Hub unit bearing for automobile wheels are comprised of bearing rings, rolling elements, and cages, similar to general-purpose rolling bearings. The first-generation hub unit(HUB I), second-generation hub unit (HUB II), and third-generation hub unit (HUB III) vary according to the integration of the bearings and their peripheral components. In addition, the forms are categorized by usage condition (rotations of outer rings or inner rings) and components location ( for driven wheels or non-driven wheels).

HUB I, NSK uses proprietary bearing-type designations such as "BWD" for ball bearing and "KWD" for tapered roller bearings. The HUB I units are double-row angular contact ball bearing (BWD) and double-row tapered roller bearings (KWD) with back-to-back duplex outer rings.

No preload adjustment (including dimensional adjustments by shims) are required on the assembly line. The initial axial clearance is properly pre-set for the preload to fall within the specified range after mounting. In addition, the integral seal seliminates the need for automotive makers to externally apply seals.