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
channel.
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. The sections on this page have
ended up being:
Tesla turbine 1: Integza's
completely 3D printed design as mentioned above, useful as an
instructional tool,
Tesla turbine 2: GravInert
(AKA Paul Townley)'s Tesla cube design, demon fast in both
acceleration and speed,
Tacho: because of the speed of Paul's
Tesla turbine I needed to design my own tacho which was also
able to plot speed over time,
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
dead.
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 next.
Tesla Turbine 2
I downloaded the STL files from the Drop Box link under
GravInert's YouTube
video and made two modifications:
the rotors wouldn't print well as a single unit, it was
very difficult to get the support material out from between
the blades afterwards, so I split them up into a rotor centre
and rotor blades that can be pushed together after printing,
the handle.stl
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
bite on.
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 turbine 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
control.
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
trust Omni
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 3.142 * 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 was
3.142 * 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
speed.
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
imported tesla_cube_nozzle_casing.stl
and tesla_cube_port_casing.stl
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.
Tacho
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
here
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
believable.
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
at this
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 was looking for was hoop stress,
the tangential force which is at a maximum at the edge of the
rotor. The equation for hoop stress 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 * 3.142 * rotations per second)
so for my 28 mm rotor this becomes (0.0142 * (2 * 3.142 * (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
2 * 3.142 * 28 = 175 mm2.
24,500 Newtons applied across 175 mm2 is
24,500 / 175 = 140 Newtons/mm2.
Checking on the
web, FDM-printed ASA seems to have a tensile strength of
30ish megaPascal, AKA Newtons/mm2.
Hence with these maximum speeds I'm likely running at more than
four times the strength of my material; Toby also mentioned that
they put concrete walls between them and their experimental
turbines.
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. But first...
Tesla Rotor
Balancing Machine For Littul Rotors
Paul pointed out that if I am going to attempt mach one,
and air bearings, I really needed to balance the
rotor. Paul and his colleague in Idaho had made a rotor
balancing machine of the type that Tesla patented so I
did the same but rather smaller. Here are the Blender and STL
files for the design. The principle is that the rotor
on its shaft/bearings is rotated while the shaft is
suspended on springs; an abrasive surface is then brought up
to the perimeter of the rotor. This is left running for a
few hours and the net effect is that material is removed
from the sides of the rotor that push out further because
they are heavier, resulting in balance.
To accommodate the very light rotor I used tension springs
with a very low Youngs modulus, 0.011 Newtons/mm,
27 mm long from www.springmasters.com
(L40350). The abrading clip in the design was intended
to accommodate those 4 mm x 4 mm sanding sticks
one can buy from Amazon
or a section of 4 mm x 4 mm Kemet Gesswein stone
one can buy from Cutwell,
cut to 33 mm in length. One of the
100 mm long M4 hex bolts I originally purchased for the
double-rotor design above
was re-purposed, thread extended with an M4 die to encompass
the entire length of the bolt, to act as the adjustment
mechanism.
The parts were printed at 0.2 mm resolution in PLA with
supports everywhere. I included two different forms of
upright: one (with the large hoop) is for the balancing process,
the other is for the testing process, see later; two copies of each and two
copies of the bearing ring were required.
The abrader plate was pressed into mating with the hinge
mounts on the base by scrunching them together in a soft-jawed
vice. Three springs each were used to suspend one of the
bearing rings in each of the hoop-shaped balancing uprights,
taking care not to stretch the spring during assembly, then the
loops on the springs were adjusted with a pliers so that the
bearing rings were visually centred with reference to each
other. A length of abrading stick was cut to size and
mounted inside the abrader clip and the clip was clipped onto
the abrader plate. The head of the modified M4 hex bolt
was pushed into the bottom cylinder and the bottom cylinder was
then clipped into its recess in the base. An M4 nut was
screwed onto the bolt and then the top cylinder was pushed on
after it and clipped into the recess in the abrader plate; thus
the height of the abrader plate could be adjusted.
Since my compressor was not very powerful and this kit would
need to run for several hours I designed myself the tiniest
nozzle, maybe 0.5 mm in diameter, providing the fastest air
flow for the smallest volume of air expended, and pushed it into
a length of 5 mm inside-diameter plastic tubing; here are
the Blender and STL files for the
nozzle, the business end of which was hand-worked with a few
pins after printing at 0.2 mm resolution (a higher
resolution would probably have been better). The nozzle
was held in the correct position with one of those "extra hands"
thingies, deliberately mounted separately from the balancing
machine to avoid passing on any vibrations and, for the same
reason, the balancing machine base was screwed down onto a nice
solid piece of wood.
Since I had no idea whether this
was going to work I arranged to measure the vibration of the
unbalanced rotor first. For this, instead of the hooped
balancing uprights, I inserted into the base two copies of the
testing uprights. With the rotor on its shaft/bearings
held in the testing uprights I could run the rotor while my
mobile phone was placed on the base running a mobile
phone app that measured the amplitude of the vibration
using the phone's accelerometer.
Right then, back to the plot to see the results...
Tesla Turbine 2 With Five
Rotor Disks
I modified Paul's design to accommodate five rotor disks and to
marginally reduce the aperture of the air inlet (more like
3 mm than 4 mm): here are the Blender and STL files, where
everything is basically 11 mm wider.
I printed the components on the right in PLA at 0.1 mm
resolution, the base in PLA at 0.2 mm resolution, the rotor
disks in the middle in ASA (ABS is also fine) at 0.1 mm
resolution (four outer rotor disks are required) and the
components on the left, the two turbine casing halves, in
polycarbonate at 0.1 mm resolution. This latter was a
trial in anticipation of moving to air bearings; polycarbonate
is very hard (the hardest material I can 3D print without a
nozzle change, and a bugger to print): I hope it will have more
chance of withstanding the load of the metal rotor shaft before
the air flow centres it. All parts were printed with
supports everywhere and, in the case of the ASA and
polycarbonate prints, a brim/skirt to keep them stuck
down. After hammering the rotor disks onto a 60 mm
length of 4 mm diameter stainless steel shaft I balanced
the rotor in the Tesla rotor balancing machine.
First, I did some runs with the balance testing uprights to
record the initial vibration: with the bearings lubed-up I
measured the vibration while sweeping the rotor speed up to
60,000 RPM, estimating the speed by running an on-line tone
generator at 1 kHz and slowly adjusting the air speed
until the sound from the rotor matched. The vibration did
not rise linearly with speed, obviously there was some form of
resonance going on with the worst case of +/- 0.5 g
occurring at somewhere around 30,000 RPM.
With that done I began the actual balancing process. After
turning on the air I adjusted the nut beneath the abrader plate
so that the turbine was just able to run and only occasionally
touched the abrasive. I left this running for three hours,
coming back every so often to add more lube and to increase the
speed of the air flow if the rotor seemed too stable.
