Originally Posted by
Brenck
Motor shaft is 1/4 and the rod will be turn to 1/4 to, my plan was to make a passing center hole in a 5/8'' alum round bar ( basically like a thick wall tube but perfectly centered ) and the fix the motor and the rod axle with thoose screws used for pulleys.
Agreed that alignment is the main thing that kills rigid couplings, but of course it's not the coupling that suffers. Machining the end of the screw is one concentricity that will be hard to hit, another is that motor shafts don't always run true either. plus or minus two thousandths of total misalignment, or a tiny, tiny bit of angular misalignment and this won't last long. Setscrews are definitely out, unless you achieve a nice transition fit (kind of a hand tight press fit). Point being that the setscrews are not pushing the shaft across a clearance hole, making it eccentric.
Oldham couplings are incredibly easy to make. You could make one in a drill press in short order. Just for fun, here's an oldham animation:
[ame="http://www.youtube.com/watch?v=acZ5_UmUIqE"]YouTube - Oldham coupling animation[/ame]
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