Rigid or flexable couplings

I built my router with rigid couplings and the encoder plug failed so
I found top speed of the machine .The ball screws that I used for the
X axis was turned down to 5mm and the coupling is a machined 2" od
alum with removable caps to tighten to the shaft with friction fit.
So when it crashed I was not worried I believed the coupling would
slip. This has only minute's time. But it twisted the shaft clean at
the neck this was machined so it would not have stress or failure.
Suggested on modifying the coupling and putting a new end on the
shaft. I bored it out and put a dowel pin in but not sure of how to
anchor so if it crashes again there will be some slip.

Thanks Randy