often supplanted by another arrangement known as Joy’s valve gear, which
leaves the crank axle unencumbered with eccentrics, and, as taking up
less space, is generally now preferred for locomotives and also for
marine engines. Its principle is very simple, and will be readily
understood from the diagram in Fig. 10_b_, where _c_ is the spindle of
the slide-valves as in Fig. 5, but capable, we shall now suppose, of a
horizontal movement only. Jointed to it at D is a rod D E attached to a
block at E, which can move only within a slot in the strong bar E F in a
circular segment, the centre of which is at D. The bar we suppose for
the moment to be immovable, and disposed symmetrically to C D. Now let
an alternate up and down motion along the circular segment be given to
block E, and the effect will be to leave the centre, D, unchanged in
position, and, therefore, in that case the valve will not be moved at
all. Now this reciprocating movement is given to the block E by a system
of levers (not here shown), jointed to the connecting-rod (K, Fig. 9) in
such a manner that the rod D E is compelled to follow the movement of
the connecting-rod, but the end E must always travel in the circular
segment. We have hitherto supposed this segmental piece to be fixed, but
the engineer has the power of so turning it as to tilt either the upper
or lower part towards D. If, for instance, the guiding segment is fixed
as at II, the block in rising will push in the valve-spindle, and in
descending draw it out, as the length of the rod D E is invariable. But
if the guides be turned over so as to bring F nearer D than E, the same
movement of the block will give the reverse motions to the
valve-spindle.
[Illustration:
FIG. 10_b_.—_Joy’s Valve Gear._
]
From the great rapidity with which the machinery of the locomotive
moves, the different parts require to be carefully balanced in order to
prevent dangerous oscillations. For example, the centrifugal force of
the massive cranks, etc., is balanced by inserting between the spokes of
the driving wheels certain counterpoises, the weights and positions of
which are finally adjusted by trial. The engine is suspended by chains
and set in motion, and a pencil attached to one corner of the frame
marks on a horizontal card the form of the oscillation, usually by an
oval figure. When the diameter of this figure is reduced to about 1/16
inch, the adjustment is considered complete.
The power of a locomotive, of course, depends on the pressure of the
steam and the size of the cylinder, &c.; but a very much lower limit
than is imposed by these conditions is set to the power of the engine to
draw loads by the adhesion between the driving wheels and the rails. By
the term “adhesion,” which is commonly used in this case, nothing more
is really meant than the friction between surfaces of iron. When the
resistance of the load drawn is greater than this friction, the wheels
turn round and slip on the rails without advancing. The adhesion depends
upon the pressure between the surfaces, and upon their condition. It is
greater in proportion as the weight supported by the driving-wheels is
greater, and when the rails are clean and dry it is equal to from 15 to
20 per cent. of that part of the weight of the engine which rests on the
driving-wheels; but when the rails are moist, or, as it is called,
“greasy,” the tractive power may be only 5 per cent. of the weight;
about one-tenth may be taken as an average. Suppose that 30 tons of the
weight of a locomotive are supported by the driving-wheels, that
locomotive could not be employed to drag a train of which the resistance
would cause a greater pull upon the coupling-links of the tender than
they would be subject to if they were used to suspend a weight of 3
tons. The number of pairs of wheels in a locomotive varies from two to
five; most commonly there are three pairs; and one, two, or all, are
driven by the engine, the wheels being coupled accordingly; very often
two pairs are coupled.
