Question 1

What does the VDI 2230 load factor Φ mean?

Accepted Answer

Φ (phi) is the bolt load factor — the fraction of an external axial force that actually reaches the bolt. Because the bolt and the clamped parts act as two springs in parallel, only Φ·F_A adds to the bolt force while (1−Φ)·F_A relieves the clamp. For a stiff steel joint Φ is often just 0.03–0.15, so a preloaded bolt typically sees only 3–15% of the applied load — the key reason preloaded joints resist fatigue. Φ_K = δ_P/(δ_S+δ_P) and, with a load-introduction factor n, Φ_en = n·Φ_K.

Question 2

What is the difference between F_Mmax, F_Mmin and F_Mzul?

Accepted Answer

F_Mmax is the maximum assembly preload the tightening must be designed for: F_Mmax = αA·[F_Kerf + (1−Φ)·F_A + F_Z]. F_Mmin = F_Mmax/αA is the lowest preload that can result from tightening scatter αA. F_Mzul is the permissible assembly preload the bolt can carry at 90% of its yield point including tightening torsion. The bolt size is adequate when F_Mzul ≥ F_Mmax.

Question 3

Why include embedding loss F_Z?

Accepted Answer

After tightening, micro-roughness on the thread flanks, under the head and at the interfaces flattens (embedding/settling) by a few micrometres f_Z. This permanently shortens the clamp and relaxes preload by F_Z = f_Z/(δ_S+δ_P). VDI 2230 adds F_Z to the required assembly preload so the joint still holds after settling. f_Z is editable here so you can match your surface finish and the number of interfaces.

Question 4

How is the tightening torque M_A obtained?

Accepted Answer

M_A = F_M·(0.16·P + 0.58·d₂·μ_G + μ_K·D_Km/2): a thread-pitch term, a thread-friction term and an under-head friction term. It is evaluated at the assembly preload you tighten to (the permissible preload F_Mzul). Friction dominates, so the achieved preload scatters with the tightening factor αA — that scatter is exactly why F_Mmin can be much lower than F_Mmax.

Question 5

What does this calculator deliberately leave out?

Accepted Answer

It implements the concentric simplification of VDI 2230-1:2015: concentric clamping and a concentric axial load, so eccentric bending (βs/βp, external moments), full transverse-slip design beyond simple friction grip, gasket creep, thermal preload change ΔF_Vth (kept at 0 with a warning) and multi-bolt patterns are out of scope. Results are a preliminary draft to be confirmed by a qualified engineer against the full standard.

Bolted Joint DesignCalculator · VDI 2230

Bolted Joint Design
Calculator · VDI 2230