Mechanical Design – Conceptual Questions (Short Answers)
1. What is the primary purpose of a bolt in a bolted joint? Answer: The primary purpose of a bolt is
to clamp the joint members together and create a preload.
2. What is bolt preload? Answer: Bolt preload is the initial tensile force induced in the bolt during
tightening.
3. Why is preload important in bolted joints? Answer: Preload prevents joint separation and reduces
load fluctuation in the bolt.
4. What happens if preload is too low? Answer: The joint may separate, causing slip, fatigue, and
eventual bolt failure.
5. Why are bolts designed to carry tension rather than shear? Answer: Because tensile loading can
be controlled by preload, while shear loading causes slip.
6. What is bolt stiffness kb? Answer: Bolt stiffness is the resistance of the bolt to elongation under
tensile load.
7. What is member stiffness km? Answer: Member stiffness is the resistance of the clamped
members to compression.
8. What does the stiffness ratio C represent? Answer: It represents the fraction of external load
carried by the bolt.
9. Why is a small value of C desirable? Answer: Because more load is carried by the members and
less by the bolt.
10. What is bearing stress in a bolted joint? Answer: Bearing stress is the compressive stress
between the bolt shank and the hole wall.
11. Why is bearing stress calculated using d × t? Answer: Because the load is distributed over the
projected contact area.
12. What is bearing failure? Answer: Bearing failure is local plastic deformation around the bolt
hole.
13. Why is bearing failure preferred over bolt shear failure? Answer: Because bearing failure is
gradual and ductile.
14. What is bolt shear failure? Answer: Bolt shear failure occurs when the bolt fractures due to
transverse shear stress.
15. Why should threads not lie in the shear plane? Answer: Threads reduce shear area and
increase stress concentration.
16. What is edge shear failure? Answer: Edge shear failure occurs when material tears out near the
bolt hole.
17. Why is edge distance important? Answer: Insufficient edge distance reduces resistance to
tear-out.
18. What is tensile failure of members? Answer: It is yielding or fracture across the net section.
19. Why is net section used in tensile failure analysis? Answer: Because bolt holes reduce effective
area.
20. What is proof strength Sp? Answer: Proof strength is the maximum stress without permanent
deformation.
21. Why is proof strength used instead of yield strength for bolts? Answer: To ensure preload is
maintained.
22. Why is 0.577 Sy used for shear strength? Answer: It comes from the von Mises criterion.
23. What is the von Mises criterion? Answer: It predicts yielding under combined stresses.
24. Why is preload beneficial for fatigue life? Answer: It reduces stress range in the bolt.
25. What causes bolt fatigue failure? Answer: Cyclic stress fluctuations.
26. What is joint separation? Answer: When external load exceeds clamping force.
27. Why is joint separation dangerous? Answer: Because the bolt carries the full load.
28. What is a governing failure mode? Answer: The failure mode occurring at the lowest load.
29. Why are washers used? Answer: To distribute load and reduce bearing stress.
30. Why hold bolt head stationary during tightening? Answer: To avoid torsional stress in the bolt.
31. What is a power screw? Answer: A device that converts rotary motion to linear motion.
32. How is a power screw different from a bolt? Answer: It transmits motion continuously.
33. What does self-locking mean? Answer: The load cannot back-drive the screw.
34. Why does high efficiency reduce self-locking? Answer: Because friction is low.
35. What is a key used for? Answer: To transmit torque between shaft and hub.
36. Why are keys weaker than shafts? Answer: To fail first and protect the shaft.
37. Why does a keyway cause stress concentration? Answer: It introduces a geometric
discontinuity.
38. What is critical speed? Answer: The speed at which resonance occurs.
39. Why operate below critical speed? Answer: To avoid vibration and failure.
40. What is the main goal in bolted joint design? Answer: To maintain preload and prevent slip.