Author Topic: What's wrong with my threaded rod drive?  (Read 782 times)

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Offline aedTopic starter

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What's wrong with my threaded rod drive?
« on: August 31, 2013, 12:01:59 AM »
Hi there - I tried to implement a threaded rod drive to move a small object along it.

However, I found it to be wobbling all the way. Here's a video https://vimeo.com/73508953  to show what's going on.

Can you help me understand what's missing from my design and why it isn't just going straight up? The object eventually gets there but follows more of a wide irregular helix than a straight line.

Things I tried:
- Moving the motor around to try to center it more (barely helped)
- Using two smooth rods instead of one (didn't help as it either got stuck completely or it wobbled anyway).
- Using one nut instead of two.

Thanks for your help!

Offline Pogertt

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Re: What's wrong with my threaded rod drive?
« Reply #1 on: August 31, 2013, 11:44:38 AM »
Try 2 guide rods fixed to a rigid base at both ends.
Make sure the rods are parallel, and that the traveling block can slide freely the entire length.
After traveling block slides freely, then add motor and threaded rod.
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Offline waltr

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Re: What's wrong with my threaded rod drive?
« Reply #2 on: August 31, 2013, 12:04:15 PM »
Yep, two guide rods would be better.

After viewing your video is see two major problems:

1- the fixed guide rod is not secure in the block. If wobbles in the hole. You will need to have this rod fit very snugly and maybe even a 'press fit' into the block. You might be able to fix this with the correct type of Loctite and ensure the rod is perfectly aligned while the Loctite cures.
 
2- The coupling between the motor shaft and the threaded rod does not keep the two in perfect axial alignment. Remove the carriage block and run the motor/threaded rod alone. If this wobbles badly then the fix the coupler until the threaded rod does not wobble. You might be able to shim the threaded rod inside the coupler and play with the set screws to get the rod to not wobble.

If you use two guide rods mounted as in #1 then use a flexible coupler so that any slight misalignment is taken up by the coupler.

Offline aedTopic starter

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Re: What's wrong with my threaded rod drive?
« Reply #3 on: August 31, 2013, 02:24:45 PM »
Thank you Pogertt and waltr for some great advice.

OK - I will start by re-designing it for 2 guide rods.

I removed the carriage as per your suggestion waltr and did see there was some wobble left (i.e. the threaded rod follows a cone shape). I played around with it a lot and was able to somewhat reduce the cone diameter but there is still a cone there. Is that something the flexible coupler can help with?

Do you have any such example of a flexible coupler that I can get ready made?

Thanks again

Offline waltr

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Re: What's wrong with my threaded rod drive?
« Reply #4 on: August 31, 2013, 03:33:52 PM »
Ok, With a rigid coupler there needs to be NO cone movement. I hope you now see where the problem is coming from.
Here is a link to flexible shaft couplings:
http://www.mcmaster.com/#flexible-shaft-couplings/=ob3x6z
This one might do what you need:
http://www.mcmaster.com/#flexible-shaft-couplings/=ob3xpb
 

Offline aedTopic starter

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Re: What's wrong with my threaded rod drive?
« Reply #5 on: August 31, 2013, 04:17:19 PM »
Thanks much waltr - I am working on the 2-guide rod build right now and will post once I have an update.

Oddly enough most of the threaded rod drives I've seen in the wild (mostly in 3D printers) all seemed to have rigid couplers and were working fine. I wonder if I'm also missing something else. Either way I'll update once the 2-rod build is done.

Thanks again!

Offline jkerns

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Re: What's wrong with my threaded rod drive?
« Reply #6 on: August 31, 2013, 06:04:17 PM »
Most other threaded rod drives such as 3D printers are constrained at both ends of the rods by being connected to a reasonably rigid structure.
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Offline waltr

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Re: What's wrong with my threaded rod drive?
« Reply #7 on: August 31, 2013, 06:26:47 PM »
Those threads rods also usually have a machined bearing surface that is concentric to the threads
and the fits are very precise.

Offline bdeuell

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Re: What's wrong with my threaded rod drive?
« Reply #8 on: September 16, 2013, 08:52:21 PM »
I agree with others adding a second rod and switching to a flexible shaft coupling would help.

You may also want to consider letting the second linear bearing float in a slot, so the function of the second rod is purely to limit rotation around the first rod. This would allow for some degree of misalignment between the two rods in your design and prevent binding.

 


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