[Illustration: FIG. 88.-Two arrangements for tube holes in multi
tubular boiler.]
The centres for the water-tubes, G and H, should be laid out, in
accordance with Fig. 88, on the tops of B and C and the lower part of
A, along lines scribed in the manner explained on p. 207. Tubes H must
be bent to a template to get them all of the same shape and length, and
all the tubes be prepared before any are put in place. If the tubes are
set 7/8 inch apart, centre to centre, instead of 1-1/4 inches, the
heating surface will be greatly increased and the furnace casing better
protected.
Assembling.—When all necessary holes have been made and are of the
correct size, begin by riveting and silver-soldering in the ends of the
drums. Next fix the cross tubes, E, taking care that they and B and C
form rectangles. Then slip the F, G, and H tubes half an inch into the
main drum, and support A, by means of strips passed between the G and H
tubes, in its correct position relatively to B and C. The E tubes can
now be pushed into B and C and silver-soldered. The supports may then
be removed, and the a and H tubes be got into position and secured.
Drum D then demands attention. The connecting tubes, KK, should be
silver-soldered in, as the boiler, if properly made, can be worked at
pressures up to 100 lbs. per square inch.
The casing is of 1/20-inch sheet iron, and in five parts. The back end
must be holed to allow A, B, and C to project 1 inch, and have a
furnace-door opening, and an airway at the bottom, 5 inches wide and 1
inch deep, cut in it. The airway may be provided with a flap, to assist
in damping down the fire if too much steam is being raised. In the
front end make an inspection opening to facilitate cleaning the tubes
and removing cinders, etc.
The side plates, m m, are bent as shown in Fig. 86, and bolted to a
semicircular top plate, n, bent to a radius of 6 inches. A slot, 1-1/2
inches wide and 11-1/2 inches long, must be cut in the top, n, to allow
it to be passed over drum D; and there must also be a 3 or 3-1/2 inch
hole for the chimney. A plate, p, covers in D. A little plate, o, is
slipped over the slot in n, and asbestos is packed in all round D. The
interior of the end, side, and the top plates should be lined with
sheet asbestos held on by large tin washers and screw bolts. To protect
the asbestos, movable iron sheets may be interposed on the furnace
side. These are replaced easily if burnt away. The pieces m m are bent
out at the bottom, and screwed down to a base-plate extending the whole
length of the boiler.
The fire-bars fill the rectangle formed by the tubes B, El, and E2. A
plate extends from the top of E2 to the front plate of the casing, to
prevent the furnace draught being “short circuited.”
Boiler Fittings.
[Illustration: FIG. 89.-Safety valve.]
Safety Valves.—The best all-round type is that shown in Fig. 89. There
is no danger of the setting being accidentally altered, as is very
possible with a lever and sliding weight. The valve should be set by
the steam gauge. Screw it down, and raise steam to the point at which
you wish the safety valve to act, and then slacken off the regulating
nuts until steam issues freely. The lock nuts under the cross-bar
should then be tightened up. In the case of a boiler with a large
heating surface, which makes steam quickly, it is important that the
safety-valve should be large enough to master the steam. If the valve
is too small, the pressure may rise to a dangerous height, even with
the steam coming out as fast as the valve can pass it.
[Illustration: FIG. 90.-Steam gauge and siphon.]
Steam Gauges.—The steam gauge should register pressures considerably
higher than that to be used, so that there may be no danger of the
boiler being forced unwittingly beyond the limit registered. A siphon
piece should be interposed between boiler and gauge (Fig. 90), to
protect the latter from the direct action of the steam. Water condenses
in the siphon, and does not become very hot.
[Illustration: FIG. 91.-Water gauge.]
Water Gauges should have three taps (Fig. 91), two between glass and
boiler, to cut off the water if the glass should burst, and one for
blowing off through. Very small gauges are a mistake, as the water
jumps about in a small tube. When fitting a gauge, put packings between
the bushes and the glass-holders, substitute a piece of metal rod for
the glass tube, and pack the rod tightly. If the bushes are now sweated
into the boiler end while thus directed, the gauge must be in line for
the glass. This method is advisable in all cases, and is necessary if
the boiler end is not perfectly flat.
Pumps.—Where a pump is used, the supply should enter the boiler below
low-water level through a non-return valve fitted with a tap, so that
water can be prevented from blowing back through the pump. As regards
the construction of pumps, the reader is referred to p. 164 and to
Chapter XXII.
