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

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 :D )

Anyone ever pondered on the idea?

Posix,

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

Lance

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

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.

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. :eek:

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!!!

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

http://www.exlar.com/products/linear/roller_screws/roller_screw_oview.html

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

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

This is a good link http://www.exlar.com/products/linear/roller_screws/pdf/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

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.

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....

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:

http://www.hobbycnc.hu/CNC/Mech12/Zanya.jpg

http://www.hobbycnc.hu/CNC/Mech11/Csuszka1.jpg

http://www.hobbycnc.hu/CNC/Mech8/2.JPG

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!!!! :wee:

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.

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:

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.

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:

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

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

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.

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

....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.

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.

Posix,

What are the conclusions so far?

Zoltan

SKF also makes roller screws. It looks like there are two designs, a Planetary setup (http://www.linearmotion.skf.com/SetLog/skfkey/ObjectKind/Solution/ObjectName/Planetary%20roller%20screws/ServiceName//url/_sl_Catalog_sl_ss.asp?FAM=SRrollerscrews_amp;Lang=EN_amp;ProfileName=US_amp;userId=0_amp;BackId1=121_amp;BackKind1=ProductRange) and a Recirculating design (http://www.linearmotion.skf.com/SetLog/skfkey/ObjectKind/Solution/ObjectName/Recirculating%20roller%20screws/ServiceName//url/_sl_Catalog_sl_ss.asp?FAM=SVrollerscrews_amp;Lang=EN_amp;ProfileName=US_amp;userId=0_amp;BackId1=121_amp;BackKind1=ProductRange).

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 :D

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. ;)

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.

Point taken posix. Please forget I mentioned delrin. (chair)

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. :D :D

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.

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

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.

Can't wait to see your result! :)

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

Found another nice design:
http://www.steinmeyer.com/english/technik/drs/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_files/pdf/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

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!!! :D

You like? :rainfro:

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

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.

Keep us posted, I carn't afford ye old ballscrews and I pondered on this one a while ago, I'm still in pondering mode LOL.

How about driving the nut instead of the shaft - long shafts may whip? if driven at the faster speeds needed? could you drive the nut in this design?.

Pat

Yes you can drive the nut and this design actually affords you higher speeds than a ballscrew.

I made some experimenting yesterday while watching TV. :)
The simplified "German version" should work with standard trapetz screws. Can't drive it with the nut and may not be as efficient as Posix's, but a lot better than a trapetz nut/screw. I'll try it out, just when I get some spare time... ;)

http://www.cnczone.com/forums/attachment.php?attachmentid=10707

Can someone buy me a lathe as an early christmass pressie? :cheers:

Nice thread! Looking at the drawings it seems you can take up any backlash by adjusting every other screw along the main screw axis. This might work better than trying to change the diameter of the outter ring or adding a second roller nut.

JR

Ok, today is the first day I was able to mount my roller screw assembly to my Y axis and compare it to the X axis.

Basically with the crummy "setup" I have at the moment I am able to squeze out additional 50% RPM more before the stepper stalls on the Y axis than on the non-roller screw X axis and they are identically built. AND the execution of my roller screw leaves a lot to be desired. Today I spoke to a different (and a lot older) metal master lathe person and he suggested some improvements on my original "design". And he'll do it even cheaper than the first bloke. Hopefully on saturday I'll have all 3 axes "roller screwed" and then we'll see.

I even spoke to him about making a real roller screw with all the little gears and things and he said it'll be difficult making the little gears with the machines he has (a big mill, a big thing that looks like a mill but has only Y and Z axes and a 1958 german-made lathe bearing a big "VDP" mark - anyone knows what it stands for?) but I told him I'm in no rush and that he can take as long as he wants with those. On saturday I'll take some printouts of what I've got to him for an oppinion. Fingers crossed my machine will fly on saturday! :bat:

Hi,

I have a question. I built one roller screw similar with yours, but despite the satellite screws are spinning freely when the main screw is not mounted, when I mount it and move it, the satellites are not spinning. Are the satellites of your construction spinning? Anyway, the screw is working more smooth and easy compared with the same screw with derlin nut, but I do not understand why the satellites are not spinning, despite not major friction is on their ends.

Thank you,

Zoltan

Hmm...if they are not spining in the opposite direction of the lead screw then it would be very difficult to turn because they would be sliding against the surface of the lead screw. In effect you would end up with just a plain nut and screw.

My rollers, or "sattelites" as you call them, are spinning in the opposite direction of the lead screw at all times, it doesn't matter if the lead screw assembly is mounted to an axis or not. If I hold the lead screw vertically and "push" the roller screw it will spin down for about 10cm before coming to a stop. It will spin up as well but not as long, only for about 3-4cm. I don't know what could be causing your rollers not to spin. Is your lead screw slightly bent perhaps? Maybe your rollers (sattelites) are not pushing against the lead screw tightly as they should? You have to calculate everything correctly. Otherwise it's :drowning:

For example I am using a normal M10 "allthread" rod. Looking from the front if the lead screw is in the middle the diameter around which lie the individual axis of rotation of the roller screws is cca. 18.5mm. How did I come at that number? Look-

M10 spec is:
outside diameter 10mm
diameter at bottom of tooth 8.376
diameter at the middle of tooth 9.02

9.02(roller on one side)+9.02(lead screw)+9.02(roller on the opposite side)=27.06 - this is diameter of the whole assembly
9.02/2=4.51 - this is the centre of a roller or its radius
27.06(whole assembly)-4.51(radius of one roller)-4.51(radius of second roller)=18.04

18 is too tight and you won't be able to assemble it. 18.5 is better - it is a bit loose and gives 0.23mm on each side but that number is good enough given that no M10 is exactly to spec since you always buy cheap iron bars with formed rather than ground threads with minor imperfections and 2/10ths of a mm is good enough.

Input your sizes into this calculation and see if your rollers are at a correct distance from the lead screw?

Hi,

Basically, I have the same figures, with one exception - 18 mm instead of 18.5, as I considered the medium diameter is 9 mm. How freely are your rollers on their bearings? Mines are rotating freely but not very easy.

Thank you,

Zoltan

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

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 :D )

Anyone ever pondered on the idea?

The idea isn't entirely new. It has popped up in technical magazines from the period 1950 - 1960 (can't say exactly when). Later it was fund having promlems with the contact surfaces due to very high pressure and a sliding movement of the contact surfaces. My opinion is that they are exellent in system that doesn't move very frequently, but of course I have been wrong before.

/jan

i find this interesting. how expensive are these things to buy anyway? Is this a case of spending all day to make a 5 cent screw? if it is inexpensive, it sounds fun to make ano matter what.

Hi,

Basically, I have the same figures, with one exception - 18 mm instead of 18.5, as I considered the medium diameter is 9 mm. How freely are your rollers on their bearings? Mines are rotating freely but not very easy.

Thank you,

Zoltan

That's because you went for 18 instead of 18.5. Mine are rotating completely freely, just like rollers in a bearing.

My opinion is that they are exellent in system that doesn't move very frequently, but of course I have been wrong before.

/jan

I don't think moving frequently is a problem, but sliding of the rollers could be. What if rollers gain a little more in friction in one direction and doesn't roll completely, instead making a tiny "sliding" movement? In this scenario the "gear ratio" of the assembly would change!

I'm thinking of making an assembly (when the time arrives, so to speak...) and make a test run, lets say moving the nut up and down on a screw a couple of hundred times and then see if it could go back to the starting position.

--Sven

I don't think moving frequently is a problem, but sliding of the rollers could be. What if rollers gain a little more in friction in one direction and doesn't roll completely, instead making a tiny "sliding" movement? In this scenario the "gear ratio" of the assembly would change!

I'm thinking of making an assembly (when the time arrives, so to speak...) and make a test run, lets say moving the nut up and down on a screw a couple of hundred times and then see if it could go back to the starting position.

