Author Topic: Rhombic drive stirling engine  (Read 2443 times)

Offline Roger B

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Rhombic drive stirling engine
« on: February 05, 2021, 08:31:53 PM »
I have decided to have a go at a Stirling engine. As ever it will be my own design and fairly small. As the rhombic drive is new to me I decided to make a test version, maybe some parts can be transferred to the final thing. It starts with an offcut of 12mm thick aluminium strip with two 10mm holes at 30mm separation with Oilite bushes to take two 30 tooth Mod 1 gears.
That seemed to go OK so then I had to mark out the gears for the crankpins. As the gears are the cranks one pin has to be aligned to a tooth and one to a gap. No room for adjustment. Next up is 4 connecting rods.
Best regards

Roger

Offline tvoght

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Re: Rhombic drive stirling engine
« Reply #1 on: February 05, 2021, 08:52:04 PM »
I'll keep an eye on this.   --Tim

Online Kim

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Re: Rhombic drive stirling engine
« Reply #2 on: February 06, 2021, 05:12:23 AM »
Sounds like a great project, Roger!   :popcorn:

Kim

Offline Bear

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Re: Rhombic drive stirling engine
« Reply #3 on: February 06, 2021, 03:29:00 PM »
I'll be following along. Always wanted to build a Sterling engine.

Offline Roger B

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Re: Rhombic drive stirling engine
« Reply #4 on: February 06, 2021, 03:54:39 PM »
Thank you all  :ThumbsUp: This is, as ever, a learning experience for me. I don't know the level of accuracy required for the various parts. Piston fits obviously have to be good but I am not sure how precise the linkages have to be. Will marked, spotted, drilled and reamed be ok or will jigs and boring be required. I am intending to use miniature ball races in the connecting rods and they will be a transition fit in a h7 reamed hole so some adhesive will probably be required.

The conrods were drilled 3mm and separated with a hacksaw. Next they were milled to the required width, the holes were opened out to 5.8mm and reamed 6mm. Finally the ends were rounded on the RT. The same mark, spot, drill, ream was used on the crossheads which doesn't look too far out.
Best regards

Roger

Offline Dave Otto

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Re: Rhombic drive stirling engine
« Reply #5 on: February 06, 2021, 03:58:25 PM »
Interesting project Roger, I will also be following along.

Dave

Offline Roger B

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Re: Rhombic drive stirling engine
« Reply #6 on: February 07, 2021, 04:21:09 PM »
Thank you Dave  :ThumbsUp:

I milled the recesses in the crossheads but unfortunately hadn't tightened the vice fully  :facepalm: There is a bit of a dig in but the part is okay for the first trials. Next I fixed the bearings into the conrods with some thread locker in the hope I will be able to remove them later (648 is probably a bit extreme). While they were curing I turned the various pivots from 3mm silver steel. Putting it together was a bit tricky as nothing was really held in place but it worked  :)

<a href="https://www.youtube.com/watch?v=hVbw7gCbWc8" target="_blank">http://www.youtube.com/watch?v=hVbw7gCbWc8</a>

Best regards

Roger

Online Kim

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Re: Rhombic drive stirling engine
« Reply #7 on: February 07, 2021, 06:29:35 PM »
That's pretty cute, Roger! Looks like a little man dancing :)

Kim

Offline Roger B

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Re: Rhombic drive stirling engine
« Reply #8 on: February 08, 2021, 03:37:40 PM »
Thank you Kim  :ThumbsUp: It is a rather fun mechanism  :)

The trial version worked well but I will need to make a couple of changes. The working piston crosshead gets rather crowded and if you move the connecting rods outwards to make space for the hollow piston rod the side forces will increase. I plan to have a pair of rods for the working piston with a single rod between them for the displacer piston. I am also not really sure of the forces involved, especially if I want to make this a power producer  ::)
Best regards

Roger

Offline Roger B

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Re: Rhombic drive stirling engine
« Reply #9 on: May 20, 2021, 05:24:04 PM »
A bit more on this one. I made another pair of rods as before and new piston and displacer crossheads to produce a more balanced system. As the crosshead pivot holes are wider spaced than the crankpins the rods have to be able to go beyond 90°. The piston rod will screw into the crosshead and have a bronze bush top and bottom to guide and hopefully seal the displacer rod. This may be an overkill and a simple 3mm reamed hole might work  :headscratch:
Best regards

Roger

Offline Bear

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Re: Rhombic drive stirling engine
« Reply #10 on: May 20, 2021, 06:01:18 PM »
Nice progress.

Offline Roger B

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Re: Rhombic drive stirling engine
« Reply #11 on: May 22, 2021, 09:58:24 AM »
Thank you  :)

The piston rod was turned from 10mm aluminium rod and threaded one end M6 x 0.5mm to fit the crosshead. It was then drilled through 3.2mm. Each end was then bored 3.8mm dia. and 8mm deep using a 3mm end mill as the boring tool. The holes for the bushes were then reamed 4mm.
Best regards

Roger

Offline Bear

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Re: Rhombic drive stirling engine
« Reply #12 on: May 22, 2021, 03:13:22 PM »
Coming along very nicely, Roger. Like your photo progression.

Offline tvoght

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Re: Rhombic drive stirling engine
« Reply #13 on: May 22, 2021, 03:19:25 PM »
Hi Roger, I'm still watching. You mentioned in earlier post about not being sure of the forces in the mechanism etc. I wonder if you have seen Meijer's original thesis paper which gives a complete analysis. I have a feeling you might appreciate his analysis (even if much of it is over my head). There are even equations for calculating the crank counterweight which will perfectly balance the engine. Maybe you've already seen it, but I include a link to the paper here for any interested.   --Tim
https://repository.tudelft.nl/islandora/object/uuid%3A8e038b9b-5c27-4ac6-bcf2-06c7a80c4da6

Offline Roger B

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Re: Rhombic drive stirling engine
« Reply #14 on: May 23, 2021, 10:40:59 AM »
Thank you both  :ThumbsUp:

Tim, Thank you for the link to the paper, I have seen excerpts from it and used them to calculate the dimensions for the cranks and rods. Plenty to read and think about.

I turned a couple of bronze bushes, boring before reaming to ensure concentricity (am I worrying too much  :headscratch:). These were reamed again after being pressed in place. A quick grubscrew to hold the displacer rod in the crosshead (apparently I should be calling them yokes) and another trial assembly.

<a href="https://www.youtube.com/watch?v=Hln4Zlk8C-8" target="_blank">http://www.youtube.com/watch?v=Hln4Zlk8C-8</a>
Best regards

Roger

 

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