temperature of the
water by means of an auxiliary heating device or by some peculiar
construction of the boiler, it is called _superheated steam_.
In order to economize fuel it is highly important that as much water
surface be exposed to the heat as possible. This may be done either by
passing the heat in tubes through the water or by passing the water
in tubes through the fire. Locomotive boilers are of the fire-tube
type. The flaming gases of the furnace pass through the boiler through
a series of tubes that lead to the stack. In the water-tube boilers
it is highly important that a good and rapid circulation of water be
maintained, otherwise there might be local generation of steam with
serious consequences.
THE GIFFARD INJECTOR
[Illustration: FIG. 45.--INJECTOR FOR INTRODUCING WATER INTO A STEAM
BOILER]
As steam is used from a boiler the water is slowly exhausted and it
must be replenished with a fresh supply. In early days of the steam
engine, water was pumped in against the boiler pressure by the use
of powerful pumps, but in 1858 a man named Giffard invented a most
ingenious apparatus by which steam of the boiler was used to force
water in directly against its own pressure. This seems like lifting
oneself by one’s boot straps. When the injector was first invented it
seemed so impossible for it to work that engineers would not accept it
until it had repeatedly demonstrated its operativeness. Even after it
was accepted and in common use its mysterious operation was a subject
of discussion for years. A sectional view of a Giffard injector is
shown in Figure 45. Steam from the boiler comes down the tube A and
passes out in a jet from the nozzle B. A needle valve C may be moved
into the nozzle to reduce or shut off the jet of steam. The jet enters
a conical chamber D which has a tube E that runs down into the water
reservoir. The steam jet blows the air out of chamber D, producing a
partial vacuum which draws water up the tube E and into the chamber D.
When the water reaches the steam jet it is driven out of the chamber
across a short open space into a slightly diverging tube or receiving
cone F and through a check valve G into the boiler. At H there is a
glass window through which the action of the water jet as it rushes
into the receiving tube may be watched. I is an overflow pipe leading
back to the reservoir. When the steam flows into the cone B it gathers
momentum and issues from the nozzle in a jet of high velocity. On
striking the water it combines with the water and condenses, but at the
same time it imparts its momentum to the water so that the water is
given more than enough momentum to drive it into the boiler against the
pressure in the boiler.
The pressure of steam is utilized to drive the piston of an engine
or the steam may be set in motion by letting it issue from a nozzle,
when its momentum may be employed to drive a steam turbine in the
same way that a water jet drives a Pelton wheel but in either case it
is heat that does the work. The steam expands as it moves the piston
and as it passes through the nozzle its expansion is accompanied by a
corresponding loss of heat.
As we have already noted, it was for the purpose of raising water that
inanimate powers were first set to work. It was with the same object in
view that steam was first employed.
HERO’S STEAM ENGINE
To be sure the first steam engine was invented by Hero, the disciple
of Ctesibius, in the second century before Christ, and it was not a
water-raising machine. It consisted of a hollow sphere (see Figure
46), mounted to rotate and fed with steam that entered it through the
journals. The steam issued from opposite sides of the sphere through
two bent tubes. The steam in issuing from the tubes reacted against the
tubes, pushing them back and causing the sphere to revolve.
Every action is accompanied by an equal and opposite reaction. A bullet
fired from a gun kicks the gun back. In order to push the bullet out of
the barrel, the powder must have something to push against and the push
against the gun is equal to the push against the bullet. Even if there
were no bullet to be pushed the discharge of the powder would react
against the gun. In order to push its own gases out of the gun, the
powder must push back against the gun.
[Illustration: FIG. 46.--HERO’S STEAM ENGINE]
In Hero’s engine, the steam, in order to push itself out of the sphere,
must push against the bent tubes. We have a similar reaction motor in
the revolving lawn sprinkler. The water issuing from the bent arms of
the sprinkler forces them back and causes them to revolve. Were the
arms radial and not bent the reaction would be there just the same, but
it would be exerted against the center of the wheel and hence there
would be no rotation.
Reaction has nothing to do with the pressure of the atmosphere as
so many people imagine. A gun would kick just as hard and the lawn
sprinkler or Hero’s engine would operate just as well and as fast in
a perfect vacuum. It was to demonstrate the principle of reaction that
Hero built his steam engine. As far as we know it was never put to
useful work and remained merely a scientific toy.
NEWCOMEN’S ATMOSPHERIC ENGINE
The first practical use of steam power was applied to the pumping
of water from the deep mines of Cornwall. Newcomen’s engine, which
antedated Watt’s by several decades, was known as an atmospheric
engine. It consisted of a cylinder fitted with a piston, but the top of
the cylinder was open. The piston was connected to one end of a lever
or walking beam, the other end of which was connected by a chain with
the plunger of a pump situated at the bottom of the mine shaft. Little
power was required to lift the piston because it was counterbalanced by
the chain and plunger attached to the opposite end of the walking beam.
The real work was done in pushing the piston down and thereby raising
the pump plunger. Steam under low pressure was let into the cylinder
under the piston to raise it and then a jet of water was sprayed into
the steam-filled cylinder. This condensed the steam, producing vacuum
and the atmospheric pressure acting on the upper surface of the piston
forced the piston down, raising the pump plunger. The engine ran very
slowly, making only about 15 strokes per minute. Later it was improved,
producing about 30 strokes per minute.
WATT’S STEAM ENGINE
Watt was the first man to build an engine in which the real work was
done by the pressure of steam as in modern steam engines. As long as
the steam engine was used for pumping water the reciprocating piston
could be connected directly to the pump plunger and it was unnecessary
to exert power on both sides of the piston, but in 1782 Watt obtained a
patent on a double-acting engine. Steam was admitted first on one side
of the piston and then on the other. The walking beam was connected
by means of a connecting rod to a crank on a shaft that carried a
flywheel, and the seesaw motion of the beam was converted into rotary
motion of the wheel. This opened up new industrial opportunities for
the steam engine.
The steam was exhausted from the cylinder into a condenser, i.e., a
chamber, in which a spray of water converted the steam into water,
producing a vacuum, thereby relieving the piston of back pressure and
virtually adding that much more power to the steam operating on the
opposite side of the piston. A pump driven by the engine drew the water
and the air liberated from the condensed steam out of the condenser.
Watt invented an ingenious governor to control the flow of steam to
the engine and insure a uniform motion. (S
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