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Does someone know a definition of this?

A very good question, Jordan..... I found the following : http://vlabs.iitkgp.ac.in/mr/exp6/index ... o%20shafts.

So......an Oldham coupling is a cylindrical device used to join two shafts that may not be in perfect axial alignment, each shaft at the point where they are required to join is machined with a transverse female slot, the female slot in the first shaft match's a similar transverse male tongue on one face of the Oldham coupling device which is also machined with a similar transverse male tongue on its other face to align with the second shaft, these two opposing face tongue's are arranged at 90 degrees relative to each other, this allows for a small amount of shaft misalignment at the point of coupling providing a limited sideways sliding action of either shaft while maintaining a secure mechanical connection.
Well that makes sense to me! but I have one in my hand looking at it! (Disclaimer I'm sorry if this comes over as nonsense) Cheers,
George

Sorry......should read above axial AND radial misalignment. Its all too hard on my brain
George

There's no ability for the coupling on a Ducati vertical drive shaft to compensate if two shafts are not in alignment.
We shouldn't really be calling them Oldham couplings.

The assembly driving the tachometer from the camshaft is an Oldham coupling by definition, but it's hardly ever referred to as such.

I personally have never considered the Ducati upper bevel shaft coupling as an Oldham, technically the Ducati coupling should be called a 'sliding muff half lap coupling' and as you rightly say Jordan has no capacity to handle radial shaft misalignment (only axial). Some older racing single cylinder overhead cam bike engines.......manx Nortons, ktt Velocettes for example use Oldham couplings in their bevel shaft drive trains, I have several of these from a Velocette ktt in my 'interesting bits' spares box.
Cheers,
George

...

blethermaskite wrote: Ducati coupling should be called a 'sliding muff half lap coupling'

Good description, but that's quite a mouthful.

Seriously Jordan that's what its called......sometimes called 'a sliding sleeve half lap coupling' ......I'm not making it up!
Cheers,
George

It could be shortened to "sleeve coupling" ?
There is no close-fitting sleeve on an Oldham type, as it would restrict its radial movement and defeat the purpose.