Axle & Woodruff Keys
Axle shaft keys or just "axle keys" are essential pieces of hardware when keeping a drive wheel tight against a revolving shaft. The key fits into a keyway, the rectangular slot that is milled into both the shaft and hub. The familiar sunken axle shaft key is always square in cross-section, although it may have rounded or bevelled ends to fit a rounded-slot axle keyway. The round ends are to make installation easier but are not essential.
The Woodruff key is a special semi-circular half-moon (sometimes with "wings") design for tapered shafts such as those on transaxle assemblies. Some people may use the name "Woodruff key" in a generic sense, applying the term to all shaft keys, but this is technically incorrect. In addition, "keyway" should only refer to the milled slot into which the key fits.