Ball bearing units consist of an insert bearing mounted in a housing. An insert bearing is a deep groove ball bearing with a sphered (convex) outside surface and an extended inner ring that includes a mechanism to lock the unit onto the shaft. The housings have a correspondingly sphered but concave bore.

The SKF assortment offers a wide variety of units with different housing shapes and bearings using different locking methods. It includes units to ISO standards, North American standards, and Japanese Industrial Standards (JIS). Ball bearing units can accommodate moderate initial misalignment, but normally do not permit axial displacement.