Material had been taken off the rotor but had this done any
balancing or had it just polished random bits of the rotor
edges? I repeated the vibration measurement using the
balance testing uprights.
The answer was "maybe". If one discounted the resonance at
around 30,000 RPM as purely some artefact of how I happened
to throw things together the first time around then the result
was actually worse than before. If the resonance was
something which the balancing had removed then it was an
improvement. I needed a more reliable vibration
measurement mechanism. And I needed to address some more
obvious problems: simply hammering the disks onto the shaft
doesn't make them square and the balancing machine isn't going
to fix that, click on the segment of video below for a closer
view.
I needed to make myself a tool to help out with that and then I
could maybe balance each disk individually in the balancing
machine for a better result.
It's rabbit-holes all the way down.
Skip ahead
if you're not interested in the construction of the turbine.
Improved Construction
I paused to make a few improvements in my construction
method. First of all I worked on the individual rotor
disks to make each of them as balanced and symmetrical as
possible.
I took a 60 mm long M4 hex bolt and screwed a nut all the
way onto it tightly so that the hex bolt could be held properly
in a lathe, then I screwed another nut a little way onto it
followed by a rotor disk and another nut to hold the rotor disk
firmly in place; I happened to have some large square M4 nuts
and used these as the latter two to help make sure the rotor
disk was perpendicular to the shaft. This assembly allowed
me to hold the rotor disks individually in my little lathe while
I finished them with abrasive sticks such that they each had a
definite and symmetrical taper.
I 3D printed (Blender and STL files here) a
rotor alignment tool in PLA at 0.2 mm resolution (no
supports required) that could be used to hold the stack of rotor
disks at 90 degrees to the 4 mm diameter shaft during
assembly.
I purchased an ADXL362
accelerometer board from Hobbytronics
and printed a mount for it that could be glued (with
cyanoacrylate adhesive) to the top of one of the bearing rings
of the Tesla rotor balancing machine. I redesigned the
Tesla rotor balancing machine base (revising the Blender and STL
files) to include fittings for a Raspberry Pi Zero W and
connected the ADXL362 accelerometer to the Raspberry Pi Zero W
as shown below. While I was at it I reduced the gap
between the uprights by 10 mm (i.e. the abrasive strip was
now 23 mm long) so that the shaft would stick out and added
a mount for the optical sensor. I could now extended my Python script (I also needed to
enable the SPI interface on the Raspberry Pi Zero W and install spidev) to
measure both peak vibration and speed (in 100 millisecond
intervals) during balancing, no need for the mobile phone app or
the testing uprights.
I took a 60 mm long M4 hex bolt and extended the thread
most of the way down it with an M4 die (just under 20 mm of
unthreaded bolt left). I painted the "marker sleeve"
component from the redesigned Tesla rotor balancing machine
length-ways half matt black (or Humbrol 32 again) and half
gloss white (Humbrol 22) to act as the marker for the
optical sensor. A bearing was pushed into each bearing
ring. It was important to ensure that the rotor disk was
square on the bolt and so I used the top-half of the rotor
alignment tool as a jig to hold the rotor disk square while I
initially screwed the bolt into it. Having done this I
unscrewed the bolt again and, with the non-accelerometer upright
held in my hand (i.e. out of the base) I pushed the M4 hex bolt
through the GRW bearing and carefully screwed the rotor disk
onto the bolt once more, allowing it to follow the same
thread. Then I pushed the upright into the base, pushed
the bolt through the other GRW bearing and finally screwed the
"marker sleeve" onto the sticky-out screw end of the bolt until
both it and, on the other end, the head of the M4 bolt were
loosely touching the bearing.
I lubed up and performed a measurement run (i.e. with the
abrader plate retracted down onto the base) on the first rotor
disk before it was balanced:
As the speed was increased, around 30 seconds in, above
about 25,000 RPM, the rotor disk started to wobble; the
spikes on the peak vibration measurement got larger and the
speed dropped. A speed of less than 27,000 RPM seemed
to be about right. Tesla mentions in his Tesla rotor
balancing machine patent that the momentum of the spinning disk
is likely what stops it bouncing off the abrasive surface in a
chaotic manner; given that each of these rotor disks weigh only
1 gm speed has to compensate for the lack of weight.
In order to be certain I repeated the measurement five times,
getting the rotor disk to stability at around 25,000 RPM
each time, and then (with a little help from here
and here)
I persuaded Excel to plot
the peak vibration distribution when the rotor disk was within
its high-but-stable RPM range (24,000 to 27,000).
The aim of my balancing process was to reduce the mean of the
peak vibration of this rotor disk from 1.83 g, moving the
bell curve to the left. After several days of trial and
error, during much of which I only made the balance worse, I
determined a good procedure to be as follows:
Clip a length of the roughest sanding stick into the
abrader.
Without any air flowing, bring the abrasive up to about
2 mm away from the rotor disk. I positioned a
light on the opposite side of the rotor disk to make this
distance easier to see.
Switch on the air flow and bring the rotor disk up to
about 25,000 RPM or as high as you can without
oscillation.
Carefully rotate the nut to increase the height of the
abrasive until it makes contact with the rotor disk: when
the speed drops as a consequence of fiddling with the nut,
rather than through natural variation, contact has been
made. Don't wiggle the uprights or move the base in
relation to the air nozzle while making the
adjustment. Take it to the limit and, if the rotor
disk begins to oscillate wildly, steady it with your
thumbnail, back the abrader off a little and try
again. See the video below for how I performed this
ticklish adjustment.
The edge of the rotor disk will become shiny; leave it to
run and, over time, the wobble will reduce. Continue for as
long as you can, at least 30 minutes, readjusting
abrader height and rotor speed as required to ensure that
contact is maintained between the abrasive and the rotor
disk but that the rotor disk doesn't oscillate wildly or
stall.
The video below shows the adjustment procedure in detail.
So far so dandy but how consistent was the improvement? I
spent a day performing more half hourish balancing runs on the
same rotor disk as above with various balancer configurations
one after the other, measuring the change in the mean of the
peak vibration in the 24,000 to 27,000 RPM range. For
each balancer configuration I plotted the effectiveness factor,
which I defined as the reduction in the mean of the peak
vibration divided by the duration of the balancing run.
This graph suggested that the method described in the video,
basically the roughest sanding stick adjusted to be as close as
possible to the rotor disk, gave the greatest likelihood of
achieving a reduction in peak vibration; not a surprise but this
sciences it.
In cases where I found I was making things worse rather than
better I returned the disk to the lathe to restore the tapered
edge and then I was able to work the vibration down
further. However, the lowest number I achieved for the
mean of the peak vibration was 1.23 g, which was still quite
high. The rest of the vibration was likely a matter of
dynamic (as opposed to static) imbalance due to yawing arising
from the rotor disk not being perfectly square on the M4
bolt. Given that, in the balancing machine, the rotor disk
is at the centre of a 60 mm long bolt this yawing probably
has quite a large effect out at the end where the accelerometer
is mounted. I was hoping it would be fixed through the
stack of rotor disks being held square by the rotor alignment
tool during assembly onto the shaft.
