Rotating Ballnut - design ideas

[Jonathan]

Moment of inertia is proportional to diameter squared so I am surprised that there is much difference between the shaft and nut? I am sure you have done the sums

The problem is mass is proportional to the radius squared, so you end up with 4th power. Hollow ballscrew would be nice!

Some more thought (sorry if all this has been covered in other threads)

Keep thinking! It's good to have it all in one place.

Is the moment of inertia significant when compared to the mass of the 'carriage' (when calculating acceleration)?

Yes it is, I'll post the numbers when I've verified them.

If you have to tension the shaft to raise its critical frequency then will have to start considering beefy shaft support bearings, I can now see why the rotating nut idea is appealing!

Just what I was thinking. I suppose tapered roller bearings would do the trick, but then you're adding friction.

For a rotating nut have you considered how much the timing belt tension would deflect the nut on a 2000mm length <25mm shaft and the effect of out of balance forces of the nut on the shaft?

Surely the bearings are going to take the radial load and stop the screw deflecting, or am I misunderstanding you? So as long as the ballnut is held on centre I should be ok. That could be an issue if the flange on the ballnut isn't concentric. Another issue I've found is that at sufficiently high rpm with a standard ballnut centripetal force will stop the balls rolling properly.

a 2000mm leadscrew poses a lot of design challenges even when cost is not an issue (which I assume is,...calibrate the travel using a dial indicator and blocks of a known dimension....survey (100mm steps?) would give you significant improvement in accuracy.

I think that method of measurement would have a cumulative error, which is especially significantly over this distance. It could maybe be done with a digital calliper and carefully fixing to consecutive points along the bed. Theoretically the process could be automated.

Obviously this would require a decent home switch.

Easily done with a cheap laser pointer I reckon.