Filling Caps.—The filling cap should be large enough to take the nozzle
of a good-sized funnel with some room to spare. Beat the nozzle out of
shape, to give room for the escape of the air displaced by the water.
The best form of filling cap has a self-seating ground plug, which, if
properly made, is steam-tight without any packing. If needed, asbestos
packing can easily be inserted between plug and cap.
Mud-holes.—All but the smallest boilers should have a mud-hole and plug
in the bottom at a point not directly exposed to the furnace. In Fig.
82 it is situated at the bottom of the barrel. In Figs. 86 and 87 there
should be a mud-hole in one end of each of the three drums, A, B, and
C. The plug may be bored at the centre for a blow-off cock, through
which the boiler should be emptied after use, while steam is up, and
after the fire has been “drawn.” Emptying in this way is much quicker
than when there is no pressure, and it assists to keep the boiler free
from sediment.
[Illustration: FIG. 92.—Steam cock.]
Steam Cocks.-The screw-down type (Fig. 92) is very preferable to the
“plug” type, which is apt to leak and stick.
Testing Boilers.—The tightness of the joints of a boiler is best tested
in the first instance by means of compressed air. Solder on an
all-metal cycle valve, “inflate” the boiler to a considerable pressure,
and submerge it in a tub of water. The slightest leak will be betrayed
by a string of bubbles coming directly from the point of leakage. Mark
any leaks by plain scratches, solder them up, and test again.
[Illustration: FIG. 94.—Benzoline lamp for model central-flue boiler.]
The boiler should then be quite filled with cold water, and heated
gradually until the pressure gauge has risen to over the working
pressure. There is no risk of an explosion, as the volume of the water
is increased but slightly.
The third test is the most important and most risky of all-namely, that
conducted under steam to a pressure well above the working pressure.
In order to carry out the test without risk, one needs to be able to
watch the steam-gauge from a considerable distance, and to have the
fire under control. My own method is to set the boiler out in the open,
screw down the safety-valve so that it cannot lift, and raise steam
with the help of a blow-lamp, to which a string is attached wherewith
to pull it backwards along a board. If the boiler is to be worked at 50
lbs., I watch the steam gauge through a telescope until 100 lbs. is
recorded, then draw the lamp away. After passing the test, the boiler,
when pressure has fallen, say, 20 lbs., may safely be inspected at
close quarters for leaks.
This test is the only quite satisfactory one, as it includes the
influence of high temperature, which has effects on the metal not shown
by “cold” tests, such as the hydraulic.
Do not increase your working pressure without first re-testing the
boiler to double the new pressure to be used.
Fuels.—For very small stationary boilers the methylated spirit lamp is
best suited, as it is smell-less, and safe if the reservoir be kept
well apart from the burner and the supply is controllable by a tap or
valve. (See Fig. 104.)
[Illustration: FIG. 95.-Paraffin burner for vertical boiler.]
For medium-sized model boilers, and for small launch boilers, benzoline
or petrol blow-lamps and paraffin stoves have become very popular, as
they do away with stoking, and the amount of heat is easily regulated
by governing the fuel supply. Fig. 94 is a sketch of a blow-lamp
suitable for the horizontal boiler shown on pp. 204, and 206, while
Fig. 95 shows a convenient form of paraffin stove with silent “Primus”
burner, which may be used for a horizontal with considerable furnace
space or for vertical boilers. In the case of all these liquid fuel
consumers, the amount of heat developed can be increased by augmenting
the number of burners. Where a gas supply is available its use is to be
recommended for small stationary boilers.
Solid Fuels.—The chief disadvantages attaching to these are smoke and
fumes; but as a solid fuel gives better results than liquid in a large
furnace, it is preferred under certain conditions, one of them being
that steam is not raised in a living room. Charcoal, coke, anthracite
coal, and ordinary coal partly burned are the fuels to use, the fire
being started with a liberal supply of embers from an open fire. Every
solid-fuel boiler should have a steam-blower in the chimney for drawing
up the fire; and if a really fierce blaze is aimed at, the exhaust from
the engine should be utilized for the same purpose.
XIX.
QUICK BOILING KETTLES.
[Transcriber’s note: Do not use lead solder on articles associated with
human or animal consumption.]
The principles of increasing the area of heating surface in model
boilers may be applied very practically to the common kettle. The
quick-boiling kettle is useful for camping out, for heating the morning
tea water of the very early riser, and for the study “brew,” which
sometimes has to be made in a hurry; and, on occasion, it will be so
welcome in the kitchen as to constitute a very useful present to the
mistress of the house.
