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shaft coupling help needed

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picclock:
I've have two 8mm shafts which I need to join. There is likely to be misalignment of both the radial and angular kind, and although small (assembly and machining tolerances) it will bind up if a fixed coupling is used. I bought a 'real' flexible shaft coupling of the Aluminium spiral spring cut type but have now found it is just too large for the design. Its length I'm short of and the purchased coupling is around 5mm too long. The shaft speed is around 500 rpm max. I was thinking of making a coupler with just a hardened cross bar and a socket with a cross slot to take the bar - if that makes any sense (an idea I pinched from an RC card wheel drive). 

Any help (and hopefully better ideas) much appreciated.

Best regards

picclock

andyf:
You could try an Oldham coupling. They cope with a good deal of radial displacement, but only up to about half a degree of angular misalignment. Arc Euro sell them, though you would have to bore out a 6mm one (or turn down the shaft ends to 6mm). Farnell sell 8mm ones. The two ends and the middle seem to be sold as separate items, so it gets a bit pricey when you add the three up.

But the cross bar would probably work just as well. 

Andy

hopefuldave:
I'm not sure about angular misalignment, but the radial could be taken care of with an Oldham coupling - Google them, but basically it's a disc with two keys at 90 degrees to the shaft axis on the faces (and at 90 degrees to each other), and a collar on each shaft with a matching key slot, I'd guess if you're short on length the centres of the keys could be cut away and the collars could be quite short? They're popular for coupling steppers to leadscrews etc. in CNC  conversions where there's slight offset to one of the shafts, might suit your application?

Dave H. (the other one)

David Jupp:
If low torque, something as simple as a piece of flexible plastic piping (hose) pushed on both shaft ends could work. 

A gear coupling would likely work well, but expensive.

You don't mention if backlash would be a problem - that could rule some options out.

picclock:
Many thanks. I think I can see a way through now  :thumbup:. I never understood oldham couplings, but now I've seen the Arc Euro page I get the idea. That is the best way of doing it as I can make a torque disk out of some nylon rod and its easy for me to fix the two end pieces to the shaft. I may have some Acetal (delrin) rod also but I would have thought nylon the better material although Arc parts are Acetal.    The problem was one of length, but with this type of coupling we are probably talking 12-14mm, well within the 20mm I've got. Radially I have lots of room so the disc can be around 20mm (not sure there is any point in going larger).

Many thanks for the info

Best Regards

picclock

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