Bolted-joint assembly engine for design and maintenance engineers: permissible assembly preload FM zul with torsion during tightening (VDI 2230 Part 1), tightening torque MA from the full friction equation, preload scatter band FM min–FM max by tightening technique (αA), nut-factor K comparison, metric coarse thread database with stress areas — every report with the full audit trail.
SectorCalc SC-035 applies VDI 2230 Part 1 assembly-level calculations to metric bolted joints: permissible assembly preload FM zul accounting for torsion while tightening, the full tightening-torque equation with thread and head friction, and the preload scatter band FM min = FM max/αA for the selected tightening technique. It also reports the traditional nut factor K so torque values can be compared with legacy shop specifications. Results are assembly reference values — the joint itself (clamped parts, embedment, external loads, fatigue) requires the full VDI 2230 joint calculation, which is a separate verification.
The torque a wrench must deliver splits into a thread-pitch term, thread friction, and head-bearing friction:
Only about 10–15% of the applied torque stretches the bolt (the 0.16·P term). The remaining 85–90% is consumed by friction — which is why two bolts at identical torque can differ by 30% or more in preload when friction differs.
Tightening loads the bolt in combined tension and torsion. VDI 2230 reduces the allowable assembly preload below the pure-tension yield load accordingly:
SC-035 then applies the yield utilization: FM max = ν · FM zul with ν = 70–90%. At ν = 90% the bolt reaches about 90% of its torsion-corrected yield at maximum assembly preload — the usual VDI design point for new bolts in controlled assembly.
No tightening method hits the target preload exactly. The tightening factor expresses the scatter between the maximum and minimum preload a method produces at the same nominal torque:
| Tightening technique | αA (guideline) | Preload scatter |
|---|---|---|
| Torque wrench (manual) | 1.6 – 2.5 | ±25–40 % |
| Impulse / impact driver | 2.5 – 4.0 | ±45 %+ |
| Hydraulic torque / multi-spindle | 1.2 – 1.6 | ±10–20 % |
| Yield-controlled (torque-angle to yield) | 1.1 – 1.3 | ±5–15 % |
| Elongation measurement / ultrasonic | 1.05 – 1.15 | ±3–8 % |
The joint must still clamp when the preload is at FM min — and the bolt must survive FM max. A torque wrench at αA = 2.0 delivers half the design preload on a bad day: that is the load case the joint verification must satisfy.
Because friction consumes most of the torque, μ is the single most influential input. Moving from μ = 0.20 (dry galvanised) to μ = 0.10 (oiled) at the same torque nearly doubles achieved preload. Practical rules:
The short formula hides geometry inside K. SC-035 reports the K implied by the full VDI equation for your exact size, friction and DKm — comparing it with a legacy "K = 0.20" specification reveals how much that legacy spec actually under- or over-tightens on your fastener.
| Size | Pitch P | Stress area As | FM zul 8.8 / 10.9 / 12.9 (μ=0.12) |
|---|---|---|---|
| M6 | 1.00 | 20.1 mm² | ≈ 9.3 / 13.7 / 16.1 kN |
| M8 | 1.25 | 36.6 mm² | ≈ 17.0 / 24.9 / 29.3 kN |
| M10 | 1.50 | 58.0 mm² | ≈ 26.9 / 39.5 / 46.5 kN |
| M12 | 1.75 | 84.3 mm² | ≈ 39.1 / 57.5 / 67.6 kN |
| M16 | 2.00 | 157 mm² | ≈ 72.8 / 107 / 126 kN |
| M20 | 2.50 | 245 mm² | ≈ 114 / 167 / 197 kN |
| M24 | 3.00 | 353 mm² | ≈ 164 / 241 / 283 kN |
| M30 | 3.50 | 561 mm² | ≈ 260 / 383 / 450 kN |
| M36 | 4.00 | 817 mm² | ≈ 379 / 557 / 655 kN |
Because 85–90% of tightening torque is eaten by friction in the thread and under the head. Plating, lubricant, surface roughness and washer condition change friction coefficient between bolts of the same batch. At fixed torque, lower friction means higher preload — a dry-spec bolt re-lubricated without changing torque can be stretched past yield.
Use the VDI equation whenever the fastener size, friction class and head-bearing diameter are known — it is geometry-exact for the specified conditions. The K short formula (M = K·F·d with K ≈ 0.15–0.20) is a screening rule only; its implied error can exceed ±25% across sizes. SC-035 reports the K implied by your exact inputs so legacy specifications can be mapped and checked.
VDI practice treats bolts tightened to 90% of torsion-corrected yield as single-use for critical joints: residual elongation and thread damage are possible and invisible. For non-critical joints with verified zero permanent set, reuse at reduced utilization (ν = 70%) is common practice. Yield-controlled (torque-to-yield) fasteners are strictly single-use by design.
No. SC-035 is the assembly-level calculation: how much preload a torque produces and how it scatters. Joint verification under external loads — clamped-part stiffness, load introduction factor, embedment/settlement, minimum residual clamp, bolt fatigue under operating load — is the full VDI 2230 joint calculation (SC-036 scope) and must be run separately for structurally significant joints.
Fine threads have different pitch diameter, minor diameter and stress area for the same nominal size, changing every term of the torque equation and the permissible preload. Using a coarse-thread table on a fine-pitch bolt under-estimates preload and over-stresses the thread. Fine-pitch support requires its own thread database entry — enter it as a custom case only with verified geometry.