--Sven

Sven in my limited grasp of this idea some of the designs - maybe the better ones for us - simpler - they use satellite rollers that don't actually have a thread form, but rather a series of grooves and ridges that match the thread form - kind of LOL. so a satellite roller slipping although quite undesirable in terms of wear will not alter any position.

now the difficult thing is that the inside of the 'nut' or outside case has internal grooves to mate with the rollers. those rollers or satellites must be of a size to pretty accurately prevent any 'slop'. that seems a little difficult to calculate/make?

am I grasping this design? or one of the types of this design :(

some designs will have simple gear teeth milled on one end of the rollers, those engage - I don't think it needs to be a particularly 'good' fit with internal teeth on the inside of the 'nut' hence making sure rollers cannot slip.

how about Delrin/acetal rollers - easy to broach teeth on?

wait :idea: internal thread on the nut - must use threaded rollers? otherwise it would jam. internal grooves must use grooved rollers. but surely the rollers cannot be allowed to slip and not roll... :drowning: heeelp

Pat

...Later it was fund having promlems with the contact surfaces due to very high pressure and a sliding movement of the contact surfaces...jan

The sliding of the satellite rollers is probably why they use the complicated sun and planet gear system with as many rollers as possible to distribute the high pressures over many contact points.

Pat; I think you have a good point "...- they use satellite rollers that don't actually have a thread form, but rather a series of grooves and ridges that match the thread form - kind of LOL. so a satellite roller slipping although quite undesirable in terms of wear will not alter any position."

I think for a low load application it should be possible to use this style and omit the outer nut. Mount the rollers in small thrust bearings to take the axial load. The bearings would have to be in adjustable mounts to space the rollers correctly around the helix.

I don't think moving frequently is a problem, but sliding of the rollers could be. What if rollers gain a little more in friction in one direction and doesn't roll completely, instead making a tiny "sliding" movement? In this scenario the "gear ratio" of the assembly would change!

I'm thinking of making an assembly (when the time arrives, so to speak...) and make a test run, lets say moving the nut up and down on a screw a couple of hundred times and then see if it could go back to the starting position.

--Sven

You got a point there... The "planets" can live e life of their own, separated from the rotation of the nut. I don't think I would use this design in a cnc design, at least not without a direct feedback of the position.
Still the problem with high contact pressure remains. Lubrication could be a problem as it is a sliding movement of the contact surfaces. In a ball-screw (or whatever the proper English word for it is...) the balls rolls into fresh lubricant all the time and also carry the load on a roling movement while the planetary nut pushes it away all the time as it slides.
/jan

:argue: (nuts) (flame2) (chair)
:tired:
:idea:

It seems that we have a bit of a language barrier here.

Nothing is supposed to slide on a roller screw assembly. That is the whole point of it. All surfaces roll against eachother in opposite directions. Lead screw rolls against rollers, rollers roll against the nut. No sliding.

Oh, and btw, there is no sliding in a roller screw. Nope, just rolling.

Did I mention that all elements in a roller screw roll against eachother rather than slide? Yep, rolling rolling rolling...

P.S. I almost forgot - lead screw and rollers roll against eachother, they don't slide. The rollers roll against the nut too. They never slide.
P.P.S. And one more thing - roller screws resemble a roller bearing in that the centre race - or lead screw - rolls and the little rollers roll around it. And the little rollers roll against the inside surfaces of the outside race - or nut.
P.P.P.S. Roller screws roll rather than slide. :cheers:

> P.P.P.S. Roller screws roll rather than slide.

I think no one had that thougt in any case, but how can you guarantee that they don't slide, even not a 0,01%?..
Of course they're supposed to roll, but I want make sure a DIY roller screw is useful.

--Sven

Visualising something without an actual model may prove to be very difficult so I urge you all to make one for yourselves. I have and my mind is made up. I wish you all made it at home and then we could all talk about ways of improving it from the same perspective rather than postulating and speculating on things that might not even pose a problem.

To directly answer your question about ensuring rollers roll rather than slide - make the "walls" of the roller screw assembly (the two pieces with little holes 120deg apart through which rollers go) out of delrin which will ensure smooth low-friction rolling of rollers around their axis and don't lubricate the lead screw as maximising friction between the lead screw and rollers is the goal to achieving rolling, as opposed to minimising it which would lead to sliding.

Gears help but can someone on here with home-shop tools make a 3.75mm diameter roller with 10 tooth on it and a 23mm ID nut with <insert the calculated number of teeth here> teeth to engage all rollers equally? Doubt it. I know I can't. So you make do with that you have. ;)

Hi, I have a quick question. I'm currently in the first semester of a tech program (manual & CNC) and my instructors think that I should go for an agressive project, like a manual bench lathe. I'm wanting to build one that I can implement at least a partial cnc system on (I already have a degree in electronics).

My question is: Would this roller-screw nut be appropriate for that application, or sheer overkill?

Thanks!

Re everyone making their own model etc... I'm going to try having the rollers turning on a single ball bearing at each end. Any use I make of the idea would only be for very very lightweight machining.
I hope to be able to drill a small dimple for the ball at each end of a short length of all-thread, as this would be easier for me to set up than turning down the ends.
Any comments ?

:argue: (nuts) (flame2) (chair)
:tired:
:idea:

It seems that we have a bit of a language barrier here.

Nothing is supposed to slide on a roller screw assembly. That is the whole point of it. All surfaces roll against eachother in opposite directions. Lead screw rolls against rollers, rollers roll against the nut. No sliding.

Oh, and btw, there is no sliding in a roller screw. Nope, just rolling.

Did I mention that all elements in a roller screw roll against eachother rather than slide? Yep, rolling rolling rolling...

P.S. I almost forgot - lead screw and rollers roll against eachother, they don't slide. The rollers roll against the nut too. They never slide.
P.P.S. And one more thing - roller screws resemble a roller bearing in that the centre race - or lead screw - rolls and the little rollers roll around it. And the little rollers roll against the inside surfaces of the outside race - or nut.
P.P.P.S. Roller screws roll rather than slide. :cheers:

Ok, ok, I wasn't thinking of sliding like in a solid brass nut, I was merely thinking of two convex surfaces in contact under high pressure. The screws with balls are a little better of as a convex surface meets a concave. Of course I wasn't thinking of sliding like a screw in a solid brass nut, but let's leave it. The contact pressure is still high as the surfaces in contact vitually have a 0 area, if not, they must slide...
Never mind, If anyone can get this device to last long it's ok with me. I see it merely as a component most useful in devices with a limited expected lifetime like in a jack that only would be used a few times but where high forces are expected at minimum weight, minimum volume and low cost. On the other hand, as I stated somewhere else at cnczone, I might be wrong. It wouldn't be the first time and probably not the last.
And - yes, I have a problem with the English language but I'm working on it. I'm how ever jolly good at Swedish :-)
By the way, I'm a test engineer by profession and looking for possible problems have became a bad habit. Sometimes i might find a problem that not really is a problem.
:cheers:
/jan

The problem with using threaded planets is one of repeatability. It's like you're driving your stage with a friction wheel, yes it rolls, but there's going to be some positional losses. Just find two same size screws and play with it for half an hour, that's what I had to do to visualize this. Now if instead there were just stacked consecutive rings of teeth with the same pitch as the drive screw, there wouldn't be a positional loss problem, but it would be highly efficient since the planets are making rolling contact like a ballscrew. If you made a similarly designed ring roller, there would be no need for a complex planet retainer with thrust bearings and such.

Unfortunately making the planets and ring roller would require a lathe and some skill. There wouldn't be the benefit of creating gear ratios either. Despite these shortcomings this could still be an inexpensive alternative to ballscrews even if someone were to have a machinist make these components.

