Machine Design MCQ Questions & Answers | Mechanical Engineering

A bench vice is a mechanical device used for holding and clamping workpieces securely in place during various machining operations. The vice is equipped with a threaded screw mechanism that allows the jaws of the vice to move closer together or farther apart. The type of threads used in the bench vice's screw mechanism determines its efficiency and load-bearing capacity.

Among the options provided, the correct answer is Option C: Buttress. A bench vice often employs a buttress thread mechanism. Buttress threads have one side with a steep angle (known as the "serrated" side) and another side with a flat or slightly angled surface. This design provides efficient load-bearing capability in one direction while allowing quick return travel in the opposite direction. Buttress threads are particularly suitable for applications where high axial loads are involved.

Therefore, the correct answer is Option C: Buttress.

Knuckle threads are used in railway carriage couplings because they are designed to withstand heavy loads and shocks.

Reason: Railway couplings are subjected to sudden jerks and vibrations during starting, stopping, and motion of trains. Knuckle threads have a rounded profile which reduces stress concentration and makes them suitable for such conditions.

Other options:
- Square threads are used in screw jacks and presses for power transmission where efficiency is required.

- Acme threads are used in lead screws of lathes due to their ease of manufacturing and strength.

- Buttress threads are used where force acts only in one direction, like in screw presses and vices.

Conclusion: Thus, knuckle threads are most suitable for railway carriage couplings.