Which key is tapered only on the side that engages the hub and has a tapered keyway?

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

Answer: (B)

In shaft-hub assemblies, keys transmit torque by creating a wedged fit between the shaft and the hub. A plain tapered key is tapered on the side that engages the hub, and the hub has a matching tapered keyway. This pairing produces a wedge-like action as the hub is pressed onto the shaft, increasing friction and preventing slipping under load. That’s why this option is the best fit for the description: it explicitly describes a key that is tapered on the engaging side and works with a tapered keyway.

The other types don’t match this behavior: a flat key has no taper and won’t provide the wedging effect; a big head key is designed for easy removal and isn’t defined by taper; rounded keys have rounded edges for different stress characteristics, not the described tapered engagement.