Woodruff Key, 5/16 x 1/2 in Key Size, D SAE Number, Steel Zinc, 4 PK, Midwest Fastener, Mfr#: 73205

A woodruff key or half-moon key, is a semicircular shaped, removable key that fits into a matching keyway cut into a shaft, leaving a protruding tab. The tab mates with a matching slot on a device mounted flush upon the shaft e.g. a pulley, thus preventing the device from freely rotating about the shaft. It is widely used in machine tools and in the automotive industry.