Calculate clamp load, torque, diameter or nut factor from any three values with unit conversions for Nm, ft-lb, mm, cm, inches, kN, N and lb.
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Clamp Load Formula
The clamp load calculation is based on the torque-tension relationship for a threaded fastener. The calculator uses torque in newton-meters, diameter in millimeters, clamp load in newtons, and a dimensionless nut factor.
Rearranged forms used to solve for each missing value:
- T = torque, in Nm
- F = clamp load, in N
- D = nominal fastener diameter, in mm
- K = nut factor, dimensionless
- 1000 = conversion factor because 1 Nm = 1000 N·mm
To calculate clamp load, enter torque, diameter, and nut factor. To calculate torque, enter clamp load, diameter, and nut factor. You can also leave diameter or nut factor blank if the other three values are known. Unit selections are converted to the base units before the formula is applied, then the result is converted back to the unit you selected.
Typical Nut Factor Values
The nut factor is an estimate of how much applied torque becomes bolt tension. Friction in the threads and under the bolt head or nut has a large effect on this value.
| Fastener condition | Typical K value | Notes |
|---|---|---|
| Clean, dry steel threads | 0.20 to 0.25 | Common rough estimate when no test data is available |
| Lightly oiled threads | 0.15 to 0.20 | Lower friction increases clamp load for the same torque |
| Lubricated or plated threads | 0.10 to 0.18 | Depends strongly on coating and lubricant |
| Prevailing torque lock nut | Varies | The simple torque-tension equation may not capture locking torque accurately |
Common Unit Conversions
| Quantity | Conversion |
|---|---|
| Torque | 1 ft-lb = 1.35582 Nm |
| Diameter | 1 inch = 25.4 mm |
| Clamp load | 1 lb = 4.44822 N |
| Clamp load | 1 kN = 1000 N |
Example Calculations
Example 1: Calculate clamp load
You apply 80 Nm of torque to a 10 mm fastener with a nut factor of 0.20.
The clamp load is 40,000 N, or 40 kN.
Example 2: Calculate required torque
You want a clamp load of 25 kN on a 12 mm fastener with a nut factor of 0.18.
First convert clamp load to newtons: 25 kN = 25,000 N.
The required torque is 54 Nm.
FAQ
What is clamp load?
Clamp load is the tensile force created in a bolt or screw when it is tightened. That tension pulls the joined parts together. In this calculator, clamp load is estimated from torque, diameter, and nut factor.
Why does the nut factor matter so much?
The nut factor accounts for friction in the threads and under the nut or bolt head. A lower K value means less torque is lost to friction, so the same torque creates more clamp load. A higher K value means more torque is lost to friction, so clamp load is lower.
Is this the same as proof load or bolt strength?
No. Clamp load is the preload created by tightening. Proof load and tensile strength are material limits of the fastener. When choosing a tightening torque, make sure the resulting clamp load stays within the allowable range for the bolt, threads, and joint.
