Stirling '60'

[Clockman]

Hi - I'm new to HMEM so I hope that I'm doing this right.
Since retiring, my hobby has been making clocks - proper ones with weights, mainsprings, pendulums etc. For a change I thought that I would try to make a Stirling engine and found the drawings for the Stirling '60' by J.Jonkman on the internet. Thanks to HMEM I was able to get the drawing page that was missing from the original internet download. Also, I found some useful information in earlier threads from people who had made the Stirling '60'.
The machining appears to be rather more complex than with clock making - there is little blind boring, for instance, in clock making so it is going to be an interesting challenge.
Two things concern me and I hope that someone who has made the Stirling '60' may be able to give me some advice. Firstly, the displacer piston is hollow with a domed end. Whilst thin-walled brass tubing is readily available, how do you form the domed end?
Secondly, I assume that the test tube has to be heat resistant glass such as borosilicate? If so, how do you cut it to length?
Any advice about constructing this engine (and the potential pitfalls) would be very welcome.
Many thanks.

 

[fcheslop]

Hi, for the displacer internal rad I simply ground one side of a twist drill to a rough rad the other side ground slightly further back and made it from bar stock in alloy. The external rad was done with a file .Not very technical but it works
To cut the glass I made a simple glass cutter along the lines of the one Jan Ridders shows on his site prior to that I used a diamond file in the lathe a wee bit more risky
There is loads of ways people cut these tubes but they never worked very well for me jjust to many scrappers until I made the simple cutter. I use a Dremil clone with a flexi shaft to power it
I can do a wee pic if needed
The engine is a good runner and I cannot remember any probs with the drawings although I broke a few tubes due to the O rings been slightly to tight
Friction is a killer for these engines as are air leaks
http://ridders.nu/index.htm
http://ridders.nu/Webpaginas/pagina_glassnijder/glassnijder_frameset.htm

 

[minh-thanh]

Hi !
one more thing : with the power piston-power cylinder, displacer bearing-displacer rod must fit, but move lightly : Especially when it's hot

 

[Clockman]

Thanks for the helpful replies. I like the idea of using my Dremel as a cutter and will follow this up. Friction is the enemy of clock making too but the point about changing clearances as the engine heats up is not something that I had thought of before. I feel that there is going to be some trial and error with this build.

 

[minh-thanh]

Friction is the enemy of clock making too but the point about changing clearances as the engine heats up is not something that I had thought of before. I feel that there is going to be some trial and error with this build.

\

It's just my experience doing some stirling engines, because it's difficult to accurately determine clearance with small piston and cylinder when cold and hot for the best

about ""some trial and error"" : I have a lot of trial and error before the first engine runs
AND : You should ask questions in :
https://www.homemodelenginemachinist.com/forums/general-engine-discussion.26/
will have more answers.

 

[fcheslop]

As Minh Tanh mentions they often take a wee bit of suck it and see
Have a look on Jans site under his hints and tips section and you will get a good idea for fits
When fitting ball bearing dont make the seats a press fit just bore to size and glue them in also for the displacer shafts give them a slight polish along the length just to remove the grinding high spots or at least thats what I do and find the bushings last a lot longer. The spindles that run in the bearing I tend to polish just to reduce the tighness of the fit just so they slide with enough resistance to still rotate the inner race or if I go a bit to fat glue them in
For the glue I just use super glue as it canbe released with a sharp tap or a bit of heat.I find loctite just to good at its job and have had problems when overhauling engines in the past.
Despite what most say about washing the grease out of ball bearings I dont have not had a problem
Good luck

 

[Clockman]

Thanks for the tips about fit. In clock making fits are generally on the loose side. There is a saying "If it rattles then it will run" which works for clocks but probably not for the Stirling. I am having problems finding test tubes of the specified 13mm diameter. I presume it would be better to get larger tubes (rather than smaller) so that there is enough air to run the Engine?

 

[minh-thanh]

Thanks for the tips about fit. In clock making fits are generally on the loose side. There is a saying "If it rattles then it will run" which works for clocks but probably not for the Stirling. I am having problems finding test tubes of the specified 13mm diameter. I presume it would be better to get larger tubes (rather than smaller) so that there is enough air to run the Engine?

