A thrust ring is a type of bearing designed to resist thrust loads or loads along the axis of rotation. Tools rotate so that they contact the face being hammered across the entire face through the subsequent impacts.
It is usally located at the flange 2/3 or 3/4 the way up the shaft of the drill
See
http://compare.ebay.com/like/300816...s&var=sbar for an example.
Niel Leon
engineering.com