I balanced the four female outer rotor disks for the five-disk
turbine, achieving mean peak vibrations (in the RPM range 24,000
to 27,000) of 1.23, 1.36, 1.52 and 1.50 g. I found it
pretty much impossible to balance the central female rotor disk:
the lack of fins on that rotor gave the air flow little to bite
on and there was no easy way I could make it stable presenting
the rotor edge to my air feed when the abrading had flattened
that edge, making the aerodynamics worse. I put the
central female rotor disk back in the lathe, restored the
tapered edge and left it at that (1.95 g).
After balancing the disks I weighed the female outer rotor disks
using a jewellers' weighing scales and made sure their weights
were identical. Borrowing a technique from my autogyro
project I added Balsaloc (but see below) to any rotor disk that
needed to weigh slightly more to ensure lateral balance, adding
it evenly around the centre of the disk; this only applied to
rotor disk one, from which about 0.08 gm of material had been
removed in all the messing about above. With that done I
slightly rounded the end of the 4 mm shaft with a file in a
lathe to make sure it could be eased into the rotor disks
without carving off any material. Then I put the stack of
rotor disks into the rotor alignment tool and forced the shaft
through them (mostly applying pressure with a vice, a hammer
only required for final adjustment) until the disks sat at the
centre of the shaft (i.e. with equal lengths of the shaft
sticking out of the rotor alignment tool at either end).
Finally I rotated the disks as necessary to align the holes in
their middles.
Out of curiosity I put the entire turbine rotor, on its shaft,
into the balancing machine and gave it a spin to measure the
vibration.
Pretty steady, though I should have
let the Balsaloc set first. In fact, splitting the data into
the X, Y and Z axes it was plain that the maximum of the mean peak
vibration in any axis was actually due to the Z axis blip when the
Balsaloc flew away, so the meaningful maximum mean peak vibration
value was 0.23 g and in the Y axis, that of the shaft itself,
a sideways motion. I cut out the Balsaloc, which never
seemed to set entirely and could have been dangerous inside the
turbine casing: better to have the slightest of imbalances, small
as compared to the weight of the rotor shaft anyway.
Tesla
Turbine 2 With Five Rotor Disks And A Shield
I screwed the five rotor disk Tesla turbine base onto a solid
piece of wood. I had some 2 mm acrylic sheets made up
by sheetplastics.co.uk,
one 140 mm x 280 mm with counter-sunk fixing holes at
each corner and another 220 mm x 100 mm. The first
I folded into an "n" shape with the aid of a blow-torch following
the advice here
and the other I attached removably to the rear of the "n" with the
aid of a couple of 3D printed slots glued
to the sides of the "n" with cyanoacrylate adhesive. The
front of the shield was left open for filming.
I installed the turbine, lubed up and away we went.
Hmph. Not so fast. From the slow roll-off once the
air was turned off it was clear that there was little frictional
drag on the rotor, yet the top speed of just 106,000 RPM was
not much over half that of the three-rotor disk turbine. Maybe the
air was not spreading out across the surface of the five rotor
disks properly? And of course I had constricted the air flow
slightly to increase the air speed. There was not much scope
for making the rotor disks thinner such that the rotor was less
wide.
Comparing the detail of Paul's original air inlet (below left)
with my modified version for five rotor disks and a slight
constriction (below right), his was clearly smoother.
So I went back and did it again.
While I was at it I made the top and bottom inlet nozzles
different sizes, one the original 3.8 mm diameter, the other
the newer 3.1 mm diameter, so that I could switch between
them using the inlet control knob. I printed the inlet and
outlet casings again in PLA at 0.2 mm resolution; printing in
polycarbonate is a pain and, when I need the extra toughness for
air bearings, I can print just an air bearing insert in
polycarbonate.
Running the revised design demonstrated that making the nozzle
inlet smaller to increase the air speed did slightly increase the
rotor speed achieved. However the improved shaping of the
inlet had no beneficial effect.
Another possibility was that air was leaking away down the sides
of the rotor. I had left a little more room to make sure
there was no contact between the rotor and the casing and
this amounted to 2 mm at either end of the rotor.
I modified the casing to reduce this to 1 mm and tried
again.
Nope, about the same. I wondered if there was too great a
gap between the outer edge of the rotor disks and the casing but I
didn't think it was any larger than the original three-disk rotor
design.
Last thing to try: split the air inlet so that there were two
separate ones; in other words rather like a pair of the original
three-disk rotors side by side, making each aperture smaller so
that the air speed within the turbine remained the same, just with
better distribution.
This was better, with a peak of 109,000 RPM, but still didn't
compete with the three-disk rotor turbine. Hmph. Where
to go next? Was there anything else I could do or should I
cut away the outer two disks and see what happens with a balanced
three-disk rotor solution? Paul wondered if the additional
weight of two more rotors was pressing on the bearings.
Time, I thought, to try making an air bearing.
Tesla
Turbine 2 With Five Rotor Disks And Air Bearings
I redesigned the "split air inlet" version of the casing halves
above and added air bearings as follows:
removed the outlets at the rear: the air isn't leaving that
way any more,
increased the space for the rotor on either side by
1 mm again to give it a little headroom,
removed the shapes that held the bearings, creating instead
a half cylinder with a small step at the inner edge,
created an air bearing to fit in that cylindrical space,
with a funnel on the inner side, a 4.5 mm diameter hole
(for the 4 mm diameter shaft) and a smaller funnel on the
outer side,
created an air buffer, designed to fit tightly onto the
rotor shaft at either end and sit loosely inside that smaller
funnel, the intention being to encourage the rotor to remain
central,
created a tool to help fit the air buffers onto the rotor
shaft.
These parts are all in the Blender and STL files. I
printed the cylindrical parts at 0.1 mm resolution, the
casings at 0.2 mm resolution, all in PLA (can switch to
polycarbonate later if required) with brim and supports
everywhere.
I sanded the paint off the end of the rotor shaft, put the air
bearings in place and then scrunched the air buffers into place
using a soft-jawed vice with the buffer fitting tool, holding the
casing in place as I did so in order to judge the distance
carefully: each air buffer needed to just prevent the rotor disks
from touching the opposite side but I also didn't want to push the
air buffers in too far as they must normally turn without touching
the sides.
I re-applied the paint, assembled the turbine and gave it a spin.
