Tightening, Loosening and Breaking Torque
The three torque values, what the standard's minimum breaking torque means, and how to set the right tightening torque.
"Tighten it firmly" is not a specification. Torque comes in three distinct values, and they are not linked by fixed ratios.
| Type | Meaning | What matters in practice |
|---|---|---|
| Tightening torque | Torque applied to achieve the target preload | Set from the required preload, friction and joint conditions |
| Loosening torque | Torque needed to undo a tightened bolt | Not a fixed proportion of tightening torque |
| Breaking torque | Torque at which failure occurs in a specified torsion test or a particular tightening condition | Depends on the test condition; not an allowable tightening torque |
How is the right tightening torque decided?
A bolt's job is to create preload that clamps the parts together. Tightening torque is therefore not set as a fraction of the breaking torque; it is worked back from the required preload and the joint conditions.
In normal joint design the target preload is chosen so that the bolt does not undergo excessive plastic deformation. While tightening, the bolt carries axial tension and torsional stress from thread friction at the same time, so both must be considered; over-tightening can lead to permanent deformation or failure of the bolt or thread.
The gap between 0.2% proof stress and tensile strength narrows as the property class rises (A2-50 210/500, A2-70 450/700, A2-80 600/800 N/mm²). The ratio of 0.2% proof stress to minimum tensile strength rises with the class. That ratio alone, however, cannot tell you the margin to plastic deformation or failure during tightening; combined tensile and torsional stress and the friction condition must be considered as well. Nor is it the ratio of tightening torque to breaking torque. (More on property classes)
To calculate tightening torque and preload for a given material, size, head type and friction condition, use the tightening torque and preload calculator, which shows the formula and assumptions.
What is the standard's "minimum breaking torque"?
ISO 3506-1:2020 gives a minimum breaking torque for austenitic stainless bolts and screws: an acceptance criterion stating that the fastener must not break below this value in the specified torsion test.
| Thread | M2 | M2.5 | M3 | M4 | M5 | M6 | M8 |
|---|---|---|---|---|---|---|---|
| Class 50 | 0.3 | 0.6 | 1.1 | 2.7 | 5.5 | 9.3 | 23 |
| Class 70 | 0.4 | 0.9 | 1.6 | 3.8 | 7.8 | 13 | 32 |
| Class 80 | 0.48 | 0.96 | 1.8 | 4.3 | 8.8 | 15 | 37 |
N·m. ISO 3506-1:2020 Table 8 (minimum breaking torque, austenitic grades). The standard notes that the drive or recess may not be able to transmit this torque, and that this is not a cause for rejection.
※ These are minimum values for austenitic stainless coarse-thread screws (classes 50, 70, 80) that meet the torsion test conditions of ISO 3506-1:2020. The standard (9.5.1) applies the torsion test to nominal diameters 1.6 mm ≤ d < 3 mm at all lengths and to 3 mm ≤ d ≤ 16 mm with nominal length l < 2.5d, with thread length b ≥ d + 2P (P = pitch) — in effect, an alternative to tensile strength for screws too short to tensile test. It applies only to fasteners with full loadability, so these values are not applied as they stand to fasteners with reduced loadability because of head geometry, such as countersunk heads.
The torsion test twists the thread to failure without preload. In real tightening, tension from the preload and torsion act together, so failure can occur at a lower torque than these values. The table verifies the torsional strength of the product; it is not a maximum allowable tightening torque on site.
Loosening torque is not a fixed proportion
Loosening torque is the torque needed to undo a tightened bolt or nut. In ordinary joints it may be lower than the tightening torque, but the value depends on what is measured.
- Breakaway torque — the torque at which a stationary joint first starts to turn in the loosening direction. It is affected by static friction, surface condition, lubrication, corrosion and thread lockers
- Running torque — the torque needed to keep turning once loosening has begun
With anaerobic thread lockers (such as Loctite), breakaway torque can exceed tightening torque. Prevailing-torque nuts (nylon insert type) keep generating their own rotational resistance after loosening begins, so breakaway torque and this prevailing torque should be considered separately; ISO 2320 evaluates the prevailing torque of such nuts in the tightening and loosening directions. Loosening torque therefore cannot be calculated as a fixed ratio, and it is difficult to judge the original preload from loosening torque alone.
What makes torque values vary
Most of the tightening torque goes into overcoming friction. One commonly cited breakdown is about 50% under the head, about 40% in the thread and only about 10% into preload (it varies with conditions). If the friction condition changes, the same torque gives a very different preload.
How much preload changes with friction is shown with numbers and a graph in Most tightening torque goes into friction.
- Lubrication — lubrication lowers friction, so the same torque can give a higher preload. Do not reuse the dry torque; confirm a torque suited to the target preload and lubricated condition
- Type of finish — each plating has a different friction coefficient
- Corrosion — corrosion can raise friction sharply, so the same torque may no longer achieve the preload
- Friction coefficient vs torque coefficient — the torque coefficient K in the simple formula T = K·F·d is not the same as the friction coefficient μ. ISO 16047 measures the overall torque coefficient, thread friction and under-head friction separately
Lubrication comes up with stainless for another reason as well: preventing galling, where threads seize. (More on galling)
Formulas and standard values are a starting point, not guaranteed values. For important joints, confirm the torque–preload relationship with the actual parts and tools. If you need tightening conditions reviewed for a specific item, please contact us.
References
- VDI 2230 Blatt 1:2015 Systematic calculation of highly stressed bolted joints — Joints with one cylindrical bolt — torque–preload relation; combined tensile and torsional stress and assembly preload
- ISO 3506-1:2020 Fasteners — Mechanical properties of corrosion-resistant stainless steel fasteners — Part 1: Bolts, screws and studs with specified grades and property classes — Table 2 proof and tensile strength by property class, Table 8 minimum breaking torque for austenitic grades (M1.6–M16); torque/clamp force characteristics not specified
- ISO 16047:2005 Fasteners — Torque/clamp force testing — overall torque coefficient, thread and under-head friction coefficients measured separately
- ISO 2320:2015 Fasteners — Prevailing torque steel nuts — Functional properties — prevailing torque and functional tests for steel nuts (ISO 898-2); for stainless nuts, confirm the applicable standard and test conditions separately
- Nord-Lock Group, Friction — a force of nature — about 50% under-head friction, 40% thread friction, 10% preload
Last revised 2026-10-09