The DC520 screwdriver doesn't use a chuck. I am guessing that you are referring to either a worn out bit holder or a worn clutch. If the bit holder, item #32, is worn out you just simply pull the old one straight out of the screwdriver and snap a new one in place. It can sometimes be stubborn to remove. It so try pulling it out with a pair of needle nose pliers.
If the clutch is slipping then you will need to replace it. There are three parts you will want to replace. The clutch, the clutch spring and the spindle. The clutch is part #603478-00, the clutch spring is part #86211-00 and the spindle is part # 96540-01.
I hope this helps with your repair!
-Mark
CORDLESS DRILL Question by DAVE
MY 18VOLT DRILL UNDER TORQUE SLIPS WHAT NEEDS REPLACING
It sounds to me like the transmission is failing. It will need to be replaced. The transmission assembly is part #152576-00 and is available from http://ereplacementparts.com.
