iv needle as displacer rod

[dev_null]

Hi,

I'm new to this forum. I have built a coke can stirling with a nylon thread for connecting the displacer to the crankshaft. While this works well, it limits the speed to how fast the displacer drops under its own weight. So my next model will have a solid connecting rod instead. I have thought that an iv needle should work very well for that. It consists of a steel needle (the connecting rod) and a tight fitting but low friction plastic tube around it. This tube would be glued into the top of the can (maybe inside a piece of brass tube for added stability).
What do you think of this idea?

[jimlarsen]

I like Music Wire. I buy it at the hardware store or at the model shop. It is cheap, finished to a fine smooth surface, and easy to work with. I am considering trying guitar string on one of my future models.

I have also heard that Home Depot sells little flags on wire rod. They are used for marking sprinklers or electrical things in the yard. The wire shafts from those make good displacer rods and crankshafts.

[Junkie]

The gravity driven engines to tend to run pretty slow compared to solid shaft versions. I use 1/16" brass rod
sliding in a 3/32 brass tube from K&S metals for my solid rod engines, works well.

[dev_null]

Junkie:
1/16'' in 3/32'' means there is a gap of 0.4 mm between shaft and tube all around the shaft. Isn't that too much leak? The iv needle would probably be almost completely airtight without having much friction (I have already used them as low friction bearings).

jimlarsen:
how do you create a seal around the music wire passing into the can?

[Junkie]

0.4mm are you sure ? I seem to remember it being 0.2mm when I started using them. It doesn't
leak too much anyway.

Anyway, the following engines I made all use this tube and rod and they work fine :

http://www.youtube.com/watch?v=J15Al2b-Aa4
http://www.youtube.com/watch?v=G9uCuGjOSHM
http://www.youtube.com/watch?v=X-W32_mcNa0
http://www.youtube.com/watch?v=Heg3xaWUMks

HTH

[dev_null]

According to my unit converter, 1/32'' is 0.79375 mm, therefore a gap of roughly 0.4 mm all around. But if it works, no problem.
Another question:
You are using displacers made from tightly rolled steel wool. Is it really necessary for efficient regeneration to make them completely from steel wool? In my first engine I used a displacer made from two coke can bottom ends glued to a cardboard toilet paper roll. This is then covered with a single layer of steel wool. Do you think this regenerator might have too little heat capacity to work efficiently? I wanted to keep the weight down.
By the way, from your videos I saw that you run the scrap to power site. I got a lot of inspiration from this site when designing my first engine!

[jimlarsen]

dev_nul,

The seal around the music wire is quite simple. I usually make a hole in the aluminum can that is a very close match in size to the wire, and then I add a second seal made of tin, with a similar hole in it. The tin piece is held on top of a spacer so there is about 1/4" between the two surfaces. I took the idea form the SFA design.

If I am using 0.062" music wire (1.57 mm) I find that it is a good fit with a 1/16" drill bit.

The music wire is hard enough that you can sharpen the end of a scrap piece and it makes a decent punch. You can make the holes my punching the tin and the aluminum can with a sharpened piece of wire.

You can see what they look like in the first picture on this page. http://sites.google.com/site/stirlingbu ... can-engine

[Junkie]

It should work fine. More surface area is better, but it'll work without a regenerator. They're not tightly wrapped though, just enough to hold themselves in shape,
as you want the air to flow through them if you can. My wire wool displacers for a coke can engine are around 10g in weight. Not too heavy.

[dev_null]

jimlarsen:
don't you get a lot of friction if you run the wire through just two holes in the can / tin?

junkie:
if the wool isn't tightly wrapped, won't all this air in the wool reduce the efficiency of the engine? My displacers are probably heavier than 10g because of the can bottoms. I'll have to try your wool-only approach.

[jimlarsen]

It seems to work fine. Check out the videos at http://www.youtube.com/16strings. I have tried a variety of options and for a simple can engine this one works the best for me. I have seen others working with just a hole in the pop can, and no second seal.

The music wire is very smooth, so friction is minimal. I like to drill it first, then punch the music wire through and expand the hole just enough to give clearance.