The various improvements described in the last chapter were
secured to Watt by patent in the year 1782. The engine now acquired an
enlarged sphere of action; for its dominion over manufactures was
decided by the _fly-wheel_, _crank_, and _governor_. By means of these
appendages, its motions were regulated with the most delicate
precision; so that while it retained a power whose magnitude was
almost unlimited, that power was under as exact regulation as the
motion of a time-piece. There is no species of manufacture, therefore,
to which this machine is not applicable, from the power which spins
the finest thread, or produces the most delicate web, to that which is
necessary to elevate the most enormous weights, or overcome the most
unlimited resistances. Although it be true, that in later times the
steam engine has received many improvements, some of which are very
creditable to the invention and talents of their projectors, yet it is
undeniable that all its great and leading perfections, all those
qualities by which it has produced such wonderful effects on the
resources of these countries, by the extension of manufactures and
commerce,–those qualities by which its influence is felt and
acknowledged in every part of the civilized globe, in increasing the
happiness, in multiplying the enjoyments, and cheapening the
pleasures of life,–that these qualities are due to the predominating
powers of one man, and that man one who possessed neither the
influence of wealth, rank, nor education, to give that first impetus
which is so often necessary to carry into circulation the earlier
productions of genius.
The method of working the valves of the double-acting steam engine, is
a subject which has much exercised the ingenuity of engineers, and
many elegant contrivances have been suggested, some of which we shall
now proceed to describe. But even in this the invention of Watt has
anticipated his successors; and the contrivances suggested by him are
those which are now almost universally used.
In order perfectly to comprehend the action of the several systems of
valves which we are about to describe, it will be necessary distinctly
to remember the manner in which the steam is to be communicated to the
cylinder, and withdrawn from it. When the piston is at the top of the
cylinder, the steam below it is to be drawn off to the condenser, and
the steam from the boiler is to be admitted above it. Again, when it
has arrived at the bottom of the cylinder, the steam above is to be
drawn off to the condenser, and the steam from the boiler is to be
admitted below it.
In the earlier engines constructed by Watt, this was accomplished by
four valves, which were opened and closed in pairs. Valve boxes were
placed at the top and bottom of the cylinder, each of which
communicated by tubes both with the steam-pipe from the boiler and the
condenser. Each valve-box accordingly contained two valves, one to
admit steam from the steam-pipe to the cylinder, and the other to
allow that steam to pass into the condenser. Thus each valve-box
contained a steam valve and an exhausting valve. The valves at the top
of the cylinder are called the _upper steam valve_ and the _upper
exhausting-valve_, and those at the bottom, the _lower steam valve_
and the _lower exhausting-valve_. In fig. 15. A´ is the upper steam
valve, which, when open, admits steam above the piston; B´ is the
upper exhausting-valve, which, when open, draws off the steam from the
piston to the condenser. C´ is the lower steam valve, which admits
steam below the piston; and D´, the lower exhausting-valve, which draws
off the steam from below the piston to the condenser.
Now, suppose the piston to be at the top of the cylinder, the cylinder
below it being filled with steam, which has just pressed it up. Let
the _upper steam valve_ A´, and the _lower exhausting-valve_ D´ be
opened, and the other two valves closed. The steam which fills the
cylinder below the piston will immediately pass through the valve D´
into the condenser, and a vacuum will be produced below the piston. At
the same time, steam is admitted from the steam-pipe through the valve
A´ above the piston, and its pressure will force the piston to the
bottom of the cylinder. On the arrival of the piston at the bottom of
the cylinder, the upper steam valve A´, and lower exhausting-valve D´,
are closed; and the lower steam valve C´, and upper exhausting-valve
B´ are opened. The steam which fills the cylinder above the piston now
passes off through B´ into the condenser, and leaves a vacuum above
the piston. At the same time, steam from the boiler is admitted
through the lower steam valve C´, below the piston, so that it will
press the piston to the top of the cylinder; and so the process is
continued.
It appears, therefore, that the upper steam valve, and the lower
exhausting-valve, must be opened together, on the arrival of the
piston at the top of the cylinder. To effect this, one lever, E´, is
made to communicate by jointed rods with both these valves, and this
lever is moved by a pin placed on the piston-rod of the air-pump; and
such a position may be given to this pin as to produce the desired
effect exactly at the proper moment of time. In like manner, another
lever, F´, communicates by jointed rods with the upper exhausting
valve and lower steam valve, so as to open them and close them
together; and this lever, in like manner, is worked by a pin on the
piston-rod of the air-pump.
