Bore Tolerances for Gears

Have you ever purchased something that cautioned, “Some assembly required” on the packaging, only to learn later that even with a master’s degree in mechanical engineering the parts don’t seem to assemble in the same fashion as the instructions describe? This is typically an issue with the tolerances of the components in the assembly. When working with gears, the clearances between the bore and shaft and the key and keyway and the location of the set screws can all contribute to an easy assembly or a nightmare.