Which keys are used for the large heavy duty shafts?

• A. Ball bearings
• B. Rounded keys
• C. Flat key
• D. Square key

Answer: (B)

When a key transfers torque from a shaft to a hub, the way its edges are formed matters for how the load is carried. In large, heavy‑duty shafts, the torque is substantial, and any sharp corner on the key or where the key meets the shaft and hub becomes a stress riser. Rounded keys have radii at the edges, which smooth the transition and spread the stresses more evenly across the contact surfaces. This reduces peak stresses, lowers the risk of cracks forming under repetitive loading, and improves fatigue life, making the connection more durable under high torque. Ball bearings are not involved in transmitting torque via a key, and flat or square keys, while common for smaller or moderate loads, create sharper corners that can concentrate stress in heavy‑duty applications, whereas rounded keys mitigate that issue.