Stirling Engine : Laura

I'm inclined to keep stum on shortening a test tube which is probably made of borosilicate. It isn't soda glass tube which breaks cleanly and easily just via a groove cut with a decent file. You'll find all sorts of ideas for cutting tube around on the net. I have used the method outlined by Maurice on tube. I've tried hot wires, chalk and other ways but stick to what a laboratory man told me to do. Something very hot will cause soda glass to crack along a groove but it needs to be rather thick wire and chalk - waste of time.

Your idea of the lathe and a file sounds fine to me. Just keep working at it but it might be better to wait for some one who has done it to come along. Diamond disc - maybe but I can see that going wrong and chipping in unwanted places unless extreme care is taken especially when the tube is nearly cut through.

If you want to try a scratch build the person who designed this has a web site some where but plans are now all over the place

**LINK**

If I remember correctly he designed and built it for a present for his father.

John

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They made the displacer solid so that the M3 grub screw could be used to adjust the length of the piston rod.....I think !

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Fair comment, Brian ... But one could also bore out the body to a much larger diameter, and then fit a suitable plug.

... I really don't know how much difference it would make.

MichaelG.

Hi MichaelG, I'm working away from home on a strange lap top. Technically the solid aluminium displacer is totally wrong, even worse than a thin walled hollow aluminium one, it will very shortly become the same temperature from end to end, at least they won't collapse like my ones did. The second reason for a hollow displacer is weight, you don't really need it, it just adds up to extra friction. I try to build my motors to run as efficiently as possible, so I try to use stainless steel in the hot end where I can, if not it is mild steel, that I usually make too thin so that after working for a while at red heat the hot cap starts to droop, and get wrinkles on the bottom side. I,m no sure of the diameter compared to the displacer that Brian has, but have a look at either an Alkaline, NiCad, or metal hydride AA batteries, the casing of these is ideal for either the hot cap, or displacer. The hot cap on this motor is the case from an AA NiCad. Ian S C

Thanks, Ian

Duly noted, for if I ever get around to building one.

MichaelG.

Bit of info here on using a Dremel with a small diamond cutting wheel to cut glass tube.

**LINK**

I don't know that a file would do the job as nicely -- but maybe the edge of a diamond file? Whichever way, if you use diamond wheel while spinning the tube in the lathe, cover the lathe bed and ways with some protective damp cloth or newspaper or tinfoil etc.

I have been making a few examples of part #15, the working piston which is made of gunmetal bronze. What sort of fit should I be aiming for here ? With a steam engine I would go for a tight fit that produces a popping sound when one end of the cylinder is plugged with my thumb and the piston withdrawn. But I think it may have to be a bit more of a loose fit for Stirling engines ? The pistons I have already made fall straight through the cylinder under gravity....just a small amount of resistance.

How essential is it to use gunmetal brass ? I think I will have to make a few more and I have no more gunmetal brass but I think ordinary brass will do.

At the moment I am reducing the diameter of the piston in the lathe then offering up the cylinder to get a good fit. But I think I may go back to my old method and make the piston slightly oversize in the lathe, then fit the piston rod (not made yet), put piston rod in an electric drill bit and apply 2000 grit sandpaper to the piston with one hand while the drill is held in the other hand. I know I can get a perfect fit this way and it gives me better control than using the lathe.

Edited By Brian John on 26/12/2015 06:20:06

I cut my borosilicate tube with it mounted in a collet on lathe with a dremel like tool on the toolpost and a diamond disc. My build is going very slowly but if I ever get to the power piston then my drawing calls for brass or graphite. I guess the graphite should have lesser inherent friction but I can see that the glass may need honing too depending on how well the tube was drawn during manufacture.

The power piston goes into the brass cylinder, not a glass one, and a well made graphite piston is just the bee's knees, brass running on brass is not really good news, although if a different grade of brass, or maybe bronze were used ok. Stainless or ordinary steel, or cast iron would be better. The Stirling Engine that I built with a brass cylinder has an aluminium piton core with a Teflon sleeve over it.

Ian S C

The ideal fit will drop through with an open end but feel springy if the other end is blocked - a challenge. Drawings I have seen suggest 0.0005" clearance on 0.6" dia. This would would behave like that.

Graphite rod seems a bit tricky to get in the UK. It is on ebay uk but only from China or the USA. I've seen suggestions that it should be polished and finally sized with photocopier type paper.

