Rotating Ballnut - design ideas

[Boyan Silyavski]

I believe in my case heat comes from friction. I use grease on ball screw nuts. When i became aware of the heating was when i was machining 8h per day nonstop. So i greased well separately the ball screws and heat lowered, but still there it was. So now i grease very well these elements very often.

But thats why i think the roller bearing is not very suitable. It will generate much more heat due to much bigger contact area for friction. I love overbuilding but i believe this is not the place to do that. Plus the price of the bearings. How much they cost? The Chinese bearings we use normally are not very expensive. 10-15$

[SafeAirOne]

Plus the price of the bearings. How much they cost? The Chinese bearings we use normally are not very expensive. 10-15$

I was surprised to see the large price range for the exact same bearing, depending on where you shop.

I initially found the bearing on the McMaster-Carr website (which I love because they have free 3D CAD models of most of the products they sell, making design in Solidworks easy). McMaster-Carr wants $48.00USD (45.00EUR/39.00GBP), but a quick search proves that this is a VERY common bearing. It looks like I can find these same bearings/races, manufactured by Timken, for as low as $19.00USD. Perhaps this isn't going to be as hard on the wallet as I originally envisioned.

But thats why i think the roller bearing is not very suitable. It will generate much more heat due to much bigger contact area for friction. I love overbuilding but i believe this is not the place to do that.

I guess this is my question, really: What are the consequences of hot bearings? I imagine the preload will increase due to expansion of the bearing parts, so the bearing life will be reduced (to several million revolutions). Grease life will be reduced (by half for each 10 degrees centegrade of temperature increase!). Other than that, I don't think there will be an impact on machine accuracy or repeatability, unless I'm looking at it wrong.

[Boyan Silyavski]

Having in mind that we are talking 3000rpm here / though some servos can reach as high as 6000, i dont think thats a design consideration. So i think they will work, assembly would be stronger but wear will be premature. In short i would use the normal angular contact bearings as they are more than strong enough for the job.

Size reduction is impossible as the nut should pass through, so that determines the size.

Size reduction in ball screw is absolutely irrelevant as normally you will buy the 3m ball screw as maximum shipping size from China and anyway you need to stretch the ball screw properly on the machine, so you dont need any gain in sizing there, see my second build from signature. As what you loose there from length is the gantry step spread, which is quite more bigger than the rotating nut assembly length.

So in short- you gain absolutely nothing


About the preloaded pair, you have the same in the normal angular bearing. But how do you fix that so it serves you? I see no way.

[SafeAirOne]

Size reduction is impossible as the nut should pass through, so that determines the size.

Hmm. In my mind, the nut attaches to the to the shaft flange and extends AWAY from the bearing housing. To have the ballnut sit INSIDE the hollow rotating shaft would make the bearings, and therefore the whole assembly, prohibitively large (in diameter), at least using a 2505 ballscrew/nut.

Shown with transparent regular ballnut, not the anti-backlash ballnut I plan to use:

Size reduction in ball screw is absolutely irrelevant as normally you will buy the 3m ball screw as maximum shipping size from China and anyway you need to stretch the ball screw properly on the machine, so you dont need any gain in sizing there, see my second build from signature.

Yes, I have been reading your 2nd build log--I've read the first page, and then skipped to the end, working my way back to the beginning. Very impressive! I see lots of good ideas in your build log. I imagine somewhere in the middle of the thread, I'll come across the change in the machine's purpose, from a CNC air hammer to a CNC router :D

As what you loose there from length is the gantry step spread, which is quite more bigger than the rotating nut assembly length.

Heh, heh...You know, I hate to say this, but I didn't really consider this. The linear rails that my gantry rides on are 3000mm long. So long as I keep the rotating ballnut assembly shorter than the length of the gantry supports, I'll have EXACTLY the same amount of travel along that axis.

Thanks for pointing this out to me! I think this alone should be cause to re-examine my design; Now that I have all this length to play with, I can explore whether using angular contact ball bearings will allow smaller bearing ODs, which in turn will reduce the distance the stationary ballscrew must be from the machine.

[Boyan Silyavski]

I meant that the ball screw has certain size and the nut attachment should pass through so this is the smallest possible bore bearing. You are right, the nut attaches to the assembly at side.