Rotating Ballnut - design ideas

[Gary]

This is something we are developing, but it will not be for hobby use.
It will start with a servo system and the 2525 and 3232 ballnut,
We will then be moving to the 4040 and 5050 ballnut.

The only steppers that could have been used for this design were the IOS motor from IMS, but these are not made anymore.

Yes that method is used on some expensive commercial laser cutters. They use large servomotors with a hollow shaft containing the ball-nut running on a tensioned ballscrew. They get feedrates measured in meters per second.

There are few stepper or servos available with hollow shafts. The common ones have too little torque or the bore is insufficient. Anything above 16mm would be useful. It may be possible to dismantle an existing motor, machine a new shaft with the hole through the middle and use bigger, angular contact, bearings. For the size of my machine (1.7*0.74*0.4m) it's just not worth it as the 3nm steppers can already run it more than fast enough.

The advantage with using a standard motor, other than simplicity, is the timing belt allows you to change the drive ratio and reduces resonance. Currently mine is on 1:1 ratio, however it does go faster with a larger pulley on the motor. I elected to leave it at 1:1 however as 10m/min is plenty and I prefer to keep the resolution than get 15m/min+, but at least I have the option which wouldn't be the case with a hollow motor. In short I'm saying you have to be very careful with the selection of the motor, which makes it even more difficult to find a suitable one.

[georgetheforge]

Hi Jonathan,

really nice design on these- they still working a treat?

i came across these badgers http://www.drives.co.uk/fullstory.asp?id=3388- but as i say on my little build log thing- bet they're bleedy expensive-!

i'm tossing up rack and pinion vs this type affair for my long axis- (2.5m) so lurking about tying to decide to my way forward!

George

[Jonathan]

really nice design on these- they still working a treat?

Thanks... yes they're still working very well indeed. Same performance as in my post a month ago. I wouldn't hesitate to use them instead of rack and pinion on a 2.5m axis. You'll get better accuracy, longevity and it'll work out cheaper. With a RM2510 (or even a higher lead) the machine will be plenty fast enough with good acceleration.

i came across these badgers http://www.drives.co.uk/fullstory.asp?id=3388- but as i say on my little build log thing- bet they're bleedy expensive-!

They do look good, but as you say they wont be cheap.

[georgetheforge]

that would indeed be interesting Jonathan- do you want to work it out and PM me- or post it's up to you-!

i'll work out the mounting of them- i was thinking of tucking them out the way under the x rails- so to minimise dust/ crap getting onto them-

thanks again

G