WATT’S ENGINE.
I have already told you that Watt suggested the use of steam alternately
on each side of the piston; and carried it out by closing the top of
the cylinder, and allowing the rod of the piston to pass through a
stuffing-box or gland. I now have to explain to you how this alternate
admission of the steam may be effected.
You evidently require first an opening at the top and bottom of the
cylinder, communicating with the boiler, one only being open at a time;
but in this case, where is the steam to escape that was on one side
of the piston when the opposite side was being acted upon? It must go
somewhere, but evidently must not return to the boiler. Hence, some
method has to be contrived by which, when one end of the cylinder is
open to the boiler, the other may be open to the air or to the condenser
(in which the steam is cooled under Watt’s plan). Fig. 61 will, I think,
render clear one or two of these arrangements.
[Illustration: Fig. 61.]
The first is the four-way cock, a very simple contrivance, easily and
frequently used in models. You must first understand how a common water
or beer tap is made. Fig. 61, A, represents one in section, turned so
as to open the passage along the pipe to which it is attached; C is the
pipe in which is the tap, a conical tube of brass set upright, and with
a hole right and left made through it, fixed into a short horizontal
tube (generally cast with it in one piece). Into this fits very exactly
the conical plug B, also with a hole through it sideways. When this is
put into place, no water or other liquid can pass, unless the hole in
the plug is in the same direction with the hollow tube forming an open
passage. If a key is put on the square part of the plug, and it is turned
half round, the passage through the pipe will be closed. A steam tap
would be made in a similar manner, if its only office were to open and
close a passage in a tube. But we now want two passages closed and two
opened, and then the alternate pair closed and opened. This is cleverly
effected by a four-way cock.
At D is shown a section of the steam cylinder and piston, with the
stuffing-box and all complete. A pipe enters this at the top and bottom,
and another crosses it in the middle, making four passages. Shaded
black is the four-way cock, the white places showing the open channels
through the plug. When this plug stands as at D, steam can pass from
the boiler to the top of the cylinder only, above the piston, which it
drives downward; the steam below the piston escapes through the other
open-curved channel into the air, or to the condenser. Just as the piston
reaches the bottom of the cylinder, the tap is turned, and the passage
stands as seen at E. Steam now passes to the bottom below the piston,
driving it upward, and the steam above it, which has done its work,
passes outward through the other open channel of the tap.
You must understand that when Newcomen first set up his engine, a man had
to turn the taps at the proper moment; and it is said that one Humphrey
Potter, a boy, being left in charge, and getting tired of this work,
first devised means to make the engine itself do this, by connecting
strings tied to the handles of the taps to the beam that moved up and
down above his head. Beighton and others improved on this, and very soon
it became unnecessary for the attendant to do anything but keep up a good
fire, and attend to the quantity of water in the boiler, and the pressure
of the steam.
In the model I gave you of Newcomen’s engine, I purposely left the taps
to be moved by hand; but F of the present figure shows how, by bringing
them near together, and adding cogged wheels or pulleys, you would make
one handle answer for both; and I shall leave you to devise an easy
method of making the engine work this one handle for itself. When Watt
made his first engine, therefore, this work had been already done, and he
only had to improve upon it, and to make it work more accurately to suit
the engine designed by himself.
If you should chance to pay a visit to the Museum at South Kensington,
you may see, I believe, Watt’s original engine, if not Newcomen’s. The
cylinders are so large and cumbrous, that the wonder is they were ever
bored by the inefficient means then in use; and the beam is a most
unwieldy mass of timber and iron, that looks as if no power of steam
could ever have made it oscillate. Yet it was in its day a successful
engine, the wonder of the age; and did good work for its inventor and
purchaser. I strongly advise my readers to try and visit Kensington, for
there are many interesting models there, besides engines and appliances
of older days. They will thus learn what rapid progress has been made
since the days of Savery, Newcomen, and Watt; not only in the improvement
of the arrangement of the parts, but in the workmanship, which last is
mainly due to the invention of the slide-rest and planing-machine.
We must now return to the double-acting or real steam engine, and
consider a second means whereby the steam can be alternately admitted and
exhausted.
The four-way cock, already explained, was found to wear very considerably
in practice, and hence work loose, and a new contrivance, called the
slide-valve, soon took its place. Of this there are two patterns, the
long D-valve and the short one, which latter is used for locomotives.
There is also a form called a tappet-valve, often used for large
stationary engines, but which is noisy and subject to rapid wear. I shall
describe the long D first, in the form in which it would be most easily
made for a model engine.
The two ports by which steam passes to the cylinder are shown at _d_,
_e_, of H, Fig. 61. C is the passage to the boiler, K is that to the
condenser. These are openings in a tube smoothly bored within, and having
at the top a stuffing-box like that on the cylinder. Within this tube
works an inner one, _b_, having rings or projections at the ends fitting
perfectly, and which are packed with india-rubber, hemp (or, in modern
days, with metal), to make a close fit. In a model, two bosses of brass,
K, soldered on the tube and then turned, make the best packing. These
packed portions of the inner tube form the stoppers to the steam ports,
_e e_, alternately, at the top and bottom. The upper part of the inner
tube has a cross arm, 3, affixed, from the centre of which rises the
valve-rod by which it is moved up and down. In the position 1, the steam
can pass from _c_ round the tube to _d_, and thence to the top of the
cylinder to which _d_ is attached. The exhaust steam passes from _e_
below the piston by _k_ to the condenser. In the second position, 2, the
steam is evidently shut off from _d_, but can pass out at _e e_ below the
cylinder, while the communication is still open to the condenser from
_d_, through the middle of the tube to K. This is a very good form of
valve, because the exhaust is always open, and the motion is smooth and
equal.
