Which of the following sections is best in torsion?
Try it first: pick an option, then read the explanation.
Correct answer: B. Hollow circular
Asked in SSC JE 01/03/2017 (Evening)
Detailed Explanation
In a shaft the shear stress grows linearly from zero at the centre. Material near the centre carries little stress and adds little to J.
Why “Hollow circular” Is Correct
A hollow circular section places material where the stress is high, giving the greatest torsional strength and stiffness per unit weight, with no warping.
Why the Other Options Are Wrong
- A. Solid circular: Solid circular works, but its core material is under-used — less efficient per unit weight.
- C. Triangular: Triangular sections warp and have low torsional constants.
- D. Rectangular: Rectangular sections warp and have stress concentration at the mid-side; weak in torsion.
Key Point
Best torsion section: hollow circular (closed, thin-walled, material far from the axis).
Key Concepts for Students
- Closed sections are far stiffer in torsion than open ones.
- Drive shafts and tubes exploit this.
- Hollow shaft with d/D = 0.5 saves 25% weight for a 6% loss of strength.
Practice more: this is question 22 in Torsion MCQs – Part 2. Try the whole set and check every answer instantly.