Which key type is most commonly used and has a square or rectangular cross section?

• A. Woodruff key
• B. Flat key
• C. Ball bearings
• D. Square key

Answer: (D)

In transmitting torque from a shaft to a hub or gear, the key sits in matching grooves and transfers rotation through shear between the key and its grooves. A square key is the most common choice because its square cross-section fits standard keyways and provides strong, uniform engagement. The equal dimensions give good shear strength and distribute load more evenly, while also being easy to machine and replace with standardized parts—this combination makes it the go-to option in many designs.

Woodruff keys are semicircular and used in some assemblies for quick, simple mounting, but they don’t offer as much torque capacity or widespread standardization. Flat keys have a rectangular cross-section with less height, which can reduce strength under load and make them more prone to wear or deformation. Ball bearings are a different component altogether and aren’t used to lock a shaft to a hub.