All parts which have to be fitted together should have matching marks
made on them with the punch. To take the parts of the valve chest as an
example. As we have seen, these should be soldered together, finished
off outside, and drilled. Before separating them make, say, two punch
marks on what will be the upper edge of the valve plate near the end,
and two similar marks on the chest as near the first as they can
conveniently be. In like manner mark the chest cover and an adjacent
part of the chest with three marks. It is utterly impossible to
reassemble the parts incorrectly after separation if the marks are
matched. Marking is of greatest importance where one piece is held up
to another by a number of screws. If it is omitted in such a case, you
may have a lot of trouble in matching the holes afterwards.
Jacket the cylinder with wood or asbestos, covered in neatly with sheet
brass, to minimize condensation. If the steam ways, valve chest, and
steam pipe also are jacketed, an increase in efficiency will be gained,
though perhaps somewhat at the expense of appearance.
Boiler.—The boiler described on pp. 211-216, or a vertical multitubular
boiler with about 800 sq. inches of heating surface will drive this
engine satisfactorily.
XVI.
MODEL STEAM TURBINES.
Steam turbines have come very much to the fore during recent years,
especially for marine propulsion. In principle they are far simpler
than cylinder engines, steam being merely directed at a suitable angle
on to specially shaped vanes attached to a revolving drum and shaft. In
the Parsons type of turbine the steam expands as it passes through
successive rings of blades, the diameter of which rings, as well as the
length and number of the blades, increases towards the exhaust end of
the casing, so that the increasing velocity of the expanding steam may
be taken full advantage of. The De Laval turbine includes but a single
ring of vanes, against which the steam issues through nozzles so shaped
as to allow the steam to expand somewhat and its molecules to be moving
at enormous velocity before reaching the vanes. A De Laval wheel
revolves at terrific speeds, the limit being tens of thousands of turns
per minute for the smallest engines. The greatest efficiency is
obtained, theoretically, when the vane velocity is half that of the
steam, the latter, after passing round the curved inside surfaces of
the vanes, being robbed of all its energy and speed. (For a fuller
description of the steam turbine, see How It Works, Chap. III.,
pp.74-86.)
The turbines to be described work on the De Laval principle, which has
been selected as the easier for the beginner to follow.
A Very Simple Turbine.
We will begin with a very simple contrivance, shown in Fig. 67. As a
“power plant” it is confessedly useless, but the making of it affords
amusement and instruction. For the boiler select a circular tin with a
jointless stamped lid, not less than 4 inches in diameter, so as to
give plenty of heating surface, and at least 2-1/2 inches deep, to
ensure a good steam space and moderately dry steam. A shallow boiler
may “prime” badly, if reasonably full, and fling out a lot of water
with the steam.
Clean the metal round the joints, and punch a small hole in the lid,
half an inch from the edge, to give egress to the heated air during the
operation of soldering up the point or joints, which must be rendered
absolutely water-tight.
[Illustration: FIG. 67.—Simple steam turbine.]
For the turbine wheel take a piece of thin sheet iron or brass; flatten
it out, and make a slight dent in it an inch from the two nearest
edges. With this dent as centre are scribed two circles, of 3/4 and 1/2
inch radius respectively. Then scratch a series of radial marks between
the circles, a fifth of an inch apart. Cut out along the outer circle,
and with your shears follow the radial lines to the inner circle. The
edge is thus separated into vanes (Fig. 68), the ends of which must
then be twisted round through half a right angle, with the aid of a
pair of narrow-nosed pliers, care being taken to turn them all in the
same direction.
[Illustration: FIG. 68.—Wheel for steam turbine, showing one vane
twisted into final position.]
A spindle is made out of a large pin, beheaded, the rough end of which
must be ground or filed to a sharp point. Next, just break through the
metal of the disc at the centre with a sharpened wire nail, and push
the spindle through till it projects a quarter of an inch or so.
Soldering the disc to the spindle is most easily effected with a
blowpipe or small blow-lamp.
The Boiler.—In the centre of the boiler make a dent, to act as bottom
bearing for the spindle. From this centre describe a circle of 5/8-inch
radius. On this circle must be made the steam port or ports. Two ports,
at opposite ends of a diameter, give better results than a single port,
as equalizing the pressure on the vanes, so that the spindle is
relieved of bending strains. Their combined area must not, however,
exceed that of the single port, if one only be used. It is important to
keep in mind that for a turbine of this kind velocity of steam is
everything, and that nothing is gained by increasing the number or size
of ports if it causes a fall in the boiler pressure.
The holes are best made with a tiny Morse twist drill. As the metal is
thin, drill squarely, so that the steam shall emerge vertically.
For the upper bearing bend a piece of tin into the shape shown in Fig.
67. The vertical parts should be as nearly as possible of the same
length as the spindle. In the centre of the underside of the standard
make a deep dent, supporting the metal on hard wood or lead, so that it
shall not be pierced. If this accident occurs the piece is useless.
