a _push_, a flexible chain, cord, or strap, is sufficient; but if
a _push_ or _thrust_ is required to be transmitted, then the
flexibility of the medium of mechanical communication afforded by
a chain renders it inapplicable. In the double-acting engine,
during the descent, the piston-rod still pulls the beam down; and
so far a chain connecting the piston-rod with the beam would be
sufficient to transmit the action of the one to the other; but in
the ascent, the beam no longer pulls up the piston-rod, but is
pushed up by it. A chain from the piston-rod to the arch head, as
described in the single-acting engine, would fail to transmit this
force. If such a chain were used with the double engine, where
there is no counterweight on the opposite end of the beam, the
consequence would be, that in the ascent of the piston the chain
would slacken, and the beam would still remain depressed. It is
therefore necessary that some other mechanical connection be
contrived between the piston-rod and the beam, of such a nature
that in the _descent_ the piston-rod may _pull_ the beam down, and
may _push_ it up in the _ascent_.
[Illustration: _Fig._ 35.]
Watt first proposed to effect this by attaching to the end of the
piston-rod a straight rack, faced with teeth, which should work in
corresponding teeth raised on the arch head of the beam, as
represented in _fig._ 35. If his improved steam engines required no
further precision of operation and construction than the atmospheric
engines, this might have been sufficient; but in these engines it
was indispensably necessary that the piston-rod should be guided
with a smooth and even motion through the stuffing-box in the top of
the cylinder, otherwise any shake or irregularity would cause it to
work loose in the stuffing-box, and either to admit the air, or to
let the steam escape. Under these circumstances, the motion of
[Pg195] the rack and toothed arch head were inadmissible, since it
was impossible by such means to impart to the piston-rod that smooth
and equable motion which was requisite. Another contrivance which
occurred to Watt was, to attach to the top of the piston-rod a bar,
which should extend above the beam, and to use two chains or straps,
one extending from the top of the bar to the lower end of the arch
head, and the other from the bottom of the bar to the upper end of
the arch head. By such means the latter strap would pull the beam
down when the piston would descend, and the former would pull the
beam up when the piston would ascend. These contrivances, however,
were superseded by the celebrated mechanism since called the
_Parallel Motion_, one of the most ingenious mechanical combinations
connected with the history of the steam engine.
(119.) It will be observed that the object was to connect by some
inflexible means the end of the piston-rod with the extremity of
the beam, and so to contrive the mechanism, that while the end of
the beam would move alternately up and down in part of a circle,
the end of the piston-rod connected with the beam should move up
and down in a straight line. If the end of the piston-rod were
fastened upon the end of the beam by a pivot without any other
connection, it is evident that, being moved up and down in the
arch of a circle, it would be drawn to the left and the right
alternately, and would consequently either be broken or bent, or
would work loose in the stuffing-box. Instead of connecting the
end of the rod immediately with the end of the beam by a pivot,
Watt proposed to connect them by certain moveable rods, so
arranged that, as the end of the beam would move up and down in
the circular arch, the rods would so accommodate themselves to
that motion, that the end connected with the piston-rod should not
be disturbed from its rectilinear course.
To explain the principle of the mechanism called the parallel
motion, let us suppose that O P (_fig._ 36.) is a rod or lever
moveable on a centre O, and that the end P of this rod shall move
through a circular arch P P′ P″ P‴ a vertical plane, and let its
play be limited by two stops S, which shall prevent its ascent
above the point P, and its descent below [Pg196] the point P‴.
Let the position of the rod and the limitation of its play be such
that the straight line A B drawn through P and P‴, the extreme
positions of the lever O P, shall be a vertical line.
[Illustration: _Fig._ 36.]
Let _o_ be a point on the other side of the vertical line A B, and
let the distance of O to the right of A B be the same as the
distance of _o_ to the left of A B. Let _o p_ be a rod equal in
length to O P, moving like O P on the centre _o_, so that its
[Pg197] extremity _p_ shall play upwards and downwards through the
arch _p p′ p″ p‴_, its play being limited in like manner by stops
_s_.