It's perhaps worth looking at a rather large pdf file from SKF on roller screws, I carn't seem to get to 'official' site link to produce the file, however I found the link from the Hungarian web site someone posted early in this thread.

http://www.hobbycnc.hu/CNC/Otletek/Gorgos_anya.pdf

it's a 5.4Mb file mind

<snip>
Gears help but can someone on here with home-shop tools make a 3.75mm diameter roller with 10 tooth on it and a 23mm ID nut with <insert the calculated number of teeth here> teeth to engage all rollers equally? Doubt it. I know I can't. So you make do with that you have. ;)

Posix, I don't know if the rollers have to be so small, but then again if you want six or so... maybe we are back to looking at less? keeping them reasonably long still gives a lot of contact and hence load carrying capability?

ok if we make them big enough it shouldn't be too difficult IMHO (ok I have some access to a small worksop at work) to perhaps broach those teeth on?

I'm reckoning that no great precision is needed just the requirement that the rollers must not be allowed to slip very far - maybe I'm kinda guessing wrong there?

well particularly if you can use Delrin it broaches real sweetly !! the internal gear teeth - I don't think that the number of teeth would be important? on some of the piccys that internally toothed ring looks like a seperate part - might be best me thinks!

Now I was dreaming the other day of how to use a single point broach to broach multiple teeth around a diameter without using a rotary table. on a 3 axis CNC mill. I must have a go sometime... take too much to try and describe will maybe try to explain on another thread.. if time permits

I have my set of 3 roller screws. But I think that the decision to go for a cheap off-the-shelf M10 "allthread" was a wrong one. I should've asked for a steel M10 allthread to be made on a lathe for me. As it is the roller screw is a bit dissapointing as the cheap allthread isn't exactly to tolerances so it wobbles a bit as the space between rollers and lead screw isn't the same throughout its travel but fluctuates somewhat. Serves me right for not having take this into consideration before I had these made. But it would now be a simple matter of having a precision M10 rod made and then use that for the rollers. The rest can be reused. I think.

Hmm... I have a set of three ballscrews without nuts, maybe should convert 'em to rollers?..

You can't as the ball screw shaft is a "female" pattern -v-v-v. You would have to make a "male" roller -^-^-^ and that would be too much effort to be honest. You need something "unisex" - i.e. it has to be the same on both sides. And the only thing that fits the bill is a seesaw thread like so /\/\/\/\

Much easier to just find a piece of hard rod and start from that.

I have my set of 3 roller screws. But I think that the decision to go for a cheap off-the-shelf M10 "allthread" was a wrong one. I should've asked for a steel M10 allthread to be made on a lathe for me. As it is the roller screw is a bit dissapointing as the cheap allthread isn't exactly to tolerances so it wobbles a bit as the space between rollers and lead screw isn't the same throughout its travel but fluctuates somewhat. Serves me right for not having take this into consideration before I had these made. But it would now be a simple matter of having a precision M10 rod made and then use that for the rollers. The rest can be reused. I think.

Thats a bit sad... the threaded shaft must be consistant throughout it's length.. you've got me wondering how they make that 'studding' I believe though I'm not sure... anyone?... that it would usually be rolled and I'm thinking does that produce a thred that is consistant throughout it's length? if you only want a shortish length then matbe you can get one turned but be careful if it's screwcut without any support on the sharft then that would cause it's own problems.

I was wondering about using Stainless steel - do they still roll that stuff?

Well now you mention the supports when I spoke to this chap about making me a threaded rod with my spec thread depth, angle etc he immediately asked how long and that it should be done with supports - I don't know their proper name but lathe people will know what I'm talking about.

And you know what the saddest part is? All that time I had a 1m piece of 20mm hardened steel rod in my car! :(

Svenakela, I have the same problem. I just bought three lengths of 1 inch precision ground ballscrew, but the nuts are all missing! I did figure out a way this could be done with them, though. If you have plenty of screw (as I do), you could cut three small lengths of the parent screw to make each nut. You then have only to fill the tracks with something round (surface ground mild steel or brass) in the proper cross section on the satellite rollers, and you would have an efficient set of mating rollers. Due to the large surface area, I would probably be able to get away with using 1.5 inch satellites on my 1 inch diameter screw.

Made a model of a recirculating nut (i think this is the one most suited for diy)! Seem very doable! Have to try it :) The problem with this design is that you can't use it as a rotating nut. But i guess you just have to make the spacers of metal and connect them to the housing in some way.

I belive you could get very good results if you use a threadmill and a rotary table on a mill! Easier than doing it on a lathe, and you could make smaller rollers and ID of the housing!

/Jay

Corvus
Any chance of finding springs of the right cross section that would grip your "rollers" as thread fillers ?

Corvus: Isn't irritating, the lack of nuts? ;)
I was thinking of making a couple of rollers that are not threaded, just with grooves, to run with the ballscrews (nope, not hard to do if someone you know have a CNC-lathe). Something like the german design I've mentioned earlier in this thread. My main question still needs an answer: What happens if the rollers stops? Even though the rollers are not threaded the gearing will change and I don't want a "dynamic gear ratio"... ;)
I was talking to a workshop that makes threaded rods, and HOLY CRAP there was a big difference in price when I questioned for better precision. That's why Im going to test my ballscrews, I already have them and they are useless...

JBV: Very nice assembly! Sure you can use it as a rotating nut, why shouldn't it work? As Posix has mentioned, the rollers are supposed to roll, and then it doesn't matter if the nut is rotating or the screw. The relative movement between the parts is the thing that counts.

Cheers,
S

Yeah but in my assembly the spacers are made of nylon or some other plastic and not connected to the housing, and if you turn the housing they almost definitively would slide :) You have to transfer the rotation of the nut to the planetary rotation of the rollers without "losses" if it's going to work predictable and with any degree of accuracy.

Should be easy to integrate the spacer in the housing (and endcap). I'll try to make a more advanced model! Think it would be a good idea to have wipers on the nut?

/Jay

What about using the same kind of leadscrew as rollers and just turn them down and the ends with a groove?

The housing is a bit harder to make but i belive you could make it in separate pieces and bolt together. Perhaps with some kind of preloading of the ridges the rollers rides on :)

Suggestions for the spacer? I have some ideas but they are hard to assemble!

/Jay

Possibly, greybeard- I still have'nt recieved the screws yet, but when I do , I intend to measure them to find out what size they really are- I'm pretty sure they are 25mm X 5mm lead, not 1 inch by 5 thread as the seller thought. That would be a very unusual size- I should know, my Index mill had 1 inch by 5 Acme screws in it, and when I had to rebuild it, I had to have them made, at about six times the price. :( I will probably just buy some nuts made for rolled screw of the same size, and hope for the best. As it sits, it looks as if these screws are going to spend some time on the shelf, next to all the other worthless parts I've accumulated in this project. Having to buy surplus kind of sucks. Its an EXPENSIVE education, but no different than any of the other five hundred other subjects I've become expert in- by doing everything WRONG the first time.

If the rollers and nut are grooved instead of threaded, it won't matter if they slip or not, your screw will advance the same amount, just not as efficiently as if they were rolling. Like JBV's first model, the second would have problems with variable gearing.

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_files/pdf/7551246328.pdf

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

Eric

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_files/pdf/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

Corvus - "Its an EXPENSIVE education" - I attend the same college !
Widgetmaster - that is some link to skn roller bearings. It's going to take a week to read, but I guess it's all there.

hi guys! have any more simple roller screw to build?

from http://www.nookindustries.com/RollerScrew/RollerScrewGlossary.cfm:

"The rollers have a gear and a cylindrical journal at each end. The gear at the roller ends meshes with the ring gear (4) at each end of the nut. This mesh prevents unwanted roller skewing by maintaining parallel axes of the planetary roller and the screw shaft."

Is the only function of the gears to prevent the skewing? Couldn't you solve that by putting the planetary rollers in a frame instead? They can't skew at all then and no need for timing. (you would have to put in the frame into the nut first and then one roller at a time and press the roller pin trough the small holes)

That's exactly why there is a planetary rolling version and a recirculating version. You just described the recirculating one. :)

--S

But the biggest difference is in the threading versus grooving!? Planetary have threaded rollers and nut, and recirculating have groved. I just think those gears seem unnecessary complex if its JUST to keep the rollers paralell to the leadscrew :D

What i try to say is, if you use threaded rollers you must have multistart thread (as many as you have rollers) in the assembly, the nut lines up all rollers and grooves perfecly, and would not mesh with a unmatched leadscrew. That rules out any simple diy versions.