As the putting in of the tubes entails some trouble, it is worth while
to select a good kettle for treatment. Get one that is made of thick
tinned sheet iron (cast-iron articles are unsuitable), or even of
copper, if you are intent on making a handsome gift which will last
indefinitely. The broad shallow kettle is best suited for tubing, as it
naturally has a fair heating surface, and its bottom area gives room
for inserting plenty of tubes. Also, the tubes can be of good length.
Let us, therefore, assume that the kettle will be of at least 8 inches
diameter.
In Figs. 96 (a) and 96 (b) are shown two forms of fire-tube kettles (a
and b) and two of water-tube (c and d). For use over a spirit or
Swedish petroleum stove the first two types are most convenient; the
third will work well on a stove or an open fire; and the last proves
very efficient on an open fire. One may take it that, as a general
rule, areas of heating surface being equal, the water-tube kettle will
boil more quickly than the fire-tube.
Fire-tube Kettles.
The tubing of Figs. 96 (a) and 96 (b) presents a little difficulty in
each case. The straight tube is the more difficult to insert, owing to
the elliptical shape of the ends; whereas the bent tube requires only
circular holes, but must be shaped on a template.
The tubing used for (a) should have at least 5/8-inch internal
diameter, for (b) 1/2 inch, and be of thin copper. Hot gases will not
pass willingly through tubes much smaller than this, in the absence of
induced or forced draught.
For convenience in fitting, the tubes should run at an angle of 45
degrees to the bottom and side of the kettle, as this gives the same
bevel at each end. Find the centre of the bottom, and through it
scratch plainly four diameters 45 degrees apart. From their ends draw
perpendiculars up the side of the kettle.
[Illustration: FIG. 96 (a).]
Now draw on a piece of paper a section of the kettle, and from what is
selected as a convenient water-level run a line obliquely, at an angle
of 45 degrees, from the side to the bottom. Measuring off from this
diagram, you can establish the points in the side and bottom at which
the upper and longer side of the tubes should emerge. Mark these off.
Next bevel off a piece of tubing to an angle of 45 degrees, cutting off
roughly in the first instance and finishing up carefully with a file
till the angle is exact. Solder to the end a piece of tin, and cut and
file this to the precise shape of the elliptical end. Detach by
heating, scribe a line along its longest axis, and attach it by a small
countersunk screw to the end of a convenient handle.
Place this template in turn on each of the eight radii, its long axis
in line with it, being careful that the plate is brought up to the
marks mentioned above, and is on the bottom corner side of it. Scratch
round plainly with a fine steel point.
To remove the metal for a tube hole, it is necessary to drill a
succession of almost contiguous holes as near the scratch as possible
without actually cutting it. When the ring is completed, join the holes
with a cold chisel held obliquely. Then file carefully with a round
file, just not cutting the scratch. As the side of the hole nearest to
the bottom corner should run obliquely to enable the tube to pass, work
this out with the file held at an angle.
As soon as a pair of holes (one in the bottom, the other in the side)
have been made, true up the side hole until a piece of tubing will run
through it at the correct angle. Then bevel off the end to 45 degrees
and pass the tube through again, bringing the bevel up against the
bottom hole from the inside. If it is a trifle difficult to pass, bevel
off the edge slightly on the inside to make a fairly easy driving fit.
(Take care not to bulge the bottom of the kettle.) Mark off the tube
beyond the side hole, allowing an eighth of an inch extra. Cut at the
mark, and number tube and hole, so that they may be paired correctly
later on.
When all the tubes are fitted, “tin” the ends with a wash of solder
before returning them to their holes. If there is a gap at any point
wide enough to let the solder run through, either beat out the tube
from the inside into contact, or, if this is impracticable, place a bit
of brass wire in the gap. Use powdered resin by preference as flux for
an iron kettle, as it does not cause the rusting produced by spirit of
salt. If the latter is used, wipe over the solder with a strong ammonia
or soda solution, in order to neutralize the acid.
As the hot gases may tend to escape too quickly through large tubes, it
is well to insert in the upper end of each a small “stop,” x—a circle
of tin with an arc cut away on the bottom side. To encourage the gases
to pass up the tubes instead of along the bottom, a ring of metal, y,
may be soldered beyond the bottom holes, if an oil or spirit stove is
to be used. This ring should have notches cut along the kettle edge, so
as not to throttle the flame too much.
[Illustration: FIG. 96—(b), (c), and (d).]
As the tubes for these require bending to shape in each case, the three
types may be grouped together. The tubes of c and d, which require
bending to somewhat sharp curves, may be of 3/8-inch internal diameter.
