US Digital Shaft Encoder and VersaChassis Hex Shaft

[vigneshv]

We are running a west coast drive with a WCP DS Gearbox, and as suggested, we attempted to use a 1/4" US Digital shaft encoder on an output VersaChassis hex shaft. However, the hole on the shaft seems slightly too big for the encoder, as the encoder shaft slides in and out of the hole in the hex shaft very easily. There doesn't seem to be enough friction to move the encoder shaft along with the turns of the hex shaft. What is the correct way to mount these encoders to those hex shafts?