Author Topic: Building another Stirling (Read 53430 times)

cidrontmg · « **on:** November 11, 2010, 10:59:09 PM »

Some time ago I built a Stirling engine, shown here http://madmodder.net/index.php?topic=3761.msg40590#msg40590
Since then I´ve mounted it to a base board, and it has been running for many hours (1/4 l of ethanol).

It now runs so nice I thought I´d build another, a bit bigger (about 1.3 scale) this time. Most of the materials will be similar, but I´m out of 4 mm ali plate and 16x16 bar, so I will use brass for those parts. I´ve also bought the difficult items (test tube, graphite, bearings, fasteners, etc.), so let´s see how it goes. There´s no real plans for this, I´m just making parts to fit together...
There should be a Stirling inside these.

And these

The flywheel is just to keep the ali bars from rolling away, it won´t be part of the engine. You will notice that I bought two test tubes, just in case. I didn´t break the tube for my first engine when cutting it, but I might, this time around... The test tubes are 30 mm dia outside, in the above engine it´s 20 mm. That´s nowhere near 1.3 scale, I know... But as any politician will tell you, there´s never too much hot air... The cord in the picture is glass fibre, to be used as a wick for the spirit lamp. There´s 1 meter of it, enough for many, many lamps.
The graphite is a lump of 20 mm dia and 42 mm long, enough for 2 pistons at least. There are also 5 bearings (of 10x6x3 mm) in the bag, only 2 will be needed for this build.
I probably won´t be making this in a hurry, so be patient...

Stilldrillin · « **Reply #1 on:** November 12, 2010, 03:58:00 AM »

Good luck Olli......

I'll be patient!

David D

cidrontmg · « **Reply #2 on:** November 12, 2010, 06:36:32 PM »

Cutting the bars to size. I have a B&D band saw that I misuse for such things.

Cutting 4 mm brass is a piece of cake. About the same as cutting 6 mm plywood.

40 and 50 mm ali bars take some time, but far less than with a hacksaw. And way easier to my right shoulder.

I also cut the 16x16 mm brass, and some 70 mm ali, for the flywheel. That´s about the max. that the B&D will take.
There were 2 pieces of the 50x4 mm brass, 113 mm long, 2 pieces of 40 mm dia ali, 53 mm and 20 mm long, 2 pieces of 50 mm ali, 60 and 20 mm long, one 16x16 mm brass, 105 mm long, and one 70 mm dia ali, 18 mm long. That´s all the "big" pieces. There will be some smaller pieces also, but they will be salvaged from scrapped stuff. More later.

Rob.Wilson · « **Reply #3 on:** November 12, 2010, 06:40:29 PM »

Looking forward to watching this build

Rob

johnbaz · « **Reply #4 on:** November 12, 2010, 08:42:37 PM »

Quote from: Rob.Wilson on November 12, 2010, 06:40:29 PM

Looking forward to watching this build

Rob

+1

Cheers, John

Brass_Machine · « **Reply #5 on:** November 12, 2010, 11:08:40 PM »

Quote from: johnbaz on November 12, 2010, 08:42:37 PM

Quote from: Rob.Wilson on November 12, 2010, 06:40:29 PM
Looking forward to watching this build

Rob

+1

Cheers, John

+2

Gonna like this one!

Eric

Bogstandard · « **Reply #6 on:** November 13, 2010, 02:28:26 AM »

Olli,

My little B&D bandsaw has been the backbone of my workshop for the last 25 years. I did make a pair of roller blade guides for it instead of the plastic ones, and it cuts all my non ferrous stuff without complaint, no matter how large. Just give it a new blade every so often and away it goes.

I am really going to enjoy this build.

Bogs

madjackghengis · « **Reply #7 on:** November 13, 2010, 09:35:52 AM »

Hi Olli, I've always wanted to build a Stirling with a glass tube for the displacer, I believe this will be the motivator to get me going on one. I'm looking forward to watching this build, and and gaining some insight into some of the things it entails. You're certainly off to a good start. mad jack

cidrontmg · « **Reply #8 on:** November 13, 2010, 06:12:18 PM »

Not much done today. Drilled the 16x16 mm bar that will serve as the bearing support. I cut the bar intentionally too long, I wasn´t sure how to make the seats for the ball bearings. There´s now an extra hole that will be cut away later, and there´s still more than enough of it left...
The idea is to mount the bearings to the two sides of the 16x16, so it would be necessary for the seats to be in line, otherwise the crankshaft will not be at 90^o to the rest of the engine.
I first tried to use a 5 mm counterbore drill. It supposedly leaves a 10 mm counterbore (the bearings are 10x6x3 mm).

