Rotating Ballnut - design ideas

[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

[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 :)