A lot of chucks are retained by a screw that is accessed with the jaws of the chuck wide open. Take care- these screws are often a LEFT HAND THREAD.
these may give you a clue:
http://www.familyhandyman.com/tools/drills/how-to-install-a-keyless-drill-chuck/view-all
- and -
https://www.youtube.com/watch?v=usC8unA5RiA
xxx, December 2016