Not so hot. After about 15,000 RPM it went into some
sort of oscillation and dropped right down to 3000 RPM;
question is, what sort of oscillation? First I tried
decreasing the diameter of the hole in the centre of the air
bearing to 4.3 mm. I ran this without the air buffers
fitted, just to see how it behaved: now the oscillation occurred
at 35,000 RPM: a positive step. So I decreased the
diameter to 4.15 mm: too tight, the rotor failed to turn and
I could feel air escaping from the diverter block rather than out
of the bearing holes. I moved up to 4.2 mm diameter:
this felt right, little play, but (without air buffers) it got no
further than 23,000 RPM and I could still feel air escaping
from the diverter block.
With the casing open I could spin the rotor with my finger and
make it emit the same oscillating noise so I reasoned that the air
probably wasn't escaping through the bearing holes as
intended. I made a version of the diverter block without a
rotary shaft to eliminate the leakage and tried again.
Up to 39,000 RPM now. I thought the "tick" sound was
likely to be the rotor really wanting to do its stuff but hitting
the sides of the casing; it was time to see if the air buffers
helped stop that. Also, there was some leakage of air
between the diverterless block and the rotor casing: I sealed that
up with some Loctite 574 sealant I happened to have lying
around. With the air buffers (slightly increased in diameter
from the originals pictured above) fitted, I tried again.
And they made no difference whatosever, same "tick" noise.
Also my sealant refused to cure, even after 24 hours.
I wondered if the air flow was too turbulent. I redesigned
the air bearing to add rifling: a pair of 0.5 mm deep,
1 mm wide spiral notches running down the inside of the air
hole. I printed the rifled air bearing in PLA at the maximum
resolution of my printer, 0.05 mm, no supports required.
In order for the rifling to have any effect I increased the length
of the air bearing to 15 mm. Of course the shaft should
be at least as long but since a new shaft would require a new
rotor I first did a test with the current rotor to see if there
was any positive effect. I had to snap the tacho tower off
as it fouled the sticky-out air bearing so I couldn't monitor
speed directly during the test. And this time I used some
Araldite (standard) around the exit air hole of the diverterless
block to seal the turbine during assembly.
Of course the answer was that rifling made no noticeable
difference in this test. At least the Araldite worked.
I decided to persist: I had a feeling that rifled air bearings
could be the thing and they would need a longer shaft to be
affective. Back to three disks, where I had most success,
and experiments with rifled air bearings.
Tesla Turbine
2 With Air Bearings
While I was waiting for a 3D print to complete I spent some time
fiddling with the open rotor in its air bearing, using the nozzle
from the Tesla rotor balancing machine to direct the air flow
freely, and noticed something interesting:
Maybe all I needed to do to keep the rotor centred was to split
the air flow, just as I had done before, but this time to point at
the gaps between the rotor disks? While I was making
modifications I made the tacho tower a separate component so that
it could be slid back and forth as required. And I modified
the design of the now diverterless block to make a better air seal
and printed myself a gasket to resolve the leakage problem without
having to resort to glue. Here are the Blender and STL files for all of
the parts, printed at 0.2 mm resolution in PLA with supports
everywhere (asking your slicer program for higher resolution
around the nozzle openings if you can) apart from the gasket which
was printed in 0.2 mm in flexible PLA (no supports required).
An Ernie Ball gauge 10 guitar string (0.25 mm in
diameter) was useful to make sure the air holes were clear.
It took several attempts to get the diameter and positions of the
inlet nozzles correct; I tried to make the nozzle holes pretty
small, like the nozzle used with the Tesla rotor balancing
machine, to obtain maximum air speed. I performed an
open-casing test run with my final configuration.
So dual inlets weren't enough, the suction effect of air escaping
out of the side of the casing brought the rotor with it, randomly
to one side or the other. I needed to concentrate on
stability over speed.
I redesigned the rifling on the air bearing to be much shallower
and have more threads to it; more like true rifling, just viable
when printed at the highest resolution of my FDM 3D printer
(0.05 mm) in PLA. In order to avoid having to balance a
new rotor/shaft I reduced the length of the air bearing once more
and tried again fitting air buffers on the ends of the shaft, this
time making them much larger in the hope that they would have a
greater effect.
That was at least interesting: the ker-chunk noise emitted from
the closed turbine didn't seem to be due to the rotor hitting the
sides of the casing after all; no wonder nothing I was doing was
making a difference. Could it simply be due to yawing of the
shaft somehow? It seemed to me that the possibilities were:
Cause
Mitigation
Outcome
1
Air bearing hole too large, permitting
yawing.
I reduced the aperture of the hole in the
air bearing again, to the point where it was initially too
stiff, and then rotated the rotor shaft in a drill while
in the air bearings for a minute to loosen it.
No difference, well maybe a slightly
higher speed before each ker-chunk but still predominantly
ker-chunk.
2
Air flow is too little to support the
weight of the rotor.
Re-try with larger air holes now that the
seals have been improved.
Using a previous iteration of the
dual-inlet block that had larger air holes the ker-chunk
no longer entirely stopped the rotation, things
kept juddering on, so an improvement of sorts.
3
Variation on the above: the rotor is too
heavy for the air flow.
Try again with an aluminium shaft.
I cut a 4 mm diameter aluminium rod
to make a shaft 100 mm long: at 3.3 gm this
weighed 30% less than the 60 mm long steel shaft at
5.7 gm. I did a better job of the rotor disk
balancing this time, getting the two outer female disks
down to 1.10 g each compared to my previous best of
1.23 g. Running this rotor with otherwise the
configuration of row 1 the ker-chunk remained; maybe not
quite entirely stopping at each ker-chunk; again a small
improvement.
4
Air flow is too large through the bearing
holes, after all it seemed to work when open.
Re-open the holes in the end-plate.
I made a version of the casings that had
room for the air bearing and had exit ports to the
rear the same size as the original design (see Blender
and STL
files). Running otherwise with the configuration of
row 1 (i.e. with the steel shaft) this produced no ker-chunk at all.
Not particularly fast but this time the buffers did seem
to be having an effect, rather than the whole thing
wanting to pull over to one side; not stable but at least
effectual.
5
The shaft is too short, allowing yawing.
Try again with a longer (aluminium) shaft.
I extended each of the air bearings by
20 mm, as long as the 100 mm aluminium shaft
would allow, and ran the turbine with otherwise the
configuration of row 1. This wouldn't start at all:
too much frictional surface between the rotor shaft and
the air bearing. I switched to the larger inlet
holes of row 2 and that did work.
Interestingly, at the ker-chunk points, the air bearing
was clearly being caught by the shaft and rotated
itself. And the turbine reached a state where it
trundled along without ker-chunk, though it really was
trundling and so maybe not fast enough to ker-chunk.
The air buffers weren't doing a great deal but I hadn't
taken a lot of care fitting them and so they may not have
been positioned correctly.
6
Rifling is the key but it needs the
precision of stereolithography/liquid resin printing, FDM
won't cut it.
Buy a resin printer.
