1. ## 8020 rotating nut

I have moved on to rack and pinion so I am offering up my rotating nut design to the group. If anyone is interested in more details I will be glad to work with you. I know some will want to use a ball screw nut insted but be sure and research it as the centrifical force of the nut turning with not allow the rotating balls to work as designed. I have designed this around the acme nut as it will not have the same iussue. I would be very interested in seeing it built by someone

2. I'm using ballscrews and thought "hmm, why not just rotate the ballnut on the long axis?"... Totally didn't occur to me that centrifugal force would come into play! Guess that's a good reason not to!

3. Originally Posted by Riceburner98
I'm using ballscrews and thought "hmm, why not just rotate the ballnut on the long axis?"... Totally didn't occur to me that centrifugal force would come into play! Guess that's a good reason not to!
I don't think it's as big of an issue as you think it is. The ballscrew would be under more force from the cutting resistance than the rotational force pushing on the bearings away from the grooves. My techno has a rotating ball nut on the 8' y axis. They have been using the same design in industrial settings for about 20+ years and I haven't heard of any problems with their ballnut design.

4. Interesting.... Do you know what the lead is on the screw? And the diameter? I run my .2" lead ballscrews at 2000RPM to get 400IPM rapids, no idea how to calculate the force that would be acting on the balls but it seems like it would be a bit? Also I have no idea of clearances / spaces between balls in a ballnut or the internal construction, so I guess I don't know much of anything about the issue.. Just something that hadn't occurred to me before.

Edit to say that my X axis is only 42" and now that I think about it, that axis is a .5" lead... But they're just 'standard' Thomson nuts so I don't know if it makes a difference.. I'm trying to squeeze as much travel as possible out of the space I have (existing enclosure), so just thinking of ways to not have the motor directly coupled to the end of the screw thereby reducing my travel by 4" or so.. I'm thinking either belt-drive at the end with the motor parallel to the screw, or rotating nut with the motor on the carriage.

BTW Pplug - I had to check out your Omis 3 build, that thing looks great! I'm going for a similar machine, but built from big 8020 and 42" x 30" travel or so.. Wish I had a way to get a 500lb slab of granite in there! Biggest thing I could get was a 100lb sheet of 3/4" aluminum for a base..

• Load on the nut depends on acceleration, mass being moved, and cutting force, not speed. If you supply those numbers (or just the first two), we could give you an idea of the force the nut has applied to it. Force on the balls themselves would be a bit more complicated, but it would be the total force divided amongst a number of the balls at any given time and probably applied to the balls at an angle to the axis of the screw. It does make a difference what kind of nut setup you have. For example if you have preloaded nuts to reduce backlash, the preload would factor into force calculations.