Hi Clockman .
With larger or longer tubes or both, it will be difficult for the engine to run or not run, because it will reduce the volume of air in the engine, resulting in reduced air expansion.
The expansion of the air is the principle of the stirling engine

 

[fcheslop]

Cromwell tools used to sell the right size . I got my last ones from fleabay they are a bit thinner walled but have worked ok
I just increased the displacor dia to give the right clearance . Bengs in Germany probably sell them
https://www.bengs-modellbau.de/materialbausaetze/flammenfresser/flammenfresser-jarne
You should be able to navigate to the spares shop from the link and yep its another future project
As Minh Thanh mentions if you go larger the power piston dia will need adjusting as well to keep to the correct ratio .
Remember one burn and you become an addict to these engines
cheers

 

[Clockman]

Thank you - that is very useful information. After much internet searching I think I have found 13mm dia. test tubes. But I will need to check the size when received as advertisers are not always that accurate.
Best wishes, Clockman

 

[minh-thanh]

Hi Clockman !
The inner diameter of hot cap ( test tube... ) - the outer diameter of the displacer = about 1,5 to 1,8 mm with small stirling engine
Do not let them touch each other when moving.

 

[fcheslop]

Test tubes are not very accuratly made and if there is a bead at the open end it tends to measure smaller that the actual tubes ID
I only found this out the hard way when building Jan Ridders Thermo pulse engine and every tube has a slightly different ID not a problem on the Stirling60
There is room for a bit of give and take around the size of the tube although you will need to tweak the O ring fit .I broke several tubes before getting it to go together .Make sure you assemble it dead square to the O rings and I ended up boring the grooves a few thou deeper than recommended as long as it seals it will be OK
Good luck

 

[Clockman]

Thanks everyone for the helpful tips. Progress has been slow because of clock repairing commitments but all the parts have now been made and assembled. A certain amount of tweaking was needed to get the proper clearances on the levers and rods. I didn't really expect it to run on its first test ( but one lives in hope, of course!) and I was proved correct. I think the problem is that I was at the limits of my machining equipment and skill and did not make the power cylinder with a good enough finish or fit. I would appreciate any advice on how to finish the internal bore of the cylinder to a high standard - similarly the fit and finish on the piston.
Making an engine, even one apparently as simple as a Stirling engine, is much harder than making a clock!

 

[Technical Ted]

I've made three of these engines now; one for my wife and one for each of my boys and their families. They can be quite fussy to get to run correctly. Everything has to be in-line, with proper fits and absolutely NO friction or rough spots in the movements. The last one I made ran at 720 RPM per my optical tach on a tea candle with a low flame. The first one I made, I ended up making some replacement parts for after I saw how well my two turned out that I made for my boys. The first was my prototype and I learned a lot making that one!

I reamed the cylinder and then ran some fine paper (1000 grit) on a piece of wood dowel up and down a few times until a gage pin I used fit nicely all the way through. Then, I made the piston to fit polishing it with the same paper. Holding the cylinder vertical, you want the piston to drop through the cylinder when the end is open, but if you hold your finger over the end you want to piston to stay in place or maybe drop down very slowly because you need a certain amount of air tightness. I did find that being free of friction is more important that the air tightness, but ever so slightly...

Here's something I had an issue with... the CI bearing. I had a hard time drilling the hole true and straight. This was my biggest problem. The tiny drill wants to run off so the hole isn't concentric on the far end after drilling. Any binding whatsoever here will kill it from running.

Another concern is alignment. The holes in the end of the main body must be exactly in-line with the hole you machine for the cylinder. Also, the slot for the beam in the upright must be perfectly in-line with the holes for the two bars that mount the main body. Any misalignment is trouble.

Manually run the flywheel with the tip of your finger and feel for rough spots. It doesn't take much of one to give you problems. Like I said, my main Nemesis were the CI bearings (until I figured out a way of drilling them true). Unhook linkages one at a time to "feel" and "hear" where your issue is. Squeaking or any other noises can be an issue. If you think it's the steel shaft and CI bearing put a tiny, and I mean tiny, drop of light oil on the shaft and run it back and forth a few times by hand and try running it. I would not recommend oiling it on a full time bases because the oil inside the hot chamber will smoke off and soot up the hot cylinder/piston and it will set up... ask me how I know.

Good luck, take your time, have patience and eliminate all issues that are causing friction. And, when first trying to get it to run, use a hotter candle than a tea candle. More heat will help get it running and with luck, after breaking in, it will run better with a tea candle.

Here's a link to my 3 running. Notice how much slower the one on top of the clock is running. This was the prototype (my wife's). I learned much making that one and improved the ones for my boys. Now, with a few replaced parts, my wife's is the best runner of them all!



