[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... :banana:

[Evodyne]

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... :banana:

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.

[GAWnCA]

Posix,

Your design looks so clean and simple. Do you think there is a need to make gears and outter threads? It seems to me that what you have in these photos would work just fine as long as ware can be adjusted for.

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... :banana:

[Evodyne]

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

[widgitmaster]

http://www.linearmotion.skf.com/en/0/products.htm
----------------------------------------------------

Here's a really detailed document on "Roller Screws" by SKF

http://skf.manager.nu/publication_fi...7551246328.pdf

I'm not to sure I understand the difference between Recirculating and Planitary type roller screws? Both are high dollar!

Eric

[svenakela]

http://www.linearmotion.skf.com/en/0/products.htm
----------------------------------------------------

Here's a really detailed document on "Roller Screws" by SKF

http://skf.manager.nu/publication_fi...7551246328.pdf

I'm not to sure I understand the difference between Recirculating and Planitary type roller screws? Both are high dollar!

Eric

The planetary roller screw has threaded rollers that not only rolls by themseleves, they also roll around the screw (like planets) and are "geared" in the end to have a fixed rotational speed.
The Recirculating screw roller has grooved rollers and the roll freely and are only forced to have their fixed position by the spacer that keeps them from each other. This version also has smaller resolutions because of the "lack" of threads.

--S

[DesignerDave]

Wow, I just read that whole thread. I think it should be a requirement that anyone who wishes to post in this thread read the whole thing... I cannot count how many times people came to the conclusion that in the designs which involve a pitch on the rollers of this design that any slippage will cause the nut to become inaccurately located. I remember at the beginning of this whole thing I was wondering how anyone was going to make it work unless they cut the rollers with straight grooves.

Anyway, this has all been beaten to death... so I'll try to bring my observations to the table hopefully to further our development on this type of screw/nut homebrew design.

First and formost, I think this design should have at least 4 rollers, and if there are more they should be an even number. Maybe I'm being too picky but if you use only two rollers and preload them laterally you essentially are twisting the nut... (when looking from the top of it, I'll do a FEA model for you guys if you want to see what I'm gabbing about) what probably doesn't matter if the rollers are of any substantial length, such as 3 times the diamater of the leadscrew or more, but still, the forces aren't even on anything in the assembly. A member posted saying that putting preload downward on the threads removes backlash but I question how much preload is required to do that and how easy can it be maintained. With only two rollers I would think that the leadscrew would really want to deflect off of center to relieve its preload, which then would transfer into unwanted friction between the leadscrew and whatever is keeping it on center. Not effecient, but this all comes back to how accurate you want everything to stay, and how cheap you are.

With four rollers the leadscrew would always keep its center location, and the forces preloading the threads would be evenly distributed thus keeping the friction between those surfaces to a minimum. ACME threads have been mentioned a few times and I cannot help but think that would be a nice way to go about this design. I figure the rollers do not have to be all that big, but the allowable pitch of the leadscrew would have to be determined using the diameter of the leadscrew to make sure the rollers wouldn't be scuffing too much as the helical thread started deviating away from the straight cut profile on the rollers.

I think I may not be describing this well. The last couple posts were about CAD. My two cents, programs like ACAD are awesome for what they are good at but if you want to get into real fun you have move onto parametric solid modeling. I design for a living and I'm currently running SolidWorks. I'll never go back. Inventor by AutoDesk is also very similar.

I should model something up and throw it on this thread so I don't get stuck in the desert with peanut butter... mmm... peanut butter.

Its my first post guys... I love this site. I've been restraining from doing the typical noob "I wanna build a mill what should I do first" post and I've been researching this site and others (such as 5bears.com) for as much as I can learn before I start requesting anything!

So far this forum has posts from some really good minds out there. I look forward to reading more of your ideas people!

Dave