Understanding Bolt Torque
Bolt torque is the rotational force applied to tighten a bolt to achieve the proper clamping force (preload) in a bolted joint. The relationship between torque and clamping force depends on the bolt diameter, thread pitch, friction coefficient, and bolt material strength. Proper torque is critical for structural integrity, leak prevention, and joint reliability.
Under-torquing leads to insufficient clamping force, which can cause joint loosening, leaks, or fatigue failure. Over-torquing can strip threads, break bolts, or damage the clamped components. This calculator uses the short-form torque equation to determine the recommended tightening torque.
Torque Formula
Where T = torque (in-lbs), K = friction factor (nut factor), D = bolt diameter (inches), F = desired preload (lbs).
Bolt Grade Reference
| SAE Grade | Metric Class | Proof Strength (psi) | Material |
|---|---|---|---|
| Grade 2 | Class 4.6 | 55,000 | Low/med carbon steel |
| Grade 5 | Class 8.8 | 85,000 | Medium carbon, Q&T |
| Grade 8 | Class 10.9 | 120,000 | Alloy steel, Q&T |
| -- | Class 12.9 | 150,000 | Alloy steel, high strength |
K Factor (Nut Factor) Values
- Dry/plain (K=0.20): Standard unlubricated steel bolts.
- Zinc plated (K=0.18): Slightly lower friction than plain.
- Oiled (K=0.15): Light machine oil or assembly lube.
- Anti-seize (K=0.12): Moly-based or copper anti-seize compound.
Frequently Asked Questions
Why does lubrication affect torque?
Approximately 85-90% of applied torque is consumed by friction (under the bolt head and in the threads). Lubrication reduces this friction, so less torque is needed to achieve the same clamping force. Using anti-seize without reducing the torque specification can over-stress the bolt by up to 60%.
What percentage of proof load should I target?
75% of proof load is the most common target for general engineering applications. For critical connections subject to vibration, 80-85% may be used. For soft gasket applications, lower values (60-70%) may be appropriate to avoid gasket crush.
Should I use torque or turn-of-nut method?
Torque wrenches are standard for most applications. The turn-of-nut method (snug plus specified rotation) is used for structural steel connections per AISC specifications and can be more reliable because it is less affected by friction variation.