Drive shaft whip

[mike hide]

Hereis a simple question.

As all know longer threaded drive screws whip at higher RPMs.Once this speed is achieved for unsupported drive screws this is the absolute limiting RPM.

I was wondering if anyone has tried driving the nuts instead of the drive screws ? perhaps it is done all the tme ,I am not aquainted with commercial machinery design....mjh

[ger21]

Search this site for rotating nuts, there should be quite a few threads. If you look at the critical spped charts at www.nookind.com, the list 4 screw mounting types. Having both ends fixed with a rotating nut only gives you about 20% more rpm than having both ends rigidly mounted with AC bearings. Although you would think it would be higher. So it's probably easier to mount the screw at both ends with better bearings, and possibly increase screw diameter rather than go to the trouble of fabricating a spinning nut. And yes, a lot of commercial machines use spinning nuts.

[lgalla]

Ger thanks for the nut review.Rotating.Commercial spinning nuts are $700 or more.Better to use the correct diameter and support bearings in the spinning screw design.There is no free lunch for rotating nuts.
Lar

[mike hide]

Search this site for rotating nuts, there should be quite a few threads. If you look at the critical spped charts at www.nookind.com, the list 4 screw mounting types. Having both ends fixed with a rotating nut only gives you about 20% more rpm than having both ends rigidly mounted with AC bearings. Although you would think it would be higher. So it's probably easier to mount the screw at both ends with better bearings, and possibly increase screw diameter rather than go to the trouble of fabricating a spinning nut. And yes, a lot of commercial machines use spinning nuts.

Thanks for the reply Ger, It was just a thought . Increasing the drive shaft diameter is a thought it would reduce the deflection due to the increased stiffness, however the off center load would be greater .I.E. centrifugal force is w/g v[squared] / r if I remember correctly.....regards mjh

[mike hide]

Ger thanks for the nut review.Rotating.Commercial spinning nuts are $700 or more.Better to use the correct diameter and support bearings in the spinning screw design.There is no free lunch for rotating nuts.
Lar

Most of the original ideas on this subject have bounced around for a long time I am sure . What i had in mind was using rotating nuts in the X axis only where the longer travel is required . I envisioned using one stepper motor each side of the gantry using a chain drive and sprockets, or perhaps a cogged belts to transfer power....mjh

[ger21]

I didn't mean to imply that it wasn't a good idea. Just not a cost effective one. Cheaper to go with bigger motors to compensate for the bigger screw.
Btw, our machine at work uses a rotating nut and it works just fine. It's about a 2" x 13' screw.

[lgalla]

Gerry,sorry to imply,that you implyed,that it was a bad idea.Can't say for everyone on the forum,but for me,being a cheapskate at heart,the cheapest road is the one to take if it gets the job done.The rotating nut just makes it more complicated and costly.If one needs 10'just get the recommended dia. screw for the RPM desired or compromise and use R&P and save a bundle.
BTW Gerry,is a 2"X10' Acme screw with Moglice nut"a screwey or nutty idea?"I have read moglice nuts on acme can acheive over 80% efficency and will backdrive like a ballscrew.
Larry

[ger21]

Larry, I think that whether or not it will backdrive has to do with the lead of the screw. High lead screws will backdrive easier.

As for moglice, supposedly it works pretty well.