The pressure at which the steam is used in the locomotive is sometimes
very considerable. A pressure equal to 180 lbs. on each square inch of
the surface of the boiler is quite usual. The greater economy obtained
by the employment of high-pressure steam acting expansively in the
cylinder, points to the probability of much higher pressures being
adopted. There is practically no limit but the power of the materials to
resist enormous strains, and there is no reason, in the nature of
things, why steam of even 500 lbs. per square inch should not be
employed, if it were found otherwise desirable. It need hardly be said
that locomotives are invariably constructed of the very best materials,
and with workmanship of the most perfect kind. The boilers are always
tested, by hydraulic pressure, to several times the amount of the
highest pressure the steam is required to have, and great care is
bestowed upon the construction of the safety-valves, so that the steam
may blow off when the due amount of pressure is exceeded. The explosion
of a locomotive is, considering the number of engines in constant use, a
very rare occurrence, and is probably in all cases owing to the sudden
generation of a large quantity of steam, and not to an excessive
pressure produced gradually. Among the causes capable of producing
explosive generation of steam may be mentioned the deposition of a hard
crust of stony matter, derived from the water; this crust allows the
boiler to be over-heated, and if water should then find its way into
contact with the heated metal, a large quantity of steam will be
abruptly generated. Or should the water in the boiler become too low,
parts of the boiler may become so heated that on the admission of fresh
water it would be suddenly converted into steam. When an explosion does
take place, the enormous force of the agent we are dealing with when we
bottle up steam in an iron vessel, is shown by the effects produced.
Fig. 11 is from a photograph taken from an exploded locomotive, where we
may see how the thick plates of iron have been torn like paper, and the
tubes, rods, and levers of the engine twisted in inextricable confusion.
[Illustration:
FIG. 11.—_Locomotive after Explosion._
]
[Illustration:
FIG. 12.—_Hancock’s Steam Omnibus._
]
Locomotive engines for propelling carriages on common roads were
invented many years ago, by Gurney, Anderson, Scott Russell, Hancock,
and others. One designed by Hancock is represented in Fig. 12. Such
engines do not appear to have found much favour, though the idea has
been successfully realized in the traction engines lately introduced.
Probably the application of steam power to the propulsion of vehicles
along common roads fell into neglect on account of the superior
advantages of railways, but the common road locomotive is at present
receiving some attention. In the tramways which are now laid along the
main roads in most large cities we see one-half of the problem solved.
It is not so much mechanical difficulties that stand in the way of this
economical system of locomotion, as the prejudices and interests which
have always to be overcome before the world can profit by new
inventions. The engines can be made noiseless, emitting no visible steam
or smoke, and they are under more perfect control than horses. But
vestries and parochial authorities offer such objections as that horses
would be frightened in the streets, if the engine made a noise; and if
it did not, people would be liable to be run over, and the horses be as
much startled as in the other case. But horses would soon become
accustomed to the sight of a carriage moving without equine aid, however
startling the matter might appear to them at first; and the objection
urged against the noiseless engines might be alleged against wooden
pavements, india-rubber tires, and many other improvements. It is highly
probable that in the course of a few years the general adoption of
steam-propelled vehicles will displace horses, at least upon tramways.
The slowness with which inventions of undeniable utility and of proved
advantage come into general use may be illustrated by the fact of some
great English towns and centres of engineering industry not having made
a single tramway until, in all the populous cities of the United States,
and in almost every European capital, tramways had been in successful
operation for many years. [1890.]
Some time has elapsed since the foregoing paragraph was written for an
earlier edition of this work, and during that period there has been an
advance in both practice and opinion; so that now it has become highly
probable that before the century ends a great change may be witnessed in
our modes of locomotion, even on ordinary roads. Already every town of
importance throughout the United Kingdom has been provided with
excellent tramways, along which, in not a few instances, horseless
vehicles roll smoothly, to the great convenience of the general public,
while not one of the difficulties and dangers to general street traffic
has been experienced that were so confidently predicted by those who
were unable to perceive that an innovation might be an improvement. The
now universally-popular bicycle has been continually receiving
improvements, of which there appears to be no end, and as the machine
and all the contrivances connected with it are so familiar to everyone,
there is no need here to do more than to refer to them, because they
have led the way to great improvements in ordinary carriages.