(61.) This method of connecting the valves, and working them, has been
superseded by another, for which Mr. MURRAY of Leeds obtained a
patent, which was, however, set aside by Messrs. Bolton and Watt, who
showed that they had previously practised it. This method is
represented in figs. 18, 19. The stems of the valves are
perpendicular, and move in steam-tight sockets in the top of the
valve-boxes. The stem of the upper steam valve A is a tube through
which the stem of the upper exhausting-valve B passes, and in which it
moves steam-tight; both these stems moving steam-tight through the top
of the valve-box. The lower steam valve C, and exhausting-valve D, are
similarly circumstanced; the stem of the former being a tube through
which the stem of the latter passes. The stems of the upper steam
valve and lower exhausting-valve are then connected by a rod, E; and
those of the upper exhausting-valve and lower steam valve by another
rod, F. These rods, therefore, are capable of moving the valves in
pairs, when elevated and depressed. The motion which works the valves
is, however, not communicated by the rod of the air-pump, but is
received from the axis of the fly-wheel. This axis works an apparatus
called an _eccentric_; the principle which regulates the motion of
this may be thus explained:–
D E (figs. 20, 21.) is a circular metallic ring, the inner surface of
which is perfectly smooth. This ring is connected with a shaft, F B,
which communicates motion to the valves by levers which are attached
to it at B. A circular metallic plate is fitted in the ring so as to
be capable of turning within it, the surfaces of the ring and plate
which are in contact being smooth and lubricated with oil or grease.
This circular plate revolves, but not on its centre. It turns on an
axis C, at some distance from its centre A; the effect of which,
evidently, is that the ring within which it is turned is moved
alternately in opposite directions, and through a space equal to twice
the distance (C A) of the axis of the circular plate from the common
centre of it and the ring. The eccentric in its two extreme positions
is represented in figs. 20, 21. The plate and ring D E are placed on
the axis of the fly-wheel, or on the axis of some other wheel which is
worked by the fly-wheel. So that the motion of continued rotation in
the fly-wheel is thus made to produce an alternate motion in a
straight line in the shaft F B. This rod is made to communicate by
levers with the rods E and F (figs. 18, 19.), which work the valves in
such a manner, that, when the eccentric is in the position fig. 20.,
one pair of valves are opened, and the other pair closed; and when it
is brought to the position fig. 21., the other pair are opened and the
former closed and so on. It is by means of such an apparatus as this
that the valves are worked almost universally at present.
The piston being supposed to be at the top of the cylinder (fig. 18.),
and the rod E raised, the valves A and D are opened, and B and C
closed. The steam enters from the steam-pipe at an aperture
immediately above the valve A, and, passing through the open valve,
enters the cylinder above the piston. At the same time, the steam
which is below the piston, and which has just pressed it up, flows
through the open valve D, and through a tube immediately under it to
the condenser. A vacuum being thus produced below the piston, and
steam pressure acting above it, it descends; and when it arrives at
the bottom of the cylinder (fig. 19.) the rod F is drawn down, and the
valves A and D fall into their seats, and at the same time the rod F
is raised, and the valves B and C are opened. Steam is now admitted
through an aperture above the valve C, and passes below the piston,
while the steam above it passes through the open valve B into a tube
immediately under it, which leads to the condenser. A vacuum being
thus produced above the piston, and steam pressure acting below it,
the piston ascends, and thus the alternate ascent and descent is
continued by the motion communicated to the rods E F from the
fly-wheel.
[Illustration: Pl. V. WATT’S DOUBLE-ACTING STEAM ENGINE.]
[Illustration: Pl. VI.]
(_c_) An improvement has been made in the United States in the mode of
working the puppet valve. It consists in placing them by pairs in two
different vertical planes instead of one. The rods then work through
four separate stuffing boxes, and the necessity of making two of them
hollow cylinders is avoided.–A. E.
(62.) There are various other contrivances for regulating the
circulation of steam through the cylinder. In figs. 22, 23. is
represented a section of a slide valve suggested by Mr. Murray of
Leeds. The steam-pipe from the boiler enters the valve-box D E at S.
Curved passages, A A, B B, communicate between this valve-box and the
top and bottom of the cylinder; and a fourth passage leads to the tube
C, which passes to the condenser. A sliding piece within the valve-box
opens a communication alternately between each end of the cylinder and
the tube C, which leads to the condenser. In the position of the
apparatus in fig. 22. steam is passing from the steam-pipes, through
the curved passage A A above the piston, and at the same time the
steam below the piston is passing through the passage B B into the
tube C, and thence to the condenser. A vacuum is thus formed below the
piston, and steam is introduced above it. The piston, therefore,
descends; and when it arrives at the bottom of the cylinder, the slide
is moved into the position represented in fig. 23. Steam now passes
from S through B B below the piston, and the steam above it passes
through A A and C to the condenser. A vacuum is thus produced above
the piston, and steam pressure is introduced below it, and the piston
ascends; and in this way the motion is continued.
The slide is moved by a lever, which is worked by the eccentric from
the fly-wheel.
(63.) Watt suggested a method of regulating the circulation of steam,
which is called the D valve, from the resemblance which the horizontal
section of the valve has to the letter D. This method, which is very
generally used, is represented in section in figs. 24, 25. Steam from
the boiler enters through S. A rod of metal connects two solid plugs,
A B, which move steam-tight in the passage D. In the position of the
apparatus represented in fig. 24. the steam passes from S through the
passage D, and enters the cylinder above the piston; while the steam
below the piston passes through the open passage by the tube C to the
condenser. A vacuum is thus formed below the piston, while the
pressure of steam is introduced above it, and it accordingly descends.
