strengthen them. Besides these, there are rods of wrought iron
extended from end to end of the boiler above the roof of the
internal fire-place. These rods are represented at _o_ in their
length in _fig._ 97., and an end view of them is seen in _fig._
102. The smoke-box F, _fig._ 97. 104., containing the cylinders,
steam-pipe, and blast-pipe, is four feet wide, and two feet long.
It is formed of wrought iron plates, half an inch thick on the
side next the boiler, and a quarter of an inch elsewhere. The
plates are riveted in the same manner as those of the fire-box
already described. From the top of the smoke-box, which, like the
fire-box, is semi-cylindrical, as seen in elevation in _fig._
103., and in section _fig._ 104., rises the chimney G, fifteen
inches diameter, and formed of 1/8 inch iron plates, riveted and
bound round by hoops. It is flanged to the top of the [Pg371]
smoke-box, as represented in _fig._ 104. Near the bottom of the
smoke-box the working cylinders are placed, side by side, in a
horizontal position, with the slide valves upwards. In the top of
the external fire-box a circular aperture is formed fifteen inches
in diameter, and upon this aperture is placed the steam-dome T
(_figs._ 97. 101, 102.) two feet high, and attached around the
circular aperture by a flange and screw secured by nuts. This
steam dome is made of brass 3/8 inch thick. In stationary boilers,
where magnitude is not limited, it has been already explained,
that the space allowed for steam is sufficiently large to secure
the complete separation of the vapour from the spray which is
mixed with it when it issues immediately from the water. In
locomotive boilers sufficient space cannot be allowed for this,
and the separation of the water from the steam is effected by the
arrangement here represented. A funnel-shaped tube _d′_ (_figs._
97. 102.), with its wide end upwards, rises into the steam-dome,
and reaches nearly to the top of it. This funnel bends towards the
back of the fire-box, and is attached by a flange and screws to
the great steam-pipe S, which traverses the whole length of the
boiler. The steam rising from the boiler fills the steam-dome T,
and descends in the funnel-shaped tube _d′_. The space it has thus
to traverse enables the steam to disengage itself almost
completely from the priming. The wider part of the great
steam-pipe _a_ is flanged and screwed at the hinder end to a
corresponding aperture in the back plate of the fire-box. This
opening is covered by a circular plate, secured by screws, having
a stuffing-box in its centre, of the same kind as is used for the
piston-rods of steam-cylinders. Through this stuffing-box the
spindle _a″_ of the regulator passes, and to its end is attached a
winch _h′_, by which the spindle _a″_ is capable of being turned.
This winch is limited in its play to a quarter of a revolution.
The other end of the spindle _a″_ is attached to a plate _e′_ seen
edgeways in _fig._ 97., and the face of which is seen in _fig._
102.: this circular plate _e_ is perforated with two apertures
somewhat less than quadrants. That part of the plate, therefore,
which remains not pierced forms two solid pieces somewhat greater
than quadrants. This plate is ground so as to move in steam-tight
[Pg372] contact with a fixed plate under it, which terminates at
the wide end of the conical mouth of the steam-pipe S. This fixed
circular plate is likewise pierced with two nearly quadrantal
apertures, corresponding with those in the movable plate _e′_.
When the movable plate _e′_ is turned round by the winch _h′_, the
apertures in it may be made to correspond with those of the fixed
circular plate on which it moves, in which position the steam-pipe
S communicates with the funnel _d′_ by the two quadrantal
apertures thus open. If, on the other hand, the winch _h′_ be
moved from this position through a quarter revolution, then the
quadrantal openings in the movable plate will be brought over the
solid parts of the fixed plate on which it moves, and these solid
parts being a little more than quadrants, while the openings are a
little less, all communication between the steam-pipe S and the
funnel _d′_ will be stopped, for in this case the quadrantal
openings in the fixed and movable plates respectively will be
stopped by the solid parts of these plates. It will be evident
that as the winch _h′_ of the regulator is moved from the former
position to the latter, in every intermediate position the
aperture communicating between the funnel _d′_ and the steam-pipe
S will be less in magnitude than the complete quadrant. It will in
fact be composed of two openings having the form of _sectors_ of a
circle less than a quadrant, and these sectors may be made of any
magnitude, however small, until the opening is altogether closed.
