Bolt Torque & Fastener Stress Calculator
Calculate recommended tightening torque, clamp preload tension, and joint shear/tensile safety factors across standard fastener grades.
Fastener Analysis Results
Recommended Torque
0.0 ft-lbs
Torque (Metric)
0.0 N-m
Clamp Preload (F_i)
0 lbs
Tensile Stress Area (A_t)
0.0000 in²
Tensile Capacity (Yield)
0 lbs
Pure Shear Capacity (0.577 Sy)
0 lbs
Joint Tension Safety Factor
0.00
Joint Shear Safety Factor
0.00
Fastener Engineering Formulas & Mechanics
Bolted joints rely on proper bolt stretch (elastic strain) generated by tightening torque to produce clamp load, preventing joint separation and fatigue failure.
1. Standard Short-Form Torque Equation
T = (K * F_i * d) / 12
T= Tightening torque (ft-lbs)K= Nut friction factor (dimensionless; approx. 0.20 dry, 0.15 lubed)F_i= Bolt preload clamp tension (lbs)d= Nominal bolt shank diameter (in)
2. Fastener Tensile Stress Area Formula
Tensile stress area accounts for the stress concentration across threaded roots:
A_t = 0.7854 * [ d - (0.9743 / n) ]²
n= Number of threads per inch (TPI)
3. Preload & Direct Shear Sizing
Target preload is generally set to 75% of fastener Proof Strength (S_p) to prevent yielding during tightening:
F_i = 0.75 * S_p * A_t
Ultimate pure shear capacity across the threaded shank is estimated via the von Mises yield criterion:
S_sy = 0.577 * S_y
4. Common SAE vs. Metric Fastener Comparison
| Grade / Class | Head Marking | Proof Stress (S_p) | Min Tensile Yield (S_y) |
|---|---|---|---|
| SAE Grade 2 | No radial lines | 55,000 PSI | 57,000 PSI |
| SAE Grade 5 | 3 radial lines | 85,000 PSI | 92,000 PSI |
| SAE Grade 8 | 6 radial lines | 120,000 PSI | 130,000 PSI |
| Metric Class 8.8 | "8.8" stamped | 85,000 PSI (580 MPa) | 94,000 PSI (640 MPa) |
| Metric Class 10.9 | "10.9" stamped | 120,000 PSI (830 MPa) | 136,000 PSI (940 MPa) |