[Illustration: FIG. 71.—Plate marked out for turbine wheel blades. B is
blade as it appears before being curved.]
The Wheel.—If you do not possess a lathe, the preparation of the
spindle and mounting the wheel disc on it should be entrusted to a
mechanic. Its diameter at the bearings should be 5/32 inch or
thereabouts. (Get the tubing for the bearings and for the spindle
turned to fit.) The larger portion is about twice as thick as the
smaller, to allow room for the screw threads. The right-hand end is
turned down quite small for the pinion, which should be of driving fit.
The Blades.—Mark out a piece of sheet iron as shown in Fig. 71 to form
32 rectangles, 1 by l/2 inch. The metal is divided along the lines
aaaa, bbbb, and ab, ab, ab, ab, etc. The piece for each blade then has
a central slot 5/16 inch long and as wide as the wheel disc cut very
carefully in it.
Bending the Blades.—In the edge of a piece of hard wood 1 inch thick
file a notch 3/8 inch wide and 1/8 inch deep with a 1/2-inch circular
file, and procure a metal bar which fits the groove loosely. Each blade
is laid in turn over the groove, and the bar is applied lengthwise on
it and driven down with a mallet, to give the blade the curvature of
the groove. When all the blades have been made and shaped, draw 16
diameters through the centre of the wheel disc, and at the 32 ends make
nicks 1/16 inch deep in the circumference.
True up the long edges of the blades with a file, and bring them off to
a sharp edge, removing the metal from the convex side.
Fixing the Blades.—Select a piece of wood as thick as half the width of
a finished blade, less half the thickness of the wheel disc. Cut out a
circle of this wood 2 inches in diameter, and bore a hole at the
centre. The wheel disc is then screwed to a perfectly flat board or
plate, the wooden disc being used as a spacer between them.
Slip a blade into place on the disc, easing the central slit, if
necessary, to allow the near edge to lie in contact with the board—that
is parallel to the disc. Solder on the blade, using the minimum of
solder needed to make a good joint. When all the blades are fixed, you
will have a wheel with the blades quite true on one side. It is,
therefore, important to consider, before commencing work, in which
direction the concave side of the blades should be, so that when the
wheel is mounted it shall face the nozzle.
To make this point clear: the direction of the nozzle having been
decided, the buckets on the trued side must in turn present their
concave sides to the nozzle. In Fig. 70 the nozzle points downwards,
and the left side of the wheel has to be trued. Therefore B1 has its
convex, B2 its concave, side facing the reader, as it were.
The Nozzle is a 1-1/2 inch piece of brass bar. Drill a 1/20-inch hole
through the centre. On the outside end, enlarge this hole to 1/8 inch
to a depth of 1/8 inch. The nozzle end is bevelled off to an angle of
20 degrees, and a broach is inserted to give the steam port a conical
section, as shown in Fig. 72, so that the steam may expand and gain
velocity as it approaches the blades. Care must be taken not to allow
the broach to enter far enough to enlarge the throat of the nozzle to
more than 1/20 inch.
[Illustration: FIG. 72.—Nozzle of turbine, showing its position
relatively to buckets.]
Fixing the Nozzle.—The centre of the nozzle discharge opening is
1-13/16-inches from the centre of the wheel. The nozzle must make an
angle of 20 degrees with the side of the casing, through which it
projects far enough to all but touch the nearer edges of the vanes.
(Fig. 72.) The wheel can then be adjusted, by means of the spindle
nuts, to the nozzle more conveniently than the nozzle to the wheel. To
get the hole in the casing correctly situated and sloped, begin by
boring a hole straight through, 1/4 inch away laterally from where the
steam discharge hole will be, centre to centre, and then work the walls
of the hole to the proper angle with a circular file of the same
diameter as the nozzle piece, which is then sweated in with solder. It
is, of course, an easy matter to fix the nozzle at the proper angle to
a thin plate, which can be screwed on to the outside of the casing, and
this method has the advantage of giving easy detachment for alteration
or replacement.
