Mechanical drive train prototyping

Pace of development has definitely slowed down, as almost all of my time these days is dedicated to Mocobe, but I was feeling lazy today so we walked down to Balompie #3 and had pupusas for lunch, and then I spent some time in the afternoon tinkering with the drive train.  I've been working on it on the weekends a bit, and instead of building it all in one go, I decided that for something this complex, I was going to try roughing it out first.  This gave me a good opportunity to try a few things out and fix problems as they arose before I settled on a final design, without having to machine a lot of parts and drill a ton of holes in steel.

I used scrap aluminum plate, and did the work in my basement, with an angle grinder to cut the aluminum and a hand drill to drill the holes, which is a lot faster than drilling holes in steel with a drill press.  Fortunately, as well, I have a good sized collection of clamps, so instead of drilling into the frame, I just clamped the pieces onto it, making it easy to adjust them, remove them, and move them around.  To attach the chain to the shuttle, I made a really janky attachment point with duct tape, some wood, and a couple of eye hooks.  Eventually I'll weld an attachment point onto the shuttle, but I wanted to test this out first.

Here's the drive train in action:

Some notes from the build: 

One thing that's different from the previous version is that I added a 15 x 20 mm cable carrier to the hose.

 

It's actually two, bolted together.  The problems I was having with the hose tracking properly are totally solved by this, the hose bends fine and doesn't get tangled in the rails at all.  Since I haven't machined the sheet cladding for the whole thing yet, I hope that it will still work okay when it's constrained.

I had to make a bunch of adjustments as I was getting the drive train to work: the tensioning gear here is critical for holding the chain against the drive gear.  Otherwise the whole thing slips, there's too much play in a chain that long.  It's also really important to get these three gears aligned properly:

Otherwise, it puts a lot of strain on the swivels.  I bought swivel hubs from Servocity for mounting the sprockets (also available at Sparkfun) and found them to be junk.  The free-play of the hubs is bad, and they repeatedly fell apart when they were used under strain.  The halves are simply held together with a nylon lock nut and  a bolt.  Presumably the idea is that the lock nut prevents the bolt from loosening (or tightening!) and the halves will just rotate against the washer, without moving the bolt.  In practice, I found that in use, the hubs either fell apart or seized up due to over-tightening, depending on the direction of rotation.  It's possible that this was due to lateral strain from misalignment of the chain, which caused pressure on the washer.  In the end, I don't care why, they're not robust enough for this application.  I wound up disassembling the ones that were failing and just putting a bolt through one half, and leaving it mostly tightened.  That was fine for the time being.

I'm also thinking that a faster motor would be good.  This is running flat out, and it's got way more torque than it needs.  I'd rather get something faster, and be able to run it at slower speed using the speed controller if I need to.  I bought this one from Servocity, which runs at 118 RPM max speed, but I could just swap it out for the 313 RPM unit and I'm pretty sure the torque will be plenty.

I haven't decided on the design of the chain attachment point for the shuttle, but I'm thinking it might be handy to have some sort of short linear tensioner, like just a screw with 1" travel.  Being able to link the chain on and then screw it down to tighten would make life a lot easier.

Also, it turns out that my bicycle chain breaker doesn't work for this roller chain because the pitch is different, which isn't surprising.  I can try doing it the old fashioned way by just pounding out a rivet, but last time I tried that with a bike chain I smashed a finger.  Or I could just cut it.  I looked for a 0.25 roller chain breaker, and found this one on Amazon, but since it's made by Koch Industries,  I won't be buying it.

Shuttle production model build notes

Hey amigos, I know it's been a while since I've rapped at ya.  I've been working on starting up a thing, which you probably already know, because if you're reading this, you're friends with me.  And if you're not friends with me, and you're reading this: welcome!  I am a handsome and accomplished man!

But, in between hacking together React widgets and swearing at JavaScript for having ever been invented, I put together a new prototype, which I am not thinking of as a prototype, it's really a test-build for a production model.  Here's the goods:

The entire thing is 8 feet long, made of two 4 foot halves.  The framed ends are 6" by 6" 0.120.  1/2" wide on the ends, and 1/4" wide on the parts in the middle.  These are held together with 1/2" square tube, welded onto the frame ends.  The rails are made of 8 foot long segments of 1/2" EMT electrical conduit.