Place the wheel in position, the longer part of the spindle upwards,
and move the standard about until the spindle is vertical in all
directions. Scratch round the feet of the standard to mark their exact
position, and solder the standard to the boiler. The top of the
standard must now be bent slightly upwards or downwards until the
spindle is held securely without being pinched.
A 3/16-inch brass nut and screw, the first soldered to the boiler round
a hole of the same size as its internal diameter, make a convenient
“filler;” but a plain hole plugged with a tapered piece of wood, such
as the end of a penholder, will serve.
Half fill the boiler by immersion in hot water, the large hole being
kept lowermost, and one of the steam vents above water to allow the air
to escape.
A spirit lamp supplies the necessary heat. Or the boiler may be held in
a wire cradle over the fire, near enough to make the wheel hum. Be
careful not to over-drive the boiler. As a wooden plug will probably be
driven out before the pressure can become dangerous, this is a point in
favour of using one. Corrosion of the boiler will be lessened if the
boiler is kept quite full of water when not in use.
A Practical Steam Turbine.
The next step takes us to the construction of a small turbine capable
of doing some useful work. It is shown in cross section and elevation
in Fig. 69.
[Illustration: FIG. 69.—Model steam turbine, showing vertical cross
section (left) and external steam pipe (right).]
The rotor in this instance is enclosed in a case made up of two stout
brass discs, D and E, and a 3/4-inch length of brass tubing. The plates
should be 1/2-inch larger in diameter than the ring, if the bolts are
to go outside. The stouter the parts, within reason, the better. Thick
discs are not so liable to cockle as thin ones, and a stout ring will
make it possible to get steam-tight joints with brown-paper packing.
The wheel is a disc of brass, say, 1/25 inch thick and 4 inches in
diameter; the spindle is 3/16 inch, of silver steel rod; the bearings,
brass tubing, making a close fit on the rod.
If you cannot get the ring ends turned up true in a lathe—a matter of
but a few minutes’ work—rub them down on a piece of emery cloth
supported on a true surface, such as a piece of thick glass.
Now mark out accurately the centres of the discs on both sides, and
make marks to show which face of each disc is to be outside.
On the outside of both scribe circles of the size of the bearing tubes,
and other circles at the proper radius for the bolt hole centres.
On the outside of D scribe two circles of 2-inch and 1-11/16-inch
radius, between which the steam pipe will lie.
On the inside of D scribe a circle of 1-27/32-inch radius for the steam
ports.
On the outside of E mark a 7/8-inch circle for the exhaust pipe.
On the inside of both mark the circles between which the ring must lie.
Bolt Holes.—The marks for these, six or twelve in number, are equally
spaced on the outside of one plate, and the two plates are clamped or
soldered together before the boring is done, to ensure the holes being
in line. If the bolts are to screw into one plate, be careful to make
the holes of the tapping size in the first instance, and to enlarge
those in D afterwards. Make guide marks in the plates before
separating, between what will be the uppermost holes and the
circumference.
Bolts.—These should be of brass if passed inside the ring. Nuts are not
necessary if E is tapped, but their addition will give a smarter
appearance and prevent-the bolts becoming loose.
Bearings.—Bore central holes in the discs to a good fit for the
bearings, and prepare the hole for the exhaust pipe. This hole is most
easily made by drilling a ring of small holes just inside the mark and
cutting through the intervening metal.
For A, B, and C cut off pieces of bearing pipe, 1/2, 1/4, and 3/4 inch
long respectively, and bevel the ends of B and C as shown, to minimize
friction if they rub. File all other ends square. (Lathe useful here.)
Bore oil holes in B and C, and clear away all the “burr.” Make
scratches on the bearings to show how far they should be pushed through
the case.
Now assemble the case, taking care that the edge of the ring
corresponds exactly with the circles marked on the discs, and clean the
metal round the bearing holes and the bearings themselves. The last are
then placed in position, with the lubricating holes pointing upwards
towards the guide marks on the discs. Push the spindle rod through the
bearings, which must be adjusted until the rod can be revolved easily
with the fingers. Then solder in the bearing with a “Tinol” lamp.
The Wheel.—Anneal this well by heating to a dull red and plunging it in
cold water. Mark a circle of 1-1/4-inch radius, and draw radial lines
1/4 inch apart at the circumference from this circle to the edge. Cut
out along the lines, and twist the vanes to make an angle of about 60
degree with the central part, and bend the ends slightly backward away
from the direction in which the rotor will revolve. (The directions
given on p. 189 for making a steam top wheel can be applied here.)
Bore a hole in the centre to make a tight fit with the spindle, and
place the rotor in position, with piece B in contact on the C side. Get
everything square (rotation will betray a bad wobble), and solder the
three parts together with the blow-lamp.
