KEVOS® MAKEOS · Fasteners
Bolt Preload Verification from Measured Stretch (and Nut Factor)
From the bolt stretch I measured, what preload did this bolt actually get, how close is it to the target, and what nut factor does the tightening torque imply?
Verification of bolt tightening from a measured elongation: bolt stiffness from shank and thread lengths and the member stiffness (as the bolt stretch tool), preload from the measured stretch, deviation from the target preload as a share of proof load, the nut factor implied by the applied torque (as the torque–preload relation of the bolt torque–preload tool), the torque that would give the target with that nut factor, and the turn angle that corresponds to the preload.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bolt diameter | mm | Nominal bolt diameter. |
| Thread pitch | mm | Thread pitch. |
| Grip length | mm | Total thickness clamped. |
| Threaded length in the grip | mm | Thread length within the grip. |
| Bolt modulus | GPa | Elastic modulus of the bolt. |
| Clamped part modulus | GPa | Elastic modulus of the clamped parts. |
| Proof strength | MPa | Proof strength of the bolt grade. |
| Target preload as share of proof load | % | Preload you aimed for as a share of proof load. |
| Accepted deviation from target | % | Plus-or-minus deviation from the target you accept. |
| Measured bolt stretch | mm | Elongation of the bolt measured (ultrasonically or by micrometer) between loose and tight. |
| Applied tightening torque | N·m | Torque applied to reach that stretch. |
Method and formulas
- k_b = 1 ÷ [(grip − L_t) ÷ (E A_d) + L_t ÷ (E A_t)]; preload F = k_b × stretch (as the bolt stretch tool).
- Target F_t = share × S_p A_t; deviation = (F − F_t) ÷ F_t.
- Nut factor K = T ÷ (F d); torque for the target = K F_t d (as the bolt torque–preload tool); turn angle = 360° (δ_bolt + δ_member) ÷ P.
- Example: 0.15 mm of stretch means 113.87 kN (3.3672% from target); the torque implies K = 0.21077.
Limitations
- Elastic stretch only; the measurement must exclude temperature change and bending.
- Stiffness uses the simplified shank-and-thread model; the nut and head add a little compliance.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check critical bolts after tightening
- Calibrate a tightening torque from stretch measurements
- Find the real nut factor for a lubricant
Who it is for
- Maintenance engineers
- Design engineers
- Quality inspectors
What you get
- Bolt stiffness and preload from the stretch
- Deviation from target
- Implied nut factor, torque for the target and turn angle
Interpreting the result
- If K is well above the assumed value, the lubricant or thread condition is soaking up torque: preload will be low.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1521–1535. Topic reference; no source tables or recommended settings are reproduced.
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Bolt Preload Verification from Measured Stretch (and Nut Factor) calculator
