KEVOS® MAKEOS · Fasteners
Bolt Stretch and Turn-of-Nut Angle
How much will this bolt stretch at the target preload, and how far must the nut turn from snug?
Bolt elongation, member compression and the nut rotation from snug-tight needed to reach a target preload, from bolt and joint stiffness.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal diameter | mm | Nominal bolt diameter. |
| Pitch | mm | Thread pitch. |
| Grip length | mm | Total clamped thickness. |
| Threaded length within the grip | mm | Length of thread inside the grip. |
| Bolt elastic modulus | GPa | Bolt modulus. |
| Member elastic modulus | GPa | Modulus of the clamped parts (one material assumed for the stiffness estimate). |
| Target preload | kN | Target preload from the joint design (see the bolt torque and preload tool). |
Method and formulas
- kb = 1 ÷ (Ls ÷ (E Ad) + Lt ÷ (E At)); km by the 30° frustum estimate.
- Bolt stretch δb = F ÷ kb; member compression δm = F ÷ km.
- Nut turn from snug = 360° × (δb + δm) ÷ P.
- Example: M20 with an 80 mm grip at 120 kN stretches 158.08 µm and needs about 27.696° from snug.
Limitations
- The snug point is uncertain and adds scatter; turn-of-nut procedures usually come from tests or the code (e.g. AS 4100 / RCSC tables).
- Elastic stiffness only; yield-controlled tightening goes beyond this calculation.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set a turn-of-nut or angle-tightening procedure
- Check ultrasonic or micrometer bolt stretch readings
- Compare joint stiffness options
Who it is for
- Design engineers
- Maintenance fitters
- Quality inspectors
What you get
- Bolt and member stiffness
- Bolt elongation at preload
- Member compression
- Nut turn from snug
Interpreting the result
- Validate any angle procedure on the actual joint with a tension-indicating method.
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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