Bolt Torque & Preload.
Calculate tightening torque for any bolt. Metric M3–M64 and UNC fasteners. Grades 4.6 to 12.9, stainless A2/A4. Adjusts for friction coefficient (dry, lubricated, anti-seize).
F_preload = util × σ_yield × As
T = F × (0.16·p + 0.58·d₂·μ + 0.625·d_head·μ)
K-factor: T ≈ K · d · F (where K ≈ 0.20 dry, 0.14 lub.)
Quick reference — 8.8 grade, μ=0.14, 75% preload
| Bolt | Tensile area (mm²) | Yield force (kN) | Torque (N·m) | Torque (kgf·m) |
|---|---|---|---|---|
| M5 (8.8) | 14.2 | 9.1 | 5.5 | 0.56 |
| M6 (8.8) | 20.1 | 12.9 | 9.5 | 0.97 |
| M8 (8.8) | 36.6 | 23.4 | 23 | 2.34 |
| M10 (8.8) | 58.0 | 37.1 | 46 | 4.69 |
| M12 (8.8) | 84.3 | 53.9 | 80 | 8.16 |
| M14 (8.8) | 115 | 73.6 | 127 | 13.0 |
| M16 (8.8) | 157 | 100 | 197 | 20.1 |
| M20 (8.8) | 245 | 157 | 385 | 39.3 |
| M24 (8.8) | 353 | 226 | 665 | 67.8 |
| M30 (8.8) | 561 | 359 | 1330 | 136 |
Bolt Torque and Preload
Bolt joints work by preload — the tension force in the bolt that clamps the joint together. Torque is just the means to create preload. Get the torque right and the joint will hold for life; get it wrong and the joint loosens, fatigues, or strips threads.
Torque Formula
T = K × F × d
Where T = tightening torque (Nm), K = nut factor (0.18-0.22 typical), F = preload (N), d = nominal diameter (m). K varies with thread condition: 0.20 dry, 0.16 lubricated, 0.12 with anti-seize.
Bolt Grade Strengths
- Grade 8.8: yield 640 MPa, tensile 800 MPa — general purpose
- Grade 10.9: yield 940 MPa, tensile 1040 MPa — high strength
- Grade 12.9: yield 1100 MPa, tensile 1220 MPa — critical fasteners
- SAE Grade 5: yield 634 MPa, tensile 827 MPa
- SAE Grade 8: yield 896 MPa, tensile 1034 MPa
Worked Example
M10 × 1.5 grade 10.9 bolt, target preload 75% of yield, dry threads (K=0.20).
- Stress area A_s for M10×1.5 = 58 mm²
- Yield force = 940 × 58 = 54,520 N
- Target preload = 0.75 × 54,520 = 40,890 N (~41 kN)
- T = 0.20 × 40,890 × 0.010 = 82 Nm
Tightening Methods Compared
- Torque only: ±25-35% scatter in preload. Standard for non-critical.
- Torque + Angle: ±5-10% scatter. Used in engine, transmission, structural.
- Torque + Yield: ±2-5% scatter. Production-line for critical joints.
- Hydraulic tensioner: ±2% scatter. Wind turbine, pressure vessels.
Why Bolts Loosen
- Vibration → use Loctite, Nyloc, or split-washers
- Insufficient preload → re-torque to spec
- Embedment (gasket compression after assembly) → re-torque after first cycle
- Thermal cycling → check that joint stiffness ratio is correct for the application
Related Tools
For pressed alternatives, see Press-Fit Calculator. For thread spec lookup, see Thread Pitch. Document torque values on AS9102 Form 3.