These are the wider (lengthwise) segments at the center of gears, pulleys, levers, etc. intended to attach them to their corresponding shafts. Hubs have several functions, as follows: they serve to prevent wobble, which in caused by loss of perpendicularity between the plane of the rotating element and the supporting shaft, by increasing the engagement length over the shaft (greater L/D, such as 2 or 3); hubs also provide a stronger attachment for keys to engage the shaft and transmit higher torque; and ultimately, they also serve to provide axial fixation either through the use of set screws or through their contact against shaft shoulders, snap rings, cotter pins, or locknuts.
In the cases where the hubs are provided with internal bearing sleeves for rotation over the shafts it will be necessary to provide for the required axial constraint, such as thrust washers etc.