Hi Jonathan,

I'd need to draw it out and look at the angles and forces etc. , but I suspect that the bearing jumping would occur on any length of leadscrew. If the bearing is mounted in a sort of gimble as you have shown and held against the screw thread via a spring, then if you push the leadscrew it will try to rotate (not spin) the bearing about the gimble pivot axis against the spring force, and ratchet the bearing over the screw threads. A bit like a handbrake ratchet on a car where the pawl runs over the teeth against the pressure of a spring.
If you zoom in close to the exact point where the bearing contacts the thread you'd see an inclined plane sliding under the angled face of the bearing. When the inclined plane of the thread moves forward (either you pushing it or the action of the stepper rotating the leadscrew) it will push the bearing upward and away. If it pushes the bearing far enough away it will jump a thread.

Anyway, I was thinking of a way around this and was thinking about a setscrew with a small rubber buffer on the end, but your belleville washer is similar and neater. Hope it all works out because it's a nice idea and will save some cash. I went with 3 ballscrews for the fit-and-forget method, but it was not cheap.

Look forward to your photos . . .