John at work had been talking for some time about the Tesla
turbine and the Tesla valve, or more properly the Tesla valvular
channel since Tesla never called it a valve and it is not a valve,
it is a channel with a valve-like property.
Christmas 2019 I looked up the Tesla turbine and found that a
nice gentleman had
created a 3D printed version, which was very convenient, though it
turned out to be only the start of my journey. The Tesla
valvular channel was more problematic; browsing the internet
suggested that efficiency wasn't very good. So I decided
that I would use the pre-designed 3D printed Tesla turbine as my
measurement mechanism for the evaluation of the Tesla valvular
Note that there is somewhat of an evolution in the designs below
as I find out what's good and what's bad so, if you intend to
reproduce any of this yourself, do make sure to read through to
the end before doing so.
Tesla Turbine 1
I downloaded Integza's Tesla turbine files from Thingiverse
and printed them on my Prusa
3D printer in grey PLA, at 0.1 mm resolution as I wasn't in a
hurry. The only problematic part was the axis which was tall
and top heavy and needed supports to stop it toppling at the stage
where the bridges were being printed.
I obtained two bearings with outside diameter 22 mm, inside
diameter 10 mm, a brushed DC electric motor with outside
diameter 22 mm (which will act as a generator) and a collar
with inside diameter 3 mm to attach to the motor shaft.
I used some 8 mm long M3 hex bolts to hold the turbine
together. I managed to fit nine disks inside the turbine
without any fowling, glueing the last one in place with
cyanoacrylate adhesive. I glued one half of the joint for
the electric motor to the end of the axis and the other to the
collar using cyanoacrylate adhesive; everything else was push-fit
or bolted. I glued a pneumatic fitting to the air inlet with
Araldite (standard, not rapid) to allow me to push-fit a 5 mm
inside diameter/8 mm outside diameter air hose.
I mounted the turbine on a piece of wood, leaving room to fit a
lamp holder and a DC power measurement unit that I ordered from
China; the wires running around the back go to a 9 Volt
battery powering the DC power measurement unit.
To make things simple to manipulate, I use quick-connect pneumatic
fittings on the rest of the tubing. There are as many types
of these as there are people on the planet and they all look very
similar so it is important to make a choice and remember it.
I used the "Series 21" type throughout as they work nicely
with 5 mm inside-diameter plastic tubing and are relatively
small and neat.
Here it is on its first test run, driven by compressed air from my
little Jun-Air compressor:
...and here's the same but with the Watt meter in circuit.
As you can see, lots of vibration and a peak of just over
2 Watts output as the stored capacity of my little compressor
is used up.
Further testing showed that the peak speed at that 2 Watts output
was around 6800 RPM. Remove the motor and that rose to
10500, much less than half the 24000 RPM peak achieved by Integza and
nowhere near achieving sync with the incoming air flow. Now
of course I'd just thrown this together with no attempt at surface
finish, vibration control, air flow, etc. but there were other
issues. My little Jun-Air compressor has a 25 litre
tank versus Integza's 100 litre tank and it is just not able
to keep up with the flow requirements of the model. And
you'll see below that putting my first-cut Tesla valvular channel
in the air flow, with its tiny orifice, stopped the whole thing
However, after posting the first video above on YouTube I was
contacted by Paul Townley, AKA GravInert.
It turns out that he and iEnergySupply,
who I guess is in the USA, are trying to build Tesla turbines
which they believe can extract energy from atmospheric pressure in
damp air. In the comments section beneath their videos
someone had posted a link to the Tesla Engine Builders
Association, which, under the heading "The Open Secret"
includes, such articles as "Conquest of Space - Before It Went
Black" and "Flying Saucers 'Explained'". I was either down
the rabbit-hole or through the looking-glass.
Back to the problem at hand: in order to have a Tesla turbine that
I can use to measure the performance of a Tesla valvular channel I
needed one that (a) required a smaller air-flow and probably (b)
employed a metal shaft. I could have tried Integza's first turbine but since
Paul Townley had been in contact I thought I'd try his first.
Tesla Turbine 2
I downloaded the STL files from the Drop Box link under
video and made two modifications:
the two rotors wouldn't print well as a single unit, it was
very difficult to get the support material out from between
the disks afterwards, so I split them up into a rotor centre
and rotor blades that can be pushed together after printing,
file seemed to be corrupted so I created my own handle; this
was designed and printed in three parts which clip together to
avoid the need for support structures when printing.