The steam-propelled vehicle for common roads has just been mentioned as
an invention belonging to the first half of the century, and the reasons
it did not find favour have been alluded to. There exists in the United
Kingdom a law concerning horseless carriages travelling on highways,
which was passed to apply to traction engines, and enacts that other
than horse vehicles are not to go along a road at a greater speed than
four miles an hour, and only two miles an hour through a town, and
moreover they are to be preceded by a man bearing a red flag, etc. But a
bill has been introduced (1895) into the legislature to amend this law,
and permit the British people to use on their common roads such light
self-propelled carriages as are becoming popular in France, as may be
seen from the following account:—
On Tuesday, 11th June, 1895, a race was started from Versailles to
Bordeaux and back, a distance of 727 miles or more for the double
journey. The first prize was the substantial sum of 40,000 francs
(£1,600), to which was attached the condition of the carriage seating
four persons, and other prizes were also to be awarded to various kinds
of automatic vehicles. No fewer than sixty-six vehicles were entered for
competition, and these were variously supplied with motive power from
steam, electricity, or petroleum spirit. The starting place was
Versailles at 12·9 p.m., and at 10·32 on Wednesday morning MM. Panhard &
Levassor’s petroleum carriage arrived at Bordeaux, whence, after a stop
of only four minutes, the return journey was begun, but shortly
afterward an accident caused a delay of one hour, but the carriage made
the whole distance at the average of 14·9 miles per hour. In this and
three other carriages belonging to the same firm, the propeller was the
Daimler motor. Though this carriage was the first to accomplish the trip
it received only the second prize, the condition of seating _four_
persons not having been complied with. The first prize fell to a
four-seated vehicle by Les Fils de Peugeot Frères, a firm who carried
off besides the third and fourth prizes. These carriages were also
driven by so-called petroleum motors. These motors are really gas
engines on the principle to be presently mentioned, but the gas is
produced by the vapourisation of a volatile constituent of petroleum
(benzoline). The Daimler motor is a compact combination of two cylinders
connected with a chamber containing the explosive mixture of gas and
air. The pistons perform their in and out strokes simultaneously, but
their working strokes alternately.
_PORTABLE ENGINES._
The application of steam power to agricultural operations has led to the
construction of engines specially adapted by their simplicity and
portability for the end in view. The movable agricultural engines have,
like the locomotives, a fire-box nearly surrounded by the water, and
horizontal tubes, and are set on wheels, so that they may be drawn by
horses from place to place. There is usually one cylinder placed
horizontally on the top of the boiler; and the piston-rod, working in
guides, is, as in the old locomotive, attached by a connecting-rod to
the crank of a shaft, which carries a fly-wheel, eccentrics, and pulleys
for belts to communicate the motion to the machines. Engines of this
kind are also much used by contractors, for hoisting stones, mixing
mortar, &c. These engines are made with endless diversities of details,
though in most such simplicity of arrangement is secured, that a
labourer of ordinary intelligence may, after a little instruction, be
trusted with the charge of the engine; while their economy of fuel,
efficiency, and cheapness are not exceeded in any other class of steam
engine.
Besides the steam engines already described or alluded to, there are
many interesting forms of the direct application of steam power. There
are, for example, the steam roller and the steam fire-engine. The former
is a kind of heavy locomotive, moving on ponderous rollers, which
support the greater part of the weight of the engine. When this machine
is made to pass slowly over roads newly laid with broken stones, a few
repetitions of the process suffice to crush down the stones and
consolidate the materials, so as at once to form a smooth road. Steam
power is applied to the fire engine, not to propel it through the
streets, but to work the pumps which force up the water. The boilers of
these engines are so arranged that in a few minutes a pressure of steam
can be obtained sufficient to throw an effective jet of water. The cut
at the end of this chapter represents a very efficient engine of this
kind, which will throw a jet 200 feet high, delivering 1,100 gallons of
water per minute. It has two steam cylinders and two pumps, each making
a stroke of two feet. These are placed horizontally, the pumps and the
air reservoir occupying the front part of the engine, while the vertical
boiler is placed behind. The steam cylinders, which are partly hid in
the cut by the iron frame of the engine, are not attached to the boiler,
which by this arrangement is saved from injurious strains produced by
the action of the moving parts of the mechanism. There are seats for
eight firemen, underneath which is a space where the hose is carried. A
first-class steam fire-engine of this kind, completely fitted, costs
upwards of £1,300.
