Chain Sprockets are fabricated from a variety of materials; this would depend upon the application of the drive. Large fabricated steel chain sprockets are manufactured with holes to reduce the weight of the chain sprocket on the equipment. Because roller chain drives sometimes have restricted spaces for their installation or mounting, the hubs are made in several different styles.
Type A chain sprockets are flat and have no hub at all. They are usually mounted on flanges or hubs of the device that they are driving. This is accomplished through a series of holes that are either plain or tapered.
Type B chain sprockets has a hub on one side and extend slightly on the other side. The hub is extended to one side to allow the sprocket to be fitted close to the machinery that it is being mounted on. This eliminates a large overhung load on the bearings of the equipment.
Type C chain sprockets are extended on both sides of the plate surface. They are usually used on the driven sprocket where the pitch diameter is larger and where there is more weight to support on the shaft. Remember this the larger the load is, the larger the hub should be.
Type D chain sprockets use an "A" chain sprocket mounted on a solid or split hub. The type A chain sprocket is split and bolted to the hub. This is done for ease of removal and not practicality. It allows the speed ratio to be changed easily by simply unbolting the sprocket and changing it without having the remove bearings or other equipment.