Now, let us suppose that the ends P _p_ of these two rods are joined
by a link P _p_, the connection being made by a pivot, so that the
angles formed by the link and the rods shall be capable of changing
their magnitude. This link will make the motion of one rod depend on
that of the other, since it will preserve their extremities P _p_
always at the same distance from each other. If, therefore, we
suppose the rod O P to be moved to the position O P‴, its extremity
P tracing the arch P P′ P″ P‴, the link connecting the rods will at
the same time drive the extremity _p_ of the rod _o p_ through the
arch _p p′ p″ p‴_ so that when the extremity of the one rod arrives
at P‴, the extremity of the other rod will arrive at _p‴_. By this
arrangement, in the simultaneous motion of the rods, whether upwards
or downwards, through the circular arches to which their play is
limited, the extremities of the link joining them will deviate from
the vertical line A B in opposite directions. At the limits of their
play, the extremities of the link will always be in the line A B;
but in all intermediate positions, the lower extremity of the link
will be to the right of A B, and its upper extremity to the left of
A B. So far as the derangement of the lower extremity of the link is
concerned, the matter composing the link would be transferred to the
right of A B, and so far as the upper extremity of the link is
concerned, the matter composing it would be transferred to the left
of A B.
By the combined effects of these contrary derangements of the
extremities of the link from the vertical line, it might be
expected that a point would exist, in the middle of the link,
where the two contrary derangements would neutralise each other,
and which point would therefore be expected to be disturbed
neither to the right nor to the left, but to be moved upwards and
downwards in the vertical line A B. Such is the principle of the
parallel motion; and in fact the middle point of the link will
move for all practical purposes accurately in the vertical line A
B, provided that the angular play of the levers O P and _o p_ does
not exceed a certain [Pg198] limit, within which, in practice,
their motion may always be restrained.
To trace the motion of the middle point of the link more minutely,
let P P′ P″ P‴ be four positions of the lever O P, and let _p_ _p′_
_p″_ _p‴_ be the four corresponding positions of the lever _o p_. In
the positions O P _o p_, the link will take the position P _p_, in
which the entire link will be vertical, and its middle point _x_
will therefore be in the vertical line A B.
When the one rod takes the position O P′, the other rod will have
the position _o p′_; and the link will have the position P′ _p′_.
The middle point of the link will be at _x′_, which will be found
to be on the vertical line A B. Thus one half of the link P′ _x′_
will be to the left of the vertical line A B; while the other
half, _p′ x′_, will be to the right of the vertical line; the
derangement from the vertical line affecting each half of the link
in contrary directions.
Again, taking the one rod in the position O P″, the corresponding
position of the other rod will be _o p″_, and the position of the
link will be P″ _p″_. If the middle point of the link in this
position be taken, it will be found to be at _x″_, on the vertical
line A B; and, as before, one half of the link P″ _x″_ will be
thrown to the left of the vertical line, while the other half _p″
x″_, will be thrown to the right of the vertical line.
Finally, let the one rod be in its lowest position, O P‴, while
the other rod shall take the corresponding position, _o p‴_. The
direction of the link P‴ _p‴_ will now coincide with the vertical
line; and its middle point _x‴_ will therefore be upon that line.
The previous derangement of the extremities of the rod, to the
right and to the left, are now redressed, and all the parts of the
rod have assumed the vertical position.
It is plain, therefore, that by such means the alternate motion of
a point such as P or _p_, upwards and downwards in a circular
arch, may be made to produce the alternate motions of another
point _x_, upwards and downwards in a straight line.
(120.) Although the guidance of the air-pump rod in a true
vertical line is not so necessary as that of the steam piston,
[Pg199] and as the air-pump piston is always brought down by its
own weight and that of its rod, the connection of the air-pump
piston-rod with the beam, by any contrivance of the kind now
described, was not so necessary. Nevertheless, by a slight
addition to the mechanical contrivance which has been just
described, Watt obtained the means of at once preserving the true
rectilinear motion of both piston-rods.
[Illustration: _Fig._ 37.]
