How to use Bolt Tightening Torque& Preload Calculator
- Enter Bolt nominal size (Metric / Imperial) and adjust the options as needed.
- Run the tool and inspect its results, evidence and limitations.
- Copy or download the result, then verify it against the actual website, source data or business records.
Try a concrete example
For an M16 Grade 8.8 bolt (stress area 157 mm², yield strength 640 MPa) loaded to 70% yield utilization, allowable clamping preload is 70.3 kN (~7.17 metric tons). Under light machine oil lubrication (nut factor k=0.15), recommended tightening torque is 168.8 N·m (124.5 lbf·ft).
How to interpret results
Tightening torque follows the Kellermann-Klein model T = k · F · d. Approximately 50% of applied torque overcomes nut bearing face friction, ~40% overcomes thread flank friction, and only ~10% converts to axial clamping preload. Lubrication condition decisively dictates torque accuracy.
Frequently asked questions
Why shouldn't bolts be tightened "as hard as possible"?
Tightening induces simultaneous axial tension and torsional shear stress. Over-tightening past the yield point produces plastic necking, initiating microcracks that result in catastrophic fatigue fracture or stress corrosion failure under cyclic loading.
What is the nut factor k, and why does lubrication cause a 2x torque variation?
The nut factor k aggregates thread geometry and friction across thread flanks and bearing faces. Dry zinc-plated fasteners exhibit k ≈ 0.20~0.22, whereas MoS₂ or anti-seize paste drops k to 0.10~0.12. Applying dry torque specs to lubricated bolts increases preload by 100%, causing bolt yield or stripping.
What mechanical properties do bolt grades 8.8, 10.9 and 12.9 designate?
The first number is 1/100 of minimum tensile strength (8.8 = 800 MPa). The second digit is the yield ratio (.8 = 80% yield strength, so 800 × 0.8 = 640 MPa yield). Grade 10.9 yields at 900 MPa, and Grade 12.9 yields at 1080 MPa.
Why must flange bolts be tightened in a cross-star or diagonal sequence?
Tightening sequentially in a circle causes elastic interaction relaxation where earlier bolts loosen as adjacent bolts compress the gasket. Best practice requires a three-pass diagonal cross pattern: 30% snugging, 70% intermediate torque, and 100% final torque with a clockwise check pass.