I designed this for use with creating an electric spinning wheel. The yarn goes into a 12mm aluminum tube. This connects to one end of this part. At the other end you can attach an oldham coupler (19mm diameter). See http://en.wikipedia.org/wiki/Coupling#Oldham
This allows me to connect a 4mm rod at the other end for the bobin and for power transfer, which is difficult to do with a 12mm tube.