All Things Nikola

This page last updated: 17 January 2021.

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

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.

Tesla Turbine parts

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.

Disks glued
Turbine bolted
Motor jointMotor Pneumatic fitting
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.

Series 21
          quick-connect pneumatic fitting

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:

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.

The parts

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.

Bought-in parts

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.

Adding a
                rotor to a rotor centre, encouraged with a soft-jawed
Male and
                female rotors half assembled
Male and
                female rotors fully assembled
                parts for assembly Peg
                inserted Knob
                on Handle

The main body parts lined up as follows:

Bodies en

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.

Shafting a
Shafted rotor in bearings

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.

                  rotor Rotor

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).

                turbine blade prints
In housing
Single stage baseplate used to close off second
                stage Assembled as single stage

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.

Improving rotor finish Loosening bearing slots
Single turbine

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.

Local valve added

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:

Air flow through
            side holes

But that's rubbish, the air was not escaping through the middle of the rotor.  What I wanted was this:

Air flow
            through end holes

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.

            air exit holes

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.

            before assembly

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.

Components for tacho
Painted shaft

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.

Optical sensor connected up
Voltage swing
                    and turbine assembled

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.

Speed curve

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.

                    analyzer acceleration
                    speed zone detail

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 megaPascals, 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.

192,000 RPM

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 (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.

                M4 thread
                spring and sticks

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.

Parts printed

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.

Squidging Springs
                loops adjusted
                stick in position
Bolt and
                cylinders mounted Assembled

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.

Tiny nozzle

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.

Printed parts

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.

Balancing test run
Vibration @ 0 to 60,000 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.

Rotor after balancing
Vibration after balancing

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.

Rotor disk on
                    M4 bolt
                    rotor finishing
                    rotor disks

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.

Rotor disk alignment

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.

                    wired to Raspberry Pi Zero W
                    in position

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.

Bolt with
                    extended thread and marker sleeve
Ensuring that
                    the rotor disk is square on the bolt
Ready to

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:

            measurement run

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).

                    five times

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:
  1. Clip a length of the roughest sanding stick into the abrader.
  2. 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.
  3. Switch on the air flow and bring the rotor disk up to about 25,000 RPM or as high as you can without oscillation.
  4. 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.
  5. 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.

Improvement with
            various different balancer configurations

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.

                    female outer rotors
Balsaloc on
                    rotor disk
Rounding end
                    of shaft
Rotor disks
                    stacked in alignment tool
Inserting shaft
Shaft inserted
Rotor completed

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.

                measurement run on whole rotor
Vibration, whole

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.

Blips due to
                Balsaloc flying away

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, 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.

Turbine and shield
I installed the turbine, lubed up and away we went.

5 rotor turbine in
Speed curve for 5
                rotor turbine

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.

Rotor in position
Air hole

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.

Paul's original air
My modification of
                the air inlet for 5 rotor disks

So I went back and did it again.

Betterly modified air

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.

Small aperture run
Large aperture run

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.

Spacing at either end of

I modified the casing to reduce this to 1 mm and tried again.

Reduced gap at rotor

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.

Five disk rotor as
          gapless as I can make it

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.

Duel inlets Speed curve with
                dual inlet

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:
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.

Air bearing
Air bearing
                inner funnel
Air bearing
                outer funnel
Air buffer
The new designs

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.

Testing for
Buffer inserted
                at the intended distance

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.

Getting better

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.

Rifled air
                bearing design
Rifled air
                bearing printed

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.

Rifled air bearings
          assembled in the turbine

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.

Dual air inlet
                for air bearing
Air's eye view
Dual air bearing
Adjustable tacho
                tower Gasket fitted

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.

Printed air
                bearing with shallow rifling
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:

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.
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.
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.
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.
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.
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.
Something else.

The bits that were

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.

Egg bearings it is.

Tesla Turbine 2 With Egg Bearings

I redesigned everything (Blender file and STL files available):

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.

Turbine casing,
                inlet side
End plate
Egg bearing, top
                view Egg bearing,
                side view Egg

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.

First and second
                air bearing design
Egg before

The other components were all printed in PLA at 0.2 mm resolution with supports everywhere as usual.

The egg bearing parts
Egg in bearing
Bearing closed

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.

Checking for air flow

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:

Weight of one
                end of turbine shaft, no air flowing
Weight of one
                end of turbine shaft, air flowing

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:

Four air exit
                  holes each 0.2 mm in diameter The printed
                four-hole bearing.

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).

                holder for graphite bearing
                bearing in place
                bearings in the turbine

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.

Maching the
                graphite bearing
Maching the
                tool Silicon
Bearing fit
Bearing in
                holder Bearings

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.

Magnet in
                position Ready to

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.

                rings sealed in place
Air inlet
                with locking ring
Tube locked
                in place

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:

Graphite air
        bearing run 1

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:

Varying air
          supply to the bearings

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 when closing-up 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.

                  holder with direct air inlet Feeds
                  from splitter Flexible
                bearing holder fitted Flexibility

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 mPa, versus the 30 mPa of ASA, and hence might actually be a better choice for printing rotor disks.

                pipe attached Resin-printed blocks and splitter

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.

PVC tubing
                and separate splitter
                to get the angles right
Wiggle is
                not independent

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:
The plan Done

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.

All resin

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.

                sealant on cylinder end Silicone
                sealant on cylinder ends
                sealant on ends of bearing holders
                hopefully, sealed up

But the silicone sealant either wasn't sufficiently evenly/generously applied or 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 and, so they were nice and organised, I added a ridge within the bearing holder cavity into which they could slot.  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.

Ready to

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 joints within the bearing holder to stop that happening.

Air leak Joints

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 requirement 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 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.

Air inlet
                  air flow Printed and

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 needed to be opened up somewhat. I got my lapping tool and F-180 (coarse) silicon carbide powder out and lapped them a little more.

To be continued...

Tesla Valvular Channel

I took the picture of the Tesla valvular channel from the patent:

Tesla valve

...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.

                    of the valve in Blender
Valve full length

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).

Solid shape
Inside the

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:

Printing the
Half way through

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.

Rounded body

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...

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