The planetary version could use ordinary threaded bar for both rollers and leadscrew but the gears in is to hard to make, so i want to get around them someway! Is that possible?

I have an ideay of making grooved rollers and a very simple nut (even more simple the German one) where the screw can be single threaded. The bad thing is that the work load on the rollers will be higher and the nut will only take the load at the ends.

Lets say I'm using a thread with 5 mm climb per rev, 5 grooved rollers needs to be moved 1 mm axially to each other. Simply it can be made with 1 mm bushings. If it's for a machine with moderate loads, it could work. Se my very simple picture.

It would also be possible to make a German version-assembly, but then the grooves on the rollers have to be placed differently. On the other hand, if the rollers are going to be made in a workshop lathe it doesn't really matter, they are all made separately.
So my conclusion is: It should be possible to make a roller screw with single threads.

--Sven

I have an ideay of making grooved rollers and a very simple nut (even more simple the German one) where the screw can be single threaded. The bad thing is that the work load on the rollers will be higher and the nut will only take the load at the ends.

Lets say I'm using a thread with 5 mm climb per rev, 5 grooved rollers needs to be moved 1 mm axially to each other. Simply it can be made with 1 mm bushings. If it's for a machine with moderate loads, it could work. Se my very simple picture.

It would also be possible to make a German version-assembly, but then the grooves on the rollers have to be placed differently. On the other hand, if the rollers are going to be made in a workshop lathe it doesn't really matter, they are all made separately.
So my conclusion is: It should be possible to make a roller screw with single threads.

--Sven

At the bottom of post#23 this is what I was suggesting without a picture. I suppose I should learn how to draw so I can include pictures. I also added that it might be possible to include backlash adjustment.

Check this out regarding the "german variant": http://www.steinmeyer.com/english/kataloge/pdf/drs.pdf

On page 6: "The rollers work under "rolling friction" to the screw and to the nut. This fact causes a certain lost motion. Therefore there is no repeatability of
the feed. In case of repeatability is required, we are in need of an additional
measuring system! On most applications, two end switches will
solve that problem."

Seem you would need an absolute linear encoder to use these! :P

In your variant Sven, do you think small missalignments would prehaps remove backlash as a bonus? :)

Ballscrews actually have the same principal, they roll with friction. I don't see that the rolling through friction idea forces me to use a linear encoder. As long as the friction is correct adjusted it should work fine.

I believe backlash is gone! :)

--S

Hello...
Check...
http://www.hobbycnc.hu/CNC/Mech8/Mechanika8.htm

From picture the rollers seems to have groves only (not thread) and does not rol around main shaft. Just rolling groves. That seems to be just like better nut. If sometime slips there is NO gearing efect just a litle more friction I think.

Hey, that looks nice, and if those rollers are grooved instead of threaded, there wouldn't be any positional losses like you said! Too bad a lathe is required to make those rollers, but it's definitely alot simpler of a design than what others have been posting, plus it's fully adjustable so you don't have to have perfect tolerances. I like it.

Hello there peoples... Long time no post :rainfro:

Anyways, I talked to some chaps at a tooling dept of a local (to me) metal working company.

They tried to make something for me and it turned out that 4732 wasn't stiff enough for what I had designed. Also it's going to be very difficult persuading someone to make you a non-standard thread shape (and I believe 90deg or even shallower is what they use on roller screws).

I've tried an M14 lead screw with 8mm "sattelites" with the same thread profile as M14. Basically an 8mm sattelite isn't tough enough and it bends on the lathe just as you try to do the final pass of the thread. So I'll either have to find a MUCH stronger material (even though 4732 should've been strong enough) or re-design with something thicker, like 10mm or so. But then the whole lot becomes very bulky indeed.

Anyone in the metals industry fancy taking a stab at guessing what materials SKF or whoever might use for their roller screws? All I know is it looks shiny but nothing other than that.

Did you use M14 fine or coarse? The look of the SKF stuff seem to be more like fine :) Parts is also is precision ground (of pretty hard steel i would guess :))

/Jay

"pretty hard steel" won't cut it this time, though :o I need names and numbers.

And skf stuff is coarse thread rather than fine and even though it may look like it it's 3 (or even 5) coarse ones packed together onto a single lead screw. And they don't seem to be standard 60 deg threads either but more like shallow 90 deg jobbies.

I have been thinking of doing some tests with the simpler variants like in the Hungarian? page... M20/M16 Fine as leadscrew and M10 Coarse as rollers! They have 1,5 mm pitch so they should mesh just fine! There is room for up to 8 rollers around the (M20) leadscrew...

Do you think it would remove backlash if i used long rollers? Like 10cm or so :)

/Jay

It's the way they mesh that is vital to removing backlash rather than the length of rollers so it's all in your thread profile really.

M16 fine and M10 coarse is an idea but you should really think about sizes of the finished thing as well. But it's worth a try anway :idea:

Posix - sorry to be a pain, but could you point me to a drawing or description of the arrangement of the rollers/screw that your going for ? I've gone back through the thred but I'm not sure of your current thinking.
Jay - are you following the same path, or a variation ? I can't spot the "Hungarian" reference.

John

Have a look at those photos at the top of 2nd page in thread. That's my current "prototype" setup. Basically an arrangement of 3 M10 "allthread" rollers at 120deg apart around a central M10 lead screw. I'm working on something more elaborate (steel lead screws and rollers being made as we speak) with lots of drawings and things.

I was thinking that if you have longer rollers, the pitch errors in the rollers makes the meshing tighter than normal in some places (and ofcourse looser in some)...

This is the page i was refering to: http://www.hobbycnc.hu/CNC/Otletek/Otletek.htm

Thanks guys, I'm up-to-date now.

re backlash in general. If the three rollers have a small degree of elasticity, ie are made from something like delrin/hdpe, and are 'preloaded' by adjusting their position end to end with reference to each other, would this be a suitable way of reaching a compromise between backlash and friction ?

Imho adding to the length will add highs and lows in proportion to length. Reducing the length to a minimum might be argued to be a better approach.

I've been struggling :rolleyes: with how to get three single balls to replace the three rollers, having aquired some 2mm ball bearings. Initially I was thinking of building a ball nut of my own design to roll round M16 studding, but then you started this thread !

I've just found those ball bearings, and measured them - they are only 1.5mm.
The little grey cells clicked, and I'll try and draw this up tonight, but for now -

M16 threaded leadscrew
thin annular ring of phosphor bronze retaining three 1.5mm diam balls, each at 120 degrees and offset to sit in the thread
outer sleeve with three grooves inside, each ball running in its own groove.

Possibly duplicate the ball/groove set up with arrangement for 'preloading'.

I've no idea how much torque this could transmit, but I have the balls/M16 studding, but so far no idea how to make the outer sleeve without a lathe set up.

That's a ball screw. Get out of this thread! :)

No,no......(cries of innocence against hard-hearted posting)... I've just shrunk the rollers dear.

But anyway there would be so much friction between retaining rings and balls (balls would slide against walls of holes on retaining rings) that it wouldn't really be usefull as a proper ball or roller screw and you would esentially end up with just a screw and nut combo. Think no friction. Think rolling. Think no sliding. :wave:

OK Boss :idea:
(exits stage left with clean drawing board, a single ice skate, and a mobius strip made of teflon)

Did you get far along that strip yet?

Started to worry when I met myself coming the other way............

Me too................... worried that is lol, Have been watching this thread since joining this community, can whole heartedly say, very interestiong ideas here :) I need these roller screws (homemade ones) ................

Dunno how I going to make em thou lol, not lathes no machining facilities etc, all by hand arggggggggggggggggggggg.

