pipe S. The throttle-valve T in that pipe, near the cylinder, is
regulated by a system of levers connected with [Pg217] the
governor. The piston P is accurately fitted in the steam cylinder
C by packing, as already described in the single-acting engine.
This piston, as it moves, divides the cylinder into two
compartments, between which there is no communication by which
steam or any other elastic fluid can pass. The upper steam box B
is divided into three compartments by the two valves. Above the
upper steam valve V is a compartment communicating with the steam
pipe; below the upper exhausting valve E is another compartment
communicating with the eduction pipe which leads to the condenser.
By the valves V and E a communication may be opened or closed
between the boiler on the one hand, or the condenser on the other,
and the top of the cylinder. The continuation S′ of the steam pipe
leads to the lower box B′, which, like the upper, is divided into
three compartments by two valves V′ and E′. The upper compartment
communicates with the steam pipe, and thereby with the boiler; and
the lower compartment communicates with the eduction pipe, and
thereby with the condenser. By means of the two valves V′ and E′,
a communication may be opened or closed between the steam pipe on
the one hand, or the exhausting pipe on the other, and the lower
part of the cylinder. The four valves V, E, V′, and E′ are
connected by a system of levers with a handle or spanner _m_,
which, being driven downwards or upwards, is capable of opening or
closing the valves in pairs, in the manner already described
(116.). The condensers, the air-pump, and the hot-water pump, are
in all respects similar to those already described in the
single-acting engine, except that the condensing jet is governed
by a lever I, by which it is allowed to play continually in the
condenser, and by which the quantity of water admitted through it
is regulated. The cold-water pump N is worked by the engine as
already described in the single-acting engine, and supplies the
cistern in which the air-pump and condenser are submerged, so as
to keep down its temperature to the proper limit. On the air-pump
rod R are two pins properly placed, so as to strike the spanner
_m_, upwards and downwards, at the proper times, when the piston
approaches the termination of the stroke at the top or bottom of
the cylinder. The pump L [Pg218] conducts the warm water drawn by
the air-pump from the condenser to a proper reservoir for feeding
the boiler. The vertical motion of the piston-rod in a straight
line is rendered compatible with the circular motion of the end of
the beam by the parallel motion already described. The point _b_,
on the beam, moves upwards and downwards in a circular arch, of
which the axis of the beam is the centre. In like manner the point
_d_ of the rod _d c_ moves upwards and downwards, in a similar
arch of which the fixed pivot _c_ is the centre. The joint or bar
_d b_, which joins these two pivots, will be moved so that its
middle point _e_ will ascend and descend nearly in a straight
line, as has been already explained (120.); [Pg219] opposite this
point _e_ is attached the piston-rod of the air-pump, which is
accordingly guided upwards and downwards by this means. The
jointed parallelogram _b d g f_ is attached to the beam by pivots;
and, as has been explained (120.), the point _g_ will be moved
upwards and downwards in a straight line, through twice the space
through which the point _e_ is moved. To the point _g_ the rod of
the steam piston is attached. Thus, the rods of the steam piston
and air-pump are moved by the same system of jointed bars, and
moved through spaces which are in the proportion of two to one.
Although this system of jointed rods forming the parallel motion,
appears in the figure to consist only of one parallelogram _b d g
f_, and one rod _c d_, called the _radius rod_, it is, in fact,
double, a similar parallelogram and radius rod being attached to
corresponding points, and in the same manner on the other side of
the beam; but from the view given in the cut, the one set of rods
hides the other. The two systems of rods thus attached to opposite
sides of the beam at several inches asunder, are connected by
cross rods, the ends of which form the pivots or joints, and
extend between the parallelograms. The ends of these rods are only
visible in the figure. It is to the middle of one of these rods,
the end of which is represented at _e_, that the air-pump
piston-rod is attached; and it is to the middle of another, the
end of which is represented at _g_, that the steam piston-rod is
attached. These two piston-rods, therefore, are driven, not
immediately by either of the parallelograms forming the parallel
motion, but by the bars extending between them.
To the working end of the beam H is attached a rod of cast-iron O,
called the _connecting rod_, the lower end of which is attached to
the crank by a pivot. The weight of the connecting rod is so made,
that it shall balance the weight of the piston-rods of the air-pump
and cylinder on the other side of the beam; and the weight of the
piston-rod of the cold-water pump N nearly balances the weight of
the piston-rod of the hot-water pump L. Thus, so far as the weights
of the machinery are concerned, the engine is in equilibrium, and
the piston would rest in any position indifferently in the cylinder.
