Rotating Ballnut - design ideas

[jcb121]

yeh, i was thinking of using a 24 tooth one as that would give me a lot of factors to change the gearing.

but it has the same amount of factors as 30 so either will be good in my design :)

[Jonathan]

But factors aren't the only factor...
Ideally you don't want to be using less than a 15T pulley as that bends the belt a lot and gets lower tooth engagement, so if you need a 1:2 ratio then you've got to go with at least 30T. However if you're happy with ratios from 15:24 and you can fit the ballscrew shaft through the 24T, then use it as it's inertia will be about 60% less than the 30T pulley. That will help get high acceleration, although you need to consider the system as a whole not just the pulley.

[Boyan Silyavski]

Hi Jonathan,
have you seen these bearings, which are meant for the purpose?

[Jonathan]

Hi Jonathan,
have you seen these bearings, which are meant for the purpose?

No, I hadn't spotted that. I see they've used bearings with a higher contact angle than most angular contact bearings, as that allows the bearing spacing to be lower for the same stiffness. I don't think this is too big a concern with the rotating ballnut bearings, as they are oversized compared to the bearings one would use if spinning the screw.

I've just finished making three more rotating nut mounts for someone who spotted the video on YouTube who's making a large machine with two SFE2525 ballscrews and one SFU2010 ballscrew. I simplified the design by changing the housing for a block machined from 4" square aluminium bar, and a separate piece cut on my router for the motor mount.


Some pictures:




A video, feedrate displayed assumes connection to 25mm pitch ballscrew:



Drawings:

RotatingNutSimple.zip


Feel free to copy or modify the drawings and use them to make your own or get the parts machined by your favorite company... I don't mind.

[Clive S]

Very nice indeed Jonathan, I can see a lot of work has gone into those. ..Clive

[paulus.v]

Hi Johnathan,

nice work! I have a question: How did you fasten the pulleys on the rotating ballnut shaft and motors?

[Jonathan]

Very nice indeed Jonathan, I can see a lot of work has gone into those. ..Clive

Thanks, yes they take forever, or at least it feels like forever when you're only able to machine things at weekends. It's been three months since I agreed to make them.

How did you fasten the pulleys on the rotating ballnut shaft and motors?

M5 and M4/M5 grubscrews respectively. So long as you threadlock them it works. The reason I didn't use a clamping method is that tends to require a larger diameter pulley, particularly on the ballnut shaft, which would increase the moment of inertia.

[Boyan Silyavski]

Basically from what i understand 24 tooth pulley / OD 37.08 at the lower point at the tooth/ can not be fit, had to be machined on the shaft directly? So the next in line is the 26 tooth pulley which is OD 40.24 at the lower tooth point, so that will give 40-35=5/2=2.5mm wall, which seems quite thin. Then 28 tooth pulley is 43.42-35=8.42/2=4.21.

So it seems if not machined directly, the minimum pulley will be 28 tooth? Or am i missing something?

[Jonathan]

The minimum pulley size that I could fit on the shaft was 30T, so that's what I used. By machining the pulley teeth directly into the shaft, as with my first version, a smaller pulley could indeed be used, but it adds a lot of time to the machining.

The shafts are 35mm diameter, so it's a bit thin even with 30T as you can see from the previous one I made, here:

[Boyan Silyavski]

Thanks.

Now wouldn't this exact RM2510 ball nut simplify things and lead to another design, even possibly with 2 of them?