Here are the Blender
and STL files for
the extras. All the parts were printed at 0.1 mm
resolution in PLA with supports everywhere aside from the
gaskets from my extras (see later) which
were printed in flexible PLA at the same resolution, and (added
later) the base plate from my extras which only needed 0.2 mm
resolution. The parts on the right in the picture below
are Paul Townley's originals, the ones on the left are my
additions; Paul's file tesla_cube_single_stage_baseplate.stl is not
required here. Two each of his "nozzle" and "port" casings
were required; note that I later modified these to improve
bearing fit, hence they moved into my extras file and this also
removed the need for the gasket once more. From my extras
five "open" rotor blades were required, one "closed" rotor
blade, two of the star-shaped rotor centres and two sets of
gaskets. After printing, aside from the usual removal of
support materials, the hole in diverter_block.stl through which rotary_shaft.stl
fitted needed some attention with a half-round file so that the
shaft fitted and turned easily.
I purchased four bearings of 9 mm outside diameter/4 mm
inside diameter/2.5 mm wide, a length of 4 mm diameter
metal rod, some 4 mm inside diameter collars, four
100 mm long M4 bolts and a pack of M4 brass inserts, though I
realised afterwards that these latter are only required for a
single-stage design; for the two stage design plain-old M4 nuts
are used instead.
I began by assembling the rotors and the handle of the diverter
block. Note the single "closed" rotor blade
positioned in the middle of one of the rotors (the right-hand
rotor in the top middle picture below) to make that the "male"
rotor. Cyanoacrylate adhesive was used to keep the rotor
blades in place on the star-shaped rotor centres, to glue the
handle parts securely to each other and to attach the handle to rotary_shaft.stl,
which allowed the direction of the air flow to be changed, being
careful not to accidentally glue the rotary shaft into place at
the same time.
The main body parts lined up as follows:
The body is held together by the long M4 bolts but, to ensure a
good air seal, I chose to attach the diverter block to its
adjacent piece permanently by gripping the two together in a vice
after applying cyanoacrylate adhesive around the air channels
between them. I cut two 50 mm long pieces of the
4 mm diameter metal rod and hammered them through the centres
of the assembled rotors, then pushed a bearing on either end of
the shaft and test fitted them in their housings. Paul
Townley had warned me to ensure that the rotor blades did not
touch the side, so I was careful how I positioned the rotor on the
shaft and did a little filing of the rotor blades here and there
to be sure.
I found that when both halves of the casing
were assembled on a rotor too much pressure was applied to the
bearing, I could feel the bearing "step" as I rotated it, hence I
printed myself the 1 mm thick gaskets out of "flexible PLA"
to fit between the two casing halves, also acting as an air seal.
The female rotor (i.e. the one with all "open" rotor blades) was
fitted into the casing nearest the nozzle, the male rotor in the
casing furthest away from the nozzle. Here is the dual
turbine fully assembled with a couple of collets attached to the
shafts to which I intended to glue something for my RPM counter to
Being impatient to see how it ran I connected it to my compressor
immediately. The first rotor span up very quickly indeed,
making a very pleasing, if slightly panic-inducing, whining
noise. The second didn't start so I increased the flow;
there was a "tick" noise and suddenly all motion stopped.
The first rotor had simply shattered; now we're talking! It
turned out that Paul Townley had his printed in carbon-reinforced
nylon, which I could do but only with a nozzle change in the
printer. And Paul balanced his rotors. And splitting
the rotor into parts would have weakened the blades. A
redesign of my rotor blade extras was in order.
I modified my rotor blades to print whole disks which were each
hammered onto the 4 mm diameter shaft, no glue
required. I printed them in the much stronger ASA (UV-safe
ABS, only because I didn't have any plain-old ABS to hand), with
support material on the print bed so that they could retain their
slightly tapered profile since Paul had mentioned that this was a
necessary feature; I hand finished the blades somewhat to make
sure that shaping survived the 3D printing process. I
learned from Paul Townley that the second rotor isn't supposed to
spin-up in the same air flow (I need to read more about how that
is meant to work) so this time I quickly assembled the whole thing
but didn't insert the second rotor and placed the solid tesla_cube_single_stage_baseplate.stl
between the two rotor-casing segments so that the second
part wasn't in use at all (see later
for why this was the wrong thing to do).
Just to make sure I had got past the smashy stage, I spun this up
immediately on the compressor with both exit air holes open.
You can just about hear, at around 35 seconds in, that it
wants to sync and it eventually gets up to about 38,000 RPM,
which is fine for an untuned first try; the little blade is
rotating 633 times per second. And no smashy, though also
not the rather exciting whine of the first version. Fiddling
was required as Paul Townley had achieved more than twice that on
his first run, though I'm not sure at what air pressure; mine was
with an initial peak of 8.5 bar.