In the last two cases the direction of the water travel is shown. The
up-flow end, which projects farther through the bottom than the
down-flow, is nearer the centre, where, if a gas stove is used, the
heat is more intense than at the circumference of the bottom. (Note.-If
type c is for use on a three-support stove, increase the number of
tubes to 9, equally spaced, 40 degrees apart, so that the kettle may be
adjusted easily.)
The copper tubing should be annealed or softened by heating to a dull
red and plunging in cold water. Cut a wooden template of the exact
outline of the inside line of the shape that the tube is to assume, and
secure this firmly to a board. Fill the tube with melted resin, to
prevent, as much as possible, “buckling” or flattening on the curves.
The tube must be kept up to the template by a stop of hard wood, at the
end at which bending commences. Don’t cut the tube into lengths before
bending, as short pieces are more difficult to handle. When a piece
sufficient for a tube has been bent, cut it oft, and remove the resin
by heating.
The fitting of the tubes is an easy matter, as the holes are circular.
Pair off a tube with its holes and number it. A fluted reamer will be
found invaluable for enlarging them to the correct size. Tin all tubes
at points where they are to be attached to the kettle.
In Fig. 96 (c) and (d) care should be taken to make all the tubes
project the same distance, so that the kettle may be level when resting
on them.
XX.
A HOT-AIR ENGINE.
The pretty little toy about to be described is interesting as a
practical application to power-producing purposes of the force exerted
by expanding air. It is easy to make, and, for mere demonstration
purposes, has an advantage over a steam-engine of the same size in that
it can be set working in less than a minute, and will continue to act
as long as a small spirit flame is kept burning beneath it; it cannot
explode; and its construction is a simpler matter than the building of
a steam-engine.
[Illustration: FIG. 97.—Vertical section of hot-air engine.]
Principles of the Hot-air Engine.—Fig. 97 gives a sectional view of the
engine. The place of what would be the boiler in a steam-engine of
similar shape is taken by an air chamber immediately above the lamp,
and above that is a chamber through which cold water circulates. In
what we will call the heating chamber a large piston, known as the
displacer, is moved up and down by a rod D and a connecting rod CR1.
This piston does not touch the sides of the chamber, so that the bulk
of the air is pushed past it from one end of the chamber to the other
as the piston moves. When the displacer is in the position shown—at the
top of its stroke—the air is heated by contact with the hot plate C,
and expands, forcing up the piston of the power cylinder, seen on the
left of the engine. (The power crank and the displacer crank are, it
should be mentioned, set at right angles to one another.) During the
second half of the power stroke the displacer is moved downwards,
causing some of the air to pass round it into contact with the cold
plate D. It immediately contracts, and reduces the pressure on the
power piston by the time that the piston has finished its stroke. When
the power piston has reached the middle of its downward stroke, the
displacer is at its lowest position, but is halfway up again when the
power piston is quite down. The air is once again displaced downwards,
and the cycle begins anew. The motive power is, therefore, provided by
the alternate heating and cooling of the same air.
Construction.—The barrel and supports were made out of a single piece
of thin brass tubing, 2-7/16 inch internal diameter and 5-5/8 inch
long. The heating end was filed up true, the other cut and filed to the
shape indicated in Fig. 98 by dotted lines. The marking out was
accomplished with the help of a strip of paper exactly as wide as the
length of the tube, and as long as the tube’s circumference. This strip
had a line ruled parallel to one of its longer edges, and 2-1/2 inches
from it, and was then folded twice, parallel to a shorter edge. A
design like the shaded part of Fig. 98 was drawn on an end fold, and
all the four folds cut through along this line with a pair of scissors.
When opened out, the paper appeared as in Fig. 98.
[Illustration: FIG. 98.]
We now—to pass into the present tense—wrap this pattern round the tube
and scratch along its edges. The metal is removed from the two hollows
by cutting out roughly with a hack saw and finishing up to the lines
with a file.
The next things to take in hand are the displacer rod D and the guide
tube in which it works. These must make so good a fit that when
slightly lubricated they shall prevent the passage of air between them
and yet set up very little friction. If you cannot find a piece of
steel rod and brass tubing which fit close enough naturally, the only
alternative is to rub down a rod, slightly too big to start with, until
it will just move freely in the tube. This is a somewhat tedious
business, but emery cloth will do it. The rod should be 3-3/8 inches,
the tube 2-1/8 inches, long. I used rod 3/16 inch in diameter; but a
smaller rod would do equally well.