The shuttle is similar in design to the last one, but the rail holders are solid tubes, and much larger diameter.  The 1/2" tube rails are much stiffer, and the rail holders are just welded onto the pipe holder in a single line.

The rails are held to the frame ends by 1/2" metal rod which is welded onto 1/2" wide segments of steel right angle.  These are screwed into the frame ends which are 6-32 tapped.  I broke my tap off in one of the frame ends at one point, and just used a cold chisel to break it off, and moved the hole.  It turns out that these tap extractors don't really work, at least not for a tap that small.

When the motor and chain drive are installed, I'll wrap the whole thing in 22 gauge steel (which is the thinnest I could get from Bay Metals that wasn't galvanized.)

Need to attach a hose and check how the running actually works, then buy and install a chain drive.  But, it's coming along.  The chain drive will be fun.

Working shuttle prototype!

Simplify, simplify...after about a year of prototyping, I finally sent out some feelers and did some networking with some other fire artists, to get some feedback on my designs.  I had a good conversation with DaveX from BMOrg, who made a couple of observations that stimulated a line of thought that I had been rolling around in my head for a while.  I hacked together a prototype out of 6" diameter HVAC conduit and off-the-shelf end-caps.  All the "machining" I had to do with it was to cut a 1" slot freehand with an angle grinder, and I cut some strips from the leftover which I screwed into it to form it back into a rounder shape:

I drilled some holes in the end caps for the rails, and used the end caps from the previous prototype to hold them in place.  Then I threaded the old shuttle onto it.  With just the slot in the top, the effect is nice, but the flame is very concentrated, and it definitely has the aesthetic of something mechanically moving the flame underneath:

BUT...(and this is where DaveX's suggestion comes in), I then fit some aluminum screen over the slot, and laid some steel wool over it to use as a diffuser.  The effect is a much more organic-looking flame:

I think this looks great, and it's incredibly easy to build.  The only welding and machining is of the shuttle.  I have some issues with the hose tracking to work out, and I need to motorize it.  But this is going to be the basic design I'm going to go with for the eventual build.  I may even extend these to 8 feet long, if I'm going to start with a pyramid instead of an icosahedron, just to make it more impressive in scale.

Tomorrow morning, I'm going to start a new venture, starting up a software company.  That may or may not work.  But it is going to take up a lot of my time and my vacation is officially going to be over.  I'm very satisfied that I have a working prototype, and even if my progress slows down, I know I can get this up and running eventually.  Onward, and upward.

Shuttle prototype v5.2: Getting Better All The Time

Spent some time testing the shuttle prototype with various air intake mechanisms, some of which produced better results than others, but none fantastic.  After the intake fan burnout I decided to go with MOAR AIR.

I had two main strategies: air mixing with the gas line, and forced air into the sheath.

My initial gas mixing strategy was to inline a vacuum ejector into the gas intake, but this had no discernable effect on the output flame color or constitution, so I got rid of that quickly; if there were significant enrichment of the flame with air, I would have seen a much bluer flame, due to more efficient burning of the propane.

To test a compressed air source, I tried to build a ghetto fabulous compressor by hooking a really cheap 12V tire compressor to an old propane tank that I had converted into an accumulator:

 This worked a bit, but it took about 30 minutes to pressurize the tank to 40 PSI, and after the third time, the poor little compressor blew a gasket.  So much for plan A.

Plan B was to rent a compressor:

I hooked this up with a T into the gas line, with a check valve at the gas line to make sure the air didn't flow back into the gas line.  The compressor went up to 120 PSI, but the regulator on it wouldn't go above 60 PSI.  This turned out to be plenty though, at least for my initial tests.  Simply mixing the gas with the air, even at 20 PSI, would blow out the flame.  The regulator didn't go much below 20 PSI, so I installed a 1/4" needle valve so I could make fine adjustments to the air flow.  I wasn't able to get any improvement over the usual observations: poor flame, mostly confined to the sheath and blowing out the ends.  This was at low gas and low air flow.  At higher air flow, the flame would blow out, and at higher gas and air flow, it was about the same.

