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

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

Answer: (A)

The idea being tested is how the shape of a key affects how it grips the shaft to transmit torque. A woodruff key uses a semicircular (half-moon) pocket in the shaft, and the key itself fits into that curved recess. That concave seat around the shaft creates a larger contact area and a self-centering fit, so friction is increased and the hub is less likely to slip under load. This curved engagement is what gives the woodruff key its secure grip.

The other keys are all rectangular or tapered in cross-section and sit in a matching rectangular keyway. They rely on flat contact surfaces and, in the case of tapered keys, an interference fit when the hub is pressed on. They do not provide the curved, concave engagement with the shaft that enhances friction and grip, so they’re not as effective at resisting slip in the same way as a woodruff key.