[Illustration: FIG. 99.]
The two plates, A and B, are next prepared by filing or turning down
thin brass[1] discs to a tight fit. (Note.—For turning down, the disc
should be soldered centrally to a piece of accurately square brass rod,
which can be gripped in a chuck. I used a specially-made holder like
that shown in Fig. 99 for this purpose.)
[Footnote 1: Thin iron plate has the disadvantage of soon corroding.]
When a good fit has been obtained, solder the two discs together so
that they coincide exactly, and bore a central hole to fit the guide
tube tightly. Before separating the plates make matching marks, so that
the same parts may lie in the same direction when they are put in
position. This will ensure the guide tube being parallel to the barrel.
The power cylinder is a piece of brass tubing 2 inches long and of
7/8-inch internal diameter. The piston is of 7/8-inch tubing, fitting
the cylinder easily, and thick enough to allow a shallow packing recess
to be turned in the outside. Brass washers turned or filed to size form
the ends of cylinder and piston. The connecting rod CR2 is a piece of
strip brass, 3-3/16 inches long, between centres of holes. This had
better be cut off a bit long in the first instance, and be fitted to
the little stirrup which attaches its lower end to the piston. The
drilling of the crank pinhole should be deferred till the cylinder and
crank are in position.
[Illustration: FIG. 100.—Exterior view of hot air engine.]
Putting in the Water-chamber Discs.—Clean the inside of the barrel
thoroughly with sandpaper; also discs A and B round the edges and the
central holes. Disc A is forced in from the crank end a little further
down than it is to be finally, and then driven up from below until at
all points its lower side is exactly three inches from the bottom edge
of the barrel. Disc B is then forced up 1-1/2 inches from the bottom
end. The guide tube— which should have been cleaned—having been driven
into place, solder is run all round the joints. If the barrel is heated
over a spirit lamp, this operation is performed very quickly. (“Tinol”
soldering paste is recommended.) Before soldering in B, drill a small
hole in the barrel between A and B to allow the air to escape.
Attaching the Cylinder.—Scratch a bold line through the centre of one
of the crank holes to the bottom of the barrel, to act as guide. Drill
a 5/32-inch hole in the barrel on this line just below plate B, and a
similar hole in the bottom of the cylinder. (The cylinder end should be
put in position temporarily while this is done to prevent distortion.)
Flatten down the cylinder slightly on the line of the hole, so that it
may lie snugly against the barrel, and clean the outside of the barrel.
Lay the cylinder against the barrel with the holes opposite one
another, and push a short piece of wood through to exclude solder from
the holes and keep the holes in line. Half a dozen turns of fine wire
strained tightly round cylinder and barrel will hold the cylinder in
place while soldering is done with a bit or lamp. The end of the
cylinder should then be made fast.
The Displacer.—This is a circular block of wood—well dried before
turning—5/8 inch thick and 3/32 inch less in diameter than the inside
of the barrel. The rod hole in it should be bored as truly central as
possible. A hole is drilled edgeways through the block and through the
rod to take a pin to hold the two together. To prevent it splitting
with the heat, make a couple of grooves in the sides to accommodate a
few turns of fine copper wire, the twisted ends of which should be
beaten down flush with the outside of the block. The bottom of the
block is protected by a disc of asbestos card held up to the wood by a
disc of tin nailed on.
The Crank Shaft and Crank.—The central crank of the crank shaft—that
for the displacer—has a “throw” of 1/4 inch, as the full travel of the
displacer is 1/2 inch. If the bending of a rod to the proper shape is
beyond the reader’s capacity, he may build up a crank in the manner
shown in Fig. 101. Holes for the shaft are bored near the tops of the
supports, and the shaft is put in place. After this has been done,
smoke the shaft in a candle flame and solder two small bits of tubing,
or bored pieces of brass, to the outside of the supports to increase
the length of the bearing. The power-crank boss is a 1-1/2-inch brass
disc. This crank has a “throw” of 1/2 inch.
[Illustration: FIG. 101.-Details of built-up crank.]
Connecting Rods.—Put a piece of card 1/16 inch thick in the bottom of
the cylinder and push the piston home. Turn the power crank down and
mark off the centre of the hole for the crank pin in the connecting rod
CR2. Solder a piece of strip brass on each side of the rod at this
point; measure again, and drill.
The top of the displacer rod D is now filed flat on two sides and
drilled. Slip a ring 1/16 inch thick over the rod and push the rod
upwards through the guide tube till the displacer can go no farther.
