Bolt preload torque
Tightening torque to reach the preload of a metric bolt, with the induced torsional/equivalent stress and the bolt-force reserve — from ISO thread geometry and the strength class.
Tightening torque, M_A77.66 N·m
Thread torque, M_G39.81 N·m
Head-friction torque, M_WD37.86 N·m
Preload force, F_i32.36 kN
Force at yield, F_0.253.93 kN
Force reserve, F_0.2 − F_i21.57 kN
Stress & geometry
Equivalent stress, σₑ531.24 MPa
Torsional stress, τ211.94 MPa
Preload stress, σₜ384 MPa
Stress area, Aₜ84.26 mm²
Yield strength, Rp0.2640 MPa
Tensile strength, Rm800 MPa
The most externally-linked calculator on the original site. Preserved literally, including the ISO thread and strength-class lookup tables.
Method & references
- ISO metric coarse thread geometry (pitch, pitch diameter d₂, minor diameter d₃).
- Proof/yield strength from the bolt strength class (e.g. 8.8, 10.9).
- Torque from thread helix and friction; equivalent stress by von Mises.
The computation is a literal port of the original client-side JavaScript from /calculators/e3_6a.htm. Results are verified equal to the original by an automated golden-master test. No formula has been re-derived or “improved”.