Thank you for your advice from last post! I am now choosing bearings for my flywheel and was wondering if anyone had any advice.
I am using a 2" diameter, 1" thick aluminum disk as my flywheel. I am trying to pick out bearings to put in the stand. I am going to use a dshaft which will be held in place by a screw going through the flywheel disk radially. My stroke length is going to be .7 inches.
The stand will also likely be made of aluminum. See the picture for the rough idea. Let me know what you all think and advice for what bearings or shafts to buy to absolutely minimize friction.
Thanks!
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There are tons of bearing and shaft options so you will have to pick something that fits well with your design. Skate board bearings are a common and readily available option, with an od of 22mm and id of 8mm. For minimizing friction, remove the lip seals and clean out the grease so you can apply a light machine oil or a dry lube. With these small models, its best to machine your stand just a touch oversized, pass the shaft through both bearings, and then secure the bearings to the stand with some loctite or glue of your choice. This will ensure the bearings are in perfect alignment and spin as free as possible.
Thanks Vincent! I was looking at some lamina flow stirling designs and many of those use very thin shafts. Is there a benefit to picking something smaller or thinner to reduce the friction area?
The bearings should be as small and light as possible for these engines. A smaller OD bearing has lower bearing speeds and so less friction there too.
These are the bearings I use for small builds. Keep the two bearings at least 1/2" apart in the stand for stability and they should do the trick. I sand down 3/16" shaft in the lathe to avoid a heavy press fit and ensure ease of assembly/ disassembly.
Hmm interesting. Do you know anything about PTFE bushings? i was considering using a dshaft through 2 ultra low friction PTFE bushings. They are only rated for 8 lbs and 120 RPM though. I will look into the bearings you sent! Thanks!
Bearings are easy enough that I don't think bushings are worth the cost savings. If you went with bushings, having the crankshaft in double shear(bushed on both sides of the flywheel), or making the crank support much wider would be beneficial. Otherwise the radial load would likely cause too much friction, IMO.
Here's a video showing a 3d printed stand with the smaller bearings I linked to.
You turned out to be totally right, Mike. I built the engine and it doesnt run. I have several theories. First is the that the piston is way too heavy. I am using a brass piston connector on a graphite piston. Thing weighs a lot. Second, my brass cylinder is not perfectly round. I am going to pivot to using a glass tube as the cylinder. Last, as you mentioned, is the flywheel. My chunky piece of aluminum is not great for this application. I am ordering a larger 3.5" diameter, 1/4" thick piece of aluminum now.
