⚡ Key Takeaways
- The core formula: T = K × F × d (torque = torque coefficient × preload × nominal diameter).
- Typical K values: 0.20 for dry zinc plating, 0.10–0.14 lubricated, 0.08–0.12 with PTFE.
- Where the torque goes: about 90% is consumed by friction and only 10% becomes preload.
- Standard: GB/T 16823.3-2010, fasteners — torque/clamp force testing.
- The catch: at the same torque a lower K gives higher preload, but also more scatter.
📑 Contents
1. Why K matters
2. Common K values
3. The torque/clamp-force test
4. Worked examples
5. How much preload to use
6. Getting a K-value report
1. Why K matters
2. Common K values
3. The torque/clamp-force test
4. Worked examples
5. How much preload to use
6. Getting a K-value report
1. Why K matters
Tightening torque follows:
T = K × F × d
where T is torque (N·m), K the torque coefficient, F the preload (N) and d the nominal diameter (mm).
The key facts:
- About 50% of the torque is lost to friction under the bearing face;
- About 40% is lost to friction in the thread;
- Only 10% becomes axial preload.
2. Common K values
| Finish / lubrication | K value | Friction coefficient μ |
|---|---|---|
| Zinc plated, dry | 0.20 | 0.15 |
| Black oxide, dry | 0.20–0.24 | 0.16 |
| Cadmium plated, dry | 0.18 | 0.13 |
| Zinc plated + machine oil | 0.14 | 0.10 |
| Molybdenum disulphide | 0.10–0.12 | 0.08 |
| PTFE coating | 0.08–0.12 | 0.07 |
| Dacromet | 0.12–0.15 | 0.10 |
| Stainless steel, dry | 0.25–0.30 | 0.18 |
3. The torque/clamp-force test
Standard: GB/T 16823.3-2010 / ISO 16047.
Method:
- Clamp the sample between a torque transducer and an axial force transducer;
- Tighten at the specified speed;
- Record the torque/axial force curve;
- Calculate K and its scatter.
Acceptance: the mean K is within ±20% of nominal and the scatter is 10% or less.
4. Worked examples
Example 1: M10 class 8.8 zinc plated, required preload F = 25,000 N, K = 0.20:
T = 0.20 × 25,000 × 0.010 = 50 N·m.
Example 2: M12 class 10.9 Dacromet, F = 45,000 N, K = 0.13:
T = 0.13 × 45,000 × 0.012 = 70 N·m.
Example 3: M6 stainless, dry, F = 8,000 N, K = 0.27:
T = 0.27 × 8,000 × 0.006 = 13 N·m.
5. How much preload to use
| Application | Preload as % of yield | Note |
|---|---|---|
| General joints | 60% | Leaves a safety margin |
| Critical joints | 70% | Automotive engines |
| Alternating load | 50% | Fatigue protection |
| Sealed joints | 75% | Keeps the gasket compressed |
| Yield-point tightening | 90% | Torque-plus-angle method |
6. Getting a K-value report
🛠️ Engineering support: Toprecision issues K-value test reports to GB/T 16823.3 on request.
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👉 Engineering services ｜ 👉 Request a technical quote