Ideas fronm you folks thou makes me think much, not sure thats such a good idea thou lolol.

Mike

I've always passed this post by, but today I decided to take a look. I didn't follow all the links and read all the PDFs but it seems to me that you could combine the original prototypes/trials by POSIX (pictured in posts #13 & 16) with JBV's design as shown in post #79.

:confused: Why can't you have two rings of 3 rollers that alternate with each other? The rollers on the inner ring would be threaded and "ROLL" to match the All Thread/ACME Threaded lead screw (POSIX) while the outer ring would be comprised of solid rollers that would "ROLL" in a groove(s) (JBV) in the outer housing. The diameter of the outer ring would only have to be slightly larger than the inner ring. Just enough to keep the threaded rod away from the outer housing and the solid rollers away from the lead screw in the center.

Does this sound plausible? Do-able? Am I missing some key concept?

Can someone with better (okay, faster) design skills mock up a drawing? This seems like it would be easier to implement for the DIYer as it doesn't involve bizarre thread sizes with multiple starts, etc.

Just a thought,

HayTay

Yo yo yo people!!!

I got some updates coming this way soon!!! AND GREAT UPDATES!

I have been at the machine shop (umpteenth by now) and have seen the first of three roller screws that they made for me.

All I can say is *THUD*


Erm...someone dropped an anvil on my head - I guess I'm supposed to keep quiet about it untill it's all done and dusted...

Can you wait till thursday

I promise this will be a cracker!

Thursday's come and gone, we're waiting with bated breath!! lol

Posix

We are looking forward to seeing what you had made. We will be here when you're ready.

I was trying to avoid this thread due to possible vigelante attacks and excessive amounts of violence that loomed upon me...

Oh well, it looked promising. Then I asked to do one more. Then everything went downhill from there....

I'm going back to the shop tomorrow with a bigger rod.

Basically, their sliding thingy couldn't cope with a 12mm rod that I wanted to make a screw from. And the lead screw came out bent, threads were misaligned (we are talking 10s or several 100ths of a mm), rollers were out of position (a 10th here, 10th there) etc etc. So I just broke down and couldn't take it anymore.

The chaps at the shop asked me if we could give it another go this time with a bigger radius rod so that's what we'll do. 20mm instead of 12 this time. And 9mm pitch. This should be fun. I will get back with pics as soon as I've got something working.

It sort of showed promise but then dissapointed big time. And it was all because of stupid fractions of a fraction of a millimetre! You know the feeling - when you're on this high, when you have a hunch that everything will work out and then everything falls apart because of a stupid 100th! GRRR!!!!

They made me a teflon nut for one of the lead screws they made for me to take home and play with till tomorrow (4.5mm pitch, 3 starts) and, albeit with some binding (remember, threads were off ever so slightly?), this thing really flies with my puny steppers! Imagine what a 9mm pitch, 5 starts, roller screw should be able to do? Wheeewwww.....

Oh and BTW - Delrin DEFINITELY CANNOT take any loads of any kind! Full stop!

I would also ask any metal guys out there to recommend an alloy for the roller screw assembly. We tried stainless steel (could it be 430?) and even though it looks pretty I have a feeling it's on the soft side. One of the numerous tool steels perhaps? 1530? 4532?

Why do I have an eerie feeling that everyone hates me right now?

Although I have nothing to contribute to your questions, I by no means hate you, and respect all the time and effort you've put into this device! :) While time is not completely on my side, I do have the patience and confidence to wait for what you come up with.

I, for one, will patiently wait until you have something to show. Thank you for all the time you have put into this project.

we are all waiting like vultures over a dehydrating and delerious posix...... will he make it to the watering hole or will it be ...... just kidding and waiting to celebrate with you on a major accomplishment. no matter how much it took to get there once you have arrived, your there!

Amen from me too. Lots of luck.

In the interim of posix getting his thing done, I present unto you a roller screw design I've been working on, and plan on prototyping if I ever come up with a CNC frame I like (don't hold your breath). While it looks complex at first, I tried to design it from the aspect of as little complex machining as possible. The three "planets" around the central 1/2-13 UNC rod use a straight grooving instead of a thread/helix, so that there aren't positional errors due to minute slipping between the planets and central screw. Of course, you don't get the speed multiplication that threaded planets provide. The planets have inlaid bearings (cheap from VXB) which ride on 1/4" rod. I couldn't find inexpensive thrust bearings, so I figure take a pair of nice metric washers (the bore on an M6 washer is a perfect fit for 1/4 rod) and sandwich some small BB's, while fabricating a retaining ring out of thin sheet metal, ideally brass. Finally sandwich it all between some angle and it's good to go. If the holes in the angle supports are oversized, it should be possible to bring the planets into contact with the screw with an additional light preload.

Hmm...zach_g you have no idea how much grief you've caused me with the statement "straight grooving instead of a thread/helix, so that there aren't positional errors due to minute slipping between the planets and central screw. Of course, you don't get the speed multiplication that threaded planets provide."

I have completely forgotten to think this through. I mean, the rollers in my mechanism roll even without pinions etc but even the slightest possibility of them slipping induses a positioning error just because the roller is threaded and no longer runs at a predefined ratio to the leadscrew but instead is standing still and slipping which makes the whole mechanism run slower (for the pitch of the roller per revolution). And that's a big issue!

Only now am I beginning to understand why is there a planetary grooved version as well - it IS slower but slippage doesn't matter.

Ugh!

Sorry to hear that. Not to come off as rude, but the problem of positional error due to slip in threaded planets has been brought up alot previously in this thread.

Maybe you could adapt your design to use straight grooves? If you had a 3 start screw you could arrange 3 planets inline with each other and use a straight grooved housing. 4 start could do 4 planets etc. Or maybe have a single start thread, and groove your housing with a pitch 1/3 that of the rollers or space out the grooves on the planets and housing and have each planet offset from the others.

While my posted design might be easier to make, it has a high part count. Yours is definitely simpler in that respect, and I hope it works out! You could always use a linear encoder too.

hey zack_g
howdy!
how many rollers could you fit on a 1" shaft? with whatever thread, course or fine would give you near 20 deg. of pitch ? and while you are at it lets make the assembly fit into a hollow center pulley so i can drive it 10' up and down my fantasy plasma table(sheet steel is 4'x10' most of the time on the turd coast). man it would be sweet to hold the shaft stationary and drive the gantry from a motor and pulley arrangement! oh and how about make half of the rollers shafts 1/8" longer with a washer and split nut to take out any extra clearences that might arise from wear. can you visualize the things i mentioned?

You could probably fit 6 rollers around a 1" shaft, but 3 is all you need really. There are plenty of points of contact for load handling. Using a wave washer would be a good idea to compensate for wear, although you could never apply a load larger than the spring force of the wave washer, which would limit the acceleration on a plasma table. Mount the whole thing between 2 big angular contact bearings and you should be good to go. I'm sure it will be a lot of fun to align everything concentric, of course if you already had a machine you could cnc some accurate plates to hold it together.

As far as what screw would give 20 deg pitch, isn't pitch usually measured in length between adjacent thread teeth?

im not knowing of the pitch question, could the angle iron be converted to a round shape(or pipe) that would fit between a pulley and the 1"shaft? would side loading from the motor torque and belt tension create drag and further slow the velocity? would reversing the direction compress the wave washer? etc?etc? ad infinum.. too much conjecture occupies the mind. remember to swallow!

I have been thinking of try to build something like this for my mill... There is very little height space for the X axis nut (23 mm) but my design is only 22 mm for a 16 mm leadscrew :)

1: Spacers/washers of different thickness to line up the rollers with the leadscrew.
2: Lock nuts to thigten the rollers so they can roll easily but not move axially.
3: Brass bushings.
4: Washer.

Think it would be possible to turn the rollers (10 mm diameter) on the mill using a endmill holder and a lathe tool held in a vise?