The axis of the fly-wheel on which the crank is formed is [Pg220]
square in the middle part, where the fly-wheel is attached to it,
but has cylindrical necks at each end, which rest in sockets or
bearings supported by the framing of the machine, in which sockets
the axis revolves freely. On the axle of the crank is placed the
fly-wheel, and connected with its axle is the governor Q, which
regulates the throttle-valve T in the manner already described.
Let us now suppose the engine to be in full operation. The piston
being at the top of the cylinder, the spanner _m_ will be raised
by the lower pin on the air-pump rod, and the upper steam valve V,
and the lower exhausting valve E′, will be opened, while the upper
exhausting valve E and the lower steam valve V′ are closed. Steam
will, therefore, be admitted above the piston, and the steam which
filled the cylinder below it will be drawn off to the condenser,
where it will be converted into water. The piston will, therefore,
be urged by the pressure of the steam above it to the bottom of
the cylinder. As it approaches that limit, the spanner _m_ will be
struck downwards by the upper pin on the air-pump rod, and the
valves V and E′ will be closed, and at the same time the lower
steam valve V′ and the upper exhausting valve E will be opened.
Steam will, therefore, be admitted below the piston, while the
steam above it will be drawn off into the condenser, and converted
into water. The pressure of the steam, therefore, below the piston
will urge it upwards, and in the same manner the motion will be
continued.
While this process is going on in the cylinder and the condenser,
the water formed in the condenser will be gradually drawn off by
the operation of the air-pump piston, in the same manner as
explained in the single-acting engine; and at the same time the
hot water thrown into the hot well by the air-pump piston will be
carried off by the hot-water pump L.
Such are the chief circumstances attending the continuance of the
operation of the double-acting engine. It is only necessary here to
recall what has been already explained respecting the operation of
the fly-wheel. The commencement of the motion of the piston from the
top and bottom of the cylinder is produced, not by the pressure of
the steam upon it upwards or downwards, which must, for the reasons
[Pg221] already explained, be entirely inefficient; but by the
momentum of the fly-wheel, which extricates the crank from those
positions in which the moving power cannot affect it.
The manner in which the motion of the crank affects the connecting
rod at the dead points produces an effect of great importance in
the operation of the engine. When the crank-pin is approaching the
lowest point of its play, and therefore the piston approaching the
top of the cylinder, the motion of the crank-pin becomes nearly
horizontal, and consequently its effect in drawing the connecting
rod and the working end of the beam downwards and the piston
upwards, is extremely small. The consequence of this is, that as
the piston approaches the top of the cylinder, its motion becomes
very rapidly retarded; and as the motion of the crank-pin at its
lowest point is actually horizontal, the piston is brought to a
state of rest by this gradually retarded motion at the top of the
cylinder. In like manner, when the crank-pin moves from its dead
point upwards, its motion at first is very nearly horizontal, and
consequently its effect in driving the working end of the beam
upwards, and the piston downwards, is at first very small, but
gradually accelerated. The effect of this upon the piston is, that
it arrives at and departs from the top of the stroke with a very
slow motion, being absolutely brought to rest at that point.
The same effect is produced when the piston arrives at the bottom
of the cylinder. This retardation and suspension of the motion of
the piston at the termination of the stroke affords time for the
process of condensation to be effected, so that when the moving
power of the steam upon the piston can come into action, the
condensation shall be sufficiently complete. As the piston
approaches the top of the cylinder, and its motion becomes slow,
the working gear is made to open the lower exhausting valve; the
steam enclosed in the cylinder below the piston, and which has
just driven the piston upwards, presses with an elastic force of
17 lbs. per square inch on every part of the interior of the
cylinder, while the uncondensed vapour in the condenser presses
with a force of about 2 lbs. per square inch. The steam,
therefore, will have a tendency to rush from the cylinder to the
[Pg222] condenser through the open exhausting valve, with an
excess of pressure amounting to 15 lbs. per square inch, while the
piston pauses at the top of the cylinder. This process goes on,
and when the piston has descended by the motion of the fly-wheel,
a sufficient distance from the top of the cylinder to call the
moving force of the steam into action, the exhaustion will be
complete, and the pressure of the uncondensed vapour in the
cylinder will become the same as in the condenser.
The pressure of steam in the cylinder, and of uncondensed vapour
in the condenser, varies, within certain limits, in different
engines, and therefore the amount here assigned to them must be
taken merely as an example.