Ted

 

[Clockman]

Hello Ted,
Thank you very much for your helpful reply. I did spend some time getting the levers, rods and flywheel to run smoothly and was careful with the alignment of the various components. This led me to the conclusion that the cylinder and piston, and my poor workmanship, were the root of the problem. I notice that you reamed the bore.
Reamers are rarely, if ever, used in clock work so I don't have any. Instead, I drilled and lapped the bore but this didn't produce a good enough result as it was my first attempt at lapping. (again, not used much in clock work). I think that the answer will be to buy a reamer! Your advice about checking how the piston behaves when dropping through the bore is very useful.
I like your video (and the good-looking clock too).
Best wishes,
Clockman

 

[Technical Ted]

I would think you could get away with a very careful drilling and then polishing/lapping. I polished (didn't lap) with a wood dowel and 1000 grit paper until a gage pin slid through with the same fit the entire length. Unhook the linkage to the displacer and rotate the flywheel by hand. If it moves very freely with no rough spots and the piston is fairly air tight (doesn't have to be perfect; I would sacrifice air tightness over friction) I think I would look elsewhere.

If the piston/cylinder seem to be OK, move on to the displacer/shaft/bearing. I made my brass displacer as thin as I dared; around 0.015" (0.4 mm) on the walls and nose. I put a radius in the nose with a ball end mill of the correct diameter. I made a steel slug with a radius on the end to insert inside while finishing the radius on the outside. It needs to be as light as possible to reduce drag/friction. The shaft was made from drill rod (silver steel) which I polished and the bearing from cast iron. I suggest disconnecting the linkage to the beam and just manually run the flywheel with the linkage connected that goes to the displacer. Do you hear any noise at all? Do you feel any faint areas where there is more resistance than other places? This would typically be in an area where the displacer is fully extended and reversing. This is where I could feel just the tiniest bit of drag on two of mine. Believe me, there wasn't much drag there at all on mine, but after putting a tiny drop of light oil on the shaft that runs in the CI bearing mine took right off!

BTW, if you have used Loctite to assemble the parts and need to get the parts apart, I used a higher wattage electric soldering gun to loosen mine. Worked very well. I cleaned all the old solder off the tip first so it didn't transfer to my parts.

Good luck,
Ted

 

[Clockman]

Thank you Ted. I will let you know how I get on.
Best wishes,
Clockman

 

[Clockman]

I spent some time chasing places where I thought there was some friction and got things moving as freely as I could - certainly much better than before. I decided to try running the engine with more heat so I added two extra wicks to my tea light and fired up. Much to my surprise and joy the engine ran! I called my wife to be a witness to this miracle. I estimate that it ran for about 3 minutes when it accelerated briefly and then stopped. When it had cooled down and I had cleared the soot off the test tube (something to be sorted out in the future) I saw that there were streaks of solder on the inside of the tube. Taking it apart, I found that the solder holding the displacer end cap had melted. Needless to say, I hadn't foreseen this as a problem as heat is not a factor in clock work. So now I know that the thing will work, I will make a new displacer and be more careful with the amount of heat that is applied. This is certainly not a boring project!
One question though, I used silver steel as the displacer bearing instead of cast iron as I didn't have any cast iron (not used in clock work). Could this be a problem?
Thanks for any advice,
Clockman

 

[Technical Ted]

There are others on this list that know a lot more about material combinations than I, but my understanding is cast iron has inherent lubrication properties in its' make up. I made my displacer rod out of drill rod (silver steel) and the bearing from cast iron and I think this combination would be better. As I've stated, the bearing was my number one nemesis with friction and alignment. I found the hole in the bearing has to be of correct size and straight/true with the OD of the bearing. Small drills like to wander and I had some first attempts at making the bearing where I could see the hole on the far end was off center from drilling it on my lathe. I did some experimenting and found the best way for me and my lathe was to hold one of my small Albrecht chucks in my tool post and indicate it dead on center with my spindle. Using the tail stock only gave me problems with drill run out.

You must have had a pretty hot heat source! I didn't solder mine. I used high temp Loctite and haven't had an issue. I've run mine for over an hour and a half with a tea candle with no problems.

If it were me, I'd make a new bearing from CI and make sure the hole is straight and concentric.

Good luck,
Ted

 

[Clockman]

Thanks for the advice, Ted.
I had not heard of high temp Loctite. I have done a bit of on-line research and it appears to be a good substitute for solder and should be a lot easier to apply. I will give it a go.
Best wishes,
Clockman