Insert bearings and wide inner ring bearings are used for mounting on shafts without the need for shoulders or adapters. The bearing is locked in place through setscrews or a locking collar. UC Style Insert Bearings have an inner ring that extends on both sides of the bearing and SB Style Insert Bearings extend on one side. 

Both designs however are intended to provide extra shaft support. Sometimes insert bearings are simply standard radial bearings that fit into a housing and these bearings may be purchased as stand-alone products to use as replacements. While standard radial ball bearings must be pressed into a machined shaft, using special installation tools, insert bearings can be slip fit directly onto the shaft.