Help On Choosing coupling

[Brenck]

Hi guys,

I need to connect a stepper to a ballscrew and i dont wanna lose the prescision so i wanna know if is there any problem to use belts and pulleys and witch kind of belt and pulley is the right one... so i dont loose prescision when the motors reverse direction... is there backlash on belts and pulleys? like i guess belts will stretch a little wont they? Or the most prescise way to conect is just using a zero backlash coupling?

Thanks

[maxvic]

Hi, lots of very accurate motion control stages use timing belts and pulleys driven by servo and stepper motors and accuracy is not lost when the motors reverse. However you will need to design in a tension adjust and like anything, the more parts you have the more attention and maintenance it will need. If you can couple directly with something like a helical or bellows style coupling that will simplify the mechanics but if you need pulleys for torque multiplication or for motor placement belts and pulleys are fine.

[Pres]

IMHO -Use a belt/pulley combination for the following reasons:
1. Wide variety of motor mounting positions - usually with less overhang & obtrusion.
2. Wide selection of gearing (pulley sizes) to match your motor specs, lead screws, and machining requirements.
(e.g. more torque or more speed)
3. Easy mods for mid range stepper resonance by changing pulley inertia.

Belt and pulley size depends on motor speed, location, lead screw size and pitch.
(for starters: get some torque/speed curves for your steppers)

HTH,
Pres

[hoga01]

Hello, Belts should work fine for your machine. They're relatively easy to set up and if the power requirements aren't excessive, they shouldn't stretch enough to cause a problem. The take-up should be designed with changing the belt in mind more than adjustment. At the speeds involved turning a screw, they won't stretch appreciably unless they're severely overloaded and there shouldn't be any problems with accuracy. You'll probably want to use XL sized trapezoidal cog belt and pulleys. If you need something bigger, you should be considering direct drive. Watch out for misallignment on all axes (timing belts don't like it at all) and don't overtighten the belt or you'll cook your steppers. The screw support bearings wouldn't like it either.

[Brenck]

Thanks for all answers,

i was wondering XL trapezoidal pulleys and belts either... do i need em made out of what? aluminium or steel is ok? there are plastic ones either.... probably alum or steel will make me change belts more often but i guess the pulleys will last longer...

[Al_The_Man]

Aluminum wears faster than steel but are much lower inertia, this makes a difference if high speeds are used, steel wears better, but much higher inertia.
So affects motor sizing for a given accel rate
Al.

[hoga01]

Al's on the money. On a hobby machine, neither aluminum nor steel is likely to wear out quickly so I'd probably opt for the aluminum: It's prettier! The steel (which is usually sintered metal) will be cheaper. Stay away from plastic.

[bobs bots]

Following on from the other posts:
There's another particular issue with stepper motors due to resonance, which requires your coupling to have some type of cushioning, so rubber timing belts are ideal in this regard. Which is why almost all commercial systems use belts. (apart from the obvious advantages of achieving gearing, and making the whole assembly more compact)
Rigid coupling of stepper motors to high inertia loads will usually cause limited acceleration and limited speed, with very noisy resonance at certain critical speeds.
If you are using microstepped drives then resonance is less of an issue.
Cheers, BobT

[smabhyan]

Dear Brenck,

Go for backlash free direct coupling. No reversal errors, no position losses due to transmission errors between pulley & belts.
You can find variety of couplings on www.ktr.com

[Pres]

Yes, you'll get more overhang at axis limits and more resonance problems.
.. but there is always another way!

Pres

[RICHARD ZASTROW]

Back to original post. Assuming rigid radial bearing support, proper belt tension etc. belts work fine. The newer belt materials seem to take a "set". Adjust belt tension, run thru recommended break-in & re-adjust tension. We used belts on 6-axis CNC gear hobbers, some axis with double reduction belt/sprocket combinations and produced AGMA class 10 and better gears. One precaution though, our application had no axis reversals.

Having said that, I believe direct coupling is the most exacting but as mentioned that does bring other problems into play. Resonance and all that. Good Luck

[hoga01]

Ahhh, the age old agrument between the proponents of direct and indirect drive. Brenck, if your still following this string, what's your machine for?