/Jay

Whenever I think about the "sliding and gearing problem" I forget to take one thing into account:

Does it actually matter if one roller slides in a 3-roller arrangement when you've got 2 other rollers that are still rolling? I know rolling resistance will grow. But will the gearing be affected?

Can someone elaborate on this a bit further?

The problem with the threaded/helical planet roller screw is that it doesn't have quantized positional steps as compared to a grooved planet. The threaded planet rollerscrew relies on rolling and the interlocking of the teeth on planets and screw to advance on the screw, while the grooved planet only relies on the interlocking between the planets and screw. The rolling of the grooved planets simply reduces friction, and doesn't advance the rollerscrew. The analogy would be using a friction wheel or a rack and pinion to drive a table. While both would position the table well, only the rack and pinion would have repeatability in position over a long period of time.

Well I would beg to differ.

Let's have an example here:

20mm threaded lead screw.
9mm lead, 6 starts, 1.5mm thread profile

8mm threaded roller
1.5mm lead, single start

Now, for every single revolution of the lead screw the roller has to make 6 revolutions in opposite direction to keep up "on thread"

Can a physicist or a mech engineer among you come up with some formulas to calculate the advance the whole assembly would be making for a single revolution of the lead screw. I know that if all rollers were sticking it would be 9mm. But what with rolling rollers?

I know that in some of my simulations on a 3-start 12mm 4.5mm pitch with 1.5mm thread profile leadscrew the actual distance travelled was 10mm for every revolution. I don't know how I came by that number but it seemed to be the only number where everything would interlock properly. All others were off somehow. I don't even see what relationship that number has to anything apart from being:

lead screw pitch+(roller pitch*number of revolutions of roller for every leadscrew revolution)

But that's not the correct answer I'm sure. Can't be because the roller rolls in the opposite direction so that would come out at -1 rather than 10.

Well I would beg to differ.

Let's have an example here:

20mm threaded lead screw.
9mm lead, 6 starts, 1.5mm thread profile

8mm threaded roller
1.5mm lead, single start

Now, for every single revolution of the lead screw the roller has to make 6 revolutions in opposite direction to keep up "on thread" ....

I think your last sentence has an error; The lead screw is 20 mm, the roller is 8mm so if slipping does not occur the roller turns 20/8 = 2.5 rev for every rev of the lead screw. Actually it will be a little different because the screw and roller make contact at some sort of pitch diameter which is a bit less than the O.D.

So for every lead screw revolution forward you move 9mm but the rollers have turned 2.5 rev back which is 3.75 mm so your net movement is 5.25 mm.

With the threaded rollers it is important that all turn at the same rate hence the fancy planetary gear train.

With parallel grooved rollers they do not have to turn at the same rate because the "lead" is zero. Actually if you locked all the rollers it would be a regular nut. So with these your motion is whatever the lead on the lead screw is.

Are you 100% on this: "the roller turns 20/8 = 2.5 rev for every rev of the lead screw". I can understand this for a single start thread on both pieces but this time we have a multiple-start threads on the lead screw and a single start on the roller.

The lead screw has a 9mm thread pitch and the rollers are 1.5mm so can you imagine the way their respective threads interlock and the resulting rotations? I can but only with the help of some motion animation.

Ok I've tried to simulate your scenario and OF COURSE it doesn't work [:D]

But what DOES work is that you actually add up the values 9 and 2.5, then it works.

As I said (on numerous occasions before) it is very difficult to visualise the way this thing works without either looking at it in flesh or having it on screen turning.

Here is a quick animation for you (divx) :cheers:

If no slippage occurs the two parts rolling against each other have to turn in the inverse ratio of their diameters. I don't see how it can be any other way; they have to maintain the same peripheral speed at the point of contact.

I think the multistart thread on the lead screw is just needed to keep the helix angle the same at the pitch diameter.

But I did have a mistake; the rollers rotate in the opposite direction to the lead screw but the two leads do not subtract they add so the net relative motion between the lead screw and nut assembly is 9 + 2.5mm = 11.5mm

Ok, fine. But now what happens if one is slipping but the remaining 2 are still rolling? Or if 2 are slipping but the 3rd is still rolling? What then?

I guess what I'm after here is for someone to tell me that there won't be any positional errors if slippage occurs and as long as at least one roller is rolling.

Or put another way - this might still work without the pinion arrangement.

There will be positional errors if slippage occurs. Think about a single roller situation: when both screw and roller are rolling the net relative motion is the sum of their individual leads; if the roller stops the net relative motion is the lead of the screw. Now move to multiple rollers; when one slips a little it starts to fall behind the others but they are in a fixed assembly and all have to move axially at the same rate therefore binding occurs.

As I mentioned earlier with parallel groove rollers the net relative motion is the same whether the roller is rolling or fixed so with these some slippage does not escalate into a binding situation.

The slippage being referred to isn't where one roller might lock up, it's the minute shifting at the point of contact that all the rollers exhibit while rolling. It's inevitable that there will be positional errors. If you don't believe me, believe Steinmeyer in their pdf on roller screws http://www.steinmeyer.com/english/kataloge/pdf/drs.pdf

"The rollers work under "rolling friction" to the screw and to the nut. This fact causes a certain lost motion. Therefore there is no repeatability of the feed. In case of repeatability is required, we are in need of an additional measuring system! On most applications, two end switches will solve that problem."

Sorry, I know it sucks to have your design that you've put alot of effort into not do what you thought it would do. All is not lost though, you can still modify your design to work, I posted a few ways earlier. The key is to use grooved planets instead of threaded, that way the lead won't depend on rolling friction, but you still get the efficiency of rolling friction like in a ballscrew. I think the binding issue Geof presented is also a problem, unless there was a retaining ring that kept all the planets equally spaced like you see in alot of commercial varieties.

I think your last sentence has an error; The lead screw is 20 mm, the roller is 8mm so if slipping does not occur the roller turns 20/8 = 2.5 rev for every rev of the lead screw. Actually it will be a little different because the screw and roller make contact at some sort of pitch diameter which is a bit less than the O.D.

So for every lead screw revolution forward you move 9mm but the rollers have turned 2.5 rev back which is 3.75 mm so your net movement is 5.25 mm.


Wow, someone agrees with what I pointed out. Gear reduction occurs when rollers are allowed to roll.

Some thoughts. 1/100 of a millimeter is getting pretty small for deviations (1/10 is getting p there, but still only .004"). Lapping compound with a run in period and then a final cleaning and lubing will normalize all/most inconsistancies. Posix you had one of your attemps traveling less than you thought it should, but with the gear reduction, it might have been as it should be. Have you managed to use a precision movement of the screw over a distance and then back so that a table contacts an indicator? This would tell you if possitional errors were building up or not.

One screw slipping on a multi roller set up is impossible if they are threaded, but not grooved. For any threaded roller to slip, "all" rollers have to slip. If they didn't, they would be out of time with each other and would bind. All lash would be removed to the point of nut lockup to the shaft. You cant adjust lash by moving threaded rollers front to back, just in or away from the main shaft. Moving the rollers forward and back would just make them roll to the new engagement point (carried over from another thread).

I'm thinking that if the roller carrier had one roller spring loaded to the screw, even if there were irregularities it'd ride through over them and maintain a steady preload. The rollers would never be loose enough to slip (hopefully).

Interesting thread, I just read all 134 posts and there are a lot of good observations.

I have to commend Posix on his enthusiasim and tenacity. In his rush to improve the state of the art he has overlooked something. As he says, visualizing how it works is difficult. The hardest to visualize is the contact path of the helix.

By the very nature of a screw thread, in order to advance a nut (of any type) there must be a thrust along the screw axis, that means that the flank of the screw thread must push against the flank of the nut thread. In an Acme thread that is the full face of the flank. In a roller screw the peak of that force is at the plane that runs through the axis of the two screws. That plane defines a line contact. That means there must be sliding contact between the flanks the threads.

