Same bolt, other friction values
How the calculator works
It follows the method behind the tightening tables in VDI 2230 Part 1 for a bolt tightened by torque:
- Thread geometry. Pitch diameter d2 = d − 0.6495P, minor diameter d3 = d − 1.2269P. Tensile stress area As = π/4 · ((d2+d3)/2)² for metric (ISO 898-1) and π/4 · (d − 0.9743P)² for UNC/UNF (ASME B1.1).
- Maximum assembly preload. The bolt is tightened until the von Mises stress from tension plus thread torsion reaches ν times the minimum yield strength: FM = As·ν·Rp0.2 / √(1 + 3[1.5·(d2/d0)·(P/(πd2) + 1.155μG)]²), where d0 is the diameter of As.
- Tightening torque. MA = FM·[(d2/2)·tan(φ + ρ′) + (DKm/2)·μK], with lead angle tanφ = P/(πd2), tanρ′ = 1.155μG for a 60° thread, and DKm = (dW + dh)/2 the mean head bearing diameter.
- Scatter. Torque control does not give an exact preload. The tightening factor αA = FM,max/FM,min from VDI 2230 Table A8 shows the lowest preload you should design the joint for.
| Check against published values (μG = μK = 0.12, ν = 90%) | This calculator |
|---|---|
| M10 class 8.8 hex: about 29.6 kN and 48–49 N·m | |
| M12 class 10.9 hex: about 63.3 kN and 123 N·m |
Friction is the biggest unknown
For a typical bolt, most of the tightening torque goes into friction, in the thread and under the head; only a small share ends up as stretch in the bolt. A 0.02 change in friction moves the preload you get from a given torque by roughly 12 to 20%, more at low friction. That is why a torque value without a friction coefficient means little, and why the table above shows the same bolt across a range of friction values. Use the friction values your fastener supplier publishes for the coating and lubricant you actually use, and if the joint matters, measure them.
Strength values used
| Class | Min. yield Rp0.2 used | Source |
|---|---|---|
| 8.8 | 640 MPa (d ≤ 16 mm), 660 MPa (d > 16 mm) | ISO 898-1 |
| 10.9 / 12.9 | 940 / 1100 MPa | ISO 898-1 |
| A2-70 / A4-70, A4-80 | 450 MPa, 600 MPa | ISO 3506-1 |
| SAE Grade 5 / Grade 8 (1/4 to 1 in) | 92 ksi (634 MPa) / 130 ksi (896 MPa) | SAE J429 |
Next step: check that the threads will not strip with the thread engagement length calculator.
Questions
What torque should I use for an M10 8.8 bolt?
With a thread and head friction of 0.12 and 90% yield utilization, the VDI 2230 method gives about 49 N·m (36 lbf·ft) and a maximum assembly preload of about 29.6 kN. With lower friction the same torque gives more preload, so the torque must be reduced; with higher friction it gives less.
Why do different torque charts give different values?
Charts assume different friction coefficients, yield utilization, head bearing diameters and strength values. A chart value is only valid for the friction it was calculated with, which is why this calculator shows the friction you are using next to every result.
Does lubricating a bolt change the torque?
Yes. Lubrication lowers friction, so the same torque produces more preload and can overload the bolt. Recalculate the torque with the lower friction value for the lubricant you use.
Is the result a full VDI 2230 joint calculation?
No. It gives the tightening torque and the assembly preload range for the bolt itself. A full calculation also checks the clamped parts, embedding, operating loads, fatigue, surface pressure and thread stripping.
Engineering estimate for guidance. Check critical joints with a full calculation and your fastener supplier's data; you remain responsible for the design. VDI 2230, ISO 898-1, ISO 3506-1, SAE J429 and ASME B1.1 are cited for the method and minimum strength values; this page is not affiliated with their publishers.