If I removed the shuttle from the sheath and ran the gas at 30 PSI, it produced a great flame.  If I slowly introduced air, I could create a nice blue flame, clearly modulating the air/fuel ratio.  By adjusting it this way, out of the sheath, I was able to ensure that I was getting the right ratio.  But putting the shuttle back in situ, the results were very similar to before, little or no sustained flame.  This suggests to me that the primary issue may not be air mixing, but ineffective ventilation of the exhaust: as soon as the gas starts burning, exhaust builds up in the chamber and snuffs the flame.  I would still expect the gas to burn under these conditions if I'm supplying enough air.  But it's possible that the "correct" ratio that I see with the shuttle in the open air is a result of the introduced air + the atmospheric air, and that in a confined space, the lack of the latter is enough to kill it.  This would suggest that it IS possible to supply enough air with the fuel, possibly.

I next tried an air manifold that inserted along the bottom of the chamber.  I took a 1/2" pipe and drilled 1/4" holes at 10" intervals, 10" from each end (so, 6 holes total).  I machined plates that fit over the fan intake fittings to hold it in place.  This also would help force the introduced air up towards the flame outlets at the top, instead of simply flowing out the open ends.

I ran these at 60 PSI, at pretty high flow rates, and fiddled with the gas to get various effects  Here's what I got at about 30 PSI gas:

The flame isn't very tight, but I'm definitely getting a translation.  At higher PSI, instead of getting a larger flame, it was simply getting spread out further, getting less localized.

I have an idea next to simply improve the ventilation of the sheath by cutting the upper half of the sides off, and seeing if this gives me the desired effect; if it does, then I can possibly reintroduce some shielding by adding sections of 6" diameter HVAC conduit.  The purpose of this is to keep the innards hidden, so that the mechanism isn't obvious.  This is the real issue I'm seeing: if I cut the top off the sheath, this would probably work, but then it would be obvious what the mechanism was, and the aesthetic would kind of be ruined.  So trying to get this to work inside the sheath, or SOME kind of sheath, seems like what I'd like to do.  Possibly going to an even larger sheath, or adding more holes to the top, or all of the above could also help with ventilation and exhaust.  Some progress, but still needs work.

Build details on shuttle prototype v5.1

After the relative success of the shuttle prototype, I've been trying to scale this up into a design that is more similar to what I imagine the eventual final design will be like, which is to say, I've been working on building something that doesn't suck, and that I can use as a blueprint for a final design. I've stopped keeping track, but I think given the gas mixing prototype, rotational valving, translational valving, multiplexing, and shuttle prototypes, this is major version 5, making this Flaming Icosahedron v5.1.  I've done some basic experiments with it, but I wanted to take the time to write up some of my build notes here.

The sheath is made from 6"x3"x5 foot rectangular tube, 0.120 wall, which I got from Bayshore Metals.

I drilled 1/4" holes at 1/2" intervals along the top.  I managed to basically destroy my 1/4" bit along the way.  I actually took it over to the bench grinder and (after watching a YouTube video) re-ground the tip by hand, badly, but well enough to get me through the end of the drilling.  But I bought a $20 bit grinding jig for next time (and also a new 1/4" cobalt tip.)

I've posted photos of the new shuttle build before,in my last post.  I moved the rail holders to the inside to reduce the width.  The rail holders are just cut up pieces of 3/8" black pipe (which is weldable, unlike galvanized pipe.  Welding galvanized metal is a great way to inhale zinc byproducts, which will kill you slowly and painfully.)  

I also made some little clips for holding the 16ga cable in place (in v5.0, I was using picture wire twisted in place, because it's what I had lying around.  None of that bush league crap this time.)

Here you can see the shuttle in place on the rails (1/4" rod from the hardware store), with the gas supply line in place on the shuttle.

The rails are held in place on the ends with 16ga sheet metal that I bent using a handbrake and drilled and tapped holes into the sheath.  I welded 3/8" pipe to the ends, and tapped holes for set screws.

Initially, I hand measured and cut the sheet metal, but for the second set I made, for the fan intakes (below) I ran these on the water jet cutter, and it saved a lot of time.  Those needed a hole in the center for the fan intake, which would have been much harder to do by hand anyway.

