Advanced Bolt Elongation Form
Example Data Table
| Bolt | Load | Length | Area | Modulus | Approx. Elongation |
|---|---|---|---|---|---|
| M10 coarse | 28 kN | 90 mm | 58 mm² | 200 GPa | 0.217 mm |
| M12 coarse | 50 kN | 120 mm | 84.3 mm² | 200 GPa | 0.356 mm |
| M16 coarse | 95 kN | 150 mm | 157 mm² | 200 GPa | 0.454 mm |
Formula Used
The calculator uses the elastic extension equation for an axially loaded fastener:
δ = F × L / (A × E)
Here, δ is elongation, F is bolt load, L is effective grip length, A is stress area, and E is elastic modulus.
Stress is calculated as σ = F / A. Strain is calculated as ε = δ / L. Bolt stiffness is calculated as k = A × E / L.
For metric thread tensile area, this page estimates At = π / 4 × (d - 0.9382p)², where d is nominal diameter and p is pitch.
How to Use This Calculator
- Enter the expected bolt load or preload.
- Enter the effective grip length between compressed joint faces.
- Select the area method. Use thread area for threaded tensile sections.
- Enter elastic modulus. Steel commonly uses about 200 GPa.
- Add yield or proof strength for safety and preload checks.
- Choose output units for elongation, stress, and force.
- Press the calculate button to view results above the form.
- Use CSV or PDF export for documentation.
Bolt Elongation Guide
Why Bolt Stretch Matters
Bolt elongation is the small elastic stretch created by tightening or loading a fastener. It is important because clamp force depends on stretch. A bolt works like a spring. When the nut turns, the bolt extends. The joint compresses at the same time. Good joints need enough stretch to hold parts together during vibration, pressure changes, and service loads.
What the Calculator Measures
This calculator estimates elongation from load, grip length, stress area, and elastic modulus. It also reports stress, strain, stiffness, yield utilization, and a target preload. These extra results help users compare a trial preload against material limits. They also make it easier to document shop checks, inspection notes, or design assumptions.
Choosing the Correct Area
The stress area has a strong effect on the answer. A threaded section stretches more than a smooth shank of the same nominal diameter. That happens because the thread root reduces the effective metal area. Use the thread tensile area option when the loaded length includes threads. Use the shank option when the loaded section is plain round stock. Use custom area when a drawing or standard gives the exact value.
Understanding the Output
A larger load increases elongation. A longer grip length also increases elongation. A larger area or higher elastic modulus reduces elongation. Stress shows how hard the bolt material is being used. Yield utilization compares the calculated stress with the entered strength. A value below one hundred percent means the elastic estimate is still within the selected strength limit.
Practical Notes
This result is an engineering estimate. Real assemblies may include thread engagement, washers, nut deformation, embedment, friction, temperature, and joint stiffness. Critical pressure vessels, lifting devices, bridges, engines, and safety parts need qualified design review. Still, elongation is a useful method because it links preload to a measurable physical change. It can support better tightening plans and more repeatable bolted joints.
FAQs
1. What is bolt elongation?
Bolt elongation is the elastic stretch produced when a bolt is tightened or pulled. It is usually very small, but it controls clamp force in many joints.
2. Which formula does this calculator use?
It uses δ = F × L / A × E. The formula assumes axial elastic loading, uniform stress area, and a material that has not yielded.
3. What is effective grip length?
Effective grip length is the stretched length of the bolt between compressed joint faces. It may differ from total bolt length.
4. Should I use shank area or thread area?
Use thread area when the loaded section includes threads. Use shank area when the stretched section is a plain cylindrical body.
5. What elastic modulus should I enter for steel?
Many steel bolts use about 200 GPa. Use the exact material value when your specification, drawing, or test certificate provides one.
6. Can this replace a bolted joint design review?
No. It gives an elastic estimate. Critical joints need checks for fatigue, relaxation, thread strength, joint separation, and safety rules.
7. Why is preload percent included?
Preload percent helps compare target clamp force with proof or yield capacity. Many designs use a controlled fraction of proof strength.
8. Why do exported PDF and CSV files help?
Exports save the input and output summary. They are useful for quality records, design notes, maintenance files, and review documentation.