I trimmed away some gasket which appeared to be touching the shaft
when the bolts were tightened and put the rotor, mounted on the
shaft, into my little Unimat lathe where I abraded the sides and
edges in an attempt to improve the uniformity/taper of the
blades. Paul Townley had pointed out to me a rotor balancing method
that Tesla had used but it was a little too complex for me to want
to set up such a thing just yet. I also found that if I spun
the rotor by hand with the bearings sitting loosely in their slots
it ran for far longer than if I pushed the bearings down into
their slots, so I used my Herzo hand abrading tool to open up the
bearing slots somewhat and I put the rotors back into the lathe
where I abraded them to reduce them in size very slightly in case
they were catching on the turbine case. I bought some
60 mm long hex bolts so that I could assemble just the first
stage turbine using those M4 brass inserts hammered into the bolt
holes in tesla_cube_single_stage_baseplate.stl
(again, see later for why this was the
wrong thing to do). A final innovation, which Paul had
mentioned but which I hadn't thought would be necessary yet given
my low speeds, was to drop some 3-in-1 oil into the shaft-hole at
either end before running.
Note also that the motor on my little Jun Air compressor can't
keep up with "open-valve" demand so while experimenting I needed
to do only timed short runs, which was fine since pulsed air
flow is where the Tesla valvular channel comes into play
anyway. I added a manually-operated valve to the tubing
delivering air to the Tesla turbine to give me convenient local
What was necessary, of course, was to know whether the rotor was
anywhere near achieving sync with the velocity of the incoming
air, a calculation which Integza talks about at this point in
his video. My compressor had a 25 litre tank and a
peak pressure of 8.5 bar with the compressor switching on
again when this drops to 6 bar. The idea is to work
out the volume of air this difference in pressure represents
once that air has escaped compression and, given the size of the
hole it has to escape through and the amount of time for which
it is escaping, work out how fast it must have been travelling.
Calculating the density of wet air is complicated so I chose to
Calculator. At 60% humidity and room temperature
this gave me 10 kg/m3 at 8.5 bar and
7.12 kg/m3 at 6 bar. A 25 litre
compressor tank is a volume of 0.025 m3 which
gave masses of air of 0.25 kg and 0.19 kg respectively
and so, between the two pressure values, 0.06 kg of air
must had been emitted. Back to Omni
Calculator: at "room" atmospheric pressure (1 bar)
the same wet air had a density of 1.18 kg/m3
which meant that my 0.06 kg of expelled air would have a
volume of 0.07 m3. Changing to more
reasonable units, 1 cubic metre contains 1,000,000 cm3
so the volume of uncompressed wet air is 70,000 cm3.
It took 20 seconds for the pressure to drop from
8.5 bar to 6 bar with the valve on the compressor
completely open so the rate of air flow was 70,000 / 20 =
3,500 cm3/s. The air entry hole in the
Tesla turbine casing, as measured at the rotary shaft in the
diverter block, was 4 mm in diameter giving a
cross-sectional area of 3.1412 * 0.22 = 0.125 cm2.
So the average linear speed of the wet air was 3,500 / 0.125 =
28,000 cm/s. The circumference of my 26 mm
diameter rotor blade was 3.1412 * 2.6 = 8 cm, so it should
on average rotate at 28,000 / 8 = 3500 times per second or
210,000 RPM. Quite a
lot, and that's the average number.
With all my improvements what did I achieve? You can see
that the rotation is still relatively poor under hand
encouragement at the start of the video below and there is quite
a lot of vibration when the turbine runs.
A peak of around 60,000 RPM (1000 times per second!),
average probably around 53,000 RPM over the period, with
both exit holes open. I'm about a quarter of the way
there, 25% efficiency in something that can theoretically get to
98%. Makes me wonder if all I'm doing here is measuring
the performance of some bearings.
After I published the video above Paul
Townley got in contact to point out (politely) that I'd been
very silly. I knew what the air flow in a Tesla turbine
was meant to be: from the outer edge of the rotor and then
towards the centre of the rotor to escape from the middle.
