Bolt Torque Calculator
Metric & Imperial Preload, Stress & Stretch Analysis
How the Bolt Torque Calculator Works
Fundamental Torque-Preload Relationship
The calculator uses the standard torque-preload formula:
Where:
- T = Torque (N·m or lbf·ft)
- K = Torque coefficient (friction factor)
- F = Preload force (N or lbf)
- d = Nominal bolt diameter (m or ft)
Bolt Stress Calculation
Bolt stress is calculated using the tensile stress area:
Where At is the tensile stress area, calculated based on bolt diameter and thread pitch.
Bolt Stretch Calculation
Elongation is determined using Hooke's Law:
Where E is the modulus of elasticity (typically 200 GPa for steel).
Standard Torque Coefficients (K-Factors)
| Surface Condition | K-Factor | Typical Applications |
|---|---|---|
| Dry, Plain Finish | 0.20 - 0.30 | Unlubricated steel bolts |
| Cadmium Plated | 0.16 - 0.20 | Corrosion resistance with moderate friction |
| Zinc Plated | 0.17 - 0.22 | General purpose corrosion protection |
| Lubricated (Oil/Grease) | 0.11 - 0.16 | Precision applications, reduced friction |
| PTFE Coated | 0.10 - 0.14 | Low friction, corrosion resistant |
Bolt Material Strength Grades
Metric Bolt Grades
| Grade | Proof Strength (MPa) | Yield Strength (MPa) | Tensile Strength (MPa) |
|---|---|---|---|
| 4.6 | 225 | 240 | 400 |
| 8.8 | 580 | 640 | 800 |
| 10.9 | 830 | 940 | 1040 |
| 12.9 | 970 | 1100 | 1220 |
Imperial Bolt Grades
| Grade | Proof Strength (ksi) | Yield Strength (ksi) | Tensile Strength (ksi) |
|---|---|---|---|
| Grade 2 | 55 | 57 | 74 |
| Grade 5 | 85 | 92 | 120 |
| Grade 8 | 120 | 130 | 150 |
Limitations & Assumptions
Important Engineering Disclaimer: This calculator provides approximate values for educational and preliminary design purposes only.
- Friction Variability: Actual K-factors can vary significantly based on surface finish, lubrication quality, and installation speed
- Thread Engagement: Assumes sufficient thread engagement (typically 1.5× diameter for steel)
- Material Properties: Uses standard material properties that may not match specific manufacturers' specifications
- Temperature Effects: Does not account for temperature variations affecting material properties
- Dynamic Loading: Calculations are for static loading conditions only
For critical applications, always consult relevant engineering standards (ASME, ISO, DIN) and perform physical testing.
Frequently Asked Questions
What is the difference between metric and imperial bolt torque calculations?
The fundamental physics are identical, but unit conversions and standard thread forms differ. Metric bolts use ISO thread standards while imperial bolts use UNC/UNF standards. The calculator handles these differences automatically.
How do I choose the right K-factor for my application?
Select the K-factor based on your surface condition and lubrication. When in doubt, use a conservative value (higher K-factor) or consult manufacturer specifications. For critical joints, measure the actual K-factor through testing.
Why is bolt preload important?
Proper preload ensures the joint remains tight under operating conditions, prevents loosening from vibration, and distributes loads evenly across joint members. Insufficient preload can lead to joint failure, while excessive preload may cause bolt yielding.
What percentage of yield strength should I target for preload?
For most applications, 75-90% of proof load is typical. For critical or dynamic applications, consult specific design standards. Never exceed the proof load in service.
How does thread pitch affect torque requirements?
Finer threads (higher pitch) generally require slightly less torque for the same preload due to better mechanical efficiency, but have reduced shear area. The calculator accounts for this in advanced mode.
Can I use this calculator for critical applications?
This calculator is designed for preliminary design and educational purposes. For critical applications, always verify calculations with physical testing and consult relevant engineering standards and qualified engineers.