By such means the admission of steam from the boiler to the
steam-pipe S may be regulated by the winch _h′_.
The steam being admitted to the steam-pipe passes through it to
the front end of the boiler, and the pipe being enclosed within
the boiler the temperature of the steam is maintained. The
steam-pipe passing through the tube-plate at the front end of the
boiler is carried to a small distance from the tube-plate in the
same direction, where it is flanged on to a cross horizontal pipe
proceeding to the right and to the left as represented in _fig._
104. This cross pipe is itself flanged to two curved steam-pipes S
(_fig._ 104.), by which the steam is conducted to the valve-boxes
V V. The lower ends of these curved arms are flanged on to the
valve-boxes of the two cylinders [Pg373] at the ends nearest to
the boiler. The opening of one of these is exhibited in the right
hand cylinder in _fig._ 99. By these pipes the steam is conducted
into the valve-boxes or steam-chests, from which it is admitted by
slide-valves to the cylinders to work the pistons in the same
manner as has been already described in the large stationary
engines.
On the upper sides of the cylinders are formed the steam-chests or
valve-boxes, which are exhibited at U (_figs._ 97. 99. 104.).
These are made of cast-iron half an inch thick, and are bolted to
the upper side of each cylinder. At the front end they are also
secured by bolts to the smoke-box, and at the hinder end are
attached to the tube-plate. These valve-boxes communicate with the
passages _m_ and _n_ _fig._ 99. leading to the top and bottom of
the cylinder: these are called the steam-ports. They also
communicate with a passage _o_ leading to the mouth of a curved
horizontal pipe _p′_ connecting the front ends of the two
cylinders, as seen in _figs._ 99. 104. These curved pipes unite in
a single vertical pipe _p_, called the _blast-pipe_, seen in
_figs._ 97. 104.: this vertical pipe becomes gradually small
towards the top, and terminates a little above the base of the
funnel or chimney G. In the valve-box is placed the slide-valve
_v_ to which is attached the spindle _l′_. This spindle moves
through a stuffing-box _k′_, and is worked by gearing, which will
be described hereafter. According to the position given to the
slide, a communication may be opened between the steam-chest, or
the waste-port, and either end of the cylinders. Thus when the
slide is in the position represented in _fig._ 97. the steam-chest
communicates with the front end of the cylinder, while the
waste-port communicates with the hinder end. If, on the other
hand, the spindle _l′_ being pressed forward, move the slide to
its extreme opposite position, the steam-port _n_ would
communicate with the waste-port _o_, while the steam-chest would
communicate with the steam-port _m_, steam would, therefore, be
admitted to the hinder end of the cylinder, while the foremost end
would communicate with the waste-port. It will be perceived that
this arrangement is precisely similar to that of the slide-valves
already described (133.). The slide-valve is represented on a
larger scale in _fig._ 106., where A is the hinder steam-port,
[Pg374] B the foremost steam-port, and C the waste-port. The
surfaces D, separating the steam-ports from the waste-ports, are
called the bars: they are planed perfectly smooth, so that the
surfaces F and G of the slide-valve, also planed perfectly smooth,
may move in steam-tight contact with them. These surfaces are kept
in contact by the pressure of the steam in the steam-chest, by
which the slide-valve is always pressed down. In its middle
position, as represented by the dotted lines in the figure, both
the steam-ports are stopped by the slide-valve, so that at that
moment no steam is admitted to either end of the cylinder. On
either side of this intermediate position the slide has an inch
and a half play, which is sufficient to open successively the two
steam-ports.
[Illustration: _Fig._ 106.]
