A different (better) kind of screw

[posix]

It is surprising to find out that no one on the forum has ever mentioned a roller screw.

They seem to be a lot more robust than ball screws and look like something that could possibly be even made at home (a home equipped with a workshop and a lathe, that is )

Anyone ever pondered on the idea?

[Evodyne]

Posix,

That's different. First time I've heard of/seen one. Any more info?

Lance

[posix]

It's pretty, though, ain't it?

Well, it works on the principle of planetary rollers - think along the lines of a rear diff on a car - the screw turns, little planetary rollers (smaller diameter screws with the same thread depth and pitch) roll in opposite direction (just like balls in a ball screw) around the circumference of the screw. The little planetary rollers also roll INSIDE a nut wich, also and predictably, has the same thread depth and pitch. So looking from the side at the whole concoction you in effect end up with a normal bearing with the shaft (screw) in the centre, balls or rollers (planetary roller screws) around the shaft and race (nut) on the outside.

I'll see if I can dig up a few drawings to make things easier to picture.

[posix]

Ok, here's another couple of cut-away drawings. Also note that little (planetary) rollers have little pinions at both ends and the nut has a corresponding large toothed ring on each end. My guess is that's to discourage the rollers from "sliding" across the screw and nut surfaces but to turn properly. I've caught mention of gearing somewhere as well.

Could any of you solidworks gurus out there make up a solid model of this which could help us visualise the whole system. Also mechanical engineers among you could come up with a spreadsheed which could be used to calculate the exact screw, roller and nut diameters and thread depths and pitch of each component since this mechanism isn't using any standard threads.

[nachini]

I already had seen it in a (Hungarian?) Web.

http://www.hobbycnc.hu/CNC/Otletek/Otletek.htm

The Web is partially translated to English, but there is no explanation in English about the screw.

Somebody can translate it?

Hey, this is my first post!!!

[Evodyne]

Originally Posted by nachini I already had seen it in a (Hungarian?) Web. http://www.hobbycnc.hu/CNC/Otletek/Otletek.htm The Web is partially translated to English, but there is no explanation in English about the screw. Somebody can translate it? Hey, this is my first post!!!

nachini,

Can't help with the translating, but welcome aboard. Watch out-this site can be addictive! Enjoy...

Lance

[motomitch1]

http://www.exlar.com/products/linear...rew_oview.html

[Dave's_Not_Here]

Here is a web site with some good explanations...
http://www.exlar.com/products/linear...ew_basics.html

[Evodyne]

It's definately neat. But it probably fits the typical model: if it looks complicated, it's expensive. If it looks expensive, it is expensive. If it looks expensive AND complicated...well like they say, if you have to ask the price you probably can't afford it. Bummer.

Lance

[swoolhead]

This is a good link http://www.exlar.com/products/linear...RolScrew12.pdf it's good some good pictures in it.

Looks like the standard nut has 0.01-0.03mm of backlash on it, but there is also a preloaded version. I am intrigued to know how it compares to something like this http://www.kerkmotion.com/ which claims zero maintenance with zero backlash, but there are no real values quoted, but does look a lot cheaper.

Stephen

[Geof]

An interesting point to note in the RolScrew12.pdf that Stephen gives the link for is that the machine tool applications that are mentioned are either high force or high speed but not high precision positioning. It could be that these screws can outperform ball screws in force and life but do not match them for accuracy and precision.

It would be interesting to try and build something of this nature using different sizes of threaded rod with the same pitch.

[ViperTX]

Well...just a quick glance and it appears to be less efficient then a ballscrew because of the increased number of parts that rub on the screw...okay, maybe they are suppose to rotate against the screw....it also has more moving parts that appear to require some precision in how they are mounted....those little planetary screws....they appear to float some....