Let the lever represented by O P in _fig._ 36. be conceived to be
prolonged to twice its length, as represented in _fig._ 37., so
that O P′ shall be twice O P. Let the points P _p_ be connected by
a link as before. Let a link P′ _x′_, equal in length to the link
P _p_ be attached to the point P′, and let the extremity _x′_ of
this link be connected with the point _p_ by another link, equal
in length to P P′, by pivots at _x′_ and _p_, so that the figure P
P′ _x′ p_ shall be a jointed parallelogram, the angles of which
will be capable of altering their magnitude with every change of
position of the rods _o p_ and O P. Thus, when the rod O P
descends, the angles of the parallelogram at P and _x′_ will be
diminished in magnitude, while the angles at P′ and _p_ will be
increased in magnitude. Now, let a line be conceived to be drawn
from O to _x′_. It is evident that that line will pass through the
middle point of the link _p_ P, for the triangle O P _x_ is in all
respects similar to the greater triangle O P′ _x′_ only on half
the scale, so that every side of the one is [Pg200] half the
corresponding side of the other. Therefore P _x_ is half the
length of P′ _x′_; but P′ _x′_ was made equal to P _p_, and
therefore _p x_ is half of P _p_, that is to say, _x_ is the
middle point of P _p_.
It has been already shown, that in the alternate motion of the
rods _o p_, O P in ascending and descending, the point _x_ is
moved upwards and downwards in a true vertical line. Now since the
triangle O P _x_ is in all respects similar to O P′ _x′_, and
subject to a similar motion during the ascent and descent of the
rods, it is apparent that the point _x′_ must be subject to a
motion in all respects similar to that which affects the points
_x_, except that the point _x′_ will move through double the
space. In fact, the principle of the mechanism is precisely
similar to that of the common pantograph, where two rods are so
connected as that the motion of the one governs the motion of the
other, so that whatever line or figure may be described by one, a
similar line or figure must be described by the other. Since,
then, the point _x_ is moved upwards and downwards in a vertical
straight line, the point _x′_ will also be moved in a vertical
straight line of double the length.
If such an arrangement of mechanism as has been here described can
be connected with the beam of the steam engine, so that while the
point _x′_ is attached to the top of the steam piston, and the
space through which it ascends and descends shall be equal to the
length of the stroke of that piston, the point _x_ shall be
attached to the rod of the air-pump piston, the stroke of the
latter being half that of the steam piston, then the points _x′_
and _x_ will guide the motion of the two pistons so as to preserve
them in true vertical straight lines.
The manner in which these ideas are reduced to practice admits of
easy explanation: let the point O be the centre of the great
working beam, and let O P′ be the arm of the beam on the side of
the steam cylinder. Let P be a pivot upon the beam, at the middle
point between its centre O and its extremity P′; and let the links
P p, P′ _x′_, and P _p_ be jointed together, as already described.
Let the point or pivot _o_ be attached to some part of the fixed
framing of the engine or engine house, and let the rod _o p_,
equal to half the arm of the beam, be attached by a pivot to the
corner of the parallelogram at [Pg201] _p_. Let the end of the
steam piston-rod be attached to the corner of the parallelogram
_x′_, and let the end of the air-pump be attached to the middle
point _x_ of the link P _p_; by which arrangement it is evident
that the rectilinear motion of the two piston-rods will be
rendered compatible with the alternate circular motions of the
points P′ and P on the beam.
Among the many mechanical inventions produced by the fertile
genius of Watt, there is none which has excited such universal,
such unqualified, and such merited admiration as that of the
parallel motion. It is indeed impossible, even for an eye
unaccustomed to view mechanical combinations, to behold the beam
of a steam engine moving the pistons, through the instrumentality
of the parallel motion, without an instinctive feeling of pleasure
at the unexpected fulfilment of an end by means having so little
apparent connection with it. When this feeling was expressed to
Watt himself, by those who first beheld the performance of this
exquisite mechanism, he exclaimed with his usual vivacity, that he
himself, when he first beheld his own contrivance in action, was
affected by the same sense of pleasure and surprise at its
regularity and precision. He said, that he received from it the
same species of enjoyment that usually accompanies the first view
of the successful invention of another person.
