Bolt Torque & Tension Preload Calculator
Calculate tightening torque (lb-ft and N·m) and axial clamping preload tension from bolt diameter, thread friction factor K, and fastener tensile grade.
Calculated Results
Frequently Asked Questions
What is the formula for calculating bolt torque from clamping tension?
The universal mechanical engineering formula is: T = (K × D × F) / 12, where T is torque in lb-ft, K is the dimensionless nut friction factor, D is nominal bolt diameter in inches, and F is desired clamping tension preload in pounds.
Why does thread lubrication dramatically reduce required torque?
Up to 90% of applied torque is lost overcoming friction between threads and under the bolt head. Using anti-seize (K = 0.12) instead of dry steel (K = 0.20) reduces friction by 40%. If you apply dry torque specifications to lubricated threads, you risk stretching or shearing the bolt!
What percentage of proof load should bolts be torqued to?
In standard mechanical assemblies, fasteners are torqued to 75% of their yield strength or proof load. This ensures maximum clamping retention without causing permanent plastic thread elongation.
How do you identify a Grade 5 vs Grade 8 hex bolt?
Look at the radial hash marks on the bolt head: Grade 5 has 3 radial lines (3 + 2 = Grade 5). Grade 8 high-tensile alloy has 6 radial lines (6 + 2 = Grade 8). A standard low-strength Grade 2 bolt has no markings.