The cylinders are inserted at one end in the plate of the
smoke-box, and at the other in the tube-plate of the boiler. They
are closed at either end by cast iron covers, nearly an inch
thick, flanged on by bolts and screws. In the cover of the
cylinder attached to the tube-plate is a stuffing-box, in which
the piston rod plays. The metallic pistons used in locomotive
engines do not differ materially from those already described, and
therefore need not be here particularly noticed. From their
horizontal position they have a tendency to wear unequally in the
cylinders, their weight pressing them on one side only; but from
their small magnitude this effect is found to be imperceptible in
practice. In the engine here described the stroke of the piston is
eighteen inches, and this is the most usual length of stroke in
locomotive engines. The piston, in its play, comes at either end
within about half an inch of the inner surface of the covers of
the cylinders, this space being allowed to prevent collision. In
the foremost cover of the cylinder is inserted a cock _q′_ (_fig._
97. 99.), by which any water which may collect in the cylinder by
condensation or priming may be discharged. A cock _r′_ (_fig._
97.), communicating with a small tube proceeding from the branches
of the waste pipe _p′_ (_fig._ 104.), is likewise provided to
discharge from that pipe any water which may be [Pg375] collected
in it. After the steam has been admitted to work the piston
through the slide-valve, and has been discharged through the
waste-port by shifting that valve, it passes through the pipe _p′_
into the blast-pipe _p_, from the mouth of which it issues, with
great force, up the funnel G. When the motion of the engine is
rapid, the steam from the two cylinders proceeds in an almost
uninterrupted current from the blast-pipe, and causes a strong
draft up the chimney. The heated air which passes from the mouths
of the tubes into the smoke-box is drawn up by this current, and a
corresponding draft is produced in the fire-box.
[Illustration: _Fig._ 107.]
The piston-rods Y terminate in a fork, by which they are attached
to cross heads Z, the ends of which are confined by guide-bars A′,
in which they are allowed to play backwards and forwards through a
space equal to the stroke of the piston. To these cross heads Z,
between the prongs of the fork in which the piston terminates, are
attached the foremost ends of the connecting rods B′. These rods
are, therefore, driven backwards and forwards by the motion
imparted to the cross head Z by the piston-rods Y. The connecting
rods B′ are attached at the hinder ends to two cranks formed upon
the axles C′ of the driving wheels D′. These two cranks are formed
upon the axles precisely at right angles to each other. The
left-hand crank is represented in its horizontal position, in
_fig._ 99., and the right-hand crank is seen in its vertical
position. A cranked axle is represented on a larger scale in
_fig._ 107., and the two cranks are seen in a position oblique to
the plane of the figure. As this axle is the instrument by which
the impelling force is conveyed to the load, and as it has to
support a great portion of the weight of the engine, it is
constructed with great strength and precision. It is made all in
one [Pg376] piece, and of the best wrought iron called Back
Barrow, or scrap iron. In the engine here described its extreme
length is six feet and a half, and its diameter is five inches. At
the centre part A it is cylindrical, and is increased to five
inches and a quarter at C, where the cranks are formed. The sides
D of the cranks are four inches thick, and the crank pins B, which
are truly cylindrical, are five inches diameter, and three inches
in length, the brasses at the extremities of the connecting rods
which play upon them having a corresponding magnitude. The
distance from the centre of the crank-pins B to the centre of the
axle A must be exactly equal to half the stroke of the piston, and
is, therefore, in this case precisely nine inches. Upon the parts
F, which are seven inches and a half long, the great driving
wheels are firmly fastened, so as to be prevented from turning or
shaking upon the axle. The axle projects beyond the wheels at G,
where it is reduced to three inches and an eighth diameter. These
projecting parts G are five inches long, having collars at the
outer ends. Brasses are fixed at the outside frame of the engine
which rest upon these projections G of the axle, and upon these
brasses the weight of the engine is supported. The entire axle is
accurately turned in a lathe, and each of the crank-pins B is
likewise turned by suspending the axle on centres corresponding
with the centres of the crank-pins, and made on strong cast iron
arms, which are firmly fixed on the ends of the axle, and project
beyond the cranks so as to balance the axle, and enable it to turn
round on the centre of the crank-pin. The axle is by such means
made perfectly true, and the cranks are made of exactly the proper
length, and precisely at right angles to each other. The corners
of the cranks are champered off, as shown in the figure, and the
ends of the cylindrical parts well rounded out.