Mount the rotor squarely by the spindle points between two pieces of
wood held lightly in the vice, and, with the aid of a gauge fixed to
the piece nearest the wheel, true up the line of the vanes. (Lathe
useful here.)
The Steam Pipe is 15 inches (or more) of 5/16-inch copper tubing, well
annealed. To assist the bending of it into a ring one needs some
circular object of the same diameter as the interior diameter of the
ring round which to curve it. I procured a tooth-powder box of the
right size, and nailed it firmly to a piece of board. Then I bevelled
off the end of the pipe to the approximately correct angle, laid it
against the box, and drove in a nail to keep it tight up. Bending was
then an easy matter, a nail driven in here and there holding the pipe
until the ring was complete. I then soldered the end to the standing
part, and detached the ring for flattening on one side with a file and
emery cloth. This done, I bored a hole through the tube at F to open up
the blind end of the ring.
Attaching the ring to disc D is effected as follows:—Tin the contact
faces of the ring and disc pretty heavily with solder, after making
poppet marks round the guide circles so that they may not be lost under
the solder. The ring must be pressed tightly against its seat while
heating is done with the lamp. An extra pair of hands makes things
easier at this point. Be careful not to unsolder the spindle bearing, a
thing which cannot happen if the bearing is kept cool by an occasional
drop or two of water. A little extra solder should be applied round the
points where the ports will be.
The Steam Ports.—These are drilled (with a 1/32-inch twist drill), at
an angle of about 30 degrees to the plate, along the circle already
scribed. If you have any doubt as to your boiler’s capacity, begin with
one hole only, and add a second if you think it advisable. As already
remarked, pressure must not be sacrificed to steam flow.
Lubricators.—These are short pieces of tubing hollowed at one end by a
round file of the same diameter as the bearings. A little “Tinol” is
smeared over the surfaces to be joined, and the lubricators are placed
in position and heated with the blow-lamp until the solder runs. To
prevent the oil flowing too freely, the lubricators should be provided
with airtight wooden plugs.
Escape Pipes.—The pipe for the exhaust steam is now soldered into disc
E, and a small water escape into the ring at its lowest point. This
pipe should be connected with a closed chamber or with the exhaust at a
point lower than the base of the turbine case.
Stirrup.—Fig. 69 shows a stirrup carrying a screw which presses against
the pulley end of the spindle. This attachment makes it easy to adjust
the distance between the rotor and the steam ports, and also
concentrates all end thrust on to a point, thereby minimizing friction.
The stirrup can be fashioned in a few minutes out of brass strip. Drill
the holes for the holding-on screws; drill and tap a hole for the
adjusting screw; insert the screw and centre it correctly on the
spindle point. Then mark the position of the two screw holes in E;
drill and tap them.
Feet are made of sheet brass, drilled to take the three (or two)
lowermost bolts, and bent to shape. Note.—A side and foot may be cut
out of one piece of metal. The difficulty is that the bending may
distort the side, and prevent a tight joint between side and ring.
Assembling.—Cut out two rings of stout brown paper a quarter of an inch
wide and slightly larger in diameter than the casing ring. In
assembling the turbine finally, these, after being soaked in oil,
should be inserted between the ring and the discs. Put in four screws
only at first, and get the ring properly centred and the bearings
exactly in line, which will be shown by the spindle revolving easily.
Then tighten up the nuts and insert the other bolts, the three lowest
of which are passed through the feet. Affix the pulley and stirrup, and
adjust the spindle longitudinally until the rotor just does not rub the
casing. The soldering on of the cap of A completes operations.
To get efficiency, heavy gearing down is needed, and this can be
managed easily enough with the help of a clockwork train, decreasing
the speed five or more times for driving a dynamo, and much more still
for slow work, such as pumping.
A More Elaborate Turbine.
[Illustration: FIG. 70.—Vertical section of steam turbine with formed
blades (left); outside view of turbine, gear side (right).]
The turbine just described can hardly be termed an efficient one, as
the vanes, owing to their simple formation, are not shaped to give good
results. We therefore offer to our readers a design for a small turbine
of a superior character. This turbine is shown in elevation and section
in Fig. 70. The casing is, as in the preceding instance, made up of
flat brass plates and a ring of tubing, and the bearings, BG1, BG2, of
brass tube. But the wheel is built up of a disc 3 inches in diameter,
round the circumference of which are 32 equally-spaced buckets, blades,
or vanes, projecting 5/8 inch beyond the edge of the disc. The wheel as
a whole is mounted on a spindle 3-1/8 inches long, to which it is
secured by three nuts, N1 N2 N3. One end of the spindle is fined down
to take a small pinion, P1, meshing with a large pinion, P2, the latter
running in bearings, BG3, in the wheel-case and cover. The drive of the
turbine is transmitted either direct from the axle of P2 or from a
pulley mounted on it.