There is another source, well below UK import duty limits.

http://www.ebay.co.uk/itm/221629899565?ru=http%3A%2F%2Fwww.ebay.co.uk%2Fsch%2Fi.html%3F_from%3DR40%26_sacat%3D0%26_nkw%3D221629899565%26_rdc%3D1

John

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Edited By John W1 on 26/12/2015 10:42:21

Edited By John W1 on 26/12/2015 11:09:41

Graphite rod can be got from College Engineering or Noggin End.

It's not shown on the CE website but if you phone them they usually have some.

Edited By herbert punter on 26/12/2015 11:00:52

Do mean like this ? How hard is it ? I thought it would be soft.

**LINK**

I've no idea how difficult it is to machine Brian but if I bought some I would think ahead and buy the largest diameter I could find within reason eg ebay 281456250113

I would have thought that brittleness was more of a problem. Elaborating on the "machining" instructions. Machine until it nearly goes in then switch to paper and then switch to paper by hand to finalise the fit. Pass - I've never done it. The bore would need to be smooth well finished and very parallel.

Could a mod arrange to do something about the way this forum often switches font size when something is pasted in from elsewhere. Also I find the normal font size just a little bit on the small size but that might just be me. I don't have this problem elsewhere though.

John

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Edited By John W1 on 26/12/2015 12:04:12

I've been gathering bits to make a 1/3 scale Robinson engine. I've been given a hand -pull beer pump, unused, made from brass and 2 1/2" bore, spot on to scale. The piston has 2 bronze rings but I think to stiff for stirling engine. I am proposing aluminium piston. What will that be like ? Ian S C-How did you make the teflon sleeve ? will it work at my sizes ?

Ian : what is wrong with brass on brass for the piston/cylinder ?

Anyway, today I tried out the new drill press and my new tap wrench. The old tap wrench would not hold 2mm taps ; they kept slipping. Everything went very well as I drilled the cylinder covers and collar for the glass tube. The collar also required two M2 threads to be cut.

I took the drill shield off the bench drill as I think they are a nuisance. After much thought I also removed the large safety switch cover. I had to keep lifting it up to turn the drill on and off all the time. I am in favour of large safety switches but it should be a separate item and not over the usual on/off switch. It made things more difficult to use without offering any real benefits that I could see....... comments are welcome !

Edited By Brian John on 27/12/2015 06:03:23

Edited By Brian John on 27/12/2015 06:04:01

Edited By Brian John on 27/12/2015 06:04:13

Edited By Brian John on 27/12/2015 06:04:49

The benifit is you can whack the big top cover in an emergency, and with the switch on the front of your machine could even headbut it if your hands are wrapped round a piece of work. My main woodworking machijne that gets daily use for work has a nice big button in line with my knee should I need to stop things quickly with my hands full, infact thats how I stop it 90% of the time.

I moved the on off switches to the front of the bench the mill sits on, rather than reaching up the right side of the mill to the rotary switch there, I keep(occasionally) thinking it would be an idea to put a long flap along the bench so the off switch can be activated with a bump of the hip, or knee.

For the last hour or so I'v been looking for the correct grade of carbon for making pistons. The web site for a supplier in Australia that I had seems to have disappeared. The place to find it (used lumps) is a foundry, or some where that does spark erosion/ die sinker. Or a place that supplies goods to those sort of places.

Gordon, The piston in that motor is 5/8" diameter, and the sleeve was made by boring out a bit of carbon impregnated Teflon bar. To make your piston, I'd make it with good amount of clearance, and use two,(one at each end)Teflon rings, these being made by getting a bit of Teflon sheet, 1 mm - 1.5 mm, or what you can get, and cut it to about 1/4" wide x the circumference, the joint is best formed as a step on each end, or it can be a diagonal cut. With the two rings the aluminium wil be clear of the bore. Make the piston hollow. In the photo you can see one of my cast iron pistons, this one is 30 mm dia, the engine is a stove top fan.

Ian S C

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Ian,

Is that to a published design, or one of your own ?

... Might be just what my son needs.

MichaelG.

Ian -thanks for that, I was wondering if that sort of set-up would work. Sorry for pinching the thread.

People might be curious about just what is the best grade of carbon for pistons Ian ?

John

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