Well, the hole was actually 10.30 mm. That´s way too much slack to fill with Loctite. Next attempt was with a 10 mm end mill.

A lot better. The hole is 10.15 mm. Just about the ticket.
These small ball bearings (and anything even smaller, of course) are best treated with like a baby´s eye. Consider that the bearing thickness is just (10-6)/2 = 2 mm (0.0785"). In that space, there are the balls and the outer and inner races. The races are 0.5 mm (0.019") at their thickest. It doesn´t take much force to distort them. Especially the inner race is very easy to expand if the shaft is too thick or irregular. It´s a very good idea to use silver steel (drill rod), but to my surprise I couldn´t find any 6 mm stock

OK, next best(?) would be stainless rod, it´s also reasonably even in thickness. As the Germans (from Joachim Steinke´s page) say "Eine Passung, die nicht geschliffen ist, ist keine Passung", or, a fit that has not been ground, is not a fit. The shaft must be ground (with 800-1000 grit emery paper) to slide with very little force through the bearings. Not to drop through, of course. But if you rotate the shaft in the bearing, and it feels "gritty", it´s no good. Not because the bearing will be destroyed prematurely, as it will, but because these small engines don´t develop much power, and it´s silly to waste a lot of the little in the bearings. It can mean the difference between a good runner, and a non-runner.
I made a flywheel, ali, 70 mm dia and 16 mm wide. When mounted temporarily to the shaft, and given a hefty push (by a finger), it should turn for at the very least 10 sec. 15 sec is all OK, the engine won´t be a non-runner because of the bearing losses. 20 sec and over is quite achievable. The more, the merrier.

I had another bronze flywheel, also 70 mm and 13.5 mm wide, made for an an engine for which it was too small. That much heavier flywheel will turn for over 30 sec.

But it´s probably too heavy, for high revs. Worth experimenting, though.
Thanks for watching!

madjackghengis · « **Reply #9 on:** November 14, 2010, 09:44:59 AM »

Hi Olli, I enjoyed the bit of German lesson, I have immense respect for lots of aspects of German engineering and that quote is a good one. I was wondering what sort of aluminum you are going to be using for the power cylinder, assuming you will be using the graphite for the power piston. I have a kit for a single cylinder Stirling with power piston and displacer operating in the same cylinder, and Jerry Howell advises lining the aluminum with brass or using steel for the power cylinder, even with the graphite for a piston. My flame sucker has a habit of scoring the cylinder every half hour or so of running, requiring lapping out the cylinder, lapping the o.d. of the bronze piston with 600 grit paper, and lapping both the outside of the cylinder valve face, and the valve. I don't mind so much, but it tends to stick when I'm showing it to someone. I'm considering making a graphite piston and valve for it, but don't want to waste the time or the graphite if it's just going to be longer intervals, and still gall up. I've made two replacement cylinders, as the old ones get out of round, and tapered, along with a couple two or three replacement pistons as well, just to keep the little engine running in its original form. Have you run graphite as a piston in aluminum before? Always looking for more knowledge

mad jack

cidrontmg · « **Reply #10 on:** November 14, 2010, 02:38:30 PM »

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

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...

cidrontmg · « **Reply #11 on:** November 15, 2010, 05:46:55 PM »

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!

cidrontmg · « **Reply #12 on:** November 15, 2010, 09:18:09 PM »

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 40-50^o, 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.

Brass_Machine · « **Reply #13 on:** November 15, 2010, 09:44:56 PM »

Very nice work so far!

Anxious to see how you cut the test tube. When I tried it, I had disastrous results

Eric

Stilldrillin · « **Reply #14 on:** November 16, 2010, 02:21:53 AM »

Beautiful work Olli!

David D

madjackghengis · « **Reply #15 on:** November 16, 2010, 12:10:22 PM »

Beautiful work Olli, and a very fine lesson on the structure of the graphite, I appreciate the fine detail you go into and will store it for the future. Your guess is right, regarding the Stirling being a beta type, and I will do a build log when I get to that project, and decide in the mean time, exactly how I will do the cylinder. I really want to cast it of bronze, having a good bit of scrap, but I have to practice my wood working techniques some as it's been some time since I had to put draft on patterns. In the mean time, I'm looking forward to seeing this engine you build come to be, and see it running.

mad jack

NickG · « **Reply #16 on:** November 16, 2010, 03:35:01 PM »

Yep, Lovely work Olli, I love stirling engines, will be watching closely!

