A different (better) kind of screw

[zoltan]

Posix,

What are the conclusions so far?

Zoltan

[wisp]

SKF also makes roller screws. It looks like there are two designs, a Planetary setup and a Recirculating design.

[posix]

Originally Posted by zoltan Posix, What are the conclusions so far? Zoltan

It's very nice and as I've said earlier on - no delrin nut can come close to this. But bear in mind that I have used a normal cheap M10 threaded rod and the thread finish on that is, well, shall we say less than optimal

I will have a custom ground 16mm threaded shaft and little rollers and THEN we'll see what this beauty can REALLY do.

But for a first try this simple arrangement of 3 pieces of M10 "allthread" is REALLY a nice thing and I would wholeheartedly recommend to anyone starting out with cnc machines to go down this route if they cannot afford/find ball screws. It will only take half an hour for an experienced machinist to make you one and you could do far worse than substitute my steel retaining rings (the "washers" that hold the 3 pieces of M10 in place) with delrin equivalents for better rolling performance of the rollers.

[posix]

Originally Posted by 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.

I think making anything apart from the positioning/retaining "washer" for the rollers out of delrin would actually be counter productive. You don't want anything sliding against anything else, you actually want things to engage other things and make them roll, just like in a ball bearing - nothing should ever slide inside a ball bearing but instead balls should engage inner and outer races and roll thus producing smooth motion. If sliding occurrs you end up damaging bits and getting less than optimal motion. So forget delrin, this should be all steel.

[greybeard]

Point taken posix. Please forget I mentioned delrin.

What I was thinking was to make the outer sleeve adjustable in diameter in order to take up any backlash. Certainly I assumed that finely adjusted, this would allow the rollers to do their stuff properly, and not slide. Would this work with steel ?
I don't want to add any unnecessary bits, only remove problems.

[posix]

It should all be steel and of course it would work perfectly. It's all in the maths required to calculate the diameters and dimensions of all parts. If you get that right everything works. If you are so worried about backlash then just put two of these babies back to back separated by a shim of some sort that would push them apart thus eliminating any backlash. But if everything is ground properly there should be no backlash to begin with. Even with my M10 "allthread" there is no PERCEIVABLE backlash - i.e. I cannot FEEL any on my fingertips but maybe a micrometer could measure some, I don't know.

[svenakela]

Posix: Are you going to make a new one with a threaded nut, or are you going to refine the prototype you've made? I'm just curious.

I really like the first example you posted, no need for bearings and the backlash could easily be adjusted with the nut. I'm thinking that by having a "splitted nut", i.e. two nuts, the backlash could easily be removed by pressing them together by hand when fixating them to the axis. How about that?

--Sven

[posix]

No refining, a proper one with threaded "nut" is next.
Yes, your picture shows the idea that I was thinking and that is your standard double preloaded nut. But you first have to build the one that I have to be able to appreciate just how little backlash this thing exhibits even though it has been made with most primitive of materials and techniques.

[svenakela]

Can't wait to see your result!

[svenakela]

Been googling a bit, really got this thing in my head now.

Found another nice design:
http://www.steinmeyer.com/english/te...s/drs_funk.htm (see the first image). No need for threading tools to make it.

http://www.about-ars.com/IHD/ARS_PS_RV1.htm and http://www.about-ars.com/IHD/ARS_PS_RVR1.htm describes the differences between satellite and recirculating rollers.
I know that SKF was mentioned earlier, but their PDF is very descriptive: http://skf.manager.nu/publication_fi...7551246328.pdf (7,84 MB)
There's also a picture on the first side where the gear ring has the teeth outwards, would be easier to make at home. The recirculating version seems to be pretty straight forward, later on I'll try to make on of those or the "german design" I mentioned...

--S

[posix]

You can google all you want but you won't find any numbers required to make them.

Well, here's one I made earlier and it really was an eye-opener and it also helped me fix some of my math.

Also I assumed only one start was required on the nut. Boy how wrong I was. You actually need 6!!!

You like?

P.S. all comments about thread collisions will be politely ignored untill someone tells me how to do patterns around a helix...

[posix]

Good or bad news first?

Well, the bad news is that no one ever told me you can't just have any thread pitch you please. It has to be one of the preset pitches on the lathe. So my 4.8mm pitch for the screw was a no-go and a show stopper.

The good news is that after a brief discussion with the master I've come to the conclusion that it would be a simple procedure of re-drawing my whole assembly with a slightly shallower thread depth in order to get a standard pitch. So 0.5mm thread depth and 3mm pitch is on the drawing board.

For example I COULD go with a 5mm pitch on the shaft but that wouldn't give me a clean number for the thread depth (0.833333333333333333) and I would end up with a non-standard pitch on the rollers (1.666666667) which, again, is a no-go. You have to end up with the same thread depth and a standard pitch on all parts.

In a few days he'll give me a ring when he's cleared up his backlog so I can come over and we can tinker all night.

Is 0.5mm enough of a thread depth? Doesn't look that substantial in my drawings that's for sure.