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Spreader bar design, friction issues

turnworks

Aluminum
Joined
Dec 12, 2018
Current bars are 3/4"-10 standard threaded rod and round nuts welded into a steel tube. Ends are just some 1"x"1 steel 6" long welded to the threaded rod. One hole in the middle of the tube for a bar to slip in to turn it.

They work ok but at times when tightening or loosening the pads on the ends start to rotate with the tube. Work is both steel and aluminum so knurling the ends of the pads for grip is out of the question. They need to be smooth as to not mark up the parts.

Thinking if I go to a steel acme threaded rod and a brass or bronze nut press fit into the steel tube ends will help but would like to know if there are more things I can do.

Spreading pressure isn't too much with these, 1-40 ft/lbs on average. Quick calculations put all of the smaller cheaper thrust bearings past the load ratings.

Think swapping from standard to acme threads and the softer nut material will make much of a difference? Prefer dry threads because I grind and weld with them in place on the parts and cleaning the threads gets old fast.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
I don't think the threads are the issue, I think you need to grease the end caps of the screw.

Another trick is to use larger end pads that are relieved in the middle. This takes much more torque to turn than a pad contacting near the middle.
 

turnworks

Aluminum
Joined
Dec 12, 2018
Never thought about dishing out the end pads. Great idea.

Should have mentioned the bearing design would only work with a single screw setup. I would prefer 2 screws setup(RH,LH).

Unless I'm missing something the friction on the threads is a major factor. If the pads to the work or screws to the pads don't provide more friction than the screw/nut does then the whole thing rotates and no spreading or contracting.

Also it might be a term issue. I've always called them and heard them called spreader bars.
 

Strostkovy

Stainless
Joined
Oct 29, 2017
Thread friction absolutely is what you have to overcome, but that's kind of dictated by the design requirements. Acme threads and a brass nut would be great, but if the thread pitch is too coarse it may take more torque to get the same force. It's absolutely a good idea to run the numbers if you can, but I don't see an easy way to get the torque down much.

A double screw setup is preferable to me as well, but you will then need a tighter pitch screw. I don't know if you are going to hit the limits of available screw pitches.

In any case, the main things you can adjust are the coefficient of friction on the pads, and the diameter of the contact area. There is some math involved to determine what's required but if you give me the torque and the force produced at that torque I'm happy to help calculate pad size.
 








 
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