[Winter]

Hello

I'm looking to bolt a pulley to my ball nut as illustrated.

Its a 25mm ballscrew so I would need a pulley with a bore of 26-27mm

There is an awful lot of belt types etc I don't really know what I'm looking for.

its a 2800mm 2505 Ballscrew, driven with a 387oz/in Nema 23 on a wood router.

I would like the belt as short as possible

Advice please!

Matt

[harryn]

Hi Matt, I am not an expert at all, but have been reading a lot about that application.

If I understand correctly:
- 25 mm diameter ball screw
- 5 mm movement per revolution
- You are making a rotating nut driven version, not a rotating screw version

All good choices for torque and high resolution.

You will not find the exact pulley you need to screw to the nut that way without some machining, but it isn't a serious issue. My perception is that machining a timing belt pulley (especially the bore) is needed very often, maybe more commonly than not.

I know that you are in NZ, but a great web site to learn more than you want to know about timing belts and pulleys is BRECOflex CO., L.L.C. - High Precision Drive Components. They also have a timing belt calculator there to make your life easy.

There are a few tooth forms and setups that over time have become really common. For your application, a good one is:

25mm wide
5 mm pitch
AT style

so commmonly written as 25AT5 belt (and the matching pulley from that series)

I will look up some pulley examples, but mostly you just want the "core" of the pulley to be large enough to take the machining you are proposing. That size will typically come with 6 or 8mm diameter pilot bore already in it.

By the way, when you get that rotating ball nut setup working, it will become a popular design around here.

[harryn]

Here is a link to the pulley options page for a 25AT5 setup from Brecoflex:

BRECOflex CO., L.L.C. - Pulleys

You can download the drawings from them and use it in your build model. Those are internationally common pulley setups, so there might be a pulley supplier closer to you.

I am not sure of the size of the bolt pattern on that ball nut, but just match it up to the pulley hub size to make sure it will fit. Brecoflex will custom machine their pulleys for you, so I assume that other firms will also.

If you don't see a pulley hub diameter large enough for your application, the switch to a 10 mm pitch design. (25AT10)

[harryn]

The router in your build thread is nearly 3 meters long. Assuming you want to cover that distance in 1 minute, (which is good for machining, but perhaps slow for no cutting work) and the nut moves 5mm / rotation

3 000 mm / minute x 1 rotation / 5mm = 600 rpm

I am not sure how much torque there will actually be in a stepper motor turning 600 rpm (maybe 1/10th of the 380 oz-in rating) so this is something to think about. You could gear it up so that the motor is turning slower, or perhaps consider a ball screw with a higher lead ?

[harryn]

By they way, have you selected bearings for this rotationg nut setup yet ? I am just curious, as I would think that tapered roller bearings would be a good choice, but it would be good to hear from people with more bearing experience.

[Winter]

Thanks for all the good replies!

RE the 600rpm / 3 m / minute. I'm not too concerned about the rapid speed. Would be nice I guess.

Remember I have dual drive screws on that axis, so even like you say 1/10th the 387 rating, that should be plenty for non cutting moves.

I would perhaps consider gearing, although I cant decide on how much.
Off the top of my head :
mm per rev / steps per rev = resolution?
5 / 2000 = 0.0025mm

If i did a 1:2 ratio
would that be

5/ 1000 = 0.005mm probably still too much resolution
1:4 would be 5 / 500 - 0.01 - getting closer and would put it about 150rpm. (or 300rpm for 30sec rapid.

Is that maths right?



And as for bearings.. I had just planned on sandwiching ball nut and timing pulley between two three-piece thrust bearings.. Sounds simple in my head.

I don't know anything about bearings really, so if anyone had suggestions on a better choice please let me know


For the timing pulley - I know of a few places locally. First step is to get an accurate model of my ballnut so I know the size I'm after. Then work out who/how to get it machined.

Thanks heaps!

Matt

[harryn]

I think your math is right, but one assumption that you are making comes up frequently for discussion. Can you really count on the 10 "micro steps" per "full step" for real world results / real, count-on-it movement.

My perception from reading is - not really.

Assuming your setup does not go into a stall, you can count on 200 steps / revolution, and maybe the 1/2 steps (400 each 1/2 steps per revolution) but the 10 micro steps are more for general motion smoothing than "I can count on it moving X amount". In other words, if you command in a 1/10th microstep motion, it may or may not actually happen.

For reasons that are still not entirely clear to me, it is also more common to find pulleys in the same series that correspond to 2:1 or 3:1 ratios. Finding useful 4:1 and 5:1 setups can drive a person crazy.

Just for reference, 500 - 600 rpm is similar to a car tire motion, so similar concepts for bearings might work.

[Winter]

OK, well all that maths is not really my style anyway.

In order to get the gearing a bit better, I need a bigger pulley on the stepper than on the ballnut right? So one rev of stepper pulley = more revs of nut pulley.

Turns out that might be the limitation of my gearing, as I can't really go too huge on the stepper pulley.

I found this pulley on the sdp-si site : part # A 6A35M042DF1008
which will be suitable for my ball nut when bored out and tapped for mounting bolts.

And this for my stepper pulley : A 6A35M060NF1008

Its only 10mm wide T5 Belt, will 10mm wide be enough? They dont seem to have much T5 stuff in wider sizes..

[harryn]

T5 is an old style belt tooth and is designed for power transmission, not motion.

There are several tooth styles that can work, but my suggestion is AT5, not T5.

You might get away with 10mm, but it will likely be springy enough that it will show up as backlash / incorrect dimensions sometimes unless it is super short. I could run an estimate calculation for you, but instead will just try to talk you out of it.

If you don't have room for a 25mm wide belt setup, then I would instead use the 16mm ATL style (thicker steel wires) or Gen III AT Redflex (no seam round belts)

Don't let the more business professional setup of Brecoflex disuade you from doing business with them, they are pretty easy to work with. I need to get some belts from them too, so I know the routine.

[Winter]

Hi Harryn,

after talking with the locals here about their suggestions,

I've ordered my pulleys.
30tooth HTD8 20mm wide for the ballnut
and 34 tooth for the stepper.
Bloody small gearing but physical size was limiting here.

Looks like I'm in for a slow machine but thats not really worrying me.

Pulleys are from Martin Sprocket. Should be here today, then I need to get on with machining them to suit.

[Winter]

Ok, Some more thought required.

Can only get the chosen pulleys with a 15mm pilot bore.
My Steppers have a 6.3mm shaft, I have been considering upgrading to nema 34 steppers with a 14mm shaft.

Any suggestions on how to attach a timing pulley with a 15mm bore to a 14mm shaft?

I guess I could always install nema 42 motors (19mm shaft) but that seems a little overkill.

[harryn]

There are a several methods of mounting a pulley onto shaft. A common method is of course set screws, which assumes that the fit is quite tight.

It might be possible to make a brass fit adapter - basically just a thin tube, that slips over your motor shaft and is small enough to fit inside of the pulley. Just don't pull your motor apart to mount it, or it will be ruined. The nice thing about this approach is that you could even fine - tune the fit by running the motor and sanding down the brass a bit as needed to make a tight fit in the pulley.

I have also seen some collet style mounting methods, example these quick-disconnect flange types at McMaster.

McMaster-Carr

In the case the link does not work, go to mcmaster.com, timing pulley, quick-disconnect.

These actually require a significant gap between the shaft and pulley.

I am glad to see your progress in this area and am following it.