I had closed off the two large rectangular holes in the back of
the turbine with tesla_cube_single_stage_baseplate.stl,
leaving the two small holes on either side of the turbine
assuming that they were the intended exit holes:
But that's rubbish, the air was not escaping through the middle
of the rotor. What I wanted was this:
Paul had included the small side holes for some other purpose
because he was sending the objects away to be 3D printed
and hence needed flexibility in the result. Those small
side holes were meant to be tapped and closed off with a screw
when not required, the intended exit holes being the large
rectangular holes. So I screwed an M2 hex bolt into each
of the small side holes, pushed M4 inserts into tesla_cube_twin_stage_baseplate.stl
(with the two rectangular holes in it) instead and reassembled
the turbine. I applied a little cyanoacrylate adhesive on
top of the M2 hex bolts for additional security; I didn't want
small metal bolts exiting the building and entering me at high
I tried again with encouraging results:
Rather than starting at 60,000 RPM and wilting down to
46,000 RPM, the turbine stayed at around 55,000 RPM
for the period. This suggested that it was now
bearing-limited, which I needed to sort out anyway as the
lilting sound is likely a result of the bearing being slightly
loose in the housing due to my fettling.
Paul had suggested that GRW bearings were the best so I went in
search; curiously the two UK agents for GRW were unable to
supply small quantities but a Dutch seller (247industries) on
Ebay could. I also purchased some turbine oil, a thin oil
used with model jet engines (from Kings Lynn model
shop) and some paraffin to mix it with, 5 parts
turbine oil to 95 parts paraffin. Paul suggested that
the bearings be immersed in this when not in use. And I
into my Blender extras file and modified them to remove the
small side holes entirely and increase the width and diameter of
the bearing slots by 0.2 mm so that they would come out the
correct size when FDM printed, no need for the gasket. I
believe Paul's casings where resin printed which has a higher
resolution and so he had no need of this tolerance.
Here are all of the parts before assembly once more: the
diverter block, rotary shaft and twin stage baseplate at either
end are from Paul's files, the rest are from my Blender
extras. Glue (cyanoacrylate) was applied solely to affix
the handle, nowhere else, and no gasket was required.
And I think this time it worked; Paul agreed: possibly the
fastest accelerating Tesla turbine in the world.
As you can see, the tachometer gave up and my measurement disk
flew into pieces that I mostly couldn't find afterwards, apart
from the one that sliced my finger. Very cool and a
wonderful sound too. I belatedly realised that my
tachometer wouldn't go above 99,999 RPM. Gonna need a
bigger measuring device. And a stronger reflective
disk. And some shielding.
During a discussion at work one lunchtime Jonathan suggested I
make my own tachometer using an optical sensor; with this I
could measure acceleration as well as speed. I purchased a
QRD1114 reflective object sensor (basically an LED and a
photo-sensitive transistor in a tiny plastic case) from Hobbytronics
for £1.20, a Raspberry Pi Zero W with the header fitted costing
£13, a power supply for the Pi costing £8 and I dug up an old
8 gigabyte SD card to use in the Pi. I designed a 3D
printable base to hold the Tesla turbine, the sensor and the
Raspberry Pi. I painted length-ways 2/3rds of the
sticky-out metal shaft of the turbine with matt black paint
(actually Humbrol 32 dark grey since I had that to hand) so
that it had reflective and non-reflective sides.
I followed the procedure here
to set up the Raspberry Pi Zero W headless on my Wifi network,
i.e. such that I could log into it from a terminal program (e.g.
PuTTY) from anywhere rather than having to connect
screen/keyboard etc. and use a GUI. I followed the advice
on how to connect the sensor to the Pi, using one of the
3.3 Volt power outputs from the Pi to supply the LED and
connecting the sensor output to a digital input pin on the Pi,
rather than an analogue input pin (so that I could use
interrupts), checking with a multimeter to make sure that it
achieved the necessary 0.8 V to 2.4 V voltage swing
for a digital input pin between the dark and light sides of the
shaft. Below you can see the voltage swing on a scope when
the rotor was spun by hand.
I wrote a Python script to run on the
Raspberry Pi which counted the number of falling edges on GPIO4
in each 100 ms interval and saved the values to a CSV file
that I could load into Microsoft Excel. For the first run
I also attached a logic analyser to the optical sensor output
pin to capture a live trace so that I could cross-check the
accuracy of the program.
I lubed up...
And away we went.
Not too shabby. A steady state of 185,000 RPM, ~90%
of my very roughly calculated theoretical maximum of
210,000 RPM given the output of my compressor. And
that kick in the curve followed by the flatness makes me wonder
if the rotor had achieved sync with the incoming air flow.
The logic analyser output agreed with the above: below are a
couple of clips (upper trace digital 3.3 V logic, lower
trace analogue), one of the startup and another from somewhere
around the peak showing a nice clean sawtooth with the number of
transitions over time to match the graph. So the data is
And, as independent proof, John at work had pointed out that I
could probably just listen to the turbine and the audible
frequency would likely be that of the rotation speed. To
test this, here is a 30 second burst of a 3.08 kHz sine wave
(AKA 185,000 RPM) as an MP3 file generated using Audacity. Try
playing it at the same time as the video and you'll see that the
tones match. I tried exporting the audio from the video to
an MP3 file and using Audacity to analyse the spectrum to see if
it would match the speed curve but unfortunately there is far
too much general hissing to pick out the wanted rotor tone; ears
are more clever.