When it has arrived at the bottom of the cylinder, the plugs A B are
moved into the position in fig. 25. Steam now passing from S through
D, enters the cylinder below the piston; while the steam which is
above the piston, and has just pressed it down, passes through the
open passage into the condenser. A vacuum is thus produced above the
piston, and the steam pressure below forces it up. When it has arrived
at the top of the cylinder, the position of the plugs A B is again
changed to that represented in fig. 24., and a similar effect to that
already described is produced, and the piston is pressed down; and so
the process is continued.
The plugs A B, and the rod which connects them, are moved up and down
by proper levers, which receive their motion from the eccentric.
This contrivance is frequently modified, by conducting the steam from
above the piston to the condenser, through a tube in the plugs A B,
and their connecting rod. In figs. 26, 27. a tube passes through the
plugs A B and the rod which joins them. In the position fig. 26. steam
entering at S passes through the tube to the cylinder above the
piston, while the steam below the piston passes through C into the
condenser. A vacuum being thus made below the piston, and steam
pressing above it, it descends; and when it has arrived at the bottom
of the cylinder, the position of the plugs A B and the tube is changed
to that represented at fig. 27. The steam now entering at S passes to
the cylinder below the piston, while the steam above the piston passes
through C into the condenser. A vacuum is thus produced above the
piston, and steam pressure introduced below it, so that it ascends.
When it has arrived at the top of the cylinder, the plugs are moved
into the position represented in fig. 26., and similar effects being
produced, the piston again descends; and so the motion is continued.
The motion of the sliding tube may be produced as in the former
contrivances, by the action of the eccentric. It is also sometimes
done by a bracket fastened on the piston-rod of the air-pump. This
bracket, in the descent of the piston, strikes a projection on the
valve-rod, and drives it down; and in the ascent meets a similar
projection, and raises it.
(64.) Another method, worthy of notice for its elegance and
simplicity, is the _four-way cock_. A section of this contrivance is
given in figs. 28, 29.: C T S B are four passages or tubes; S leads
from the boiler, and introduces steam; C, opposite to it, leads to the
condenser; T is a tube which communicates with the top of the
cylinder; and B one which communicates with the bottom of the
cylinder. These four tubes communicate with a cock, which is furnished
with two curved passages, as represented in the figures; and these
passages are so formed, that, according to the position given to the
cock, they may be made to open a communication between any two
adjacent tubes of the four just mentioned. When the cock is placed as
in fig. 28. communication is opened between the steam-pipe and the top
of the cylinder by one of the curved passages, and between the
condenser and the bottom of the cylinder by the other curved passage.
In this case the steam passes from below the piston to the condenser,
leaving a vacuum under it, and steam is introduced from the boiler
above the piston. The piston therefore descends; and when it has
arrived at the bottom of the cylinder, the position of the cock is
changed to that represented in fig. 29. This change is made by turning
the cock through one fourth of an entire revolution, which may be done
by a lever moved by the eccentric, or by various other means. One of
the curved passages in the cock now opens a communication between the
steam-pipe and the bottom of the cylinder; while the other opens a
communication between the condenser and the top of the cylinder. By
these means, the steam from the boiler is introduced below the piston,
while the steam above the piston is drawn off to the condenser. A
vacuum being thus made above the piston, and steam introduced below
it, it ascends; and when it has arrived at the top of the cylinder,
the cock being moved back, it resumes the position in fig. 28., and
the same consequences ensue, the piston descends; and so the process
is continued. In figs. 30, 31. the four-way cock with the passages to
the top and bottom of the cylinder is represented on a larger scale.
This beautiful contrivance is not of late invention. It was used by
Papin, and is also described by Leupold in his _Theatrum Machinarum_,
a work published about the year 1720, in which an engine is described
acting with steam of high pressure, on a principle which we shall
describe in a subsequent chapter.
The four-way cock is liable to some practical objections. The quantity
of steam which fills the tubes between the cock and the cylinder, is
wasted every stroke. This objection, however, also applies to the
sliding valve (figs. 22, 23.), and to the sliding tube or D valves
(figs. 24, 25, 26, 27.). In fact, it is applicable to every
contrivance in which means of shutting off the steam are not placed at
both top and bottom of the cylinder. Besides this, however, the
various passages and tubes cannot be conveniently made large enough to
supply steam in sufficient abundance; and consequently it becomes
necessary to produce steam in the boiler of a more than ordinary
strength to bear the attenuation which it suffers in its passage
through so many narrow tubes.
[Illustration: Pl. VII.]
One of the greatest objections, however, to the use of the four-way
cock, particularly in large engines, is its unequal wear. The parts of
it near the passages having smaller surfaces, become more affected
by the friction, and in a short time the steam leaks between the cock
and its case, and becomes wasted, and tends to vitiate the vacuum.
These cocks are seldom used in condensing engines, except they be
small engines, but are frequently adopted in high-pressure
steam-engines; for in these the leakage is not of so much consequence,
as will appear hereafter.
