Motor to leadscrew coupling help

Hi all,
Hope this is the right forum for this. I would like to know how strong the couplings that attach the motors to the screws should be. Motors are 425 oz nema 23's screws are 10tpi 1/2" acme's machined to 1/4" at one end. I made some with 1/4" id 3/8" od hdpe plastic tubing with 1/2 x 1" x 3/8 id high density plastic bushings tapped for set screws to attach at the ends (too weak) then I sleeved them inside with a very thin aluminum tubing and hdpe (again a little weak). I don't want to make them too strong as I would rather replace these than the alternatives. What if I were to use steel thumb shrews (drilled out and tapped for set screws) Would these be okay? Also, I've considered drilling out a piece of 3/8 or 1/2 aluminum rod (set screws again)? any help will be greatly appreciated.
Thanks, Jeff

The easiest option sounds like Lovejoy connectors. One metal hub is attached to the motor plus another for the screw. A rubber spider connects them and allows a slight amount of mis-alignment. The hubs can be ordered individually with 1/4" thru 5/8" holes.
Total cost at Enco is around $2.00 per hub plus $1.50 for the spider.

By strong, i think you mean torsional stiffness (oz-in/degree). Your required stiffness will depend on your max. expected torque (~425 oz-in.) and your required accuracy. The torsional stiffness of any good coupling will be specified. Convert your desired displacement accuracy to rotational accuracy (Ex. 0.001" desired accuracy on a 10TPI acme screw = rotational accuracy=0.001"*10TPI*360=3.6 degrees...3.6 degree rotation will move the axis by 0.001"). Required torsional stiffness is 425oz-in/3.6degree=118oz-in/deg. Of course there is also torsional deflection in your screw, so the coupling stiffness would have to be higher so that the total error from torsional deflection is less than 3.6 degrees. This gives you a ball park idea what sotsional stiffness you need. A very good resource for choosing what type (and finding where to is an option for some of the distributors) of coupling to use based on cost, rigidity, maintence, backlash, etc. is at

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