During the initial jump to 100,000 RPM in just one second
the surface speed of the rotor (diameter 28 mm) went from 0
to 147 metres/second, so an acceleration of
147 metres/second/second, Now I thought that was
15 g (147 / 9.8) but after another lunchtime
conversation David, who once worked on centrifuges, pointed me
link about converting RPM to g force which says that
relative centrifugal g force (RCF) is (RPM)2 * 1.118 * 10-5 * r,
where r is the radius of the rotating thing in
centimetres. That calculation makes the force
53,000 g at max RPM.
Then I remembered that one of my brother-in-laws, Toby, works in
the design of fans and he confirmed that, though the RCF number
is interesting, the number I should be looking for was hoop
stress, the tangential force, which is: (density * r2 * w2) / 3,
where r is the radius of the thing in metres and w
is the speed in radians/second. The density of ASA is
virtually one, the speed in radians/second is
(2 * 3.1412 * rotations per second)
so for my 28 mm rotor this becomes (0.0142 * (2 * 3.1412 * (185000 / 60))2) / 3 = 24,500
Newtons. For a rotor with disks of thickness 2 mm the
surface area over which that force is applied would be
3.1412 * 28 * 2 = 175 mm2.
24,500 Newtons applied across 175 mm2 is
24,500 / 175 = 140 Newtons/mm2.
Checking on the web, ASA seems to have a tensile strength of
30ish megaPascals, AKA Newtons/mm2.
So with these maximum speeds I'm running at more than four times
the strength of my material. Toby also mentioned that they
put concrete walls between them and their turbine fans.
Anyway, the rotor survived without flying apart, thankfully, as
I hadn't installed a shield. The surface speed of the
rotor at max RPM was 271 metres/second. This
article suggests that a .38 calibre pistol, apparently a
popular small hand gun, has a muzzle velocity of
286 metres/second and, interestingly, fires a bullet
weighing 6 gm, just twice the weight of the rotor, though
it is only able to inflict as much damage as a hockey puck at
full pelt. But I'd better make that shield.
To reinforce that point, I thought I'd try a second run and, two
seconds in, there was a bang as the disks of the rotor suffered
rapid unplanned disassembly; 192,000 RPM and rising this
time. Maybe the dip and then rise in the acceleration on
the first run was fortuitous? I will open the air inlet valve
more slowly in future.
While I had been sorting the tachometer out, Paul had suggested
a number of additional experiments: increase the number of disks
and try using an air bearing. I thought it was worthwhile
seeing if I could make the rotor edge go supersonic;
340 metres/second (232,000 RPM for my rotor diameter)
looked like it could be feasible.
To be continued...
I took the picture of the Tesla valvular channel from the
...and followed the procedure described in this
YouTube tutorial to import it into Blender as a usable
shape; basically, from a fixed camera viewpoint, trace around
the salient parts of one section of the shape with Bezier
curves and then duplicate/join that three times to make a set
Finally I extruded the 2D form vertically and made it into a
solid that could be printed, 115 mm long. I did try
a version smaller than this (90 mm long), as I had the
impression that the pressure exerted on the valvular channel
is a factor and the smaller the geometry the less external
pressure would be required, however 115 mm long is about
as small as my 3D printer could print the features of the
valvular channel distinctly. Here are the Blender and STL files, which need
to be printed with supports on the print bed only (see below
Here's a cross-section of what it looked like in my printer's
slicer program and during printing at my 3D printer's highest
possible resolution, 0.05 mm, in PLA:
I wanted to be able to push on the 5 mm inside-diameter
plastic tubing so I modified the outside of the valvular
channel to be round, like a pencil, and tapered the
ends. Here's a quick tour, going through it the "wrong"
way, courtesy of a couple of days rendering from Blender.
This had to be printed with supports (on the print bed
only). Even then I had some trouble stopping leaks from
the connection to the tubing so cyanoacrylate adhesive was
applied to the tapers as the tubing was pushed on.
I placed my Tesla valvular channel in the air-flow to the
turbine and... nothing happened. Either way around, the
tiny aperture of my valvular channel constricted the air flow
so much that the turbine would not rotate at all. Either
I needed to make the valvular channel bigger or I needed a
turbine which could work with a reduced air flow; possibly
both. To be continued once I've bottomed-out the Tesla turbine