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Bolt Torque Spec Calculator

Recommended Torque Formula: T = K × d × F

Where T is torque, K is the friction coefficient, d is bolt diameter, and F is bolt preload tension (typically 75% of proof strength).

A professional estimator for machine shops, automotive technicians, and structural engineers. Calculate target tightening torque and preload parameters for standard fasteners.

Parameters

Calculation Outputs

Recommended Torque 9.0 Nm
(6.7 ft-lb)
Target Preload Force 9.0 kN
Stress Area 20.1 mm²

*Disclaimer: Calculated values are estimates based on standard formula coefficients. Actual structural clamp forces vary with surface condition, thread quality, and installation tools.

The Friction Factor K

The torque coefficient $K$ (friction factor) accounts for the friction in the threads and under the bolt head:

  • Dry Steel (0.20): Uncoated steel bolts with dry threads. Friction is high, requiring more torque to reach target clamp preload force.
  • Lubricated (0.15): Threads coated with motor oil, graphite, or anti-seize. Lubrication lowers friction, meaning a smaller torque value generates the same clamp load. **Warning: Torquing a lubed bolt to dry specs can snap it!**
  • Zinc Plated (0.22): Zinc coating increases surface friction slightly, needing higher installation torque values.

Target Preload Tension F

Preload is the tension created in a fastener when tightened. It is designed to be high enough to hold joints together, but safely below the plastic deformation limit of the bolt:

Preload Force = 0.75 × Tensile Stress Area × Grade Proof Strength