The size of the valves by which the steam is allowed to pass from
the cylinder to the condenser should be such as to cause the
condensation to take place in a sufficiently short time, to be
completed when the steam impelling the piston is called into
action.
Watt, in the construction of his engines, made the
exhaustion-valves with a diameter which was one fifth of the
diameter of the cylinder, and therefore the actual magnitude of
the aperture for the escape of the steam was one twenty-fifth of
the magnitude of the cylinder; but the spindle of the valve
diminished this so that the available space for the escape of
steam did not exceed one twenty-seventh of the magnitude of the
cylinder. This was found to produce a sufficiently rapid
condensation.
It was usual to make the steam valves of the same magnitude as the
exhausting valves, but the flow of steam through the former was
resisted by the throttle-valve, while no obstruction was opposed
to its passage through the latter.
The rapidity with which the cylinder must be exhausted by the
condenser will, however, depend upon the velocity with which the
piston is moved in it. The magnitude, therefore, of the exhausting
valves which would be sufficient for an engine which acts with a
slow motion would be too small where a rapid motion is required.
In the single-acting steam engine, where the moving force always
acted downwards on the piston, the pressure upon [Pg223] all the
joints of the machinery by which the force of the piston was
conveyed to the working parts, always took place in the same
direction, and consequently whatever might be the mechanical
connection by which the several joints were formed, the pins by
which they were connected, must always come to a bearing in their
respective sockets, however loosely they may have been fitted. For
the same reason, however, that the arch head and chain were
abandoned as a means of connecting the steam piston with the beam,
and the parallel motion substituted, it was also necessary in the
double-acting engine, where all joints whatever were driven
alternately in opposite directions, to fit the connecting pins
with the greatest accuracy in their sockets, and to abandon all
connection of the parts by chains. If any sensible looseness was
left in the joints, a violent jerk would be produced every time
the motion of the piston was reversed. Any looseness either in the
pivots or joints of the parallel motion of the working beam, the
connecting rod, or crank, would, at every change of stroke, be so
accumulated as to produce upon the machinery the effects of
percussion, and would consequently be attended with the danger of
straining and breaking the moveable parts of the mechanism.
To secure, therefore, the necessary accuracy of the joints, Watt
contrived that every joint in the engine should admit of the size of
the socket being exactly adapted to the size of the pin, so as
always to make a good fitting by closing the socket upon the pin,
when any looseness would be produced by wear. With this view, all
the joints were fitted with sockets made of brass or gun-metal,
capable of adjustment. Each socket was composed of two pieces,
accurately fitted into a cell or groove, in which one of the brasses
can be moved towards the other by means of a wedge or screw. Each
brass has in it a semi-cylindrical cavity, and the two cavities
being opposed to each other, form a socket for the joint-pin. One of
the two brasses can always be tightened round that pin, so as to
enclose it tight between the two semi-cylindrical cavities, and to
prevent any looseness taking place. The brasses, and other parts of
such a joint, are represented [Pg224] in _fig._ 44. These joints
still continue to be used in the engines as now constructed.
[Illustration: _Fig._ 44.]
The motion of the working beam, and the pump-rods which it drives,
and of the connecting rod, ought, if the whole were constructed
with perfect precision, to take place in the same or parallel
vertical planes; but this supposes a perfection of execution which
could hardly have been expected in the early manufacture of such
engines, whatever may have been attained by improvements which
have been since made. In the details of construction, Watt saw
that there would be a liability to lateral strain, owing to the
planes of the different motions not being truly vertical and truly
parallel, and that if a provision were not made for such lateral
motion, the machinery would be subject to constant strain in its
joints and rapid wear. He provided against this by constructing
the main joints by which the great working lever was connected
with the pistons and connecting rod, so as to form universal
joints, giving freedom of motion laterally as well as vertically.
The great lever, or working beam, was so called from being
originally made from a beam of oak. It is now, however,
universally constructed of cast-iron. The connecting rod is also
made of cast-iron, and attached to the beam and to the crank by
axles or pivots.
The mechanism by which the four valves are opened and closed, is
subject to considerable variation in different engines. They have
been described above as being opened and closed simultaneously by
a single lever. Sometimes, however, they are opened alternately in
pairs by two distinct levers driven by two pins attached to the
air-pump rod. One pin strikes the lever, which opens and closes
the upper steam valve, and lower exhausting valve; the other
strikes that which opens and closes the lower steam valve and
upper exhausting valve.
