Displacer Thickness

[himavad]

Hi,

I'm attempting to fabricate a Gamma type Stirling Engine with a stainless steel displacer and a cast iron power piston, and I needed a bit of help. I noticed in some of the posts here that the displacer should be thin SS with a wall thickness of .25mm or less (some posts said .007" to 01"). Is this number correct? I consulted my manufacturing lab technician and he claims that it's impossible to turn solid SS down to that thickness. Now of course, he might be mistaken, but I wanted to be a little more sure before I disagreed with him.

Also, he said that with a thickness so low it's impossible to do any sort of welding. So I was wondering how I'm going to make the ends of the displacer airtight. Can someone give me some suggestions on this?

As for the displacer cylinder, I'm sourcing for some Pyrex tube of the appropriate diameter, and if I can't find one I'm planning to make one from thin SS just like the displacer.

Thanks for your help guys!

Cheers,
Him.

[Ferraccio]

imavad,
Talking about "good techology level engine".
This is my personal idea: for displacer is good stainless steel, has quite low heat transmission coefficient, and may be is found in thin plates. Except if found in very thin pipes I think is good 0.8 mm tick rolled plate, hand bended in tube shape; less than this range of thicknesses is difficult to seal by TIG welding, and turned from solid, but I've no idea about more high-tech solutions.
E.g.: minor thick may be are electrically continuous welded by particular systems.
Was proposed in use tubes come from stainless steel bottles for water.
For cylinder is good austenitic stainless steel, (more thick- aisi 310, 316) (eg: from stainless steel pipe) or, if you have more possibility to find: carbon steel in good alloy for construction, or if found !!!(/and paid) Nickel high alloy.
Cylinder tickness depends from pressurization, if not pressurized for diameters 50-100 mm may be are good in range of 2-4 mm thick, only for stability.
Other depends from the engine complexity and architecture (separate regenerator, cylinder finning, ability of weldings).
In general smaller thicknesses means better heat exchange, and in the case of diplacer: minor heat stored.
Is also (for displacer) quite important (for good level engine) reduce the weigh (mass) of the moved (in alternate way) parts.
Glass is not good becouse is not a good heat conductor, the use glass is only good for a "dermonstrative" prototype. All elements about yeld in this case are abandoned.
All friends are invited to carry their ideas.
Ferraccio (bad Iron).

[Ian S C]

I'v been lucky in finding thin walled stainless steel tube, but it is also possible tomake it from solid, done that, or heavier walled tube. Starting from the inside, get it round and smooth, then make a tight fitting mandriland fit the tube on this (without taking it from the lathe), now turn the outside down to the mandrel size plus the thickness of the required tube, have patience, take your time. Ian S C

[Ian S C]

I,v just gone and measured some of my stainless tube,,the tube for the displacer is .014", and the displacer cylinder is .02165". A steel one turned on a mandrel is .0115", I think i was aiming for .010", so I suppose +.0015" is near enough. The steel displace has the end brazed in, the stainless has the end TIGed on, The bloke I get to do the welding does not think he can go much thinner than the .014" with his current equipment, he's a comercial welder with his own company manufacturing stainless steel goods of all sorts.Ian S C

[himavad]

Thanks.

I was not able to find stainless plate. But I did manage to get a 1m long SUS tube of inner diameter 28.7mm and outer diameter 31.7mm. I'm going for a bore of 30mm, so I can machine it inside then outside like Ian SC said. But my lab technician says that the lowest he can machine it down to is 0.5mm, and even that he says will be too hard to weld so he recommends 0.8mm thickness. Is that a no go? Will it make my displacer too heavy/too conductive?

[Ian S C]

.5 mm should be OK, I imagine that you'v got to be pretty good at welding, and doing it every day, all day. Sorry I don't know what SUS tube is, but one of the tubes I use is the extention on the hose of a vaccuum cleaner, they are made from a number of materials, plastic, aluminium, steel, and stainless steel, I,v used the last two with success. The stainless for displacers came from the stand for solar powered garden lights, I thought they all were plastic, but someone was throwing out some old ones, and I wanted the solar cells, it was then I discovered the stainless tube. Ian S C

[himavad]

Sorry, SUS is Stainless Use Steel, no different from stainless steel. The supplier told me SUS so I thought it might be a commonly used term. Yes I'm going to machine it down to 0.5mm thickness, or even less if possible. The only thing I'm scared about is that the welding person in my school lab may not be able to weld it afterward.