"Among the parts composing the steam engine, you have doubtless,"
says M. Arago, "observed a certain articulated parallelogram. At
each ascent and descent of the piston, its angles open and close
with the sweetness—I had almost said with the grace—which charms
you in the gestures of a consummate actor. Follow with your eye
alternately the progress of its successive changes, and you will
find them subject to the most curious geometrical conditions. You
will see, that of the four angles of the jointed parallelogram,
three describe circular arches, but the fourth which holds the
piston-rod is moved nearly in a straight line. The immense utility
of this result strikes mechanicians with even less force than the
simplicity of the means by which Watt has attained it."
The parallel motion, of which there are several other varieties,
depending, however, generally upon the same [Pg202] principle,
formed part of a patent which Mr. Watt obtained in the year 1784,
another part of which patent was for a locomotive engine, by which
a carriage was to be propelled on a road. In a letter to Mr.
Smeaton dated 22d October, in the same year, Watt says,—
"I have lately contrived several methods of getting entirely rid
of all the chains and circular arches about the great levers of
steam engines, and nevertheless making the piston-rods ascend and
descend perpendicularly, without any sliding motions or
right-lined guides, merely by combinations of motions about
centres; and with this further advantage, that they answer equally
well to push upwards as to pull downwards, so that this method is
applicable to our double engines which act both in the ascent and
descent of their pistons.
"A rotative engine of this species with the new motion which is
now at work in our manufactory (but must be sent away very soon)
answers admirably. It has cost much brain work to contrive proper
working gear for these double engines, but I have at last done it
tolerably well, by means of the circular valves, placed in an
inverted position, so as to be opened by the force of the steam;
and they are kept shut by the working gear. We have erected an
engine at Messrs. Goodwyne and Co.'s brewery, East Smithfield,
London."
[Illustration: _Fig._ 38.]
(121.) By the contrivance which has been explained above, the
force of the piston in ascending and descending would be conveyed
to the working end of the beam; and the next problem which Watt
had to solve was, to produce by the force exerted by the working
end of the beam in ascending and descending a continuous motion of
rotation. In the first instance he proposed to accomplish this by
a crank placed upon the axle to which rotation was to be imparted,
and driven by a rod connecting it with the working end of the
beam. Let K (_fig._ 38.) be the centre, to which motion is to be
imparted by the working end H of the beam. On the axle K suppose a
short lever K I to be fixed so that when K I is turned round the
centre K, the axle must turn with it. Let an iron rod, the weight
of which shall balance the piston and piston-rod at the other end
of the beam, be connected by joints with the working end H of the
beam, and the extremity I of the [Pg203] lever K I. As the end H
of the beam is moved upwards and downwards, the lever K I will be
turned round the centre K, taking successively the positions
represented by faint lines in the figure; and thus a motion of
continued rotation will be imparted to the axle K.
This simple and effectual expedient of producing a continued
rotatory motion by a crank was abandoned by Watt, as already
explained, by reason of a patent having been obtained upon
information of his experiments surreptitiously procured. To avoid
litigation, he therefore substituted for the crank the sun and
planet wheel already described; but at the expiration of the
patent, which restricted the use of the crank, the sun and planet
wheel was discontinued in Watt's engine, and the crank restored.
(122.) Whether the crank or the sun and planet wheel be used,
there is still a difficulty in the maintenance of a regular motion
of rotation. In the various positions which the crank and
connecting rod assume throughout a complete revolution, there are
two in which the moving power loses all influence in impelling the
crank. These positions are those which the crank assumes when the
piston is at the top and bottom of the [Pg204] cylinder, and is
just about to change the direction of its motion. When the piston
is at the bottom of the cylinder, the pivot I (_fig._ 38.), by
which the connecting rod H I is attached to the end of the crank,
is immediately over the axle K of the crank, and under the pivot
H, which joins the upper end of the connecting rod with the beam.
In fact, in this position the connecting rod and crank are in the
same straight line, extending from the end of the beam to the axle
of the crank. The steam, on entering the cylinder below the
piston, and pressing it upwards, would produce a corresponding
downward force on the connecting rod at H, which would be
continued along the connecting rod and crank to the axle K. It is
evident that such a force could have no tendency to turn the crank
round, but would expend its whole energy in pressing the axle K
downwards.
