Pros and cons: acme threaded rod

[dsirl]

Can someone give me the pros and cons of using an acme threaded rod vs. an R&P system?

I am considering the ACME rod for my y axis which is about 4'. I understand that the stepper can cause the rods to vibrate or resonate when used over long distances.

So: Multiple start rods require slower RPM for similar speeds. What is the drawback in that approach besides loss of resolution?

Thanks for the help.

[dkirtley]

Well, the pros of ACME threaded rods are:

• It gives a mechanical advantage.
• The thread is designed for linear loads.
• It doesn't lose tension.
• It is relatively cheap.
• It can be placed out of the way.
• It is easy to support for short spans

The cons:

• Backlash
• Slower motion
• Friction
• Hard to support for long spans
• Precision (for rolled threads)

Overall, it is pretty common and relatively cheap. It can get expensive in long pieces but the 4' you mentioned is not a problem. Multiple start threads have less mechanical advantage but are faster. Balls help, but you will need more powerful motors the less mechanical advantage you have.

I think the resonance or vibration is not that much of an issue if you are using properly sized threaded rod.

[ger21]

Backlash is probably less of an issue with acme than rack and pinion. You just nuy anti-backlash nuts.

And Acme is usually easier to install.

I am considering the ACME rod for my y axis which is about 4'. I understand that the stepper can cause the rods to vibrate or resonate when used over long distances.

It's not the stepper that causes it. It's the speed at which it's spinning, and depends on how well it's supported. 4ft shouldn't be a problem at all with 5 start screws.

So: Multiple start rods require slower RPM for similar speeds. What is the drawback in that approach besides loss of resolution?

There are no drawbacks.
Slower rpm gives you more torque.
And compared to R&P, you're gaining resolution. When using 5 start screws, you normally have .5" of travel per motor revolution. With R&P, you usually have 1" (or more) of travel per motor revolution, even with a 3:1 reduction.
So 5 start screws will have double the resolution of R&P.

Any time you use steppers, there's a tradeoff. The faster you want to go, the lower the resolution you'll get. The reason is that steppers lose torque as rpm's increase. So, to move you're machine faster, you need to spin the steppers slower, which reduces resolution.

[studysession]

Gery - that helped some questions I was currently having. Thanks!!!