How torque becomes bolt preload
Applied torque advances the thread and overcomes friction in the thread and at the rotating bearing surface. Only part of that torque produces bolt tension. Diameter and strength class alone therefore cannot define an installation torque.
The Tightening Torque model uses the following relationship, with dimensions in millimetres, force in newtons and torque in N·m:
F = Aₛ × σy × (yield percentage / 100)
T = F × (0.16P + 0.58μd₂ + μDₖₘ / 2) / 1000
Aₛ is tensile stress area; σy is yield strength; P is pitch; d₂ is pitch diameter; Dₖₘ is the assumed effective bearing diameter. This model uses the same nominal friction coefficient μ for the thread and bearing surface. Equivalent K expresses the result in the shorter form T = K × D × F, with D in metres.
Example torque values: class 8.8 bolts
Metric coarse threads, dry clean steel (μ = 0.1469), and target preload at 70% of yield load. These calculated examples are not recommended installation settings.
| Thread | Pitch (mm) | Preload (kN) | Torque (N·m) |
|---|---|---|---|
| M6 | 1 | 9.02 | 11.0 |
| M8 | 1.25 | 16.40 | 26.6 |
| M10 | 1.5 | 25.98 | 52.0 |
Model assumptions and engineering limits
Tightening Torque does not verify tapped-hole depth, parent material strength, joint stiffness, fatigue or thermal loading. Bolted Joint adds simplified static checks for bolt strength, thread stripping, bearing, slip and separation, but does not replace a complete joint design assessment.
ISO 898-1, ISO 3506-1 and SAE J429 identify fastener strength properties; a listed grade is not confirmation that every size and application falls within that standard's scope. ISO 16047 concerns torque/clamp-force testing, while VDI 2230 addresses systematic calculation of highly stressed bolted joints. Neither is a universal torque chart.
Validate friction, geometry and material properties for the actual assembly. Results are estimates, not certified torque specifications. No supplier listing or product selection is connected to these calculations.
Bolt torque questions
How is bolt tightening torque calculated?
GyverNet first estimates preload from tensile stress area, yield strength and the selected percentage of yield load. It then estimates torque from thread pitch, pitch diameter, friction and an assumed effective bearing diameter. The simple alternative is T = K × D × F. Both methods need friction data representative of the actual assembly.
What is the difference between tightening torque and bolted joint analysis?
Tightening Torque estimates torque and preload for the chosen thread, grade and friction condition. Bolted Joint also considers clamped thickness, head style, washers, thread engagement, wrench accuracy and service loads. A tightening torque result alone does not check whether a tapped block can resist thread stripping.
Does the calculator support metric and inch threads?
Yes. Tightening Torque includes metric coarse and fine threads and inch UNC and UNF threads. Its strength choices include ISO metric classes, SAE grades, ASTM designations and A2/A4 stainless classes. When a selected size has no fine thread in the data, the calculator identifies its coarse-thread fallback.
How does lubrication change the torque value?
For the same target preload, lower friction requires less tightening torque. Reusing a dry torque setting on lubricated threads can increase preload. The listed conditions are nominal assumptions, not measured friction for a particular coating or lubricant; use supplier data or torque-tension tests where available.
Can I use these results for nuts or screws in a tapped hole?
They are starting estimates, not interchangeable installation specifications. Tightening at the head or nut changes the rotating bearing surface. A tapped hole also requires checks of parent material and engagement. Use Bolted Joint to explore those assumptions; the through-bolt model assumes a matching-class nut rather than verifying an independently selected nut.
Are the results a certified torque specification?
No. These are engineering estimates using simplified models and nominal material and friction data. They do not constitute a complete VDI 2230 assessment or an ISO 16047 torque-tension test. Follow the fastener and equipment manufacturer's instructions, and validate safety-critical assemblies with appropriate testing.