A different (better) kind of screw

[posix]

I don't know but all material I've managed to come across lists roller screws as having superior accuracy and repeatability to ball screws and due to the large number of contact points also exhibits better stiffness and load resistance.

But all this is way beside the point. What I wanted this thread to concentrate on is making a DIY version of it as ball screws are nigh on impossible to make at home and these suckers look a lot less complicated.

And here are some examples of what people have allready done, albeit much simpler and cruder versions of what the real deal looks like:

[posix]

Well, hello everyone.

Today I had a productive day. For roughly $15 and in 15 minutes of cutting time I had my own roller screw. And I can tell you this thing beats any delrin nut any day. Ok it has some backlash, it does have some axial play as well but considering it's been made out of M10 "allread" and with only an idea in my head and no drawings of any kind, planning or anything else for that matter (even the cuts are as rough as they get - no polishing or finishing of any kind) this thing rolls really nice.

Now on thursday I'm going to put some plugs in my ears and go to the same guy with the drawings of a real roller screw and he told me that from what he made today and from what I explained to him about the real roller screws he could do something.

I'll try and upload a few pics of what I had made today. This is fantastic! Cheap homemade better-than-ballscrews roller screws are coming!!!!

[DieGuy]

I could envision putting a gear train on the rollers and driving that instead of the main screw as well, just look at that last picture and think gear driven.

[posix]

Some images of what I've got:

P.S. sorry about the resolution; I have a feeling my digital ixus is going to pack up soon due to the lack of use... :frown:

[swoolhead]

All the home made designs seem to be missing the planetry gear that slaves all the screws together. I wonder what difference this makes?

Nice work BTW!

Stephen.

[posix]

I haven't forgotten the planetary gear or the outer threaded shell but this was a test just to see how easy it turns and if the concept is ok.

The next phase is to take one of the cutaway drawings of a real roller screw together with all the elements and make the real mccoy, custom threaded rod, little threaded and toothed rollers, outer threaded shell, the works. Can't wait 'till thursday...

[Evodyne]

Originally Posted by swoolhead All the home made designs seem to be missing the planetry gear that slaves all the screws together. I wonder what difference this makes? Nice work BTW! Stephen.

If you look closely the gears DON'T tie the screws together-directly. They mesh with a ring gear. Here is my take. The little gears can't be of a larger diameter than the threaded piece they are attached to: the main screw would never fit though. But they could be the same or smaller diameter. If they were smaller, the ring gear would need to be resized to mesh properly. But changing the relative diameters affects he "gear ratio" between them. Thus, for a single turn of the body, you could get more or less rotation of the inner screws. If they spin faster, then the main lead screw will travel further for a given amount of nut rotation. So the ones without lose nothing-they are just operating at a "fixed" design point.

Posix, I posted earlier and said I thought this looked complicated/expensive. What you have done has changed my mind. I'm really impressed by the way you dove right in and tried it. Way to go!

Lance

[Evodyne]

Originally Posted by posix I haven't forgotten the planetary gear or the outer threaded shell but this was a test just to see how easy it turns and if the concept is ok. The next phase is to take one of the cutaway drawings of a real roller screw together with all the elements and make the real mccoy, custom threaded rod, little threaded and toothed rollers, outer threaded shell, the works. Can't wait 'till thursday...

Thursday, huh? You ambitious little devil, you. Question: how have you resolved the threads, pitches, etc? Inquiring minds want to know....

Lance

[posix]

That's why I asked for the mechanical engineers among you to make some solidworks (or any other 3d) drawings and figure out all the dimensions. I have some simple and crude formulas for calculating diameters based on the drawings and photos I have so far.

The diameter of little rollers is ROUGHLY 34.09% of the screw diameter
the INSIDE diameter of the outer shell is roughly 163.18% of the screw diameter
thread depth on the screw is roughly 5% of its own the diameter
thread depth on one roller is roughly 14.66% of its own diameter
thread depth on (in?) the outer shell is roughly 3.06% of its own diameter.

So assuming a 16mm screw we have following:

screw ~16mm, thread 0.8mm, 5mm thread lead
roller ~5.45mm, thread 0.8mm, 5mm thread lead
shell ~26.11mm, thread 0.8mm, 5mm thread lead

Now, can someone with a bit of high-school trigonometry left in their memory confirm/deny this.

[svenakela]

The DIY versions seems to run with the rollers only and the heavy duty versions have an outer "nut".
BUT, there also seems to be two different styles of production designs. One with the outer nut fixed as seen earlier in this thread, and one with the outer nut rotating, moving the axial load to the flange via roller bearings. Check the picture (or http://www.itwspiroid.com/rolrscrw.htm).

--Sven

[Geof]

Originally Posted by svenakela ....BUT, there also seems to be two different styles of production designs. One with the outer nut fixed as seen earlier in this thread, and one with the outer nut rotating, moving the axial load to the flange via roller bearings.... Sven

There is an additional difference:

The picture attached to post #4 has a caption that reads:

"Threaded rollers are the basis of SR/BR/TR/PR planetary toller screws."

But the link from post #22 has this description:

"What is a Spiracon® Roller Screw?
Our unique, patented Spiracon Roller Screw consists of a screw engaged by rollers which have annular grooves. Full line contact exists between the threads of the screw and the annular grooves of the roller."

For a DIY version it could be easier to do the annular roller version. It could remove the need to do precise measurements or calculations and also incorporate adjustment to remove backlash. A housing could be made to take six rollers with the bearings for the rollers axially adjustable. The adjustment is needed because annular rollers cannot all be in the same plane around the screw they have to follow the helix. Because the rollers do not have a thread but just grooves it would be fairly simple to machine samples to determine the particular diameter needed for a close fit. With six rollers all axially adjustable it would be possible to adjust three in one direction and three in the other to remove backlash.

[greybeard]

In the same way that delrin anti-backlash nuts are designed, could the outer 'barrel' be made in the form of a delrin split tube which could be tightened down with a further sleeve/clamp.