Universal Torque Equation
Tightening torque is governed by T = k · d · F, where T is torque (N·m), k is the torque friction factor, d is nominal diameter (m), and F is target clamping preload force (N).
VoltCheck24 Mechanical Fastener Calculator
Bolt Tightening Torque Calculator
Calculate precision tightening torque, clamp preload force, and safety margins for metric fasteners (M4 to M36) across Property Classes 4.8, 8.8, 10.9, and 12.9.
Friction Coefficient (k-factor)
Dry, unlubricated black oxide fasteners typically have k ≈ 0.20. Lightly oiled threads drop to k ≈ 0.15, while anti-seize or PTFE coatings drop to k ≈ 0.10 - 0.12. Using the wrong k-factor can create a 30%+ error in clamping force!
Preload Safety Guideline
Industrial standards (VDI 2230 / ISO 898-1) recommend tightening to 90% of fastener yield strength (0.9 × Rp0.2 × As) during elastic assembly to prevent vibration loosening while avoiding permanent plastic deformation.
Frequently Asked Questions
Why should you never tighten high-strength bolts by feel with an impact wrench?
Impact wrenches apply non-linear hammering torque spikes that easily exceed the ultimate tensile strength of Grade 10.9 and 12.9 bolts, leading to delayed stress fracture or stripped threads.
What is the difference between Grade 8.8 and Grade 10.9 bolts?
Grade 8.8 has a nominal tensile strength of 800 MPa and yield ratio of 80% (640 MPa). Grade 10.9 has a tensile strength of 1000 MPa and yield of 900 MPa, delivering 40% higher clamp load for identical thread diameter.