An ordinary rule is to make the outside diameter of the hub twice the diameter of the shaft. The two half hubs (one on each side of the arms) are usually loose and are held central by a single dowel pin. Their diameters are adapted to the size of the shaft upon which the pulley is to run, and the length is proportioned to the width of the rim as wall as its diameter. The length of the hub should be about two-thirds the width of the rim except in the cases of tight and loose pulleys, where the hub should be a trifle longer than the width of the rim. It may then project about 1/8 inch on the sides in contact, and inch on the outside.