The strength and accuracy of construction indispensable in these
cranked axles, in order to make them execute their work, render
them very expensive. Those which are here described cost about
50_l._ each. When properly constructed, however, they are seldom
broken, but are sometimes bent when the engine escapes from the
rails.
The proper motion to admit and withdraw the steam from [Pg377]
either end of the cylinder is imparted to the slide-valves by
eccentrics, in a manner and on a principle so similar to that
already described in large stationary engines, that it will not be
necessary here to enter into any detailed explanation of the
apparatus for communicating this motion, which is exhibited in
plan and section in _figs._ 97. 99. The eccentrics are attached to
the cranked axles at E′ E″. The eccentric E′ imparts motion by a
rod _e″_ to a lever _h″_, formed on an axle extending across the
frame of the engine. This conveys motion to another lever _l″_,
projecting from the same axle. This lever _l″_ is jointed to
horizontal links _m″_, which at the foremost ends are attached to
the spindle _l′_, by which the slide is driven. By these means the
motion received by the eccentric from the great working axle
conveys to the spindle _l′_ an alternate movement backwards and
forwards, and the points at which it is reversed will be regulated
by the position given to the eccentric upon the great axle. The
eccentric is formed in two separate semicircles, and is keyed on
to the great axle, and consequently any position may be given to
it which may be required. The position to be given to the
eccentrics should be such that they shall be at right angles to
their respective cranks, and they should be fixed a quarter of a
revolution behind the cranks so as to move the slides to that
extent in advance of the pistons, since by the position of the
levers _h″_ and _l″_, the motion of the eccentric becomes reversed
before it reaches the valve spindle.
The performance of the engine is materially affected by the
position of the eccentrics on the working axle. The slide should
begin to uncover the steam-port a little before the commencement
of the stroke of the piston, in order that the steam impelling the
piston should be shut off, and the steam about to impel it in the
contrary direction admitted before the termination of the stroke.
Through this small space the steam, therefore, must act in
opposition to the motion of the piston. This is called the _lead_
of the slide, and the extent generally given to it is about a
quarter of an inch. This is accomplished by fixing the eccentrics
not precisely at right angles to the respective cranks, but a
little in advance of that position. The introduction of the steam
to [Pg378] the piston before the termination of the stroke has
the effect of bringing it gradually to rest at the end of the
stroke, and thereby diminishing the jerk or shock produced by the
rapid change of motion. In stationary engines, where the
reciprocations of the engine are slow, the necessity for this
provision does not arise; but in locomotive engines in which the
motion of the piston is changed from four to six times in a
second, it becomes necessary. The steam admitted to the piston
before the termination of the stroke acts as a spring-cushion to
assist in changing its motion, and if it were not applied, the
piston could not be kept tight upon the piston-rod. Another
advantage which is produced by allowing some lead to the slide is
that the waste steam which has just impelled the piston begins to
make its escape through the waste-port before the commencement of
the next stroke, so that when the impelling steam begins to
produce the returning stroke, there is less waste steam on the
other side of the piston to resist it.
When the motion of the engine is very rapid, the resistance of the
waste steam, as it escapes from the blast-pipe to the piston, has
been generally supposed to be very considerable, though we are not
aware of any direct experiments by which its amount has been
ascertained. In the account of the locomotive engine which has
been here described, supplied by Mr. Stephenson for the last
edition of Tredgold on the Steam Engine, he states, that the
average resisting pressure of the waste steam throughout the
stroke is 6 lbs. per square inch, when running at the usual rate
of from 25 to 28 miles an hour, and that at greater velocities
this negative pressure has been found to increase to more than
double that amount. No experiments are, however, cited from which
this inference has been drawn.
It has been also thought that the pressure of steam upon the
piston in the cylinder, at high velocities, is considerably below
the pressure of steam in the boiler; but this has not been, so far
as we are informed, ascertained by any satisfactory experimental
test. Mr. Stephenson likewise states, that this loss of pressure,
causes the negative pressure or resistance of the waste steam to
amount to [Pg379] from 30 to 40 per cent. of the positive
pressure upon the piston when the engine is running very fast, and
that therefore the power of the engine is diminished nearly one
half.
