What is the term called for the arrangement using D-shaped shafts with small flats cut into them and set screws?

• A. Positive mechanical engagement
• B. Friction fit
• C. Interference fit
• D. Keying

Answer: (A)

Positive mechanical engagement describes a drive where torque is transferred by a physical interlock rather than by surface friction alone. In the arrangement with a D-shaped shaft and small flats, the flats provide defined contact surfaces, and the set screws clamp onto those flats. That clamping creates a positive lock: the pieces rotate together because the shape and the screws prevent any slip, even under load. This is different from friction fits, which rely purely on surface pressure, or interference fits, which use deformation to lock parts without any clamping fasteners. It’s also not a traditional keyed drive, which uses a separate key and keyway to transmit torque. The flats plus set screws give a direct, positive connection, hence the term.