This page has
my notes on model steam turbines. I haven't built one yet, but plan to
Be careful with steam and especially steam turbine engines. Engines operate
off energy and can generate significant forces. Steam explosions, bearing
failures, and other maladies can be extremely hazardous!
A full on gas turbine is on
my list as well, but model steam turbines are much simpler to build. I
believe Bogstandard was to first to make me aware they even existed through
his posts on the HMEM board, so I dedicate this page as a tribute to him
and his wonderful "blingy" projects.
There are two
kinds of turbine, the impulse and reaction varieties:
Two kinds of
Steam Turbine. Most models will be "impulse" turbines...
Most models will
be of the impulse variety due to the difficulties of producing the complex
curves for a reaction turbine. Most commercial turbines are reaction.
With an impulse turbine, steam velocity is everything, but there are tradeoffs.
An air powered engine will generate the most velocity with a converging
nozzle such as is described on the three rotor turbine below. However,
steam is heated and can expand. In that case, providing more volume just
before injection to the rotor blades converts as much heat as possible
to velocity. So, a diverging nozzle (such as the "trumpet" nozzle
on de Laval's turbine below) works better there.
is operating efficiency versus RPM. One of Parson's essential insights
was to use successive stages that each expanded the steam a bit more to
extract maximum energy from the steam expansion as well as designing a
turbine that could run at a little slower rpm's than his predecessor's
turbine Dr de Laval:
The de Laval
turbine used trumpet shaped nozzles to extract most of the expansion energy
from the steam, but it ran at velocities that were too high for the materials
of the time except in very small applications. Might make for an interesting
In fact, it is
the use of multiple stages and more gradual steam expansion among the
stages that leads to the greater efficiency at slower speeds of Parson's
design. The different diameters at each stage are what allow for expansion
on this 70,000 HP steam turbine rotor:
diameters allow for steam expansion...
Here is a rotor
from Parson's first turbine:
rotor must have been difficult to construct without CNC!
The blades were
made of drawn brass. Each blade was fitted into a groove with a wedge,
called a "distance" piece in the drawing. Here is some more
on the labyrinth passages that connect each stage:
redirect the steam direction for the next stage...
In modern times
I'm sure the shape of the blades is carefully predicated on aerodynamics
similar to those used to design airplane wings, but in Parson's time,
no such understanding was available. His shapes were trial and error combined
with intuition. Nevertheless, they were not bereft of sophisticated knowledge
in other ways. For example, they determined that the optimal velocity
of the blade was 1/2 to 3/4 of the velocity of the steam, hence the very
high operating speeds of these turbines. Parson's first model ran at 18,000
Isn't this a
lovely curvaceous Parsons' turbine?
The model steam
turbines I've seen are not Parson's turbines, they are impulse turbines,
and hence the wind up to ridiculously high velocities. Enough to blow
the bearings, so be careful! It would be fascinating to see a model Parsons,
but the issue would be in the much increased complexity of many rotors
for gradual expansion as well as complex shapes to machine on the rotor
and stator blades. Certainly not impossible, as we see model gas turbines,
just not easy.
Single Rotor Impulse Steam Turbine
Bogstandard is (was, he may
have left the forum) one of the real geniuses on the HMEM board. I learned
a lot of things from him and always admired the fine craftsmanship of
his model engines.
Here are photos from the
first thread, a single rotor turbine that's really cool.
and machining the "buckets" on the rotor...
the steam ports on the housing. The first version had perspex on both
I like the final
version better. The back side is perspex. There is a little Swiss motor
set up as a generator, and as the rpm increases, you see the LEDs light
up one by one. The model is beautiful, and shows Bogstandard's trademark
"bling" touches to make it more decorative.
This one was made "on
spec" for a model boater. Ultimately they concluded that the space
available on the boat and the power characteristics made it impractical.
These turbines don't make much power until they get over 10K rpm, and
the more rpm you give them, the more prone to runaway they are. A 15 or
20:1 gear reduction would be needed as well as some form of speed governor
to prevent the turbine from blowing up its bearings. Here is what it looks
like when a model turbine blows:
Note the shredded
housing as the rotor blew. Excercise care running these little engines
to avoid exceeding their limits!
Getting back to
the three rotor, here are some photos (many more on this one!):
how the leftmost rotor is a reversing rotor, so the "buckets"
go the other way?
basic shape in phosphor bronze
is drilled to lighten it...
Areas in green
on the shaft are knurled to hold the shaft and rotor together...