Having found a good (?) reason at last, I
purchased a Prusa
SL1 liquid resin 3D printer to complement my Prusa
MK3 FDM 3D printer. I printed both the normal length and
the long air bearings with this new machine and re-ran
configurations 4 and 5. They worked maybe slightly
but not significantly better: no banana.
7
Something else.
So it looked like the ker-chunk noise was the shaft catching in
the air bearing. What improved matters was increasing the
amount of air, having air flow rapidly through the turbine and out
the other end, and a lighter/longer shaft. However I didn't
really have an air bearing, what I had was metal running on
plastic: ker-chunk.
While I was trying all of the above Paul Townley got in contact
and showed me a drawing he had obtained of Tesla's turbine with
its top open. This showed that Telsa's "steam bearings"
(Tesla used steam, benefiting from the higher viscosity) were egg
shaped: the shaft was swollen like an egg at each end and the eggs
sat in egg shaped hollows. This made perfect sense, the egg
shape being able to provide lateral as well as vertical
support. Also there was separate steam injection to the
bearing itself. Paul suggested I dedicate my compressor to
that job and pull air through the turbine by attaching a normal
vacuum cleaner to the exit holes.
The diverterless block was modified to direct air into the
lower hole of the turbine casing, leaving the upper hole of
the turbine casing as just a hole through which air could
enter. The diverterless block was also modified to
accept 6 mm outside diameter/4.5 mm inside diameter Nylon
pneumatic tubing since that was easier to assemble and cheaper
to obtain the required valves for.
The turbine casing was modified so that the air inlet to
the rotor chamber was a single hole again; the air from the
diverterless block was fed into two small holes in the egg
bearing mounts.
The egg bearings were designed with a central egg-shaped
hole and split into two halves, the bottom half having a
"spout" sticking out of the side which fitted into the small
air-feed holes in the turbine casing and then distributed the
air underneath the egg.
An egg was created.
The turbine end-plate was modified so that the air exit
holes fed into a 32 mm diameter cylinder onto which I
could shove a vacuum cleaner hose.
Click on the video below to see an animation of the plan where
the blue balls represent air pushed into the egg bearings and
the green balls represent air sucked through the turbine.
The egg bearings and the egg included detail too small to
print on an FDM printer, requiring instead the 0.025 mm
resolution of my new liquid resin 3D printer. The design
took a little while to iterate, you can see below that my first
attempt had individual tubes for the air flow within the
bearing: I found that the tubes usually didn't form correctly,
being just 0.5 mm in diameter, hence the "air gap" design
you see above with the larger exit holes. Note also that
the supports on the egg had to be removed carefully with a
scalpel to avoid leaving pimples.
The other components were all printed in PLA at 0.2 mm
resolution with supports everywhere as usual.
Next came the subject of air flow. I needed to be sure
that the flow was sufficient to float the shaft on its eggs or
I'd just have another ker-chunk on my hands. I assembled
everything with just the lower half of the bearing so that I
could see what was happening.
The air flowed through the correct paths but there was no sign
of any floating. To get a very approximate measurement I placed
my jewellers' weighing scales, which happened to be just the
right height, under one end of the shaft and took a reading with
the air flowing and without:
2.06 gm from 2.46 gm is a definite effect but it
needs to be about five times better. The first simple
change I could make was to reduce the size of the holes beneath
the egg in order to increase the exit speed of the air.
Here there are four holes, each 1.2 mm in diameter:
This brought the weight on my scales down to
1.36 gm. I amped it up, reducing the holes to
0.6 mm in diameter: at this size the alcohol and water from
the washing process used on a resin-printed part, visible in the
picture above, couldn't easily escape from the cavity and
blocked the furthest holes; I had to hold the part to my mouth
and blow the fluid out through the entrance hole.
But then I had to concentrate on something else for a month,
during which time Paul sent me a link to the Dan Gelbart video of
his home-made high-precision lathe complete with air bearings
where, 50 seconds in, he demonstrates just how good an air
bearing can be; however the implication is that micron-level
precision is required. YouTube then took me to a Machine
Tech Video blog where a simple air bearing is made on a
lathe, which led me to the David
Preiss video of much more basic DIY methods and finally to
the prize, the Applied
Science vacuum pre-loaded air bearings video where they
make the darned things pretty much by hand.
I had already been beginning to doubt that I could achieve the
required surface finish even with my higher resolution resin 3D
printer and this inspired me: time to change tack.
Tesla
Turbine 2 With Graphite Bearings
As the video links above explain, to achieve an air bearing
you need teeny-tiny tolerances and smooth air flow. It
turns out that solid graphite is (a) very porous to air, (b)
smooth, a "dry lubricant", and (c) ridiculously easy to shape by
hand. It gets absolutely everywhere, so one must take some
care when shaping it, but the results are amazing: see for
instance 12 minutes into the Applied Science video link
above.
I ordered some 100 mm long, 10 mm diameter graphite
rods from Amazon. My intention was to make a graphite
cylinder bearing that I could trap inside 3D printed bearing
holders. I intended to achieve that micron-level accuracy
on the inside of the graphite cylinder by finishing it by hand
using some of the same aluminium rod that forms the shaft of the
turbine rotor.
I re-designed the 3D printed parts with this in mind (Blender and
STL files
available).
My only worry was alignment of the two bearings, they needed
to float somehow. Anyway, suck it and see.
I machined the graphite rod down to 9.2 mm diameter, to fit
into the Blender-sized 9 mm-wide top of the bearing holder,
which came out at about 8.6 mm when printed, so that it was
gripped tightly around the top of the bearing holder, making a
reasonable air seal and preventing the rod from touching the rest
of the bearing holder wall (further out at 10 mm diameter),
leaving an air gap. I drilled the centre of the graphite rod
in the lathe with a 3.9 mm drill. While machining I
kept a vacuum cleaner running with the nozzle positioned beside
the cut. I formed myself two 10 mm long graphite
cylinders this way.
To make a lapping tool I cut a piece of
the 4 mm diameter aluminium rod that is used as the rotor
shaft and machined a kind of taper into the final 10 mm,
going down to about 3.7 mm diameter, so that the end of the
aluminium rod would fit into the hole in the graphite rod. To
make a handle I stuffed the other end of the aluminium rod into
a hole drilled into a piece of dowel and fixed it in place with
a jubilee clip. I scored, length-ways, the aluminium rod
with a file, got myself a dish of water, dipped the aluminium
rod into it and then into some F-180 (coarse) silicon carbide
powder, the kind used for polishing stones. Holding the
tool in one hand and the graphite rod in the other, I worked the
tool into the hole in the graphite rod, switching ends
frequently, ensuring that I kept it turning (the graphite rod
might otherwise crack open, which happened on two occasions) and
refreshing the water/silicon carbide coating frequently: it is
the silicon carbide powder that is doing the cutting [lapping]
here. This was a messy process requiring some patience: I
tried mounting the graphite rod inside the bearing holders while
doing it as an easy way of holding the graphite rod without my
fingers getting covered in graphite but that didn't work: the
graphite rod tended to slip inside the bearing holder at just
the point when cutting should begin. Once the graphite rod
could run completely freely along the length of the aluminium
rod I tested it on the rotor shaft: it should drop without
hindrance (and then some, see below).
