BOLT TORQUE & PRELOAD

Bolted joints rely on preload (clamp force) to hold parts together. The relationship between applied torque T and preload F is T = K·D·F where K is the nut factor (typically 0.15-0.25 depending on lubrication). A more accurate model splits friction into thread-helix and head/nut-face components. Preload scatter is significant: torque-controlled bolting typically achieves ±25% of target preload, while angle-controlled and stretch-controlled methods are tighter.

• T = K·D·F (nut-factor)
• T = F·[(d_p/2)·tan(α+ρ) + (µ_c·d_c)/2]
• σ_t = F / A_t
• τ = T_thread / Z_p
• σ_eq = sqrt(σ_t² + 3τ²)
• F_target = 0.75 · Sy · A_t (typical)

References & StandardsVDI 2230, ISO 898-1, ASME B1.1, NASA RP-1228

Use this module to size fasteners for any preloaded joint: pressure-vessel flanges, structural connections, engine head bolts, or bracketed equipment. Set the target preload first (typically 70-90% of bolt yield), then back-calculate the torque. Verify that bolt stress under combined preload + external load stays below yield with adequate margin. For critical joints, prefer hydraulic tensioning or angle-of-turn methods over torque alone.