From Team 449 Wiki
Shafts are long, narrow pieces of metal, generally steel, nickel, or chromium alloys. They are used for rotational motion within a mechanism. They come in various shapes and sizes, and can be cylindrical or hexagonal in shape. Cylindrical shafts generally have keyseats to allow shaft keys to be placed in them. While shafts are purchased at very long lengths, they can be cut down using a band saw or a chop saw, depending on their material, size, and the wanted precision. An example of a shaft in use would be the ball intake on ROUS.
Shaft keys are small rectangular prisms made out of metal. They are designed to fit in the keyseats on a shaft. Due to the fact that a shaft could just spin in the cylinder it is inserted into without making the outer one spin, a shaft key can be placed in the keyseat of the inner cylinder and in the keyway of the outer one, making them both spin together. This is useful when the shaft has to be expanded at some point, or for mounting various additions to the shaft that have a cylindrical hole in them.