Turn the displacer crank up and measure from the centre of the hole in
the rod to the centre of the crank. The top of the connecting rod
should be filed to fit the under side of the crank, against which it
should be held by a little horseshoe-shaped strap pinned on. (Fig.
102). (Be sure to remove the ring after it has served its purpose.)
The Water Circulation.—The water chamber is connected by two rubber
tubes with an external tank. In Fig. 97 the cooling water tank is
shown, for illustrative purposes, on the fly-wheel side of the engine,
but can be placed more conveniently behind the engine, as it were. Two
short nozzles, E1 and E2, of 1/4-inch tube are soldered into the water
chamber near the top and bottom for the rubber pipes to be slipped
over, and two more on the water tank. For the tank one may select a
discarded 1 lb. carbide tin. Cut off the top and solder on a ring of
brass wire; make all the joints water-tight with solder, and give the
tin a couple of coatings of paint inside and outside.
[Illustration: FIG. 102.]
Closing the Hot-air Chamber.—When all the parts except the lamp chamber
have been prepared, assemble them to make sure that everything is in
order. The lower end of the hot-air chamber has then to be made
air-tight. Soldering is obviously useless here, as the heat of the lamp
would soon cause the solder to run, and it is impossible to make a
brazed joint without unsoldering the joints in the upper parts of the
engine. I was a bit puzzled over the problem, and solved it by means of
the lower part of an old tooth-powder box stamped out of a single piece
of tin. This made a tight fit on the outside of the barrel, and as it
was nearly an inch deep, I expected that if it were driven home on the
barrel and soldered to it the joint would be too near the water chamber
to be affected by the lamp. This has proved to be the case, even when
the water is nearly at boiling point. If a very close-fitting box is
not procurable, the space between box and barrel must be filled in with
a strip of tin cut off to the correct length.
The Lamp Chamber.—Cut out a strip of tin 4 inches wide and 1 inch
longer than the circumference of the lower end of the hot-air chamber.
Scratch a line 1/2 inch from one of the sides, a line 3/4 inch from the
other, and a line 1/2 inch from each of the ends.
A lamp hole is cut in the centre, and ventilation holes 1 inch apart,
as shown in Fig. 103. If the latter holes are made square or triangular
(base uppermost), and the metal is cut with a cold chisel so as to
leave the side nearest the edge unsevered, the parts may be turned up
to form supports for the barrel.
[Illustration: FIG. 103.—Plate for lamp chamber cut out ready for
bending.]
The slit lower side of the plate is splayed out into a series of
“feet,” by three or more of which, the chamber is secured to the base.
Bend the plate round the barrel and put the two screws and bolts which
hold the ends in place, and tighten them until the barrel is gripped
firmly. Screw the engine to its base, fit on the rubber water
connections, and fasten down the tank by a screw through the centre of
the bottom. The screw should pass through a brass washer, between which
and the tank should be interposed a rubber washer to make a water-tight
joint.
The Lamp.—The lamp shown in Fig. 104 was made out of a truncated brass
elbow, a piece of 5/16-inch brass tube, and a round tin box holding
about 1/3-pint of methylated spirit. A tap interposed between the
reservoir and burner assists regulation of the flame, and prevents
leakage when the lamp is not in use.
Running the Engine.—The power and displacer cranks must be set exactly
at right angles to one another, and the first be secured by soldering
or otherwise to the crank shaft. The fly wheel will revolve in that
direction in which the displacer crank is 90 degrees ahead of the
other.
[Illustration: FIG. l04.-Spirit lamp for hot-air engine, with
regulating tap.]
The packing of the piston should be sufficiently tight to prevent
leakage of air, but not to cause undue friction. When the packing has
settled into place, an occasional drop of oil in the cylinder and guide
tube will assist to make the piston and slide air-tight.
The engine begins to work a quarter of a minute or so after the lamp is
lit, and increases its speed up to a certain point, say 300 revolutions
per minute. When the water becomes very hot it may be changed. The
power might be applied, through demultiplying gear, to a small pump
drawing water from the bottom of the tank and forcing it through the
water chamber and a bent-over stand pipe into the tank again. This will
help to keep the water cool, and will add to the interest of the
exhibit by showing “work being done.”
XXI.
A WATER MOTOR.
FIG. 105 is a perspective view of a simple water motor which costs
little to make, and can be constructed by anybody able to use
carpenter’s tools and a soldering iron. It will serve to drive a very
small dynamo, or do other work for which power on a small scale is
required. A water supply giving a pressure of 40 lbs. upwards per
square inch must be available.
