our team has always used keys, we never shear them, but we loose them, all the time. however we dont want to buy a hex broaching kit. if possible use a hex broach, but keys work just as well. something i would look at is staying away from gearboxes all together, jester-drive uses only belts to gain gear reduction. picture here.
http://www.chiefdelphi.com/media/img...09401a44_l.jpg