shaft coupling help needed

[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  . 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

[HS93]

 I got some of these recently from germany they post quick and take pay pal they are only about 25/28mm long I will mesure if you are interested they come 5mm but will go out to 8mm they have a pin drive in to the rubber on bothe sides so 4 pins in all they seem very sturdy. http://www.jet-drive.de/index.php?page=shop.product_details&flypage=shop.flypage&product_id=235&category_id=83&manufacturer_id=0&option=com_virtuemart&Itemid=17

also http://www.prop-shop.co.uk/index.php?id=26&section=store&sub_section=products&product_type=26   bit expensive but strong
there are two types all s/steel I think

http://www.prop-shop.co.uk/index.php?id=26&section=services&sub_section=products&product_type=26

Peter

[Steamer5]

 Hi Picclock,
 Bit late but you know what they say...better that then never.
I modified a 3/8 flexible coupling from a socket set as a coupling between the motor & gearbox on my electric tram. A full noise the motor does 5000 rpm. This was the MK 3 version. The first 2 had little / no flexible movement or any up / down movement. The current version has been on the go now for several year & quite a few kilometers.
I have since found that you can get the flex coupling with a 3/8 drive one end & the socket of your choice at the other. never checked but you maybe able to get 1/4 drive version

Cheers Kerrin