We begin operations by fashioning the case, which consists of three
main parts, the centre and two sides, held together by brass screws.
For the centre, select a piece of oak 1 inch thick. Mark off a square,
7 inches on the side; find the centre of this, and describe a circle 5
inches in diameter. A bulge is given to the circle towards one corner
of the square, at which the waste-pipe will be situated.
Cut out along the line with a keyhole saw. Then saw out the square of
wood. A 5/8-inch hole is now bored edgeways through the wood into the
“bulge” for the escape, and in what will be the top edge is drilled a
1/4-inch hole to allow air to enter.
[Illustration: FIG. l05.—Simple water turbine.]
Cut out the sides, and screw them on to the centre at the four corners,
taking care that the grain runs the same way in all three pieces, so
that they may all expand or contract in the same direction. Plane off
the edges of the sides flush with the centre.
The parts should now be separated, after being marked so that they can
be reassembled correctly, and laid for a quarter of an hour in a pan of
melted paraffin wax, or, failing this, of vaseline, until the wood is
thoroughly impregnated. Reassemble the parts, and put in the rest of
the holding screws, which should have their heads countersunk flush
with the wood.
[Illustration: FIG. 106.—Water turbine, with pulley side of casing
removed.]
For the shaft select a piece of steel rod 5/32 inch in diameter, and 3
or 4 inches long; for the bearings use two pieces, 3/4 inch long each,
of close-fitting brass tube. Now take a drill, very slightly smaller in
diameter than the bearings, and run holes right through the centres of,
and square to, the sides. Both holes should be drilled at one
operation, so that they may be in line.
With a wooden mallet drive the bearings, which should be tapered
slightly at the entering end, through the sides. Push the shaft through
them. If it refuses to pass, or, if passed, turns very unwillingly, the
bearings must be out of line; in which case the following operation
will put things right. Remove the bearing on the pulley side, and
enlarge the hole slightly. Then bore a hole in the centre of a metal
disc, 1 inch in diameter, to fit the bearing; and drill three holes for
screws to hold the disc against the case. Rub disc and bearing bright
all over.
Replace the bearing in its hole, slip the disc over it, and push the
shaft through both bearings. Move the disc about until the shaft turns
easily, mark the screw holes, and insert the screws. Finally, solder
the bearing to the disc while the shaft is still in place.
The wheel is a flat brass disc 4 inches in diameter. Polish this, and
scratch on one side twelve equally spaced radii. At the end of each
radius a small cup, made by bending a piece of strip brass 1/4 inch
wide and 1/2 inch long into an arc of a circle, is soldered with its
extremities on the scratch. A little “Tinol” soldering lamp (price 1s.
6d.) comes in very handy here.
To fix the wheel of the shaft requires the use of a third small piece
of tubing, which should be turned off quite square at both ends. Slip
this and the wheel on the shaft, and make a good, firm, soldered joint.
Note.— Consult Fig. 107 for a general idea of the position of the
wheel, which must be kept just clear of the case by the near bearing.
[Illustration: FIG. 107.—Plan of water turbine, showing arrangement of
nozzle.]
The nozzle should be a straight, tapered tube of some kind—the nose of
a large oil can will serve the purpose. The exit must be small enough
to allow the water to leave it at high velocity; if too large, the
efficiency of the wheel will be diminished. To the rear end of the
nozzle should be soldered a piece of brass tubing, which will make a
tight fit with the hose pipe leading from the water supply. A few small
brass rings soldered round this piece will prevent the hose blowing off
if well wired on the outside.
Now comes the boring of the hole for the nozzle. Fig. 106 shows the
line it should take horizontally, so that the water shall strike the
uppermost bucket just below the centre; while Fig. 107 indicates the
obliquity needed to make the stream miss the intervening bucket. A
tapered broach should be used to enlarge the hole gradually till the
nozzle projects sufficiently. If the line is not quite correct, the tip
should be bent carefully in the direction required. One must avoid
distorting the orifice, which should be perfectly circular; clean it
out with a small twist drill of the proper size.
A brass elbow, which may be purchased for a few pence, should be driven
into the waste hole, and a small shield be nailed under the air hole. A
couple of screwed-on cross pieces are required to steady the motor
sideways and raise the elbow clear of the ground.
The motor may be geared direct to a very small dynamo, if the latter is
designed to run at high speeds. If a geared-down drive is needed, a
small pulley—such as is used for blinds, and may be bought for a
penny—should be attached to the shaft, and a bootlace be employed as
belt. Avoid overloading the wheel, for if it is unable to run at a high
speed it will prove inefficient.