The other position in which the power loses its effect upon the
crank is when the piston is at the top of the cylinder. In this
case, the working end of the beam will be at the lowest point of
its play, and the crank-pin I will be immediately below the axle
K; so that K will be placed immediately between H and I. When the
steam presses on the top of the piston, it will expend its force
in drawing the end H of the connecting rod upwards, by which the
crank-pin I will likewise be drawn upwards. It is evident that
this force can have no effect in turning the crank round, but will
expend its whole energy in producing an upward strain on the axle
K.
If the crank were absolutely at rest in either of the positions
above described, it is apparent that the engine could not be put
in motion by the steam; but if the engine has been previously in
motion, then the mass of matter forming the crank, and the axle on
which the crank is formed, having already had a motion of
rotation, will have a tendency to preserve the momentum it has
received, and this tendency will be sufficient to throw the crank
K I out of either of those critical positions which have been
described. Having once escaped these dead points, then the
connecting rod forming an angle, however obtuse or acute, with the
crank, the pressure or pull upon the former will have a tendency
to produce rotation in the latter. As the crank revolves, however,
the influence [Pg205] of the connecting rod upon it will vary
according to the angle formed by the connecting rod and crank.
When that angle is a right angle, then the effect of the
connecting rod on the crank is greatest, since the force upon it
has the advantage of the whole leverage of the crank; but
according as the angle formed by the crank and connecting rod
becomes more or less acute or obtuse in the successive attitudes
which they assume in the revolution of the crank, the influence of
the connecting rod over the crank varies, changing from nothing at
the two dead points already described, to the full effect produced
in the two positions where they are at right angles. In
consequence of this varying leverage, by which the force with
which the connecting rod is driven by the steam is transmitted to
the axle on which the crank revolves, a corresponding variation of
speed would necessarily be produced in the motion imparted to the
crank. The speed at the dead points would be least, being due
altogether to the momentum already imparted to the revolving mass
of the crank and axle; and it would gradually increase and be
greatest at the points where the effect of the crank on the
connecting rod is greatest. Although this change of speed would
not affect the actual mechanical efficacy of the machine, and
although the same quantity of steam would perform the same work at
the varying velocity as it would do if the velocity were
regulated, yet this variation of speed would be incompatible with
the purposes to which it was now proposed that the steam engine
should be applied in manufactures. In these a regular uniform
motion should be imparted to the main axle.
(123.) One of the expedients which Watt proposed for the
attainment of this end was, by placing two cranks on the same
axle, in different positions, to be worked by different cylinders,
so that while one crank should be at its dead points, the other
should be in the attitude most favourable for its action. This
expedient has since, as we shall see, been carried into effect in
steam vessels; but one more simple and efficient presented itself
in the use of a _fly-wheel_.
On the main axle driven by the crank Watt placed a large wheel of
metal, as represented in _fig._ 43., called a _fly-wheel_. This
wheel being well constructed, and nicely balanced on its [Pg206]
axle, was subject to very little resistance from friction; any
moving force which it would receive it would therefore retain, and
would be ready to impart such moving force to the main axle
whenever that axle ceased to be driven by the power. When the
crank, therefore, is in those positions in which the action of the
power upon it is most efficient, a portion of the energy of the
power is expended in increasing the velocity of the mass of matter
composing the fly-wheel. As the crank approaches the dead points,
the effect of the moving power upon the axle and upon the crank is
gradually enfeebled, and at these points vanishes altogether. The
momentum which has been imparted to the fly-wheel then comes into
play, and carries forward the axle and crank out of the dead
points with a velocity very little less than that which it had
when the crank was in the most favourable position for receiving
the action of the moving power.
By this expedient, the motion of revolution received by the axle
from the steam piston is subject to no other variation than just
the amount of change of momentum in the great mass of the
fly-wheel, which is sufficient to extricate the crank twice in
every revolution from the mechanical dilemma to which its peculiar
form exposes it; and this change of velocity may be reduced to as
small an amount as can be requisite by giving the necessary weight
and magnitude to the fly-wheel.