Understanding Bolt Torque and Clamp Load
When you tighten a bolt, you are essentially stretching it like a very stiff spring. This stretching creates a tension, or "preload," that clamps two or more parts together. Torque is simply the rotational force needed to create this tension. The goal is to apply enough torque to achieve the correct clamp load without exceeding the bolt's elastic limit.
This calculator uses the standard industry formula: T = K × D × P, where T is Torque, K is the Torque Coefficient (nut factor), D is the nominal bolt diameter, and P is the desired clamp load (preload).
Common Torque Coefficients (K)
| Condition | K-Value |
|---|---|
| Non-plated, black finish (dry) | 0.30 |
| Zinc-plated (dry) | 0.20 |
| Lubricated (e.g., engine oil) | 0.15 - 0.18 |
| Cadmium-plated (dry) | 0.16 |
How to Use the Calculator
- Select the diameter of your bolt.
- Select the grade of your bolt (e.g., Grade 5, Grade 8). This determines its strength.
- Select the appropriate Torque Coefficient (K factor) based on the condition of your threads (dry, zinc-plated, lubricated).
- Click "Calculate Torque" to see the target clamp load and the recommended tightening torque in foot-pounds.
Frequently Asked Questions
What is "Clamp Load" or "Preload"?
Clamp load is the force that the tightened bolt exerts on the parts it is holding together. Achieving the correct clamp load is the entire purpose of tightening a bolt. It's what keeps the joint from separating or shifting under load.
Why does lubrication change the required torque?
A significant portion of torque is used to overcome friction between the threads and under the bolt head. Lubrication reduces this friction. This means for the same amount of applied torque, a lubricated bolt will produce much more stretching (clamp load) than a dry bolt. Using the wrong K factor can lead to severely under-tightened or over-tightened (broken) bolts.