Thread: Rotating Ballnut - design ideas
Is the 50% figure for static or dynamic loading?
You only really need to use taper bearings when dealing with high loads. For the majority of applications, ball bearings work fine, they have less resistance, and are more tolerant should dirt get into them.
According to the SKF site the axial internal clearence for a 40mm bore double row angular contact bearing is 11um. So that's like having 11um end float on the screw if I used the design with the single bearing. I'm not sure how bad that amount is - it doesn't sound like much. C2 clearance bearings only have 2um clearance.
Last edited by Jonathan; 11-04-2011 at 12:42 AM.
4th, and hopefully final, design:
That's one double row angular contact bearing to take the thrust load, and one standard bearing. Stepper will bolt on to the 15mm plate. I can use 4" aluminium bar for the bits on the other side, and 2.5" for the rest purely because those are the sizes I've found cheap!
Edit: Disregard that, not thinking... this one still does not preload the bearing properly. I'll add another angular contact bearing, but that's going to cost too much :sad::sad: Now I'm a bit stuck, perhaps a thrust bearing would do.
Last edited by Jonathan; 11-04-2011 at 11:05 PM.
If you're using a double row angular contact bearing, then it doesn't need preloaded, as they come preloaded.
Provided the orange and red bits tighten together onto the inner race, and the blue and green bits clamp the outer race, then that design will work fine.
The only purpose the second bearing has, is to provide support to the shaft to stop it being pulled/deflected by the stepper/belt.
On a related topic, do you think 2 ballnuts like I drew in #13 is sensible for a router? It should enable backlash, and endfloat from the above, to be eliminated I think.
You'll have far bigger sources of play than 11 microns in a bearing!
Whether you use two ball nuts or nut, will depend entirely on how much play you're willing to allow (or money you're wanting to spend!), and there will also be an increase in friction using two preloaded ballnuts.
It's always best to eliminate play, but there are costs to that, as you're discovering.
Most accurate way to machine bearing bores so they're in perfect alignment, is ideally do them in one go, but for that design wouldn't be too easy.
However, as the double row bearing gets clamped in position, you could machine it's bore to allow the bearing to move around a bit, so when you finally assemble it, the bearings get lined up via the shaft, and it's just a case of clamping the doublerow in place.
If you would like some ideas or a rotating nut you can look at my mill build log. I have just uploaded some pictures showing the assembly of the rotating nut. I used 2 AC bearings and preloaded with a nut. I bought a double AC bearing from EBAY but was not happy with the amount of clearance between the balls and the race so I resorted to 2 seperate AC bearings. The nut is not a lock nut but the pulley below has a grub screw over the key. The nut stack is preloaded then clamped with the grub screw. If this causes problems the nut is thick enough to slit radially and add an M3 clamp screw. Hope this gives food for thought
I'm hoping if I press fit the bearing onto the shaft that should expand the inner ring, and tighten the bearing up nicely. Similarly with the outer ring, except if I follow m_c's excellent suggestion of boring it slightly oversize and clamping it then that's not going to happen. Did you get the single row angular contact bearings on eBay, and what size are they?
The problem I've got is that I don't have the constant force applied to my bearings due to gravity, unlike on the various milling machines that have a rotating nut.
I'm really not at all sure what to do now. I've said already two single row angular contact bearings is a lot of money...and yet it looks like that is certainly the best way to do it. Having said that there's someone on CNC zone who has done it with two standard deep groove bearings.
If I could get away with a 16mm. 10mm pitch, ballscrew then I would save so much on the screw, that I could easily afford the cheaper smaller bearings. However I would prefer to have the 25mm screw just in case it fails completely and I have to spin the screw.
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