Question 1

How is bolt tightening torque calculated?

Accepted Answer

Tightening torque is calculated as T = K × d × F_pre, where K is the nut factor (torque coefficient), d is the nominal bolt diameter in meters, and F_pre is the target preload in Newtons. For standard dry steel bolts K ≈ 0.18–0.22; with lubrication K drops to 0.12–0.15.

Question 2

What is the load-sharing ratio Φ in a bolted joint?

Accepted Answer

The load-sharing ratio Φ = k_b/(k_b + k_j) gives the fraction of external load carried by the bolt, where k_b is bolt stiffness and k_j is joint (clamped member) stiffness. For typical metal flanges Φ ≈ 0.1–0.25, meaning most of the external load is absorbed by the joint, which is favorable for fatigue.

Question 3

How do you use the Goodman diagram to assess bolt fatigue?

Accepted Answer

Plot the operating point (mean stress σ_m, alternating stress σ_a) on the Goodman diagram. The safety factor is n = 1/(σ_a/σ_e + σ_m/σ_u). High-strength bolts (Grade 12.9) have a relatively low fatigue limit of σ_e ≈ 100–120 MPa despite high ultimate strength, so fatigue must always be checked.

Question 4

What are effective strategies to improve bolt fatigue life?

Accepted Answer

Effective strategies include: (1) increase preload to raise mean stress while keeping the alternating range small; (2) reduce bolt stiffness k_b relative to k_j (longer bolt, smaller shank diameter) to lower Φ; (3) shot-peening the thread root to introduce compressive residual stresses; (4) rolled threads instead of cut threads to reduce stress concentration.

Bolt Preload & Fatigue Design Calculator

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