Which key is semicircular and fits partially into a circular segment keyway?

• A. Plain Tapered keys
• B. Rounded keys
• C. Woodruff key
• D. Ball bearings

Answer: (C)

A semicircular, half-disk key is designed to fit into a circular segment keyway, which is a curved pocket cut into the shaft and hub. The Woodruff key matches that curved geometry, centering the hub on the shaft and distributing the transmitted torque smoothly with minimal stress concentrations. This shape also makes assembly easy and helps keep alignment accurate. Plain tapered keys have a rectangular cross-section and require a rectangular keyway, so they don’t fit the circular segment pocket. Rounded keys and ball bearings don’t provide the same secure, positive drive in a circular keyseat, so they aren’t used for this application.