[Illustration: Fig. 62.]
There are many modifications of the long D-valve, but the principle of
all is the same; I shall therefore describe the short slide-valve which
is nearly always used in the models which are purchased at the shops.
This, too, is the usual form of valve in locomotives, traction-engines,
and the majority of those in use for agricultural and similar purposes.
A, Fig. 62, is the cylinder as before in section with piston. A thick
piece is cast with the cylinder, on one side of it, having steam ports
also cast in it, which are here left white. The two as before go to
the top and bottom of the cylinder, and have no communication with the
central one, which is bored straight into the boss, and generally is
turned at right angles and connected with the condenser, or with a pipe
opening into the chimney of the engine to increase the draught by means
of the jets of steam, as is the case always in locomotives, or into the
air, which is less usual. Seen from behind, these ports are like B,
being cast and cut rectangular; and the face, B, is planed quite level,
which is absolutely necessary to the proper action of the slide-valve
which has to work upon it. This valve is a box of iron, C, with a wide
flange or rim, this flange being of sufficient width to close either
port. If this valve is placed as it stands when the engine is at rest,
_b_ covers the upper steam port, and _a_ the lower; while the exhaust or
middle port is open to the hollow part of the box. Now, if we slide the
valve downwards until the upper port is open, the other two will be in
communication, being united by being both together in the inside of this
box or valve. Suppose the valve then cased in, and that steam is admitted
from the boiler into the case, it is evident that such steam could freely
pass to the top of the cylinder above the piston to force it downwards,
while that which was below would escape by the lower port into the box,
and thence pass to the condenser. If, instead of pushing down the valve,
we had drawn it upwards, the lower port would have been opened, and the
upper and middle would have been brought into communication inside the
valve, and the contrary effect would have been produced upon the piston.
This is the arrangement adopted, and which will be clearly understood
from the following sectional drawing, D. _a_, _a_, is the thick casting
upon the cylinder, with the upper and lower steam ports, which end
towards the middle of the cylinder, with the third port lying between;
then _b_ is a section of the valve, in such a position that the flange
of it no longer covers the lower steam port, while the other two are
open together on the inside of the valve. The latter is cased in by the
valve-box, _e e_, in the back of which is the steam pipe _f_ coming from
the boiler. The valve-rod, which is moved by the engine, passes at _o_
through a stuffing-box. It is evidently necessary that this slide-valve
should fit, and work very smoothly and correctly against the face of the
ports, so as not to allow any escape of the steam. It is not, however,
packed in any way at the back (although springs have been sometimes
added), because, as the back is subjected to the full pressure of the
steam from the boiler, this keeps it quite close to its seat. The rod,
however, by which it is worked, might prevent this close contact of the
two surfaces if it was screwed into the valve; it is therefore made with
a cross, E, at the end, which falls into a notch in a boss cast upon the
back of the valve as seen at F. This allows a certain degree of play in
one direction, and permits the steam to press it close even after it has
become worn by use.
You will, I think, now clearly understand how steam can be admitted
alternately to the top and bottom of a cylinder, and how the exhausted
steam that has done its work escapes. I must therefore now tell you how
the rod of the slide-valve is moved up and down by the engine, but to do
this, I must draw such engine complete.
[Illustration: Fig. 63.]
The cylinder, A, is screwed down on its side upon the bed-plate, R R, out
of which are cut two holes, one for the fly-wheel, P, of which part only
appears for want of space, the other for the crank, L, on the end of the
axle, M M, running through bearings, N N. The slide-valve-box is at B,
C being the steam-pipe from the boiler. The piston-rod has necessarily
to move only in a straight line in the direction of its length, but
the crank which it has to work to turn the fly-wheel must needs move
round in a circle. Hence, a poker-and-tongs joint, F O F, is arranged.
The connecting-rod, H, which is attached to the crank by brasses at K,
divides or is attached to a forked piece, at the lower end of which are
a pair of bearings or brasses, F F. The piston-rod carries the piece O,
the cross-bar of which is turned, being, in fact, the pin which passes
into these bearings at F F. This forms, therefore, a hinge-joint at this
place, so that although the piston-rod cannot leave the right line, and
can only slide in the guide, E, the rod, H, has an up-and-down motion
upon this hinge, allowing the revolution of the crank-pin to take place.
D is the valve-rod, in which is a hinge at S, which suffices for the
slight movement required in the rod, as it rises and falls by the action
of the eccentric, T, the motion and effect of which I now have to explain.