Fully assembled (without the fan intakes), you can see the shuttle running along the rail very smoothly, dragging the looped-back gas line with it:

This is another one of those things where, after a week of construction, I was so happy with the results that I sat there playing with it for at least 10 minutes, just pulling the shuttle back and forth, because it worked so smoothly.  It's not a complex machine.  It's just great to imagine a thing, sketch it on the back of a donation insert at high holiday services, and then make it.

I ran a preliminary test without the intake fans, to see how it would work without any additional air, and the results were pretty predictable; at about 30 PSI, I could get okay functioning near the ends, but towards the center the flame snuffs out due to too little air:

  At 50 PSI (closer to where I'd like to run it), it doesn't really work at all.  I attached the intake fans and tried again, and the results were at first better, but there was a flame-out inside the sheath.  I see this pretty often: if not enough air gets into the middle, the fire can go out.  When propane reaches the open end, it flares up and comes out the end in a jet.  This is what you see at the beginning of this demo from v5.0.  There's no video of this run, but in this case, the flame jet came out through the intake (which was providing the air that the fire was seeking), destroying the intake fan:

So, from a build perspective, it's solid, and it works well.  I'm going to disassemble it today to check the hose, and make sure the flame-out didn't damage it.  But getting enough air in is going to require a different design.  Luckily, there are several design I'm considering: adding forced air through a compressor, adding more ventilation to the sheath, and also, carburation.  I found this design online, and reached out to the author:

I have an SMC vacuum ejector that I plan to use as my next test after I check for hose damage.  Progress!

Building industrial grade fire art

I was able to make the shuttle prototype functional, at least marginally, by simply slapping a fan on one end to push more air into the chamber.  Here are the results:

There's some wind, and I'm not running the gas at very high volumes, but you can see it clearly translate from one end to the other as I pull the shuttle through.  If you look at the second video closely, you'll also notice that the tygon tube has burned off the end by the time I get it back through: stress + heat = failure.  I'm very happy with this design overall, though, and am now trying to scale it up to fix the problems.  First, I came up with a new, much larger design spec, using a 3" x 6" x 5 foot sheath.  The 6" height should give me enough space to use a low-pressure braided LPG hose, which is much less flexible, and keep it well out of the way of the shuttle.  I've also redesigned the shuttle, which will run along rails

It came out a little too wide, I'm going to have to grind off the tubes which run along the rails (actually 3/8" black pipe) and move them closer to the center.  But the concept is there.

The rail will run along the top half of the 6" height, leaving the bottom half for things like hose clearance (mostly), but also having a fan on either end to push air in, some room for gearing and a motor to move the shuttle back and forth, etc.  I also started machining the ends of the sheath, which will hold the rails in place (and the fans):

These are 16 ga steel sheet which I cut and bent, then tapped holes in the sheath to hold them in place.  Fun stuff.  I screwed up the bending the first couple of times, took some practice to get it right.  But this one fits nicely.

So, this is not a "proof-of-concept" prototype.  This is intended to be an actual model for what I might eventually build to bring to the Playa, and is industrial-grade.  No joke.

Carburetion vs forced air

Some really interesting reading this morning on strategies for mixing air and gas to the right ratio.  Based on last night's experiments, this will be key to getting this to work, and I've seen gas/air mixing as critical to almost every prototype I've worked on.

Basically, you need about 96% air and 4% propane to get a good burn.  In a confined space, like the sheath tube I'm using, getting enough air in there requires an active strategy.  One strategy is to just pump air into the sheath.  This is the simplest thing, and probably what I'll try first: drill some holes in the side of the sheath, and hook up an air pump.  It's inexact, but it should work.  If I go to a pressurized air tank, I can get higher flows, and use a needle valve to tune the ratio.

But an interesting alternative is to build a carburetor that pre-mixes the propane and air.  A carburetor basically uses the Venturi effect to draw air into the stream actively before the gas reaches the outlet.  This is a good example of such a design, for a propane burner that can burn much hotter than just burning raw propane:

The writing is unfortunately pretty bad, but the gist is that the teeny hole in the middle pipe lets out propane, and it draws air in through the giant hole in the reducer.  There's not much in the way of tuning, but it works pretty well apparently, possibly just based on the specific hole sizes used.  Using a design like this in the shuttle would be complicated; it's not clear how I would adapt this.  But it's a starting point for some thinking on how to make this work.