Which key secures a member to a machine shaft and is concave to grip the shaft by friction?

• A. Woodruff key
• B. Flat key
• C. Plain Tapered keys
• D. Big head key

Answer: (D)

Key idea: how a key shape affects torque transmission and how it grips the shaft. A Woodruff key is a semicircular, half-moon shaped piece that sits in a matching recess on the shaft. Its curved underside matches the shaft surface, creating a concave contact that increases the contact area and holds the hub in place by friction, while also helping with alignment. Flat keys are rectangular and rely on straight contact in keyways, not a concave gripping surface. Plain tapered keys use wedging from a taper rather than a curved, friction-based grip. A big head key has a head for easier handling, but it doesn’t provide the concave frictional grip on the shaft. So the feature described is characteristic of a Woodruff key.