With the shaft
press fitted, skim cuts are taken to ensure balance relative to the shaft...
Now the buckets
are being cut. It comes out looking quite exotic and beautiful, no?
A squared block
is placed in the lathe and aligned with the pump center...
First the big
drill bits, then boring. Here is a trick if you don't keep your boring
bars all in holders. Use a regular tool that is known to be set correctly
to center to mark the workpiece...
Now line up
your bar on that mark. Clever!
fits just right after boring. Note the balancing holes on that rotor.
are 1/8" followed by a 3/16" endmill to countersink an area
for the connector to seat in. Next is the dividing head work. The face
marked "top" was aligned along the rotational axis with a machinists
square. The dial indicator is swept to be sure the axis is parallel to
the mill's X-axis...
Now the mill
is lined up on the original holes, the table is moved 3/8", and the
head is rotated 20 degrees to achieve the correct tangential angle for
the inlet port....
Here are the
exhaust ports, which are much larger than the inlets to keep back pressure
Next up are
the endcaps. The housing is scribed to the endcap material and then a
circle is drawn around with a dividers...
are trimmed close to the outline on the bandsaw to save too much interrupted
cutting on the lathe...
Watch this interesting
trick to secure the workpiece. First, masking tape on the back side of
the workpieces provides some friction with the chuck. Put a piece of stock
in the jaws large enough so that when the jaws are tightened on it they
are slightly smaller than the OD of the workpiece...
Now trap the
workpiece against the chuck with a live center and you're ready to go!
This is another reason Bog avoided a nasty interrupted cut: that masking
tape only provides so much friction!
End pieces are
close. About 1/8" too thick. Next up is machining the bearing pockets.
This is done with softjaws on the 4-jaw chuck (this one is self-centering,
an independent jaw wouldn't need the soft jaws) to make it as accurate
is good, so the workpiece is flipped and now a spigot is turned that will
be a "wringing" fit to the bore...
fit is good and things are starting to take shape...
Next step are
the bearing caps. Round insert tool makes a pretty radius...
Parted off and
you can see how they look...
They need to
be the same thickness, so the difference is measured on the surface plate...
faced off in the collet chuck...
Trial fit looks
good. Here is the hardware to mount the end caps and bearing caps...
It's quite sophisticated
looking, isn't it? I'd hate to try to carry one past airport security!
all that painstaking layout and setup would be trivial with a CNC...
These are the
rotor spacers. These are set up with 0.002" clearance at either end
to allow for thermal expansion...
Here is Bog's
converging nozzle, which increases the velocity of air or steam entering
the turbine. The actual nozzle is tiny!
Time to modify
the end plates some more. First they're marked off on the inside (where
it won't show!). Then the outside gets a final facing to get rid of the
old scribe marks and leave a nice blingey finish. This creates a nifty
boss for appearance sake.
wacks off the sides in the bandsaw. I keep thinking, why didn't he do
more work in the mill and start out with a square piece, but I suppose
he wanted the accuracy and finer finish of the lathe...
A nice flycut
makes it look all better. In fact, it very nearly looks like a single
block of metal there...
and a slightly radiused end mill result in mounting blocks...
result look sweet? Needs a shot of brake cleaner to get rid of the remaining
blue in the holes...
Now the exhaust
manifold is a somewhat complex part...
for the pocket. Bog was at pains to machine without damaging the parallels.
A better solution would be a set of softjaws with a step in the vise...
good with those big exhaust pipes! The blued piece is a brass plate: more
Holes are carefully
drilled while the plate is held in place with double sided tape...
the control valve...
Housing is turned
to size, parted, and inserted in the collet block for edge finding...
3 Holes are
drilled and then it's back to the lathe for boring. Hmmm, why not bore
first? I guess it keeps burrs out of the bore...
The two end
caps are being cut...
Last piccy shows
how an o-ring groove will be cut...
Shaft fits and
all looks well...
drill blank rod held in place with a wooden wedge allows indication relative
to the holes. A straight piece in the drill chuck locates the center hole
to 0.002" under the lathe axis. I'd be using my Blake Coax for that,
but I don't think Bog's mill has that much Z room...
Final dial in
to clean up that 0.002 and the mounting holes can be drilled...
is a sharp little piece of plumbing, no?
without the spool!
Groove on either
side to join passages....
A bit of dry
fit up and it's time for the silver solder...
Comes out all
blotchy, but it can be cleaned up quickly with a little Scotchbrite pad...
A marble makes
an excellent control knob for the valve!