A different (better) kind of screw

[svenakela]

Originally Posted by jans123 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

[Pat2000]

Originally Posted by svenakela 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 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... heeelp

Pat

[Geof]

Originally Posted by jans123 ...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.

[jans123]

Originally Posted by svenakela 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

[posix]

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.

[svenakela]

> 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

[posix]

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.

[rallen19]

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!

[greybeard]

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 ?

[jans123]

Originally Posted by posix 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.

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.

/jan

[Zach_G]

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.

[Pat2000]

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