When the mating thread is rolling (as in a roller screw) the contact is smaller ( a line versus a helical plane) so there is considerably less friction than say, An Acme screw. However there will be more friction than a ball screw.

When a ball screw advances a ball nut, the contact path along the flank of the ball screw is a point contact. Any plane tanget to a sphere results in a point. So the compression force between the flanks of a ballscrew/nut increases the force on the balls, causing the balls to rotate, resulting in less friction. Any intermittent contact between adjacent balls will not cause any measurable drag due to the low forces involved.


Having said all that, I think persuing the roller screw idea as a DIY project is great and I would like to help. I just want to make sure we know what the results will be - a leadscrew that is less efficient but has more load capacity than a ball screw.


Posix, If you want to forward me your design info, I will see if I can make up some prototypes for you in my shop. We are currenly running a batch of small leadscrews and I might be able to squeeze in some for you.

BTW, I have designed roller actuators and it took me weeks to get a grip on the whole helical contact path problem.

I, too, commend Posix for his tenacity. But I now wonder if he and all the contributors to this thread have been overtaken by a new development in the UK.
Marchant Dice is advertising on ebay various lengths/ sizes of "trapezoidal acme ballscrew leadscrews".
This is obviously a universal answer to all the problems - trapezoidal balls, anyone ? :rolleyes:

..erm....maybe the trapezoidal refers to the rod being bent into a trapezoid so you get instant x-y motion with just one leadscrew?

Not 100% sure. I think that accross the pond in England trapazoidal screws meen acme thread.

Here in England Trapazoidal Screws are indeed ACME Threads, we always like to make the words as long as possible ;->

Here in England Trapazoidal Screws are indeed ACME Threads, we always like to make the words as long as possible ;->

Probably because The Acme Pewter Tuning Fork Company had tied up the tradename.

Why do I have an eerie feeling that everyone hates me right now?
Are you kidding me? I think I might have more respect for you than anyone on the plannet right now! You had an idea, tried it, spent big money on it, failed, kept trying, spent money, kept trying. That's awsome! I have learned more from this thread than any other on this board!

Geof - is it one of these we need to tune up our servos ? :D

Hi everybody.... Well, I will try to help from other point of the world.

I don't Know if anyone asked it, the helix of the roller and the screw must be in different direction? ...if not, will it have friction problems?...:confused:

Try to put two screw together you will realised that move in an angle to get the tread fix correctly.


I was playing with CATIA And get this pictures..

Glossary: comment Spanish, (English)
right thread = clockwise
left thread = counter clockwise

also I made a formula to get the correct fix for the thread angle, considering the different diameters.

Geof - is it one of these we need to tune up our servos ? :D

Who was the cartoonist who created The Acme Pewter Tuning Fork Co? I am under the impression he was British (English)?

Posix;
There are many trite sayings about he who never dares never gains and all that stuff; mostly becasue in general they have a large element of truth. Sometimes after chasing an idea with lots of time and sometimes lots of money you realise not only is it not going anywhere it never could go anywhere and feel somewhat chagrined. However, if you never pursue anything you never get anywhere. I know on balance I am ahead with the return from ideas that did go places far outweighing what went into the ones that stalled. Probably most people who persisted are.

ggg: your roller thread is wrong way, both lead screw and rollers have normal, clockwise screw (right-hand?) threads.

geof thanks for the kind words of support but you did hit the nail on the head there by assuming I'm somewhat discouraged by this whole malarky.

It gets very VERY frustrating having to travel 10-20 miles to get something done on a lathe only to discover that the holes are 0.1mm out or somesuch when you need 0.01mm accuracy. Well, ok maybe 0.01 is too much to ask for but 0.05 is the biggest mistake this whole shebang will tolerate. More than that and it begins to wobble allover the place.

Yesterday I gave up completely and ask those guys to just make me 3 stupid acme rods and nuts so I can get this machine finished. Once I'm (hopefully) up and running I'll look into getting a 4th axis (somehow) and experiment myself. It'll be cheaper (at least no petrol costs and wasted materials at least) and quicker (cnc).

I, however, do believe my design works. I have built-in some adjustment to get rid of slip completely obviating need for teeny-weeny pinions but I need parts machined to exact specifications rather than 0.1mm out here and 0.1mm out there. 0.2mm is lots of slip, lots of play, lots of backlash, lots of waste above all, both money and time-wise.

I will be putting this on a back-burner for a while, at least untill I have a working machine to work with. But even THAT is proving difficult. I think these acme leadscrews will prove to be too much for my steppers, 20mm steel rod (their linear support can't take rods less than 20mm diameter!) 500mm long, weigh a ton. They did promise to make me an aluminium lead screw if this one can't be moved by my steppers and I suspect that will be the case...

ggg: your roller thread is wrong way, both lead screw and rollers have normal, clockwise screw (right-thread?) threads.


But if you have both normal , clockwise screw, you will have two contact points not axial aligne . The roller thread will try to get an angle to correct fix with the lead screw thread. is that a problem?
If you have different ways you roller an the lead screw forces work aligned so you will have less friction.

Originally Posted by posix
ggg: your roller thread is wrong way, both lead screw and rollers have normal, clockwise screw (right-thread?) threads.
----------------

If i'm understanding this correctly (perhaps i'm not but i think i am), having opposite threads, two equal sized rods whose threads were meshed would result in no linear movement as the driving rod would drive the other rod in one direction, but the driven one would "climb" the driver at the same rate it is being driven. (hope that made sense)

if we used this arrangement with satellite rollers that were smaller than the driving rod it would result in linear motion in the opposite direction of what we're used to. and if the satellites were larger it would go in the traditional direction, but with a speed REDUCTION. Also ggg's "backward" threads will will have forces axially along the rods in BOTH DIRECTIONS ALL THE TIME. Whereas traditional designs adjust out the backlash, this design FUNDAMENTALLY HAS NO BACKLASH.

So in theory, we could set our ratio of rod diameters to give us a very fine feed for a highly accurate machine.

A minor problem that i see with this arrangement would be the slippage between rods as always. I say "minor" because the greater friction would reduce slippage, and if our ratio is set to give us 1/10,000" feed and we moved 1/10,000" we'd have less slippage than that.

the major problem i see is finding someone to calculate the exact diameter of contact between the rollers so we'd know what our "gear ratio" was. And then the tolerances to which they would need to be machined to get anywhere close to that.

I hope i didn't post any wrong information, my head is starting to spin from thinking about how helixes interact :)

Hi Guys

What a thread! My eyes are burnt after reading all of these posts. I have been investigating the roller screw for a while now and have considered the following points which may be of interest to those considering making their own roller screw. The attached file was given to me by a friend with about 25 years on me. He knew I was into hobby CNC so he thought it may be of use to me. Sorry I can't quote the mag it is out of but the article was written by John Jardine. Thanks John wherever you are!

Using a roller screw which is as per attached file ie same thread used for rollers and main screw result in a lead multiplication of theoretically 2. Practically if you were to make this design you would use phosphour bronze rollers so as to minimise wear between the roller and main screw.

Using "All Thread" which is commonly available from hardware stores have relatively fine pitches (16mm, 2mm pitch for example) which when used with a simple nut severly restricts your rapid speeds. Having the roller design with a 2 x multiplication factor bring this back to 4mm (as a example using the 16mm, 2mm pitch thread) which gives reasonable rapid speeds.

"All Thread" is not well know for its pitch accuracy.

"Proper" roller screws (likewise for ball and leadscrews) are made from engineered steels made for high surface pressure use and are hardened with processes other than quenching when it goes cherry red. Most of us aren't going to be able to build a decent roller screw which will, performance wise, compare with a rolled ball screw.

You can buy precision rolled ballscrews from homeshopcnc very cheap (I'm not plugging them, probably other sources as well???) which have a lead accuracy of 0.1mm per 300mm which for hobby cnc is more than acceptable.

Go through this exercise and you will realise that the pain of researching this subject is for the fun of it alone (I did it, I had fun and learnt something new) and not a practical cost effective solution if you consider your own time and the finishing of your machine in a shorter time period valuable.