Also, any suggestions on how I should seal the cold end, and fit the connecting rod there? I'm planning to machine the connecting rod using Stainless steel as well, cylindrical, about 5mm thick.

Thanks a lot for your advice!

[Ian S C]

The way I do the cold end is to make an aluminium plug with a hole in it to take the shaft. On the shaftI fit a disc on the end, this will be situated halfway up inside the displacer (no need for measurements). The plug is glued on the shaft, and the plug is then glued into the end of the displacer. I use high strength/ high temperature shaft lock Loktite, I used to braze the end in, and I did solder one or two, they all work, but the glue is best, well easiest anyway. One thing that helps is that on most of my motors, the shaft is stainless tube, so the internal pressure is atmospheric, if it were solid the pressure increases as the temp increases, and can pop the end off. The only time I'v had a problem ( 3 actually), on my free piston motor I used aluminium aerosol cans, three of them collapsed, it now has a stainless one, no more problems (aluminium looses its strength at a moderately low temp. The displacer cylinder on that motor is the outer casing of a shock absorber from a car, so its at least 2 mm thick, it takes a time to warm up, I'm thinking I should rebuild the hot end in stainless. Ian S C

[himavad]

Thanks Ian, that makes a lot of sense. I wouldn't have thought to use hollow shaft. I'm doing just that now.

Could you also give me suggestions on how to connect the cast iron piston to the connecting rod?

[Ian S C]

I'll tell you how I make mine: before parting the piston off the bar that it is made from, I hollow the piston to give a skirt thickness of 1 to 1.5 mmto a depth that will leave about 3 mm, or enough to put a short thread in, I then drill and tap the crown of the piston, then part it off. I then make a little fork with a thread to suit the thread in the piston. Fit the con rod to the fork, then screw it into the piston. Having such a thin skirt means that you must be careful if you put grooves in the skirt that you don't go deep, .25 mm is about it if you manage a 1 mm thickness, most of my very thin ones manage quite OK without grooves. You need a very sharp lathe tool and after boring the inside, take the finishing cut on the outside, take it to size in cuts of about .25 mm (.005"), the tool pressure is enough to distort the metal, but you'll end up with a very light piston. Ian S C

[himavad]

Sorry Ian, I might be a noob but I could not understand what you meant. Do you have example pictures that I can see to understand what you mean?

Currently I have a solid cast iron cyilnder of 30mm dia and 50mm length. I intend to make a piston with 25mm bore, 30mm long. I have flat SS bars that I planned to use to machine the connecting rods. Can I still use your method now? If so, please tell me and I will try to understand again.

Thanks a lot!

[Ian S C]

I'll try again, I'v been trying to up load a sketch of a piston, from one of my stove top fans, it has a diameter of 35 mm, and is 18 mm long, its in the form of a hollow shell with the skirt thickness of 1.25 mm, the crown is 4 mm thick with a 3/8" UNF threaded hole, into this the fork for the con rod is screwed. Your piston could be a little shorter, this would reduce friction, so 20 to 25 mmwould be OK for the length.
My piston as described has a stroke of 25 mm via a 80 mm long con rod. Ian S C

[himavad]

Hi Ian,

I think the reason I'm not able to understand what you're saying is because in my head I'm only picturing a hollow piston with a piston pin and a connecting rod like in gasoline engines. So I don't know what you mean by 'fork with a thread', or 'drill and tap the crown'. I guess a picture would help, or if you could show me a link to a picture online that would better help me understand. I'm currently still working on the other parts of the engine, I'll be doing the piston right at the end. Thanks for helping, and sorry for not picking up more easily.

Regards,
Himavad.

[Ian S C]

Test 046.jpg (152.07 KiB) Viewed 8072 times

I'v managed to get a photo to my gallery(not the sketch I wanted, but I think it shows most , if the piston was up the other way you would see that it is hollow. The extra bits are a leather cup seal, and the flat nut that holds it on the top of the piston.

[himavad]

Thanks a lot Ian, now it makes complete sense. And this seems easier than having a piston pin going through the sides of the piston. Thanks!

Also, how important is that leather cup seal?