V is a round disc of metal with a recess on its edge, so that it is like
an ordinary pulley, but large in proportion to its thickness. A hole
for the main crank axle, to which it has to be firmly keyed, is made
through it, but _not in its centre_ (hence its name, eccentric—out of the
centre). As the axle revolves, it is evident that this disc revolving
with it will carry any point, Y, of its surface round in a circle; the
centre of which is on the central line or axis of the crank-shaft. I have
drawn such circle as described by the point Y, farthest from the axis;
but any and all points describe larger or lesser circles round the same
centre. The point Y may, therefore, be considered as the centre of a
crank-pin; and the eccentric might, so far as its effects are concerned,
be replaced by a crank. Now, if you turn the fly-wheel of your lathe by
hand, the crank will revolve, but the treadle will rise and fall only
in a straight line; and you will presently see how the eccentric, in
its revolution, gives just such a to-and-fro motion to the rod D, and
consequently also to the slide-valve, which it has to move.
Round the disc V, closely encircling it, is a flat ring, shown separately
at X, with a rod, W, attached to and part of it. This ring is generally
made in separate halves, united by bolts passing through projecting lugs
or ears. The ring also fits into the groove turned on the edge of the
disc V, so that it cannot slip off sideways. This outer ring is turned
quite smooth and true on the inside, so that the eccentric disc can
revolve within it. In doing so, it is plain that the whole ring will
rise and fall, and that the rod W will move up and down, or to and fro,
like the treadle of the lathe, thereby giving motion to the valve-rod,
which is a continuation of the rod W. As the upper end, however, of this
rod has an oscillating, or up-and-down motion, this is imparted, in a
certain degree, to its other end, at the farthest distance from the
eccentric; and hence the necessity for a hinged joint at S, to prevent
the valve-rod from partaking of this movement. It is, however, very
slight, so that the rod of the valve is not often made to pass through
guides like the piston. The whole movement of the valve-rod is very
limited, its traverse only being required to be sufficient to shift
the valve the width of one of its ports at each stroke. The length of
_stroke_ or traverse which can be obtained by the eccentric is always
equal to twice the distance between its real centre, and that on which it
turns, which will always be a guide to you in making an engine.
[Illustration: Fig. 64.]
The drawing here described is a plan, _i.e._, a drawing viewed directly
from above; therefore I cannot show you the perspective view of the
parts, which are, indeed, in many cases only suggested by the shading.
I have, therefore, added a second drawing of the several details. This
engine is, in construction, the simplest that can be devised with
a slide-valve, there being no additions beyond what are absolutely
necessary to make it work; the exhaust-port is below, opposite to the
letter B on the valve-box. A, Fig. 64, is the forked connecting-rod,
marked H in the previous drawing. This is cast with forked ends, _x_, and
_x_ Y (the latter being F F of Fig. 63). These ends receive brasses in
the following way, the end _x_ being represented on a larger scale at B,
with such brasses in place; of these there are two shaped like D. One of
these lies in the fork of the connecting-rod end. A second similar one
lies in the strap of iron C, which reaches beyond the first. A cotter or
key, which is, in fact, a wedge of iron, is then passed through a slot
in the strap, and a similar one in the rod; and being driven home, draws
the two brasses tightly together, causing them to embrace the crank-pin,
L, Fig. 63, or any similar bearing. All shafts that revolve in bearings
are made to pass through brasses, and whenever these occur at the end
of a rod, they are fitted as here described. E is another bearing of
cast-iron, also fitted with brasses; but in a case like this, a plate
lies on the upper one, and is screwed down by bolts and nuts as required.
This bearing would do very well at E, Fig. 63, as a guide for the
piston-rod; but in models such guide is commonly made without brasses,
like F or G of the present drawing.
At H, I have shown the part F O F of the drawing 63. The middle is
of brass or iron; if of the former, _g g_ must be separate, as these
gudgeons would not be substantial enough, unless of iron or steel. It
is essential that K L, the piston-rod, should be in one right line;
but, if this is attended to, they need not necessarily be one piece;
and frequently the piston-rod, L, is fixed into one end of the central
casting, and another rod, K, is screwed into the other. In a model, the
piston-rod should pass quite through, and _g g_ should be two separate
gudgeons screwed in, and then turned together in the lathe, to insure
their being exactly in one line. These go into the brasses in the forked
ends of the connecting-rod, to form a hinge at that part, as will be
understood by a reference to Fig. 63.
At M, I have shown another simple eccentric and rod, which is less
trouble to make in a model than the other. In this the ring is made in
one piece, with a round rod screwing into it. The disc has a slight
groove turned in its edge, and a small screw, P, passes through the
ring and falls into this groove. This suffices to prevent the ring
from falling off sideways, and of course is not screwed down so tight
as to prevent the disc from revolving. This is a very easy way to fit
the eccentric, and is generally followed in small engines. The lattice
eccentric rod is nearly always used in large beam engines.
I do not think the reader will now have any difficulty in understanding
the precise arrangement of the various parts in the simple horizontal
engine of which I have given a sketch. It is a neat and convenient form,
easily arranged as a model, and I shall proceed at once to the practical
work of constructing this, and engines in general, presupposing a
knowledge of the use of the lathe, and of the few tools required.