Thread Stripping Strength⚠ unverified
Mechanical / Fasteners · Compute the approximate thread-stripping load of an engaged thread
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| d | d | m | 1.0 | Nominal bolt diameter |
| Le | Le | m | 1.0 | Length of thread engagement |
| Sut | Sut | Pa | 1.0 | Ultimate tensile strength of the weaker material |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | Fstrip | N | Thread-stripping load, in newtons (N) |
The science & history
Understanding the Parameters
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Nominal diameter $d$ — sets the circumference $\pi d$ of the shear cylinder along which the threads would strip. Larger threads present more shear area.
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Engagement length $L_e$ — how much thread is engaged (nut height or tapped-hole depth). Stripping strength grows linearly with engagement — the direct reason deeper engagement resists stripping and why minimum thread-engagement rules exist.
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Ultimate tensile strength $S_{ut}$ — of the weaker of the two mating materials (often the internal thread / tapped part, e.g. an aluminium casting under a steel bolt). The shear strength is taken as $\approx 0.5\,S_{ut}$ — the standard shear-to-tensile ratio (close to the distortion-energy value $0.577$, reduced for thread geometry). The weaker material strips, so its $S_{ut}$ governs.
Registry note: the shear area is approximated as the plain cylinder $\pi d\, L_e$. Rigorous stripping analysis multiplies by a thread-form factor (typically ~0.5–0.6, from ASME/FED-STD-H28 or Alexander's model) that accounts for the actual thread overlap and the internal-vs-external strip path. This estimate is therefore optimistic; treat it as a first-order screen. Noted in Known Issues.
Derivation (Approaching a Proof)
Thread stripping is a shear failure across the cylindrical surface where the internal and external threads engage. The stripping load is the shear area times the shear strength of the failing material:
$$F_{\text{strip}} = A_{\text{shear}} \times \tau_{\text{ult}}.$$
Two approximations give the registry form:
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Shear area. The engaged threads shear along a cylinder of diameter $\approx d$ over the engagement length $L_e$, so $A_{\text{shear}} \approx \pi d\, L_e$. (A rigorous treatment multiplies by a thread-form factor $\le 1$ for the actual overlap and distinguishes external- vs internal-thread stripping — see note.)
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Shear strength. For ductile metals, the ultimate shear strength is roughly half the ultimate tensile strength, $\tau_{\text{ult}} \approx 0.5\,S_{ut}$ (the distortion-energy criterion gives $0.577\,S_{ut}$; $0.5$ is a common conservative-for-tensile / typical-for-thread value).
Combining:
$$F_{\text{strip}} = \pi d\, L_e \times 0.5\, S_{ut}.$$
Because both engagement length and diameter enter linearly, thread strength scales with the engaged cylinder area — the basis of thread-engagement and nut-height rules (Nut Height Minimum).
Dimensional check. $[F_{\text{strip}}] = \text{m} \cdot \text{m} \cdot \text{Pa} = \text{m}^2(\text{N/m}^2) = \text{N}$. ✓
History and Development
Thread-stripping analysis is standardised in ASME/FED-STD-H28 and detailed in Machinery's Handbook and Alexander's classic thread-strength model (1977). The design goal — engage enough thread that the bolt fails in tension before the threads strip — governs nut heights, tapped-hole depths (especially in soft materials like aluminium, where extra engagement or thread inserts are needed), and the property-class matching of nuts to bolts.
Related Concepts: Nut Height Minimum, Bolt Proof Load, Bolt Tensile Stress Area, Shear Stress, Bolt Yield Torque
Notes: First-order estimate — the plain-cylinder shear area is optimistic; apply a thread-form factor (~0.5–0.6) for design. Use the weaker material's $S_{ut}$. Soft internal threads (aluminium, plastic) need extra engagement or inserts.