KEVOS® MAKEOS · Fasteners
Bolt Tightening Torque and Preload
What preload does this torque give, or what torque gives the target preload?
Relation between tightening torque and bolt preload for metric bolts using the nut factor (T = K F d), with stress in the tensile stress area and the share of proof load.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Calculate | choice | Choose torque from a preload target, or preload from a torque. |
| Nominal diameter | mm | Nominal (major) diameter of the bolt. |
| Thread pitch | mm | Thread pitch (coarse M12 is 1.75 mm). |
| Nut factor K | Nut factor for the actual surface finish and lubricant, from the fastener supplier or a torque–tension test (about 0.2 for plain dry steel, lower when lubricated). | |
| Proof strength of the bolt | MPa | Proof stress for the property class from the bolt standard (e.g. ISO 898-1) or the supplier. |
| Target preload as % of proof load | % | Target preload as a percentage of proof load from your joint design (often 65–75%). |
| Tightening torque | N·m | Specified tightening torque (preload mode). |
Method and formulas
- Tensile stress area As = π/4 (d − 0.938194P)².
- Proof load = Sp × As; target preload F = % × proof load.
- Torque T = K F d (d in metres gives N·m); preload from torque F = T ÷ (K d).
- Example: M12 × 1.75 at 75% of a 600 MPa proof stress needs 37.92 kN, about 91.008 N·m with K = 0.2.
Limitations
- The nut factor scatters widely (often ±25%); preload from torque is approximate. Use tension control, turn-of-nut or direct tension indicators for critical joints.
- Coarse and fine metric threads only; prevailing-torque nuts add torque that does not produce preload.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set a tightening torque for a joint
- Check what preload a specified torque produces
- Compare lubricated and dry assembly
Who it is for
- Design engineers
- Maintenance fitters
- Quality inspectors
What you get
- Tensile stress area
- Preload and bolt stress
- Share of proof load
- Tightening torque
Interpreting the result
- Specify torque with the lubricant condition; changing lubrication changes the preload at the same torque.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1535. Topic reference; no source tables or recommended settings are reproduced.
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