Since the date of the earlier double-acting engines, constructed
by Boulton and Watt, a great variety of mechanical expedients have
been practised for working the valves, by which the steam is
admitted to and withdrawn from the [Pg225] cylinder. We shall
here describe a few of these methods:—
(128.) The method of working the valves by pins on the air-pump
rod driving levers connected with the valves has been, in almost
all modern double-acting machines, superseded by an apparatus
called an _eccentric_, by which the motion of the axle of the
fly-wheel is made to open and close the valves at the proper
times.
[Illustration: _Fig._ 45.]
An eccentric is a metallic circle attached to a revolving axle, so
that the centre of the circle shall not coincide with the centre
round which the axle revolves. Let us suppose that G (_fig._ 45.),
is a square revolving shaft. Let a circular plate of metal B D,
having its centre at C, have a square hole cut in it, corresponding
to the shaft G, and let the shaft G pass through this square
aperture, so that the circular plate B D shall be fastened upon the
shaft, and capable of revolving with it as the shaft revolves. The
centre C of the circular plate B D will be carried round the centre
G of the revolving shaft, and will describe round it a circle, the
radius of which will be the distance of the centre C of the circular
plate from the centre of the shaft. Such circular plate so placed
upon a shaft, and revolving with it, is _an eccentric_.
Let E F be a metallic ring, formed of two semicircles of metal
screwed together at H, so as to be capable, by the adjustment of
the screws, of having the circular aperture formed by the ring
enlarged and diminished within certain [Pg226] small limits. Let
this circular aperture be supposed to be equal to the magnitude of
the eccentric B D. To the circular ring E F let an arm L M be
attached. If the ring E F be placed around the eccentric B D, and
that the screws H be so adjusted as to allow the eccentric B D to
revolve within the ring E F, then while the eccentric revolves,
the ring not partaking of its revolution, the arm L M will be
alternately driven to the right and to the left, by the motion of
the centre C of the eccentric as it revolves round the centre G of
the axle. When the centre C of the eccentric is in the same
horizontal line with the centre G, and to the left of it, then the
position of L M will be that which is represented in _fig._ 45.;
but when, after half a revolution of the main axle, the centre C
of the eccentric is thrown on the other side of the centre G, then
the point M will be transferred to the right, to a distance equal
to twice the distance C G. Thus as the eccentric B D revolves
within the ring E F, that ring, together with the arm L M, will be
alternately driven, right and left, through a space equal to twice
the distance between the centre of the eccentric and the centre of
the revolving shaft.
If we suppose a notch formed at the extremity of the arm L M, which
is capable of embracing a lever N M, moveable on a pivot at N, the
motion of the eccentric would give to such a lever an alternate
motion from right to left, and _vice versâ_. If we suppose another
lever N O connected with N M, and at right angles to it, forming
what is called a bell-crank, then the alternate motion received by
M, from right to left, would give a corresponding motion to the
extremity O of the lever N O, upwards and downwards. If this last
point O were attached to a vertical arm or shaft, it would impart to
such arm or shaft an alternate motion upwards and downwards, the
extent of which would be regulated by the length of the levers
respectively.
By such a contrivance the revolution of the fly-wheel shaft is
made to give an alternate vertical motion of any required extent
to a vertical shaft placed near the cylinder, which may be so
connected with the valves as to open and close them. Since the
upward and downward motion of this vertical shaft is governed by
the alternate motion of the centre [Pg227] C to the right and to
the left of the centre G, it is evident that by the adjustment of
the eccentric upon the fly-wheel shaft, the valves may be opened
and closed at any required position of the fly-wheel and crank,
and therefore at any required position of the piston in the
cylinder.
Such is the contrivance by which the valves, whatever form may be
given to them, are now almost universally worked in double-acting
steam engines.
* * * * *
Having described the general structure and operation of the steam
engine as improved by Watt, we shall now explain, in a more
detailed manner, some parts of its machinery which have been
variously constructed, and in which more or less improvements have
been made.
OF THE COCKS AND VALVES.
(129.) In the steam engine, as well as in every other machine in
which fluids act, it is necessary to open or close, occasionally,
the tubes or passages through which these fluids move. The
instruments by which this is accomplished are called cocks or
valves.