A cheap and very convenient prime mover has lately come into use, which
has certain advantages over even the steam engine. Where a moderate or a
very small power is required, especially where it is used only at
intervals, the _gas engine_ is found to be more convenient. It is small
and compact, no boiler or furnace is required, and it can be started at
any moment. As now made, it works smoothly and without noise. The piston
is impelled, not by the expansive force of steam, but by that of heated
air, the heat being generated by the explosion of a mixture of common
coal gas and air within the cylinder itself. Thus a series of small
explosions has the same effect as the admissions of steam through a
valve. A due quantity of gas and air is introduced into the cylinder,
and is ignited by the momentary opening of a communication with a
lighted gas jet outside. But the machine is provided with a regulator or
governor, which so acts on the valve mechanism that this communication
is made at each stroke only when the speed of rotation falls below a
certain assigned limit, and thus the number of the explosions is less
than the number of strokes, unless its work absorbs the machine’s whole
energy, which, according to the size of the engine, may be from that of
a child up to 30–horse power.
_THE STEAM HAMMER._
Before the invention of the steam hammer, large forge hammers had been
in use actuated by steam, but in an indirect manner, the hammer having
been lifted by cams and other expedients, which rendered the apparatus
cumbersome, costly, and very wasteful of power, on account of the
indirect way in which the original source of the force, namely, the
pressure of the steam, had to reach its point of application by giving
the blow to the hammer. Not only did the necessary mechanism for
communicating the force in this roundabout manner interfere with the
space necessary for the proper handling of the article to be forged, but
the range of the fall of the hammer being only about 18 in., caused a
very rapid decrease in the energy of the blow when only a very
moderate-sized piece of iron was introduced. For example, a piece of
iron 9 in. in diameter reduced the fall of the mass forming the hammer
to one-half, and the work it could accomplish was diminished in like
proportion. Besides, as the hammer was attached to a lever working on a
centre, the striking face of the hammer was parallel to the anvil only
at one particular point of its fall; and again, as the hammer was always
necessarily raised to the same height at each stroke, there was
absolutely no means of controlling the force of the blow. When we
reflect on the fact that the rectilinear motion of the piston in the
cylinder of the engine had first to be converted into a rotary one, by
beams, connecting-rod, crank, &c., and then this rotary movement
transformed into a lifting one by the intervention of wheels, shafts,
cams, &c., while all that is required in the hammer is a straight
up-and-down movement, the wonder is that such an indirect and cumbersome
application of power should have for so many years been contentedly
used. But in November, 1839, Mr. Nasmyth, an eminent engineer of
Manchester, received a letter from a correspondent, informing him of the
difficulty he had found in carrying out an order received for the
forging of a shaft for the paddle-wheels of a steamer, which shaft was
required to be 3 ft. in diameter. There was in all England no forge
hammer capable of executing such a piece of work. This caused Mr.
Nasmyth to reflect on the construction of forge hammers, and in _a few
minutes_ he had formed the conception of the steam hammer. He
immediately sketched the design, and soon afterward the steam hammer was
a _fait accompli_, for Mr. Nasmyth had one at once executed and erected
at his works, where he invited all concerned to come and witness its
performances. Will it be believed that four years elapsed before this
admirable application of steam power found employment outside the walls
of Mr. Nasmyth’s workshops? After a time he succeeded in making those
best able to profit by such an invention aware of the new power—for such
it has practically proved itself, having done more to revolutionize the
manufacture of iron than any other inventions that can be named, except,
perhaps, those of Cort and Bessemer. The usual prejudice attending the
introduction of any new machine, however obvious its advantages are
afterward admitted to be, at length cleared away, and the steam hammer
is from henceforth an absolute necessity in every engineering workshop,
and scarcely less so for some of the early stages of the process of
manufacturing crude wrought iron. Whether blows of enormous energy or
gentle taps are required, or strokes of every gradation and in any
order, the steam hammer is ready to supply them.
