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| Building another Stirling |
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| cidrontmg:
Hi mad jack, I wish I knew what sort of aluminium it is! The seller said it comes from France, and that´s all he knew. It´s probably something similar to 6061, very easy to machine anyway. In the other (smaller) machine, there are aluminiums from (at least) three different sources. The power cylinder is from a bar I bought in Hamburg, Germany, the flywheel is from the same local bought French bars, and the displacer cylinder is made from AFAIK a Norwegian bar bought from Sweden. None of the sellers knew much about their properties. They all machine quite well, and polish also well, but the French bars rapidly lose their lustre and turn dull. In the first picture, if you compare the power cylinder and the flywheel, you see what I mean. When I finished polishing them they were equally shiny. The effect is even stronger in nature than in the photos. The power piston is almost like chrome plated, the flywheel is like the bottom of a Coke can. I´ve had a graphite piston in an ali cylinder in one engine before these two, a steam (= compressed air) engine. I think they work just fine - if the cylinders are rather cool, as they are with compressed or even hot air. I think even in a flame sucker the temperatures would be low enough. I have no experience about steam or I.C., and graphite/aluminium, and I´m very skeptical until proven wrong :scratch: Graphite doesn´t expand nearly as much with heat as aluminium (graphite=0.5-6.5 when aluminium=23.1), so if the cylinder gets quite hot, the graphite piston won´t follow, and will start to leak. BTW, graphite is "two-directional", it has a laminar structure, like a stack of paper. The expansion across the stack is just 0.5, the 6.5 is along the "grain". A graphite bar is "amorphous", meaning that there´s graphite particles in every direction. Usually in a graphite bar there are more particles oriented along the bar than across it. So the piston lengthens several times more than it grows in thickness, per every degree of temp. The ali cylinder would expand 46(!) times more with each degree, if all the graphite particles were oriented along the bar. Even if in fact they´re not, a graphite piston won´t expand practically at all with heat. So cylinders for graphite pistons should not get very hot, or they will start to leak, no matter what they´re made of. Steel has a thermal expansion coefficient of 12 (half that of ali), stainless is 17.2, and brass is about the same as aluminium at 20.3. Anything won´t come even close to cross-grain graphite in that respect. Pure chromium, and certain glasses are near to amorphous graphite. Another thing is that a graphite piston will slide freely in just about any cylinder material. It doesn´t much matter, if the cylinder is cool, what it is made of. Graphite will almost certainly be far softer anyway, and slightly scratch against the cylinder walls, and thus lubricate it. And you should never oil graphite. It will not make it more slippery, just the opposite. If you absolutely MUST lubricate a graphite piston, the only alternative is graphite powder :) I guess Jerry Howell´s Stirling is so-called beta type, where both the pistons are in line. His recommendation is good in the sense that it´s easier(?) to replace a worn graphite piston than a cylinder. Of course there´s wear in both, no matter how slippery graphite is, and in an aluminium cylinder, it would be worse. A steel liner would be far more wear resistant, and expand a little less with heat than ali. Also it would be a worse heat conductor, so it would remain cooler for longer, and expand less than ali. I´m not so sure about using brass, it is harder than ali, of course, but just about as good a conductor of heat, and will expand just as much with it. The flame sucker might benefit from a graphite piston. They´re notoriously hard to lubricate. A graphite piston would solve that. The problem would be keeping the "cold" end of the cylinder cool enough that the piston won´t leak. If the piston is tight in the cold end, it will be sucked towards the flame (hotter) end, and there it would be more leaky. The flame (=hot gases) will get sucked inside even if it is, but it shouldn´t leak much near the bottom dead center. In my Stirling, there are two narrow grooves in the piston, they´re trying to imitate a labyrinth seal. Not sure if they do anything useful really, but every little helps... And at the very least, the grooved piston weighs a little less... :D :wave: |
| cidrontmg:
Started with the cylinders, doing the cooling fins. First the displacer cyl. continued with the work cyl. There´s lots of ali ribbon in the swarf bucket already... The original work cylinder has a blind bore, no separate head. That´s good against leaks, but doesn´t somehow look right... This time I´ll make a proper separate cylinder head, leaky or rather not, and make some fins there also. Cooling fins here are more for the looks, they don´t serve any useful purpose. Who cares. Just some more milling for aesthetics... Then there´s the piece that unites the glass cylinder with the displacer cyl. There´s of course a hole for the test tube. Starting the hole. The test tube is a nominal 30 mm, but in reality it´s 28.9 mm +/- 0.01. I bored the hole 29.3 mm, so there´s 0.2 all around it. It must not be too snug a fit in the hole, or it is likely to develop a hairline crack. Seems to fit OK. Then there´s the screw holes for fixing the heads. I plan to use similar M3 hex cylinder heads to the original. It has 4 holes, this time I´ll do 6. I also want to sink the screw heads somewhat. The heads are very concentric with the threads, and actually meant to be sunk, they provide some guidance to the piece to be fixed. Often they are not, there´s not enough thickness in the piece, or they´re left protruding because they enhance the "technological" appearance... Whatever, I first milled the screw head seats (6.5 mm), and then drilled the 3 mm holes. And here´s today´s catch. Not much, but there´s a bucket of ribbony swarf at least. The O-ring in the middle is 30 mm outside dia. and 4 mm thick. This is how it will be around the tube when the engine is assembled. A little over half of it will be in the chamfer. More when there´s more to show. Thanks for watching! :wave: |
| cidrontmg:
Hm. I forgot one picture that´s rather relevant. "A little over half of it will be in the chamfer." What chamfer?? The chamfer is here. Done by turning the top slide to something like 40-50o, the angle is not very important, neither is the width of the chamfer, something like 5 mm. It will probably need some adjusting when the engine is assembled. The idea is that most of the O-ring will be inside the chamfer, and it will be pressed against the cylinder top. That way, it will seal the glass tube both along its side, and against the displacer cylinder. :wave: |
| Brass_Machine:
Very nice work so far! Anxious to see how you cut the test tube. When I tried it, I had disastrous results Eric |
| Stilldrillin:
Beautiful work Olli! :clap: David D |
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