Cocks or valves may be classified by the manner in which they are
opened: 1st, they may be opened by a motion similar to the lid of
a box upon its hinges; 2d, they may be opened by being raised
directly upwards, in the same manner as the lid of a pot or
kettle; 3d, they may be opened by a sliding motion, like that of
the sash of a window or the lid of a box which slides in grooves;
4th, they may be opened by a motion of revolution, in the same
manner as the cock of a beer-barrel is opened or closed. The term
_valve_ is more properly applied to the first and second of these
classes; the third class are usually called _slides_, and the
fourth _cocks_.
(130.) The single clack valve is the most simple example of the
first class. It is usually constructed by attaching to a plate of
metal larger than the aperture which the valve is intended to
stop, a piece of leather, and to the under side of this leather
another piece of metal smaller than the aperture. The leather
[Pg228] extending on one side beyond the larger metallic plate,
and being flexible, forms the hinge on which the valve plays. Such
a valve is usually closed by its own weight, and opened by the
pressure of the fluid which passes through it. It is also held
closed more firmly by the pressure of the fluid whose return it is
intended to obstruct. An example of this valve occurs in the steam
engine, in the passage between the condenser and the air-pump. The
aperture which it stops is there a seat inclined at an angle whose
inclination is such as to render the weight of the valve
sufficient to close it. In cases where the valve is exposed to
heat, as in the example just mentioned, where it is continually in
contact with the hot water flowing from the condenser to the
air-pump, the use of leather is inadmissible, and in that case the
metallic surface of the valve is ground smooth to fit its seat.
The extent to which such a valve should be capable of opening,
ought to be such that the aperture produced by it shall be equal
to the aperture which it stops. This will be effected if the angle
through which it rises be about 30°.
[Illustration: _Fig._ 46.]
The valve by which the air and water collected in the bottom of
the air-pump are admitted to pass through the air-pump piston is a
double clack, consisting of two semicircular plates, having the
hinges on the diameters of these semicircles, as represented in
_fig._ 46.
(131.) Of the valves which are opened by a motion perpendicular to
their seat, the most simple is a flat metallic plate, made larger
than the orifice which it is intended to stop, and ground so as to
rest in steam-tight contact with the surface surrounding the
aperture. Such a valve is usually guided in its perpendicular
motion by a spindle passing through its centre, and sliding in
holes made in cross bars extending above and below the seat of the
valve.
The conical steam-valves, which have been already described
(116.), usually called spindle-valves, are the most common of this
class. The best angle to be given to the conical seat is found in
practice to be 45°. With a less inclination the valve has a
tendency to be fastened in its seat, and a greater inclination
would cause the top of the valve to occupy [Pg229] unnecessary
space in the valve-box. The area, or transverse section of the
valve-box, should be rather more than double the magnitude of the
upper surface of the valve, in order to allow a sufficiently free
passage for the steam, and the play of the valve should be such as
to allow it to rise from its seat to a height not less than one
fourth of the diameter of its upper surface.
The valves coming under this class are sometimes formed as spheres
or hemispheres resting in a conical seat, and in such cases they
are generally closed by their own weight, and opened by the
pressure of the fluid which passes through them.
(132.) One of the advantages attending the use of slides, compared
with the other form of valves, is the simplicity with which the
same slide may be made to govern several passages, so that a
single motion with a slide may perform the office of two or more
motions imparted to independent valves.
In most modern engines the passage of the steam to and from the
cylinder is governed by slides of various forms, some of which we
shall now explain.
[Illustration: _Fig._ 47.]
(133.) In _figs._ 47. and 48. is represented a slide-valve
contrived by Mr. Murray of Leeds. A B is a steam-tight case
attached to the side of the cylinder; E F is a rod, which receives
an alternate motion, upwards and downwards, from the eccentric, or
from whatever other part of the engine is intended to move the
slide. This rod, passing through a stuffing-box, moves the slide G
upwards and downwards. S is the mouth of the steam pipe coming
from the boiler; T is the mouth of a tube or pipe leading to the
condenser; H is a passage leading to the top, and I to the bottom,
of the cylinder. In the position of the slide represented in
_fig._ 47., the steam coming from the boiler through S passes
through the space H to the top of the cylinder, while the steam
from the bottom of the cylinder passes through the space I into
the tube T, and goes to the condenser. When the rod [Pg230] E F
is raised to the position represented in _fig._ 48., then the
passage H is thrown into communication with the tube T, while the
passage I is made to communicate with the tube S. Steam,
therefore, passes from the boiler through I below the piston,
while the steam which was above the piston, passing through H into
T, goes to the condenser. Thus the single slide G performs the
office of the four valves described in (116.).
[Illustration: _Fig._ 48.]