Rotating Ballnut - design ideas

[Boyan Silyavski]

3) Note that on the drawing I posted the motor mount is 88mm wide, not 90mm which is the height of the main block. I intentionally left a 1mm gap so that when the block is bolted to a flat surface, you wont end up with the assembly partially resting on the motor mount (if the motor mount isn't attached accurately). The gap is on both sides since I put mounting holes on both sides of one block so they can be mirrored.

I see, will correct it.

There are 2 minor changes from the original design:

- i made the motor mount from the wider side, as many will like maybe 34 nema and it looks more sturdy. I wanted to make a combined nema 23 and 34 mount, but i feel sleepy now. when ready will see if it will stay like this or follow your original design.

-i believe i shortened the rotating shaft by making its wide part go through the motor mount plate. It is now 2mm to place the ball screw nut in its place,10mm for the ball screw nut holding thread, 2mm ring to push only the small bearing ring and 90mm the rest.

I think that all can be further shortened and lightened :
-2mm protruding thread instead of 5mm , when the nuts are tightened
-6mm /shortening the whole assembly/ eating from the clearance near the shaft pulley
-The area where the ball screw nut fastens could be recessed 8mm with diameter 40mm and some more holes could be done / 45 degree following the pattern/

This could lower considerably rotating mass

[EddyCurrent]

I know you had the housing made from several pieces originally but they looked a bit complex so is it feasible to make the rectangular housing from 4 or 5 just rectangular pieces bolted together with the motor mounting plate being one side ?

[Jonathan]

Is it feasible to make the rectangular housing from 4 or 5 pieces bolted together with the motor mounting plate being one side ?

That's pretty much how I did it earlier, see posts #50 and #61. One thing to bear in mind is it's harder to maintain the accuracy of the bearing mounts if they're separate parts.

[EddyCurrent]

Damn, the very pages I must has skipped
Yes I realise about alignment, it just seemed easier for someone without a decent machine shop.

[Boyan Silyavski]

Damn, the very pages I must has skipped
Yes I realise about alignment, it just seemed easier for someone without a decent machine shop.

Actually if Jonathan or sb from the forum doesn't have the time to make the parts for me, i have to go to my local guy, who is way off with the 0.01mm, he works in the 0.5 zone definition
So i would update the drawings with extra drawing , made from OD100 and OD80 round bar. That way it would be more easy for him not to make a mistake an find the center. Further benefit will be that it could be mounted in cut in 2 pieces 85mm or 100mm spindle mount/not perfect but will work/ . anyway, i will mount it directly on its face, not using the bottom holes.

[paulus.v]

One grubscrew is enough for clamping the pulley to the motor shaft or should I use two at 90 degrees? Are there other simple methods that I could use to be able to change the pulley (ratio) more easily?


Here are my machined parts, still not assembled and tested:

[Boyan Silyavski]

For the sake of experiment, found some providers of Servo motors with hollow shaft where a ball nut can be mounted. Usually the 3nm motors accept only16xx sizes, for 25xx sizes a 9Nm servos are offered.

I wanted to make sure Hybrid Closed Loop Stepper driver + motor+shipping + this assembly will not equal a proper servomotor price.

No prices on the sites/ means "in the thousands" to me/ . Well , one called me , wow, i got even more suspicious about the price. Now received the quote.

Wow, again. 1600eur the motor and the driver and cables. driver costs 420, so rest is the motor.

[Boyan Silyavski]

Guys, now the most stupid question: How to order the ball screws machined at both ends? Simple thread, long enough for 3 nuts? One inside, then mount plate then another plus locking one?

[Jonathan]

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.

For the sake of experiment, found some providers of Servo motors with hollow shaft where a ball nut can be mounted.
[...]
Wow, again. 1600eur the motor and the driver and cables. driver costs 420, so rest is the motor.

Make your own.

Guys, now the most stupid question: How to order the ball screws machined at both ends? Simple thread, long enough for 3 nuts? One inside, then mount plate then another plus locking one?

I got mine machined the same as the standard end machining for the angular contact bearings at both ends, in case the rotating nut didn't work. All you really need is a cylindrical portion and a thread on both ends. On one end simply use the thread to clamp the screw, ideally into a hole the same diameter as the cylindrical portion so it's held rigidly. Something similar on the other end, except you can use the thread to tension the screw a bit if your frame is strong enough. Also, design the end mounts in two parts so you can adjust the ends of the screw to be concentric with the bearings.

If you look earlier in the thread you'll see the end mounts I made are very simple - essentially just a block with a clearance hole. They work, but having a hole the same diameter as the screw instead of using it for adjustment will be better.

Edit: Idea for silvaski - to make the housing easier to machine, instead of boring the bearings from both sides you could bore from one side almost all the way through, then machine a spacer tube the right thickness to go in between the outer rings of the bearings.

[Boyan Silyavski]

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.

I knew this is important. I was sleepy the other night when i made the Sketchup drawing and wondered why you separated them so much, but overlooked it. I even gave ideas how to be made shorter