What a Torque Wrench Is Really Controlling
A torque wrench does not directly measure bolt preload. It applies a twisting moment to the fastener. The preload is only inferred from the relationship between torque, diameter, and friction.
This is why two bolts tightened to the same torque can still end up with different clamp forces if lubrication, plating, washers, or thread condition are different.
The Unit Fix That Matters
Yield stress or proof stress in MPa is the same as N/mm². That means the bolt tensile stress area should be calculated in mm².
The old version converted diameter from mm to cm, then multiplied MPa by cm². That produces inconsistent units. This version keeps the stress calculation in N/mm² and mm², so the preload force comes out in Newtons.
Effective Tensile Stress Area
For ISO metric threads, a common tensile stress area approximation is:
where d is nominal diameter and p is thread pitch, both in mm.
Torque Estimate
After choosing a target preload force, the calculator estimates tightening torque with:
In this equation, d must be in meters if torque is reported in N·m.
Typical Nut Factor Ranges
| Condition | Typical K estimate |
|---|---|
| Controlled lubrication | 0.10 to 0.15 |
| Light lubrication | 0.15 to 0.18 |
| Dry steel estimate | Around 0.20 |
| High friction or rough condition | 0.25 to 0.30+ |
Common Mistakes
- Using nominal shank area instead of tensile stress area.
- Mixing MPa with cm² instead of mm².
- Assuming torque gives exact preload.
- Ignoring lubrication and coating effects.
- Using one torque value for every bolt grade and joint condition.
- Subtracting external load directly from preload without a full joint-stiffness model.
Frequently Asked Questions
Is bolt preload the same as tightening torque?
No. Preload is the axial clamping force in the bolt. Torque is the twisting moment used to create that preload.
Why does this calculator ask for thread pitch?
Thread pitch is used to estimate tensile stress area. A threaded bolt does not carry tensile stress over the full nominal diameter area.
What preload percentage should I use?
Many preliminary bolted-joint estimates use a percentage of proof or yield capacity, but the correct value depends on the joint, standard, bolt grade, tightening method, and application.
Can I use this for engine, pressure vessel, or structural bolts?
Not by itself. Those applications require approved procedures, standards, qualified engineering review, and often calibrated tightening or direct tension-control methods.