It was at this point, while fiddling around with different
lapping techniques, that I tried holding the end of the
aluminium rotor shaft in a stand drill and this showed quite
clearly that the aluminium rotor shaft had a bend in it: not a
very visible one but it was clear when rotating at speed.
This likely contributed to my ker-chunk problem and probably
didn't help with rotor disk centring. So I reverted to the
three disk stainless steel shaft/rotor set. Here's
a test run to check that the bearings basically worked; no air
being injected into the bearing in this case, it is simply a dry
graphite bearing.
Shifting to the much shorter stainless steel shaft I needed to
attach something to one end of the shaft so that the tachometer
could see it: I drilled a 1.6 mm hole in one end of the
shaft using the lathe and tapped it with an M2 thread so that I
could screw in a bolt. Then, again with no air to the
bearing, I checked that my domestic vacuum cleaner (Bosch GL-30)
would at least pull sufficient air to cause the rotor to move.
It did but centring of the rotor remained an issue. I
had anticipated this and, inspired by the use of magnets in the
videos linked above, I had bought some 4 mm diameter
neodymium magnets and had intended to glue them to the ends of
the rotor shaft such that the shaft would be repelled by some
adjustable stops mounted on the turbine base that also held
neodymium magnets; you can see the stops in the video
above. However I had found the magnets difficult to attach
reliably without fowling the bearing hole and I didn't want tiny
metal projectiles flying across the room so I tried making
another M2 threaded hole in the other end of the shaft into
which I could screw another bolt, a bolt with an outer-hex-head
this time, intending to affix the magnet to the head of that,
but my tap broke in the hole and even a diamond coated boring
tool would not get it out. Darn.
I cut myself a new stainless steel rotor shaft, 80 mm
long this time, made myself three new rotor disks for it (1.09
and 1.05 g balancing achieved on the outers), then I had
sufficient length of shaft to push on 3D printed sleeves, into
the ends of which the neodymium magnets could be reliably glued
with cyanoacrylate adhesive.
I attached my compressor to the air inlet for the graphite
bearings and tried spinning the shaft by hand to see if there
was "float", i.e. were they really air bearings or just
[ker-chunk free] dry graphite bearings; it was difficult to
tell: they certainly weren't completely free and forever-spinny
as I had hoped for, air seemed to be escaping from the seal
between the graphite cylinder and the bearing holder on one
side, and also from where the inlet hose attached to the
turbine.
I used thick cyanoacrylate adhesive to fill any air gaps
between the graphite rings and the edges of the lower bearing
holders (allowing this to dry overnight before reassembly so
that everything wouldn't get stuck together) and I modified the
nozzle where the inlet hose attached to include a 3D printed
outer locking ring in the hope of giving it a better seal.
That seemed to do the trick: here's another quick test
rotating the shaft by hand.
So I attached the vacuum cleaner to pull air through the
turbine, as well as attaching the compressor to float the
bearing, and gave it a first run. Not very impressive: it
didn't even go as fast as the turbine blades did while they were
being balanced:
For comparative purposes I did a run where I started the vacuum
cleaner with no air to the bearings, increased the air to the
bearings up to the maximum then decreased it again before
switching the vacuum cleaner off:
Looking at the moving average, there might be some initial
improvement but maximal air to the bearing actually made things
worse: maybe the shaft was being pushed up against the top of the
bearing, where there is no air cushion? And here is a plot of the
roll-off in acceleration while the air to the bearing was left on
(at a level which seemed optimal) after the vacuum cleaner was
switched off:
The roll-off is about 14,000 RPM in 5 seconds, as
compared to about 20 seconds for the same loss with the GRW
bearings, though admittedly the latter was likely with a residue
of air still flowing through the turbine from the compressor.
In summary the graphite air bearings were making a difference but
they needed work. Out of curiosity I tried using the rotor
with the aluminium shaft and that showed a best speed of only
14,000 RPM; dumping that was the right thing to do.
To determine if there was enough air flow I temporarily redesigned
the two lower halves of the bearing holders to take direct air
feeds from the compressor, arriving via a splitter: the flow can't
get better than this. However, as I was fiddling with
mounting the bearing holders back into the turbine body, it became
quite clear that the position the bearing holders happened to
take-up when closing the turbine had a huge influence on how the
shaft was held; the bearing holders really did need to float and,
with the relatively short lengths of stiff Nylon pneumatic tubing
I was using, they were easily pulled out of square.
So I 3D printed a nice thick washer out of flexible PLA, modified
the turbine body to allow the washer to sit just behind the flange
of the bearing holders in a gutter and opened up the holes in the
sides of the turbine casing so that the bearing holders were no
longer held so tightly by the casing.
I couldn't find tubing that was more flexible in the correct form
factor so I 3D printed short lengths of tubing out of flexible PLA
and went ahead and modified the diverter block to become the
splitter, something I would have to do anyway to accommodate this
"floating bearing" model in the final design (Blender and
STL files
available). I had real trouble, though, with the
protrusions from the diverter block snapping off as I pushed the
tubing into place and so I switched to printing the turbine casing
parts on the resin 3D printer instead for additional
strength. Out of interest, the tensile strength of the
"transparent tough" resin was 52 megaPascal, versus the
30 megaPascal of ASA, and hence might actually be a better
choice for printing rotor disks.
However, even with the very flexible PLA tubing, the shortness of
its length meant I'd lost the float again. So in order to
workaround the tube form-factor issue a different way I found some
quite flexible PVC tubing with a similar internal diameter
(5 mm, 8 mm outside diameter) and designed a separate
splitter, printed out of resin for strength (Blender and
STL files
available) that could sit further away from the turbine block,
putting the lengths of tubing the correct distance apart, allowing
the flexibility of the tubing to have an effect and then
connecting onwards to the required 4.5 mm internal
diameter/6 mm outside diameter Nylon pneumatic tubing.
However that still didn't do it: the angle at which the bearing
holders ended up joining the tube remained too influential, was
difficult to get right, and though there was flexibility it wasn't
sufficiently independent that each bearing holder could take up
its ideal position. By this time I had seen that IEnergySupply
was following the same graphite air bearing idea: he had the
advantage of large (AKA normal) scale, something which made
floating a possibility. By contrast my small scale and the
wholly 3D printed machine should bring the advantage of
precision. I decided to take two different tacks:
order some silicone tubing of the same dimension as the PVC
tubing, can't get more flexible than that,
redesign the turbine once more (Blender
and STL
files available) to take advantage of precision: the two
centre blocks were modified to include the bearing holders
(for zero slop) and were printed on the resin 3D printer,
rather than the FDM 3D printer, for 0.025 mm precision
(supports on the build plate only). The bearing holder
portions of the blocks were thickened to improve the seal
between them and to allow the air flow to be internal once
more. The graphite cylinders were held in place within
the bearing holders by compression from either end via
silicone O-rings (8 mm internal diameter, 1.5 mm thick) to act
as an air seal and to permit the graphite cylinders just a
small amount of movement.
