LTD Stirling

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Tim Wescott

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If things work out, this is going to be a really excruciatingly slow build thread for a low temperature differential (AKA "coffee cup") Stirling. If not it'll be yet another abandoned thread.

It's going to be a "Plans? Yes, I'm planning to build an engine!" sort of build. But I am looking for folk's thoughts on two things: first, how necessary is the glass cylinder/graphite piston idea, and second, what do people use to seal the displacer piston shaft?
 

gartof

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I built one years ago the power piston is graphite in an acrylic cylinder seems to work.. the displacer shaft is just a hole in long (1 inch) acrylic, brass shaft. Runs best on a bowl of ice. (Built from a photo)
 

Tim Wescott

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I just realized this should go into the "Engines" forum. Oops. If any moderator reads this -- feel free.

At any rate -- decades ago I bought a 3" long piece of 4" diameter acrylic tubing for this engine. It's been patiently waiting. Today I turned a tool to turn a tool, then I turned a tool (a wooden disk to hold the tubing round when I put it into a chuck), and I put the tool into the stock. Soon I'll part off a section of the tube to make the displacer-piston cylinder.
 

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a41capt

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Wherever it ends up, I’ll be following Tim! :)

I built my LTD last year and gave it to my 8 year old grandson. I had it running on ice packs, and when I finally got it going, it was quite a celebratory moment!


I look forward to seeing your plans and the finished engine running!

John W
 

doc1955

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I also built and had them running on ice rather than heat. They sit and run untill the ice is all melted.

 

Tim Wescott

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It's a displacer cylinder! It was supposed to come out one inch long, but it's about five mils short. I'm going to keep it.
 

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Tim Wescott

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So where you you source a hunk of graphite for a power piston? I'm tempted to start out with an aluminum piston and some graphite lubricant.
 

gartof

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So where you you source a hunk of graphite for a power piston? I'm tempted to start out with an aluminum piston and some graphite lubricant.
Go to a electric motor repair/rewind shop tell them what you want they may give/sell you a brush large enough to get a piston out of or in my case 10 pistons.
Gary
 

a41capt

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So where you you source a hunk of graphite for a power piston? I'm tempted to start out with an aluminum piston and some graphite lubricant.
Better still, go on fleabay and look for a graphite and glass dashpot. I bought about 30 of them in a package deal years ago in several different diameters and lengths. Just do a little surgery to suit your needs and voila’, power cylinder with NO leaks!

John W
 

Tim Wescott

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Top and bottom plates. Made from some scrap aluminum sheet that I got God knows where.

I'm going to see if I can turn these into nice round four and a half inch diameter circles. I don't know if I'm going to keep the wood grain or if I'm going to try to remove it. I'm sure it's going to insulate them a little bit, and that's bad, but then there's an awful lot of surface area there. I think I'll try and see how hard it is to remove that stuff and base my decision on that.
 

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Tim Wescott

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One plate, 4.497" in diameter -- I keep hitting these things a bit small!

The other plate, ready to be tossed into the 4-jaw. I'm using double-stick tape to hold the plate to the round bit of plywood that I'm using as a chuckable piece.
 

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Tim Wescott

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Two nominally nice plates. I say "nominally" because the second one is a bit - uh - not flat. It's probably within about 0.01 or maybe even 0.005 now, though. The sheet that I cut this from was banged up in one corner, and I think that wow extended into the circle there.

Fortunately, I have body & fender tools, and I managed to make it flatter. Also fortunately, I'm planning on sealing these with calking compound on assembly, which should cover up (or fill in) a number of sins.

I need to step back and actually think about design parameters now. I got that nice suggestion from @a41capt to buy a dashpot, and then decided it would be cheating to buy a piston-cylinder assembly already made. So I'm going to make one myself. I'm going to keep that thought in mind though, if making my own doesn't work out.

It turns out that folks use graphite rods for stirring in jewelry making -- I got myself a 1" diameter rod one foot long, that should be enough to get into a whole lot of trouble with. My lathe will be dirty (but slippery, if it's pure graphite).
 

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a41capt

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Two nominally nice plates. I say "nominally" because the second one is a but - uh - not flat. It's probably within about 0.01 or maybe even 0.005 now, though. The sheet that I cut this from was banged up in one corner, and I think that wow extended into the circle there.

Fortunately, I have body & fender tools, and I managed to make it flatter. Also fortunately, I'm planning on sealing these with calking compound on assembly, which should cover up (or fill in) a number of sins.

I need to step back and actually think about design parameters now. I got that nice suggestion from @a41capt to buy a dashpot, and then decided it would be cheating to buy a piston-cylinder assembly already made. So I'm going to make one myself. I'm going to keep that thought in mind though, if making my own doesn't work out.

It turns out that folks use graphite rods for stirring in jewelry making -- I got myself a 1" diameter rod, that should be enough to get into a whole lot of trouble with. My lathe will be dirty (but slippery, if it's pure graphite).
Having made an earlier piston out of graphite, I found a nicely rounded piece of HSS with about 1 degree of face relief and no top relief got me close to a good finish. To get the perfectly smooth finish I was looking for in and almost airtight sliding fit, my final “sanding” was done with a piece of notebook paper and followed with a piece of newsprint. Polishes it up perfectly!

John W
 

Tim Wescott

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I'm quite happy. I inherited a bunch of mechanical and electrical and electromechanical doodads from my Uncle, who passed away 30 years ago. In all the stuff is a little tube with some very sweet little bearings, 3/16" ID, 5/16" OD, and about 0.110" long. I'm going to use these to support the crankshaft. They're very free running, even though they were probably made before 1970.

I had wanted to make up a crankshaft with cranks in the middle, but decided that the path of sanity lies in using some 3/16" drill rod that I have on hand and that fits the bearings nicely, and fitting the power and displacer cranks onto the ends of that.

I'm making a 1" long bearing carrier out of 1/2" square aluminum. I wasn't sure if I could get the bearing fit right, but I managed to do it on one side -- the bearing's a nice light press fit. I think it's one of the best fits I've achieved. I'm happy about that. I'm slightly less happy about removing the bearing now that I have it in. I don't want to damage the thing. I'm going to see if warming the assembly up to get it out will work. Even if I have trouble there, I'm still happy, dammit!
 

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Tim Wescott

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Hah! I put it in an electric skillet set to 350F (probably way hotter than necessary, and only as I write this do the words "temper" and "draw" zip through my brain), and it popped right out. Next time I won't get it so warm, if there even needs to be a next time.
 

a41capt

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Buying one of the cheap heat guns from HF was one of the best things I’ve done. No more torch, no more waiting until my wife is away to use the kitchen oven, and all the convenience of just plugging into the wall for power!
 

Tim Wescott

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So, to solve a problem with figuring out how to hold the work in my Smithy in milling mode, I decided I'm going to trepan a slot in the bearing block, and make a matching dingus in the post. I've just trepanned something for the first time ever! And made a trepanning tool for the first time ever! And it works!

And if you're not going "WTF?" yet - after thinking it over multiple times, carefully mapping out in my head how the tool was going to work, etc., etc. -- I cut it mirrored from what it should be. So I have the perfect trepanning tool for a left-handed lathe. Rather than do that again, I just reversed the direction of the lathe to make the cut 😮.

The tool itself is basically carved out of the end of a 1/4" square bit of tool steel with a Dremil Dangerous Disk, with just a bit of touch-up on the bench grinder where it needs to be sharp and flat.

Next time I'll get it right. Really. Honest.
 

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