Sheared coupler through bolt

[scottie]

Key in coupling was perfectly normal indeed standard parallel or tapered shaft
unsplit or non clamped coupling would have a taper pin through side of shaft similar to bike

 

[Caraway]

Key in coupling was perfectly normal indeed standard parallel or tapered shaft
unsplit or non clamped coupling would have a taper pin through side of shaft similar to bike

Like a cotta pin. That would make more sense. I think this coupling is 2nd generation bodge. (Not the OP, but his 'engineer' seems to have carried on with a quick fix made previously). I can't imagine the original set up had a massive coupling like that which relied on a weedy little bolt through the centre of the shaft.

 

[scottie]

Indeed boring a hole across a shaft centre is an effective way to weaken it I believe!

 

[colhel]

Trying to think of a solution, it was suggested earlier to clean the threads of the clamp bolts, another thing to try is to put some washers below the nut or bolt head. If the 2 faces of the clamp are touching that's there limit and would show a sizing problem ( but you'd have to try really hard to make them touch).
It might be possible to wrap some shim around the shaft, don't overlap it or you'll put the balance out. Its a bit of a bodgy solution but could work.