Nick

cidrontmg · « **Reply #17 on:** November 17, 2010, 09:59:13 PM »

Not much done today, but something.
The two 50x4 mm brass platters will make the cylinder supports. I drilled plenty of holes to them and screwed them together and drew the center line. The cylinder centers will be 65 mm so I marked them, drew a couple of circles for the cylinder outlines and bolt holes, and drilled two M4 holes. I took some pictures, but managed to get them way off focus. I don´t much like the (Chinese) camera I recently bought, it´s really a camcorder, and does pretty awful stills. Min. focus seems to be about 50 cm... A new (Japanese) camera is on the way...
Anyway, I used the B&D to remove most of the excess brass, and put the platters into the rotab. And (slowly and carefully) milled the outlines of the cylinders. And drilled the fixing holes (M3 for the time being). An M4 cylinder head is not much to hold on in the chuck. But it went rather well.

And here they are still together.

I will use M4 screws to fix the cylinders, M3 seems rather skimpy. The cylinder central holes will need to be opened up quite a bit, and I´ll fly cut all the surfaces, although the brass stock was rather clean and not too scratched. But the brass might be a bit warped, and the sides that will go together when assembled, will need to be true and flat. And the platters are unnecessarily thick. I´ll use M4x16 stainless screws because I have a huge stock of them. The thread length in M4x16 is 12 mm, so it leaves just 4 mm to go in the cylinder. It would surely be enough, but if I fly cut 0.25 mm from each surface, it will be another mm, for the six screws. Certain is certain...

There will also be a milled channel in the middle of (at least) one of the pieces, to pass the air (pressure) between the cylinders. That´s for maybe tomorrow. Weather forecast is rain, time to stay indoors.

NickG · « **Reply #18 on:** November 18, 2010, 03:48:19 AM »

Nice work again olli. I think I am getting tool envy as it seems a great deal more can be achieved with rotary tables, I have to stick to basic shapes. I guess you still need experience and the right techniques though, if I tried it it would probably end up like a dogs dinner!

Nick

cidrontmg · « **Reply #19 on:** November 18, 2010, 08:03:51 PM »

Nothing done today, it didn´t rain as promised, and the new camera arrived. This thing (Canon SX20) focuses from 28 mm on. That´s practically touching the lens. Some learning curve ahead, it is a point-and-click, but there´s lots of ways to make bad pictures even so. One sample that wasn´t a total failure.

Blade · « **Reply #20 on:** November 18, 2010, 09:55:18 PM »

Nice work!

NickG · « **Reply #21 on:** November 19, 2010, 03:43:18 AM »

Nice work and great pic!

cidrontmg · « **Reply #22 on:** November 19, 2010, 10:41:11 PM »

Some more work done. Drilled some holes, tapped a little fewer, fly cut the brass pieces a bit thinner, scrounged the scrap bin for some more parts, tried some polishing (no real success, but the scratches show better), and mocked up what I have now. I need to measure and mark some lengths and distances from the assembled parts, so it´s not just for showing up. But that´s the main reason, of course...

Cylinder heads in place.

Flywheel, again with 5 holes. The bottom plate started life as a CPU cooler from a scrapped computer. I sawed the fins away and fly cut the surface, leaving it slightly thicker in the middle, because the cooling fins were cut that way. Also there´s a bit missing from the left corner. That´s not my doing, it (and the fin) was cut away already in the computer. Some electrolytes in the main board apparently came too close to comfort. I guess I´ll round the whole corner later on.

Taking shape.

There are some Sharpie marks in the cylinders+heads, to keep them always oriented the same way. Later on, they won´t be needed. After the air passages are bored, the cylinders can be oriented just one way in the support. You might notice that the bearings will not go entirely in their milled cavities. They´re some 0.2 mm proud. That´s to make the distance between them as big as possible - every little helps, etc.
You also might notice that this engine will be a mirror image of the smaller model.
Have to take some measures now, or the rest of the parts won´t fit together...
Thanks for watching, more in the pipeline.

Brass_Machine · « **Reply #23 on:** November 19, 2010, 10:55:29 PM »

Wow Olli That is coming along very quickly. Very impressive.

Eric

NickG · « **Reply #24 on:** November 20, 2010, 09:35:02 AM »

Beautiful work Olli, I know how good it must be if it's showing up like that on close ups, the camera always makes stuff look worse than it is.

I can't work out where the air passages are going yet so I'll have to keep watching

Nick

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Author Topic: Building another Stirling (Read 53430 times)

Rob.Wilson