After the pictures above were taken the central blocks were
further modified to include the magnets that centre the shaft
laterally, the holder for the tachometer sensor and, as a plug-on
extra, a seat for the Raspberry Pi Zero W: a self-contained
resin-3D-printable Tesla turbine with graphite air bearings.
But back to that first tack: switching to silicone tubing (and
doubling the length of the tubing from 5 to 10 cm) seemed to
do the trick in that the tubing no longer dictated the attitude of
the bearing holders. I had some trouble sealing the much
softer silicone tubing at joints and so applied cyanoacrylate
adhesive liberally to give myself the best chance of
success. But no matter what I did the shaft/holders never
positioned themselves such that the shaft ran loosely inside the
graphite bearings: either there was insufficient free movement or
insufficient air supply/sealing. It wasn't going to get any
better than this; onwards to taking advantage of precision.
With the freedom of movement thing in mind I decided to put the
O-rings to one side and instead adopt the approach IEnergySupply
took in their video and use silicone sealant as my mounting
compound. I coated both ends of each graphite cylinder and
the holder generously with silicone sealant, assembled everything
and then left it to dry for 24 hours with the shaft pulled upwards
by an elastic band so that the graphite cylinders would end up at
the top of the bearing holder cavity leaving room for the air
supply, coming in at the bottom of the bearing holder cavity, to
circulate.
But either the silicone sealant wasn't sufficiently
evenly/generously applied or it just wasn't good enough to form an
air seal (something which wasn't relevant in the IEnergy Supply
case since he had otherwise sealed his graphite air bearings, the
silicon sealant was purely for mounting): air was escaping almost
immediately out of the sides of the bearing holder. I tried
the O-rings instead but, as they were just shoved in on the ends
of the cylinders, they held the cylinders in some random position
that wasn't sympathetic to the attitude of the rotor shaft.
Since the O-rings were flexible enough to fit around the body of
the graphite cylinder I decided to fit them that way instead; so
they were nice and organised, I added a ridge within the bearing
holder cavity into which they could slot, designed such that the
~1.5 mm thick O-rings were squished into slots half that
size. While I was at it I modified the magnet holders such
that they were removable as I thought it might be useful to be
able to "see" down the length of the shaft-axis without
obstruction.
When air was applied I could tell this was now definitely forming
an air seal as I had allowed slightly too much space in the
bearing holder for the graphite cylinder and there was a "thock"
noise as the O-ring was pushed off into the gap.
I re-printed again with that gap closed, which solved the issue,
then air was leaking from the gaps at the top and bottom of the
bearing holder; it was stopping at the joints I had carefully
inserted but then escaping out of the sides instead, d'oh. I
added further jointing within the bearing holder to stop that
happening.
This worked. However, now when I switched on the air flow to the
bearings I found that the shaft became tighter and not
looser in the bearing. The only explanation I could think of
for that was the force of air flow, which was coming up from below
and to one side in the bearing holders, was pushing the graphite
cylinders askew. I really wanted to keep the air flow as
part of the structure, rather than having the complication of a
separate air supply to a sealed bearing which could then be left
to float, but that meant that I was unable to separate the issue
of forming a seal from that of allowing float. So I
redesigned again: I positioned the air tube outlets centrally in
the sides of the bearing holders, pushed the air opening back from
the cylinders somewhat to let the air disperse and also split the
air feed, tunnelling tubes through the body of the turbine beneath
the shaft to enter centrally on the opposing side of the bearing
holder as well (Blender
and STL
files available); can't get much evenererer than that.
But the result was the same: switch on the air flow to the
bearings and the shaft became more difficult to rotate, not less
difficult. However this time when I switched off the air
flow the ability to rotate the shaft returned to normal whereas
last time it did not. I wondered if maybe I had solved the
problem of the graphite cylinders being moved off-centre by the
air flow and now the problem was that the graphite cylinders were
being compressed by the air pressure and the holes in the centre
of the graphite cylinders needed to be opened up somewhat. I was
about to get out my lapping tool and F-180 (coarse) silicon
carbide powder and lap them a little more but, since that is
irreversible, I thought I'd first try adding baffles to the air
inputs (Blender
and STL
files available): there is no reason for a speedy air flow,
uniform air pressure is all we need and so baffles might remove
pressure points if that was the cause of my problem.
It didn't help; didn't do any harm either, so I retained the
feature. While I was fiddling I noticed that air was
escaping from the seal where the air tube tunnelled beneath the
bearing on the left hand side: maybe the reduced air pressure on
one side was the problem? As one would when searching for a
puncture in a bicycle inner-tube, I decided to submerge the
turbine in a bowl of water to find out where air was leaving it.
This showed that I was worrying about the correct leak.
There was no obvious sign of a problem on that side versus the
other so I tried simply tightening up the bolt on that side a
little more. This helped somewhat but, with the extra
tightening, the shaft was definitely not properly loose; time for
that additional lapping. I spent
15 minutes on each graphite cylinder using the same lapping
process as above with plenty of F-180
(coarse) silicon carbide powder, then I held the middle portion of
the turbine in a vice with a free length of shaft pushed through
the cylinders to check that, with the vice wound up tight, the
shaft moved easily.
Afterwards, when fully assembled, thoroughly tightened, and with
air flowing, things were slightly better in that the shaft was now
loose both when air was flowing and when it was not flowing, but
it still didn't have the "float" that I was expecting. Air
was only escaping out of the shaft-ends of the assembly, so no
leaks from elsewhere, but I wondered if my O-rings were doing
their job: was the air really being forced through the graphite
cylinders rather than simply escaping immediately out of the
sides, or into the inside? I noticed that the inner O-ring
was positioned a little close to the end of the graphite cylinder
so I re-worked the design once more (Blender
and STL
files available) to re-centre the O-ring slots, giving the
graphite cylinder 0.5 mm stick-out from the O-ring on each
side, rather than 1 mm at the outer end and nothing at the
inner end.
That seemed to make a difference, more of a "feeling of
float". I assembled everything once more and gave it a spin
to see how airy the air bearings were now.
The answer seemed to be "slightly". Below is a graph of the
rotor speed over the period of the video above:
The familiar "ker-chunk" indicated that I didn't have an
air-bearing, 'cos that's true at the start of the graph, but, when
air was supplied to the air bearing, self-recovery from the
"ker-chunk" and the potential lack of "ker-chunk" as I got the
sleeves/magnets to centre the rotor were positive signs. The
rather poor maximum speed and the fact that "ker-chunk"s were
still there implied to me that I didn't have a very good
air bearing or, possibly, didn't have sufficient air flow, either
to the air bearing or through the turbine.
