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Bellcrank Beta Stirling Engine Project

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tvoght:


After a long time away from the shop, I'm finally ready to start a new build.


I like designing and building Stirling engines, so I've decided to build a beta Stirling. The beta type has a power piston and displacer sharing the same cylinder.


There are a variety of mechanisms that have been used to actuate the displacer
in a beta engine, and for a few reasons, I chose a bellcrank mechanism.


My first inspiration came from seeing this video on Youtube:


(riderericsson channel: "World's finest hot air fan")
http://www.youtube.com/watch?v=aEuoWWXbGrs


I don't intend to build a fan like in the video, it's the mechanism that's of interest.


I started by "prototyping" a mechanism in CAD (onshape) and animating it. I wanted to get a quick idea if I could make something work. That's in the first clip below. The cylinder has been hidden to observe the actions of the power piston and displacer, and you'll notice that none of the joints have pins in them. Pins weren't necessary to make the simulation work.


After I assured myself with the quick mockup, I started on a buildable design.
The current state of that design is shown in the second clip.


The third clip shows how an animated sectioned view can be helpful for checking interferences.


--Tim


I hope the attached animated gifs work...

Dave Otto:
Hi Tim

That is an interesting mechanism, looking forward to seeing this one being built.
I do like hot air engines, :)

Dave

tvoght:
Hi Dave!  I know you like hot air engines :). You make some really beautiful fans.


 My design is not completely locked down, but there are a couple of ideas I have to expedite the build.
 First, the base plate and the standards that support the crankshaft, bellcrank and cylinder are intended to be ordered laser cut from 1/4" steel. It's something I want to try. I'll accept any accusations of laziness.
 Second, I intend to have the cooler 3d printed in aluminum. To justify the outsourcing, the cooler should be difficult or impossible to make by traditional home shop methods. That's why I have drawn it up as a pin-type heat exchanger. I think it would be an effective cooler, and something I couldn't possibly machine. I'm not committed to that particular style though. It's where I am at the moment.


Thanks for looking,
--Tim

crueby:
That  pin type cooler would have a LOT of surface area, should work well! Does the 3d printed aluminum come out dimensionally accurate to use as is, or will it need further machining (I am assuming some other piston rides against its inner surface?)

tvoght:
Hi Chris. The cooler will fit over a cast iron cylinder. The dimensional constraint will be the bore size where it slides over the cylinder. I am thinking (always gets me into trouble) that I can have the bore a little small and bore it to size at the mill. Ideally, it would be lapped to the cylinder for best heat transfer. What do you think? It would be my first experience with printed metal. I do know that the design I'm showing does not provide for any clamping for a boring operation, so something must be done about that.
--Tim

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