Balancing the Wheel.—As the wheel will revolve at very high speed, it
should be balanced as accurately as possible. A simple method of
testing is to rest the ends of the spindle on two carefully levelled
straight edges. If the wheel persists in rolling till it takes up a
certain position, lighten the lower part of the wheel by scraping off
solder, or by cutting away bits of the vanes below the circumference of
the disc, or by drilling holes in the disc itself.
Securing the Wheel.—When the wheel has been finally adjusted relatively
to the nozzle, tighten up all the spindle nuts hard, and drill a hole
for a pin through them and the disc parallel to the spindle, and
another through N3 and the spindle. (Fig. 70.)
Gearing.—The gear wheels should be of good width, not less than 3/16
inch, and the smaller of steel, to withstand prolonged wear. Constant
lubrication is needed, and to this end the cover should make an
oil-tight fit with the casing, so that the bottom of the big pinion may
run in oil. To prevent overfilling, make a plug-hole at the limit
level, and fit a draw-off cock in the bottom of the cover. If oil ducts
are bored in the bearing inside the cover, the splashed oil will
lubricate the big pinion spindle automatically.
[Illustration: FIG. 73.—Perspective view of completed turbine.]
General—The sides of the casing are held against the drum by six screw
bolts on the outside of the drum. The bottom of the sides is flattened
as shown (Fig. 70), and the supports, S1 S2, made of such a length that
when they are screwed down the flattened part is pressed hard against
the bed. The oil box on top of the casing has a pad of cotton wool at
the bottom to regulate the flow of oil to the bearings. Fit a drain
pipe to the bottom of the wheel-case.
Testing.—If your boiler will make steam above its working pressure
faster than the turbine can use it, the nozzle may be enlarged with a
broach until it passes all the steam that can be raised; or a second
nozzle may be fitted on the other end of the diameter on which the
first lies. This second nozzle should have a separate valve, so that it
can be shut off.
XVII.
STEAM TOPS.
A very interesting and novel application of the steam turbine principle
is to substitute for a wheel running in fixed bearings a “free” wheel
pivoted on a vertical spindle, the point of which takes the weight, so
that the turbine becomes a top which can be kept spinning as long as
the steam supply lasts.
These toys, for such they must be considered, are very easy to make,
and are “warranted to give satisfaction” if the following instructions
are carried out.
A Small Top.—Fig. 74 shows a small specimen, which is of the
self-contained order, the boiler serving as support for the top.
[Illustration: FIG. 74.-Simplest form of steam top.] [1]
[Footnote 1: Spirit lamp shown for heating boiler.]
For the boiler use a piece of brass tubing 4 inches or so in diameter
and 3 inches long. (The case of an old brass “drum” clock, which may be
bought for a few pence at a watchmaker’s, serves very well if the small
screw holes are soldered over.) The ends should be of brass or zinc,
the one which will be uppermost being at least 1/16 inch thick. If you
do not possess a lathe, lay the tube on the sheet metal, and with a
very sharp steel point scratch round the angle between tube and plate
on the inside. Cut out with cold chisel or shears to within 1/16 inch
of the mark, and finish off carefully—testing by the tube now and
then—to the mark. Make a dent with a centre punch in the centre of the
top plate for the top to spin in.
[Illustration: FIG. 75.—Wheel of steam top, ready for blades to be
bent.
A hole is drilled at the inner end of every slit to make bending
easier.]
Solder the plates into the tube, allowing an overlap of a quarter of an
inch beyond the lower one, to help retain the heat.
The top wheel is cut out of a flat piece of sheet iron, zinc, or brass.
Its diameter should be about 2-1/2 inches, the vanes 1/2 inch long and
1/4 inch wide at the circumference. Turn them over to make an angle of
about 45 degrees with the spindle. They will be more easily bent and
give better results if holes are drilled, as shown in Fig. 75.
The spindle is made out of a bit of steel or wire—a knitting-needle or
wire-nail—not more than 1 inch in diameter and 1-1/2 inches long. The
hole for this must be drilled quite centrally in the wheel; otherwise
the top will be badly balanced, and vibrate at high speeds. For the
same reason, the spindle requires to be accurately pointed.
