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

• A. Flat key
• B. Square key
• C. Big head key
• D. Woodruff key

Answer: (D)

Key shapes must match the slot or keyway they sit in to reliably transmit torque. A circular segment (semicircular) keyway is a partial circular groove, so the mating key needs a matching curved profile. The Woodruff key is designed exactly for this: it has a semicircular cross-section with a flat base, forming a half‑circle that fits into the circular segment keyway on the shaft and hub. This fit aligns the parts and distributes loading smoothly around the arc, reducing stress concentration and making assembly easier while preventing slip.

The other options don’t fit a circular segment keyway. A flat key and a square key have rectangular cross-sections and require rectangular keyways, not semicircular ones. A big head key isn’t the semicircular type and isn’t used for circular segment keyways.