[Illustration:
FIG. 13.—_Nasmyth’s Steam Hammer._
]
A steam hammer of the smaller kind is represented in Fig. 13. The
general mode of action will easily be understood. The steam is admitted
below the piston, which is thus raised to any required height within the
limits of the stroke. When the communication with the boiler is shut off
and the steam below the piston is allowed to escape, the piston, with
the mass of iron forming the hammer attached to the piston-rod, falls by
its own weight. This weight, in the large steam hammers, amounts to
several tons; and the force of the blow will depend jointly upon the
weight of the hammer, and upon the height from which it is allowed to
fall. The steam is admitted and allowed to escape by valves, moved by a
lever under the control of a workman. By allowing the hammer to be
raised to a greater or less height, and by regulating the escape of the
steam from beneath the piston, the operator has it in his power to vary
the force of the blow. Men who are accustomed to work the valves can do
this with great nicety. They sometimes exhibit their perfect control
over the machine by cracking a nut on the anvil of a huge hammer; or a
watch having been placed—face upwards—upon the anvil, and a moistened
wafer laid on the glass, a practised operator will bring down the
ponderous mass with such exactitude and delicacy that it will pick up
the wafer, and the watch-glass will not even be cracked. The steam
hammer has recently been improved in several ways, and its power has
been more than doubled, by causing the steam, during the descent, to
enter above the piston and add its pressure to the force of gravity.
Probably one of the most powerful steam hammers ever constructed is that
recently erected at the Royal Gun Factory at Woolwich, for the purpose
of forging great guns for the British Navy. It has been made by Nasmyth
& Co., and is in shape similar to their other steam hammers. Its height
is upwards of 50 ft., and it is surrounded with furnaces and powerful
cranes, carrying the huge iron tongs that are to grasp the glowing
masses. The hammer descends not merely with its own weight of 30 tons;
steam is injected behind the falling piston, which is thus driven down
with vastly enhanced rapidity and impulse. Of the lower portion of this
stupendous forge, nothing is visible but a flat table of iron—the
anvil—level with the floor of the foundry. But more wonderful, perhaps,
than anything seen aboveground, is the extraordinarily solid foundation
beneath. Huge tablets of foot-thick castings alternate with concrete and
enormous baulks of timber, and, lower down, beds of concrete, and piles
driven deep into the solid earth, form a support for the uppermost
plate, upon which the giant delivers his terrible stroke. Less than this
would render it unsafe to work the hammer to its full power. As the
monster works—soberly and obediently though he does it—the solid soil
trembles, and everything movable shivers, far and near, as, with a
scream of the steam, our ‘hammer of Thor’ came thundering down, mashing
the hot iron into shape as easily as if it were crimson dough, squirting
jets of scarlet and yellow yeast. The head of the hammer, which of
course works vertically, is detachable, so that if the monster breaks
his steel fist upon coil or anvil, another can be quickly supplied.
These huge heads alone are as big as a sugar-hogshead, and come down
upon the hot iron with an energy of more than a thousand foot-tons. By
the courteous permission of Major E. Maitland, Superintendent of the
Royal Gun Factories, we are enabled to present our readers with the view
of the monster hammer which forms the Plate III.
Mr. Condie, in his form of steam hammer, utilizes the mass of the
cylinder itself to serve as the hammer. The piston-rod is hollow, and
forms a pipe, through which the steam is admitted and discharged, and
the piston is stationary, the cylinder moving instead—between vertical
guides. A hammer face is attached to the bottom of the cylinder by a
kind of dovetail socket, so that if the striking surface becomes injured
in any way, another can easily be substituted. The massive framework
which supports the moving parts of Condie’s hammer has its supports
placed very far apart, so as to leave ample space for the handling of
large forgings.
[Illustration:
FIG. 14.—_Merryweather’s Steam Fire-Engine._
]
[Illustration:
PLATE III.
THE GREAT STEAM HAMMER, ROYAL GUN FACTORY, WOOLWICH.
]
[Illustration:
FIG. 15.—_A Foundry._
]
IRON.