Studying the graphite cylinders, I realised that I had gone
slightly too far with the hole in one of them:
I needed a better technique for getting the hole diameter just
right. The original Applied
Science method, which iEnergySupply
also employed, was to use a length of bar that the air bearing was
to be mounted on scored with an angle-grinder, effectively the
burr doing the job, but I wasn't sure that would work at my scale
and I didn't possess an angle-grinder. I decided to try a
more direct approach: the particle size of F-180 silicon carbide
powder is about 0.06 mm so I could expect the gap between the
shaft and the graphite cylinder to be about that much. I
have drills of 0.1 mm step size, so using a 4.1 mm drill
should give me 0.05 mm clearance between the shaft and the
graphite cylinder without any messing [literally] about. I
returned to the lathe to make a new pair of graphite cylinders.
Another thing which likely wasn't helping was that the rotor
disks, though balanced, I had managed to fit to the rotor shaft
somewhat askew. Given the newly-discovered capabilities of
resin 3D printing I decided to balance a new set of "transparent
tough" resin (the 52 megaPascal tensile strength stuff) rotor
disks. I re-designed and resin-printed the rotor alignment tool to make
sure the rotor disks were pushed together during fitting, adding
prongs to line up the holes in the rotor disks as they were
mounted.
Then fate intervened: my little Jun-Air 6-25 compressor sprung a
leak, a leak from the body which would be difficult to repair; I
had to pause to increase
my compressor capability.
Four months later I had a rather more capable
10 bar/100 litre compressor, had cut a new pair of
graphite cylinders 9.2 mm in diameter, 10 mm long with a
4.1 mm diameter drilled centre hole and had three resin-based
rotor disks, mounted on the usual 80 mm long, 4 mm
diameter, stainless steel shaft. I finished the disks in the
lathe to ensure a tapered edge but skipped the lengthy rotor
balancing process for this run; I will invest in that once I
actually have an air bearing.
I also opened-up the slots where the O-ring sits in the casing (Blender and
STL files
available) so that the O-ring had a little more room to move, not
squishing the graphite cylinder while hopefully still forming an
air seal.
While playing with this I tried pulling air through the turbine
via the vacuum cleaner and also tried injecting air from the
compressor, both without supplying any air to the bearing; there
was a considerable difference: 10,000 RPM versus 90,000 RPM.
Of course supplying air to the bearing is the point but it
occurred to me that maybe I should experiment with using the
compressor for both propelling the rotor and supplying air to the
bearing. I bought myself some pneumatic T joints, two
in-line pressure regulators, two valves and two pressure gauges,
allowing me to split the incoming air, adjust each branch
independently and measure the pressure in each. I modified
the diverterless block to add a nozzle to the hole where air
enters the turbine so that I could connect tubing to that inlet.
When I tried running this, though there appeared to be some float
in the air bearing, the turbine didn't run at all no matter how I
split the air (even solely into the turbine rotor air intake)
whereas it did the moment I used my air gun to inject the air into
the turbine rotor air intake. So I returned to air intake
nozzle design but, this time, I made the nozzle an insert to fit
between the diverterless block and the inlet casing; this way I
could play around with the dimensions without having to re-print a
whole turbine section. Following the experiments above I
made the exit hole 0.25 mm in diameter and, of course, I was
now printing this on the resin 3D printer which allowed somewhat
greater accuracy (though a quick run-through with the Ernie Ball
gauge 10 guitar string was still required to open up the
hole).
It was while testing this arrangement that I realised where I had
been going wrong all along. Watch this:
It was the magnets at either end that had been "gripping" the ends
of the rotor shaft, pushing them out of line; or maybe, you know
how if you push the same poles of two bar magnets at each other
they tend to want to slide off to one side? That could be
deflecting the rotor shaft; remove them and I had float. I
HAD FLOAT!!! How stupid of me not to have realised
this. But of course, without the magnets, I had side-to-side
shudder:
What to do to fix this? Since there was now a lot more air
in the system I decided to have another go at the approaches above
(Blender
and STL
files available), in combination this time, i.e. split the air
intake at the nozzle into two so that it was aimed into the gaps
between the rotor disks rather than directly at the edge of the
central rotor disk and also modify the sleeves on either end of
the shaft so that they could come up much closer to where air was
escaping from the bearing with a slight lip, intending to form an
air buffer. I also opened the hole(s) in the nozzle
somewhat: each about 0.5 mm in diameter to avoid needing the
Ernie Ball treatment.
Here's a slow-motion (1/8th real time) video of it running (the
clue is in the subtitle):
As you can see, as happened last time I tried it, and despite the
increased air flow, the shaft sleeves are as much sucked back onto
the casing as they are pushed away from it. The air flow
was, it seemed, directional, pulling in from one side and out the
other. I opened up the turbine and, holding it in my hand,
ran air just through the inlet nozzle to see how the rotor behaved
(it wasn't leaning to one side, that is just how I'm holding the
camera):
It definitely wanted to pull to one side and, if the air was
flowing only one way, using it as a buffer was not going to
work. Maybe the issue with the magnets was the arrangement I
had chosen: the "point against point" nature would always lead to
a deflection of the rotor shaft as they slid off each other.
I ordered some ring magnets to have another go. By
experiment, the poles of the ring ferromagnets (12 mm outside
diameter, 7 mm inside diameter, 3 mm thick) were
arranged as below; provided I could get distance "x" correct,
which seemed to be somewhere between 2 and 5 mm, I might achieve
the effect I was looking for. I redesigned (Blender and
STL files
available) to bring the magnet support towers back.
However, playing with the magnet position and the air flow showed
me a few things:
I suppose that made sense: the air flowing into the air bearing
will be exiting, partly, into the body of the turbine and hitting
the rotor disks, flowing in the opposite direction to that of the
incoming air from the nozzle that is trying to escape, hence it
will push the rotor around the opposite way. How
annoying. And the ring shape of the magnet wasn't doing me
any good, though maybe the fact that the edge of the magnet worked
well enough meant that I just needed a "buffer" magnet with a
larger, flatter, surface to avoid it causing deflection of the
opposing magnet attached to the rotor shaft. I redesigned
again (Blender
and STL
files available), this time to introduce an air escape channel
between the housings for the bearings and the rest of the turbine
casing, and I also ordered some larger/flatter magnets (15 mm
diameter by 1 mm thick); only the inlet side of the casing is
pictured below but an identical air escape channel was made in the
port casing.
To be continued...
Tesla Valvular Channel
I took the picture of the Tesla valvular channel from the
patent:
...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 of eight.
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 for
why).
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 part...