Rotating Ballnut - design ideas

[paulus.v]

Paulus - I like the way you've designed it such that the housing is one part, however I'm slightly concerned by the apparent small spacing of the angular contact bearings. According to NSK, you need to have the angular contact bearings spaced by 1.5 times their external diameter, but it looks like in your design they're almost touching, or a double row bearing which wont qualify? I expect you'll still get a reasonably high critical speed from this setup, but not as high as it could have been. The reason the bearings need to be so rigid is that the ballnut will inevitably not be perfectly concentric to the screw. Ideally the error will be small, but any error will cause energy to be transferred from the ballnut to the screw causing them both to oscillate. Both the nut and screw need to be rigidly held to suppress this.

Thanks Jonathan for your comments,

I needed a simple design as I don't have access to precision machining. I wanted one part housing without the need of bearing pair alignment. So I chose a double row? -or how it's called- bearing. Will see how it will perform.



Your last design is great, I will be able to modify mine if necessary :)

[paulus.v]

Jonathan, from your experience with rotating ballnut, what is the best way of lubrication? As continuous flow lubrication is excluded is it better to use grease instead of oil to last longer? What type of grease/oil do you recommend?

thanks

[vre]

Very nice the idea of rotating nut!
I want to convert a conventional lathe to cnc and Z axis leadscrew has 1900mm length 30mm diameter.
If i use L1900mm 3210 rotating ballscrew with 2 bk25 end supports i will have 1300rpm citical speed & column strength 28477 newton
The critical speed is ok to me because i will get 10m/min rapids with 1000rpm in ballscrew i have concern about column strength because 28477 newton (2900kg / Z axis servo can give 3400kg ) isn't to big force and also i have fear about sag to 3210 ballscrew and runout at rotation that will do a lot of vibration.

In my situation would be better a rotating nut design like this to overcome these 2 problems (column strength & ballscrew sag/buckling) ?
Also with rotating nut design can i use a lower diameter ballscrew without having whipping ?

[mutzy]

Jonathan and all of the determined mad scientists and hobbyists of this forum. This is great work. I really appreciate you sharing the setbacks and successes of hard work. Keep up the great efforts...I learn so much from this. Thank you again......Mutzy

[mutzy]

So boyan, 22 teeth will not work with the 1 in ball screw. Anything larger will not let the bearing slide over the gears.

[Boyan Silyavski]

There is a reason we do it like we do it. Now you know it.

[magicniner]

There is a reason we do it like we do it. Now you know it.

You should just have designed it for him!
Oops! You Did ;-)

[mutzy]

Still working on it Boyan, and thanks for all your help as well as Jonathan. I made the Inner diameter of the RBN to be 1.0787. That would be 2mm of space for the ball screw as you suggested. Then I made the outer diameter of the RBN to be 1.3720 in which is about .006 smaller for the 7207 to fit nicely. Still as you can see, at the HTD gear part, The wall is about .0421 in in thickness. I think since both sides of the gear are attached to 1.3720 it will be strong enough. What do you think. Mutzy

[m_c]

Just over a mm wall thickness. You might get away with it in steel, but I'd expect aluminium to sheer.

By the time you consider axial loading due to bearing preload/dynamic loading, then the belt/nut causing some sheer/twist/radial loading, that part is going to be under a fair bit of stress under high loads. And that's before you consider stress points due to not being a smooth shaft.

[mutzy]

Thanks for the input M_C. How much minimum thickness do I need in Alluminum vs steel? If I bring the numbers to what Boyan suggested in the previous posts to 27mm ID it would leave the thickness at about .054. not too much more, but it helps. mutzy

Would it make sense to use a slighly smaller Ball Screw?