[Illustration: FIG. l08.-Water motor working a photographic
dish-rocker.]
Lubrication.—The water will keep the bearings cool, but the bearings
should be well lubricated. The most convenient method of effecting this
is to bore holes in the bearings, and from them run small pipes to an
oil reservoir on the top of the case (as in Fig. 70), where they are
fed on the siphon principle through strands of worsted.
Alternative Construction.—If an all-metal case is preferred, the reader
might utilize the description given of a steam turbine on pp. 170-178.
The details there given will apply to water as well as steam, the one
exception being that a nozzle of the kind described above must be
substituted for the steam pipe and small ports.
XXII.
MODEL PUMPS.
Every steam boiler which has to run for long periods and evaporate
considerable quantities of water should be in connection with a pump
capable of forcing water in against the highest pressure used. On a
previous page (p. 158) we have described a force pump driven directly
off the crank shaft of an engine. As the action of this is dependent on
the running of the engine, it is advisable, in cases where the boiler
may have to work an engine not provided with a pump of its own, to
install an independent auxiliary pump operated by hand or by steam, and
of considerable capacity, so that in an emergency water may be supplied
quickly.
[Illustration: FIG. l09.-Vertical section of force pump.]
Making a Hand pump.—Fig. 109 shows the details of a hand pump which is
easy to make. The barrel is a length of brass tubing; the plunger a
piece of brass or preferably gun-metal rod, which fits the tube
closely, but works easily in it. The gland at the top of the barrel, E,
is composed of a piece, D, of the same tubing as the barrel, sliding in
a collar, C, soldered to E. The bottom of D and top of E are bevelled
to force the packing against the plunger. The plates A and B, soldered
to D and C respectively, are drawn together by three or more screws. A
brass door-knob makes a convenient top for the plunger. When the knob
touches A, the bottom of the plunger must not come lower than the top
of the delivery pipe, lest the water flow should be impeded and the
valve, V, injured. Round off the end of the plunger, so that it may be
replaced easily and without disarranging the packing if pulled out of
the pump.
The valves are gun-metal balls, for which seats have been prepared by
hammering in steel cycle balls of the same size. Be careful to select
balls considerably larger than the bore of the pipes on which they
rest, to avoid all possibility of jamming. An eighth of an inch or so
above the ball, cross wires should be soldered in to prevent the ball
rising too far from its seat.
[Illustration: FIG. 110.]
A convenient mounting for a hand pump is shown in Fig. 110. The plate,
F, of the pump is screwed to a wooden base resting on a framework of
bent sheet zinc, which is attached to the bottom of a zinc water tray.
The delivery pipe, G, will be protected against undue strains if
secured by a strap to the side of the wooden base.
The same pump is easily adapted to be worked by a lever, which makes
the work of pumping easier. Fig. 111 gives details of the top of the
plunger and the links, B. A slot must be cut in the plunger for the
lever, A, to pass through, and the sides bored for a pivot pin. The
links are straddled (see sketch of end view) to prevent the back end of
the lever wobbling from side to side.
[Illustration: FIG. 111.—Details of lever for force pump.]
A Steam Pump.—The pump illustrated in Fig. 112 belongs to what is
probably the simplest self-contained type, as no fly wheel, crank, or
eccentric is needed for operating the valve.
The steam cylinder and the pump are set in line with one another (in
the case shown, horizontally), and half as far apart again as the
stroke of the cylinder. The plunger is either a continuation of the
piston rod, or attached to it.
[Illustration: FIG. 112—View of steam pump, showing details.]
An arm, S, fixed at right angles to the piston rod, has a forked end
which moves along the rod. This rod is connected with the slide valve
through the rocking arm, R1 and the rod, R2. On it are two adjustable
stops, T1 T2, which S strikes alternately towards the end of a stroke,
causing the valve to shift over and expose the other side of the piston
to steam pressure. The absence of the momentum of a fly wheel makes it
necessary for the thrust exerted by the piston to be considerably
greater than the back pressure of the water, so that the moving parts
may work with a velocity sufficient to open the valve. If the speed
falls below a certain limit, the valve opens only part way, the speed
falls, and at the end of the next stroke the valve is not shifted at
all.
The diameter of the plunger must be decided by the pressure against
which it will have to work. For boiler feeding it should not exceed
one-third that of the piston; and in such case the piston rod and
plunger may well be one.
A piston valve, being moved more easily than a box valve, is better
suited for a pump of this kind, as friction should be reduced as much
as possible.