BTW my current machine uses 12mm x 1.75 all thread with 2 nuts in a wood housing separated by a spring. I'm not anti "All Thread" I just know that the next step is rolled ball screw and not making my own roller screw.

Cheers

Derek

I was really trying to get away from the bulky rectangular arrangement but that seems to be the only "diy"-able solution. Everything else requires a lathe. Not saying that this doesn't require one either but at least it looks a lot less forgiving!

I just found these intersting videos of the internal working of SKF roller and ball screws. I thought that others might like to take a look:
http://www.linearmotion.skf.com/en/0/cat-cat1829-Roller_screws.html
http://www.linearmotion.skf.com/en/0/cat-cat1830-Ball_screws.html
Under "related links" near the bottom of the page.
Check out the recirculating one. The rollers accually shift a thread each revolution!

Neatman

Thanks Neatman, cool videos. Don't forget to check out the PDF's as well. Heaps of interesting reading.

Cheers

Derek

Is there an explanation for the "jump"?

Is there an explanation for the "jump"?

:cool: if you look carefully in the "reciculating roller screw" the roller hasn't thread, only are "V" slots and the external thread have only 1 lead.

Looks like the "jump" corrects the relation between the roller an the external piece thread and the roller never goes out.

doesn't make sense. grooved rollers don't "go out" they don't have the gearing problem as threaded rollers. why would they complicate something needlessly?

It's so that the outer ring can be threaded instead of just grooved, which would allow gear ratios. There's probably other benefits, but yeah it does seem somewhat overly complex when it could be so much simpler. I'd imagine that design would have problems with high speed operation vibration and noise levels. Kinda neat how it works though.

doesn't make sense. grooved rollers don't "go out" they don't have the gearing problem as threaded rollers. why would they complicate something needlessly?

....could be a construction simplification. .... with the thread in the ring allow the ends nuts to be adjusted to avoid backslash.

remember also that with the tread in the ring the advance is higher.

The designs of the assorted commercial roller nuts discussed here can perhaps best be understood by considering them as what they are: planetary systems, directly analogous to planetary gearing.

The rollers are the planetary gears, the screw is the central pinion, and the nut/casing is the ring gear.

Sliding contact is inescapable since the three elements are all achieving the same pitch with different diameters, which means different "unrolled length" for the thread.

If we have a screw with a pitch diameter that has 1 inch of surface per revolution and planetaries that have .5 surface inches per revolution, and both elements have the same thread pitch, then in one full thread revolution there is .5 inch of sliding contact between the two members.

Sliding contact becomes much more extreme between the .5 surface inch planetaries and the outer nut, since the nut *must* be larger at pitch diameter than either the screw or the planetaries, but the thread pitch locks all three elements into rotating at the same "pitch speed" regardless how many inches of surface are involved in achieving that pitch length.

This is the reason why the higher grade, higher cost commercial units use not threaded rollers, but rollers with mating annular profiles. The planetaries become "decouplers" that unlock that thread pitch coupling.

The tiny gear teeth on the ends of the rollers (and the ring gears that drive them) serve no purpose other than to distribute the inescapable sliding contact so as to regularise wear and optimise part life.

To get full rolling contact with no component of sliding (apart from tiny sliding values as your two meshed arcs engage and separate), you would have to use rollers the same diameter as your screw and eliminate entirely the outer nut/casing.

Personally I'm rather amazed that no one has mentioned at all using rolled thread acme rod. The low 14.5 degree pressure angle would be a natural for this application, wasting far less input torque into deflection forces as compared to any 60 degree thread form.

The acme rod would also be far easier to support within the nut assembly since the flat crest would make possible use of independent plain rollers for support that could spin as fast as needed to match the surface inches of the screw major diameter.

I'm fiddling about with a CAD design for a compact acme unit, may post it later if there's interest and nothing interrupts my play time ;)


EDIT> had to correct the acme pressure angle. it's 14.5, not the full 29 included angle (duh)

Tiger

Go on, grandma, hit me with your best shot! :D

btw, that's grandpa :P

Barring snafus, renderings should be up in a few hours



Tiger

The Exlar stuff is quite expensive but you can get big forces and long life. Kerkmotion is low cost, low force. For similar sizes you are probably looking at a 10 to 1 or greater difference in cost between Exlar and Kerkmotion.

Personally I'm rather amazed that no one has mentioned at all using rolled thread acme rod. The low 14.5 degree pressure angle would be a natural for this application, wasting far less input torque into deflection forces as compared to any 60 degree thread form.

Tiger


Hey! Reread the thread, will ya?! ;)
Acme/Trapetz was one of the main discussions in the beginning of this thread. But I'm glad you're putting it back in focus. :)

Cheers,
Sven

btw, that's grandpa :P

Barring snafus, renderings should be up in a few hours



Tiger


few hours, few days, what's the difference, right?! (nuts)

Barring snafus...

The riders and codicils often determine ;)

Had a malf, lost all the work, starting over from scratch.



Tiger

Well, life got in the way and it took longer than it should have, but herewith the low grade renderings of the roller nut.

These are from a dimensioned dwg sized to fit under the table of my old bench mill. Had to reproduce (mostly from memory) an 8 yr old file that got irretrievably mangled when I tried to load the autocad 12 file into turbocad ;)

The idea is pretty clearcut: the hardened shoulder screws become off the shelf pinions for the rollers, the bearings are push (not press) fit in the bores and allow the screws to apply axial tension in opposite directions, taking up all backlash.

The rollers are from LaSalle ETD 150 which has a RC 32 hardness, 150k yield and machines very cleanly. They're annular, not threaded. The acme was 3/4" 8 tpi.

I know, the setup is not suitable for extreme loads or speeds but my goal was purely zero backlash in a design that was adjustable to maintain that condition. At the time I was doing nothing but specialty one-off and prototype work, the sort of thing you do slow, careful, and precise.

http://www.cnczone.com/gallery/data/500/medium/pinion.jpg
http://www.cnczone.com/gallery/data/500/medium/assembly.jpg

But this would call for very precise holes for those two bolts to be drilled in relation to the centre of rotation of the leadscrew. Very difficult to do DIY, wouldn't you say? Or are those bronze bearings eccentric off-the-shelf?

Also do you not need at least one more roller to keep everything paralell or are these two enough?

It was DIY for me. I already had the mill and lathe and was doing precision one off work :)

Didn't have room for the third roller, hence the odd design. The bushings pivot under load and the advance of the chord segment brings it into contact with the crest of the acme threads. Of course they pivot in opposite directions, so there is a total of four points of support for the acme. Passive self adjusting containment to keep the acme centered between the rollers.

This isn't really intended to be made with a hacksaw and an electric drill. Since so much else discussed here has been farmed out to local machine shops, this didn't seem particularly out of step with the thread.

The parts involve some tedious lathe and mill work, but there are only the housing, bearings and rollers that are complex at all, and should be well within the capabilities of anyone competent to be doing business as a machinist (imo, anyway).

I only had three things in mind with the design: low cost, zero backlash, and fitting it in the available space on my mill.



Tiger

ok ok show off grandpa!

so no biggie then. I'd only suggest either eccentric bushings or perhaps a slot cut for one of the rollers so it can be adjusted in case holes are misaligned or something?

one more thing, how are the bronze bushings when it comes to face-to-face contact between roller face and bushing face and bushing face and case face? this is where some would call for a thrust bearing since these surfaces are for transferring loads from leadscrew to the table. Would delrin do ok in this area? Or stick with bronze? finding a thrust bearing to fit in that space would be either a long search or an expensive one, pick your favourite...

That's the beauty of it. As long as the holes are parallel there's a few thousandths wiggle room on the spacing. All you lose if your hole spacing is off a bit is a couple or three thousandths of root/crest clearance or a bit of tooth depth engagement.

The housing is only 3 inches end to end. Anyone with a dec