The steam ports are next drilled in the top of the boiler. Three of
them should be equally spaced (120 degrees apart) on a circle of 1-inch
radius drawn about the spindle poppet as centre. The holes must be as
small as possible—1/40 to 1/50 inch—and inclined at an angle of not
more than 45 degrees to the top plate. The best drills for the purpose
are tiny Morse twists, sold at from 2d. to 3d. each, held in a pin vice
rotated by the fingers. The points for drilling should be marked with a
punch, to give the drills a hold. Commence drilling almost vertically,
and as the drill enters tilt it gradually over till the correct angle
is attained.
If a little extra trouble is not objected to, a better job will be made
of this operation if three little bits of brass, filed to a triangular
section (Fig. 76 a), are soldered to the top plate at the proper
places, so that the drilling can be done squarely to one face and a
perfectly clear hole obtained. The one drawback to these additions is
that the vanes of the turbine may strike them. As an alternative,
patches may be soldered to the under side of the plate (Fig. 76, b)
before it is joined to the barrel; this will give longer holes and a
truer direction to the steam ports.
[Illustration: FIG. 76. Steam port details.]
Note that it is important that the ports should be all of the same
diameter and tangential to the circle on which they are placed, and all
equally inclined to the plate. Differences in size or direction affect
the running of the top.
Solder the spindle to the wheel in such a position that the vanes clear
the boiler by an eighth of an inch or so. If tests show that the top
runs quite vertically, the distance might be reduced to half, as the
smaller it is the more effect will the steam jets have.
A small brass filler should be affixed to the boiler halfway up. A
filler with ground joints costs about 6d.
A wick spirit lamp will serve to raise steam. Solder to the boiler
three legs of such a length as to give an inch clearance between the
lamp wick and the boiler. If the wick is arranged to turn up and down,
the speed of the top can be regulated.
A Large Top.—The top just described must be light, as the steam driving
it is low-pressure, having free egress from the boiler, and small, as
the steam has comparatively low velocity. The possessor of a
high-pressure boiler may be inclined to make something rather more
ambitious—larger, heavier, and useful for displaying spectrum discs,
etc.
The top shown in Fig. 77 is 3 inches in diameter, weighs 1 oz., and was
cut out of sheet-zinc. It stands on a brass disc, round the
circumference of which is soldered a ring of 5/32-inch copper tubing,
furnished with a union for connection with a boiler.
[Illustration: FIG. 77.—-Large steam top and base.]
The copper tubing must be well annealed, so as to bend quite easily.
Bevel off one end, and solder this to the plate. Bend a couple of
inches to the curve of the plate, clamp it in position, and solder; and
so on until the circle is completed, bringing the tube snugly against
the bevelled end. A hole should now be drilled through the tube into
this end—so that steam may enter the ring in both directions-and
plugged externally.
By preference, the ring should be below the plate, as this gives a
greater thickness of metal for drilling, and also makes it easy to
jacket the tube by sinking the plate into a wooden disc of somewhat
greater diameter.
Under 50 lbs. of steam, a top of this kind attains a tremendous
velocity. Also, it flings the condensed steam about so indiscriminately
that a ring of zinc 3 inches high and 18 inches in diameter should be
made wherewith to surround it while it is running.
If a little bowl with edges turned over be accurately centred on the
wheel, a demonstration of the effects of centrifugal force may be made
with water, quicksilver, or shot, which fly up into the rim and
disappear as the top attains high speed, and come into sight again when
its velocity decreases to a certain figure. A perforated metal globe
threaded on the spindle gives the familiar humming sound.
A spectrum disc of the seven primary colours—violet, indigo, blue,
green, yellow, orange, red—revolved by the top, will appear more or
less white, the purity of which depends on the accuracy of the tints
used.
XVIII.
MODEL BOILERS.
A chapter devoted to the construction of model boilers may well open
with a few cautionary words, as the dangers connected with
steam-raisers are very real; and though model-boiler explosions are
fortunately rare, if they do occur they may be extremely disastrous.