Free Engineering Tool
Bolt Tightening Torque Calculator
Calculate the required torque to achieve proper bolt preload force. Supports coarse & fine pitch metric threads, imperial UNC, and multiple lubrication conditions.
This calculator was developed and verified by the Vibromera engineering team, makers of the Balanset portable balancing instruments.
Results
- 1Hand-tighten all bolts until snug
- 2Tighten to — (30% torque) in pattern
- 3Tighten to — (70% torque) in pattern
- 4Tighten to — (100% torque) — smooth motion
Torque Calculation Formula
The required tightening torque is calculated using the simplified VDI 2230 nut-factor method:
- T — tightening torque (N·m)
- K — torque coefficient / nut factor (0.10–0.25)
- F — target preload force (N)
- d — nominal bolt diameter (m)
Preload Force
- S — strength basis: Rp (yield) or Sp (proof stress)
- As — tensile stress area (mm²)
- η — utilization factor (0.50–0.90)
Kellermann-Klein Equation
K-Factor Reference
| Surface Condition | K-factor | Notes |
|---|---|---|
| Dry | 0.20–0.25 | High scatter |
| Light oil | 0.14–0.18 | Most common |
| MoS₂ | 0.10–0.12 | High loads, stainless |
| PTFE | 0.08–0.10 | Caution: over-torque |
| Zinc plated | 0.17–0.20 | Depends on coating |
| Anti-seize | 0.11–0.13 | High-temperature |
| Wax patch | 0.09–0.11 | Automotive OEM |
Bolt Property Classes (ISO 898-1)
| Class | Rm | Rp | Sp | Application |
|---|---|---|---|---|
| 4.6 | 400 | 240 | 225 | Non-critical |
| 5.6 | 500 | 300 | 280 | General |
| 8.8 | 800 | 640 | 580/600 | Standard structural |
| 10.9 | 1040 | 940 | 830 | High-strength |
| 12.9 | 1220 | 1100 | 970 | Critical, max load |
Practical Examples
K=0.16, F=640×84.3×0.75=40,464 N → T=0.16×40464×0.012=77.7 N·m
K=0.12, F=900×245×0.75=165,375 N → T=0.12×165375×0.020=397 N·m
- Over-torquing strips threads or fractures bolt
- Under-torquing → loosening, leaks, fatigue failure
- Calibrate torque wrenches annually
- Clean threads before assembly
- Do not reuse class 10.9+ bolts
- This is an engineering estimate — test per ISO 16047 for critical joints
T = K × F × d, where K is the torque coefficient (0.10–0.25), F is preload force (N), d is nominal diameter (m).
For detailed analysis: Kellermann-Klein equation separates thread and bearing friction.
K-factor combines thread and bearing friction. Determined per ISO 16047 at 75% proof load.
- Dry: 0.20–0.25
- Oil: 0.14–0.18
- MoS₂: 0.10–0.12
- PTFE: 0.08–0.10
Standard: 75% of Rp. Range: 50% (light duty) to 90% (critical). For strict design, use Sp instead of Rp.
Lubrication dramatically reduces K-factor. Dry K≈0.22 vs PTFE K≈0.09. Using dry torque on lubricated bolt = severe over-tightening.
Multiple passes with cross/star pattern: snug → 30% → 70% → 100%. Re-check after 24h operation.
T = F × [P/(2π) + 1.154·μth·d₂ + μb·(Do+dh)/4]. Separates thread friction from bearing friction.
Fine pitch = larger stress area, higher preload, better vibration resistance, but higher strip risk. Used in automotive, aerospace.
Class 10.9+ should not be reused after design preload. Torque-to-yield bolts are single-use. Class 8.8 may be reused if undamaged.
Tool: click ±5%, digital ±2%. But friction variation causes ±25-30% preload scatter. For critical joints: torque-angle or hydraulic tensioning.
ISO 16047 = testing (measure K-factor on real fasteners). VDI 2230 = design (calculate required bolt, preload, torque). Complementary.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
