The outer edge of the shot guard is curved and fits into the bore of the tube along with the bearing. The curved shaped deflects dirt away from the bearing. Any dirt that makes it past the shot guard can fall out through a drain hole in the bottom. By using shot guards you can increase the life of your roller in applications where dirt, sand, weld spatter, machine shavings, and other material is repeatedly dropped onto the bearings. Shot guards offer limited protection against small airborne particles, like dust, that can be blown upwards into the drain hole.