Tightening, Loosening and Breaking Torque
Three different torque values, and a practical basis for how tight is tight enough.
"Tighten it firmly" is not a specification. Torque comes in three distinct values.
| Type | Meaning | Approximate relation |
|---|---|---|
| Tightening torque | The design value to tighten to | Reference (100%) |
| Loosening torque | Torque needed to undo a tightened bolt | About 70–80% of tightening torque |
| Breaking torque | Torque at which the bolt fails | About 2–2.5× tightening torque (A2-50 basis) |
For example, with A2-50 M6 at a friction coefficient of 0.2, if the tightening torque is about 4.1 N·m, loosening torque is roughly 2.8–3.3 N·m and breaking torque roughly 8.2–10.3 N·m.
Tightening torque is set from proof stress, while fracture occurs in the tensile strength region. The narrower the gap between the two, the smaller the margin.
| Property class | Proof / tensile (N/mm²) | Yield ratio | Margin to fracture |
|---|---|---|---|
| A2-50 | 210 / 500 | 0.42 | Wide — about 2–2.5× |
| A2-70 | 450 / 700 | 0.64 | Narrow |
| A2-80 | 600 / 800 | 0.75 | Very narrow — down to about 1.5× |
Why tighten to less than half the breaking torque
A bolt's job is to generate clamp load (preload) that holds the joint together. Tighten beyond the proof stress and the bolt stretches permanently, and that much clamp load is lost. A stretched rubber band no longer grips what it is holding. Clamp load is then not maintained under vibration or thermal cycling — the joint looks tight but is effectively loose.
Design preload is therefore based on proof stress, with fracture left above as margin.
What makes torque values move
Most of the applied torque is consumed overcoming friction; only around 10–15% converts into clamp load. Change the friction condition and the same torque produces a very different preload.
- Lubrication — clamp load rises sharply at the same torque. Reduce the torque
- Surface finish — friction coefficients differ by plating type
- Corrosion — corrosion raises friction substantially, so the same torque no longer secures the intended clamp load
- Thread lockers — with prevailing-torque nuts (nylon insert type) or anaerobic thread-locking adhesives, loosening torque can exceed tightening torque
In stainless work, lubrication is discussed for more than clamp load control. It also serves to prevent threads from seizing. (more on galling)
The figures above are examples for a specific condition (material, size, friction coefficient) and are not guaranteed values. We recommend verification under conditions matching your actual application. Contact us if you need recommended tightening torque for a specific item. Request a quote