But it will be perceived that besides the uncertainty which
attends the estimate of the actual amount of pressure on the
piston compared with the pressure of steam in the boiler, the
inference here drawn does not appear to be compatible with what
has been already proved respecting the mechanical effect of steam.
No change of pressure which may take place between the boiler and
the cylinder can affect the practical efficacy of the steam. As
the steam passes through the engine, whatever change of pressure
it may be subject to, it still remains common steam; and though
its pressure may be diminished, its volume being increased in a
nearly equal proportion, its mechanical effect will remain the
same. The power of the engine, therefore, estimated as it ought to
be, by the whole mechanical effect produced, will not be altered
otherwise than by the effect of the increased resistance produced
by the blast-pipe. What that resistance is, we repeat, has not, so
far as we know, been ascertained by direct experiment, and there
are circumstances attending it which render it probable that, even
at high velocities, it is less in amount than Mr. Stephenson's
estimate.
The position of the eccentrics which is necessary to make the
pistons drive the engine forward must be directly the reverse of
that which would cause them to drive the engine backwards. To be
able, therefore, to reverse the motion of the engine, it would
only be necessary to be able to reverse the position of the
eccentrics, which may be accomplished by either of two expedients.
_First_, The eccentrics may be capable of revolving on the great
working axle, and also of sliding upon it through a small space.
Their revolution on the axle may be checked by letting a pin
attached to a collar fastened on the axle fall into a hole on the
side of the eccentric. Such a pin will drive the eccentric round
with the axle, and the position of this pin and the hole will
determine the position of the eccentric with reference to the
crank. At a short distance [Pg380] on the other side of the
eccentric may be a corresponding collar with a pin in the opposite
position. By moving the eccentric longitudinally on the axle, the
former pin may be withdrawn from the hole, and the latter allowed
to fall into the hole on the other side. Proper mechanism may be
provided by which the position of the eccentric may thus be
reversed in reference to the crank, and by such means the motion
of the engine may be reversed.
_Secondly_, Supposing the eccentrics which drive the engine
forward to be immovably fixed upon the axle, two other eccentrics
may be provided attached to other parts of the same axle, and
having a position exactly the reverse with reference to the
cranks. Proper mechanism may be provided, by which either or both
pairs of eccentrics may be thrown in or out of gear. Such are the
means adopted in the engine which has been already described. The
eccentrics for driving the engine backwards are placed outside the
cranks at F′ F″. A hand lever _w″_ _fig._ 101. is provided, by
which the engine man may throw either pair of eccentrics into or
out of gear, so as to make the engine work either backwards or
forwards.
[Illustration: 108.]
[Illustration: 109.]
[Illustration: 110.]
As all the moving parts of the engine require to be constantly
lubricated with oil to diminish the friction, and keep them cool,
oil-cups for this purpose are fixed upon them. In some engines
these oil-cups are attached separately to all the moving parts: in
others they are placed near each other in a row on the boiler, and
communicate by small tubes with the several parts required to be
lubricated. One of these is requisite for each end of the
connecting rods, for each of the guides of the piston-rods, for
the piston-rod itself, the spindle of the slide-valve, and other
parts. An elevation of one of these oil-cups is shown in _fig._
108., a vertical section in _fig._ 109., and horizontal plan in
_fig._ 110. The cup A is made of brass with a cover B. This cover
has a piece projecting from it turning upon a pin in a socket C at
the side of the cup A, and square at the end, resting upon a small
spring at the bottom of the socket to hold it either open or shut.
In the bottom of the [Pg381] cup is inserted an iron tube D
extending nearly to the top. This tube projects from the bottom of
the cup, where it is tapped for the purpose of fixing the cup on
the part of the engine which it is intended to lubricate. The hole
into which the cup is screwed communicates with the rubbing
surface, and some cotton thread is passed through the tube dipping
into the oil in the cup at the one end and touching the moving
part at the other. This thread acts as a siphon, and constantly
drops oil on the rubbing surface.
[Illustration: _Fig._ 111.]