The reason my spinning wheel didn’t work seemed to be that the wheel supports weren’t tight, so the wheel wobbles all over the place when you apply real force with the treadle. Then the belt pops off.
I had heard of some material that’s supposed to make wood swell so joints fit tight. The main application is to fix wobbly chairs. On my way to work on Friday I stopped at National Lumber to look for the stuff. After not finding it myself, I asked someone who worked there, “Do you have some material called something like Chair-Loc that’s supposed to fix loose joints in furniture?” He said, “Yes, that’s exactly what it’s called,” and found me a little bottle – two ounces for $4.65 or thereabouts. “Does it work?” I asked. “Yes, at least I think so. I’ve seen it around for 40 years,” he said. Late that night I took the loose supports out of the wheel, applied some Chair-Loc to the inside and outside of the joints, put it back together, and let it sit. By morning the whole thing felt much more solid. When I tried using the wheel, it didn’t wobble all over the place any more, and I was able to spin on it without the belt coming off. So there’s my unsolicited testimonial to Chair-Loc.