Nexen Roller Pinion Drive

[Bobbyr70]

Just wanting to know if anyone out there has set up the Nexen roller rack and pinion on any of their applications. I was rather impressed with the design and it's simplicity. It's kind of pricey but so is anything that is precision and accurate,
I am starting out with a standard cheap 20 deg. rack and pinion set up for the cnc table I am building being it is a money thing.
I plan to set up a split pinion to eliminate the back lash. I am hoping it will last long enough to be able to afford the upgrade to the nexen rack set up later.

[Mike Everman]

I haven't, but I'm replying because I made one quite a while ago, like '96. I wasn't satisfied with how large the pitch diameter would be on this concept when there are over 3 rollers, so we NC'd a rack similar to the nexen, but generated for two rollers per pinion, and then belt connected two pinions 90 deg out of phase.
Probably not what you're wanting to know, but I was feeling chatty. ;-)

[Bobbyr70]

Thanks, could you pass along the design? It sounds like just what i am wanting to do as well. Do you have some dxf files of the rack? I want to replicate the profile and make my own and give it a try.

[Mike Everman]

Sure. I have to poke around the server at the office. I'll get you something by tonight.

[Mike Everman]

It was cool going through a bunch of old ACAD drawings. Designs from years ago...

This is made for .75" cam followers. Effective pitch diameter of the "wave" rack drivers is .825", which translates into far more force per torque than the nexen.
The motor was mounted to one, and a belt drove the other. Worked pretty great. I am going out to one of the sheds this weekend and see if I can find all of the hardware and take some pics.
This file has all of the part details. I think that the linear bearing was a THK SSR 25mm.
I did a fancy Lisp program to generate the wave geometry, which I think is lost to me now. I don't know how I would do it in solidworks these days!

Fun fun fun!

[ahren]

Mike,

Really cool design! Did you ever measure the backlash of the system? Would be interesting to know how it compared to the claimed 20 micron backlash on the Nexen.

Thanks,

Ahren

[Mike Everman]

Thanks! As ever, extreme low backlash will be about fit and finish. The one I made was neat, but not great. I'd estimate the backlash on it was maybe .002-.004".

You need to build in backlash to account for geometrical inaccuracies in several elements, or more precisely, the fidelity of the hardware you make will limit the backlash you can achieve.

You need:
1. Internally preloaded rollers, if there's slop in the cam followers, then they are not the diameter they say they measure, they act as if they are smaller in diameter. Mine were cheap needle cam followers.
2. A true interpolated tooth form. if someone wants to make one of these racks in a DIY router, forget it, or at least, don't expect much. Ideally, a special purpose machine would do this, with two cutters at the cam follower spacing, and the same diameter, and they are rotated just like the driver would as it travels along the material. I'd expect that the nexen guys are pre-cutting this with an NC, then putting it in a fabulously expensive profile grinding machine.
3. High parallelism between the linear bearing and the pitch line of the rack.

Having built one, I'd do it a bit different next time. I'd have a drive pinion between the two spinners and a way to adjust the "phase" of the two to exactly 180 deg (and maybe do 3-roller spindles). Belt flex may rear it's ugly head somewhere in the load regime.

Still fun, any way you slice it!

[Bobbyr70]

Thanks for the insight, I will take the advise and stick with nexens design if I ever can afford it. For now I am going to work with the gear rack with split pinions and hope I can make a buck or two to be able to afford the nexen. Last I inquired it was about 100 per foot and 375 for a pinion. If it lasts as long as they suggest it would pay for itself. The option of running things dry contact on the exterior with wood dust sounds much better than any oil or grease filled with would dust.

[Mike Everman]

I can't imagine that the Nexen is worth it, though a neat thing.
Two things,
Consider a single pinion, but double up your rack and spring one of them. Split pinions, in my o-pinion are limited by the stiffness of spring you can fit in there, so if it's not enough spring torque to move your whole carriage, you're better off just going with some interference and chase down the wear as necessary.
Secondly, stay tuned, by the end of this week I'll be able to show you something amazing. I just need it to be pat. pend. first, which is hopefully tomorrow. It'll make racks obsolete and is zero backlash, and inexpensive to do!
Sorry I can't say more...

[Bobbyr70]

You have my attention!

[Mike Everman]

I started a thread to tell you all about it, with video.

[Bobbyr70]

I'm hunting for it as I type this,