Which key is most commonly square or rectangular in cross section?

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

Answer: (B)

Keys transmit torque between a shaft and a hub, so the cross-section shape matters for how well the connection handles twisting forces and how easy it is to manufacture and assemble. The square key is the most common form because its square cross-section fits neatly into matching square keyways in both the shaft and the hub, giving a large contact area and predictable shear behavior. This standard geometry works well across a wide range of sizes and loads, making it economical and reliable for general use. Other shapes exist for specific reasons—Woodruff keys have a semicircular profile that can ease assembly and reduce stress concentrations but typically carry lower torque; tapered keys rely on wedging to lock parts together and can complicate alignment and machining; big head keys mainly offer easier removal but don’t change the fundamental cross-section advantage of the square key.