Bolt Torque Calculator Formula
Understand the math behind the bolt torque calculator. Each variable explained with a worked example.
Formulas Used
Required Torque
torque_nm = k_factor * f_n * d_mVariables
| Variable | Description | Default |
|---|---|---|
preload | Target Preload Force (F)(kN) | 50 |
bolt_dia | Nominal Bolt Diameter (d)(mm) | 16 |
k_factor | Torque Coefficient (K) | 0.2 |
f_n | Derived value= preload * 1000 | calculated |
d_m | Derived value= bolt_dia / 1000 | calculated |
How It Works
Bolt Torque-Preload Relationship
Torque applied to a bolt creates axial preload that clamps the joint together.
Formula
T = K F d
where T is the tightening torque, K is the nut factor (torque coefficient), F is the desired preload, and d is the nominal bolt diameter. K depends on lubrication: dry steel K = 0.20, lightly oiled K = 0.15, anti-seize compound K = 0.12.
Worked Example
An M16 bolt needs 50 kN preload with K = 0.2 (dry).
- 01F = 50 kN = 50,000 N
- 02d = 16 mm = 0.016 m
- 03T = 0.2 x 50,000 x 0.016 = 160 N-m
Frequently Asked Questions
What is the K factor for different conditions?
Dry steel: K = 0.20. Lightly oiled: K = 0.15. Cadmium plated: K = 0.13. Anti-seize: K = 0.12. Molybdenum disulfide: K = 0.10. Always verify with the fastener manufacturer.
How much of the bolt strength should the preload be?
For structural bolts, preload is typically 70% of the bolt proof load. For non-critical joints, 60-75% of proof load is common. Never exceed proof load as permanent deformation will occur.
Why is lubrication important for bolt torque?
Up to 90% of applied torque is consumed by friction under the nut and in the threads. Lubrication reduces friction, so the same torque produces higher preload. Always adjust K when changing lubrication.
Learn More
Guide
How to Calculate Torque: Formulas, Units, and Real-World Applications
Master torque calculations for engineering applications. Covers torque formulas, moment arms, units conversion, motor torque, bolt torque, and the relationship between torque, speed, and power.
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Open Bolt Torque Calculator