About this calculator
Thread stripping is the shear failure of the engaged threads themselves — the bolt pulls out of the nut or tapped hole, leaving the sheared thread helix behind. It matters most when a bolt is screwed into a material weaker than the bolt: aluminium housings, cast iron, or low-strength steel. This calculator estimates the axial force needed to strip a metric coarse thread for a given engagement length Le and shear strength τ of the weakest part, and converts it into an equivalent tensile stress so you can compare directly against the bolt’s own strength.
The design rule behind it: the bolt should always break before the threads strip. A bolt fracture is obvious and detectable; stripped threads fail progressively and invisibly inside the hole.
Theory and equations
The engaged thread is modelled as a cylindrical shear surface at the mean thread diameter. With d2 and d3 from ISO 724, the mean diameter and shear area are:
The factor ½ accounts for the fact that only about half of the cylindrical surface is actually load-bearing thread material (the rest is the thread gap and clearance). The stripping force and the equivalent tensile stress on the bolt core section are then:
If the equivalent stress σ exceeds the bolt’s tensile strength Rm, the bolt shank will break before the threads strip — the desired failure hierarchy. The calculator pre-fills Le = 0.75 d as a starting point, which corresponds roughly to the engagement of a standard-height nut.
Worked example: M12 in a tapped steel hole
Inputs: M12 coarse thread, engagement length Le = 9 mm (0.75 d), shear strength of the weaker material τ = 400 MPa (roughly 0.6 Rm for a 650–700 MPa steel).
- Mean thread diameter: d0 = (10.863 + 9.853)/2 = 10.358 mm.
- Shear area: Ath = 0.5 × π × 10.358 × 9 = 146.4 mm².
- Stripping force: F = 400 × 146.4 = 58.6 kN.
- Equivalent tensile stress: σ = 58 570 / 84.27 = 695 MPa.
Assessment: for a class 8.8 bolt (Rm = 800 MPa, breaking load ≈ 67 kN) the threads would strip at 58.6 kN — before the bolt breaks. The engagement is too short for this material pairing: increasing Le to about 10.5 mm raises the stripping capacity above the bolt’s breaking load and restores the correct failure hierarchy. For a class 5.6 bolt (Rm = 500 MPa) the same 9 mm engagement would already be sufficient.
Assumptions and limitations
- Uniform shear distribution along the engagement — conservative for short engagements, optimistic beyond about 1.5 d because the first threads carry most of the load.
- The 50% load-bearing fraction is a standard simplification; exact values depend on thread tolerance class and nut dilation.
- Applies to ISO metric coarse threads; both internal and external stripping are covered by using the shear strength of the weaker part.
- Static loading only; cyclic loading of partially stripped threads is far more severe.
Frequently asked questions
How much thread engagement do I need?
Rules of thumb: 0.8–1.0 d in steel of similar strength to the bolt (a standard nut), 1.5–2 d in cast iron or aluminium, 2–2.5 d in soft light alloys — then verify with the numbers, as in the worked example above.
How do I estimate the shear strength τ?
For steels, τ ≈ 0.6 Rm. Class 8.8 bolt material: ~480 MPa; S355 structural steel: ~300 MPa; cast aluminium: often below 150 MPa. Always use the weaker of the two threaded parts.
Why should the bolt break before the threads strip?
Bolt fracture is sudden, visible, and occurs at a predictable load. Stripping is progressive and hidden inside the hole — a joint can lose its preload without any external sign. Size the engagement so stripping capacity exceeds the bolt’s breaking load.
Does doubling the engagement double the strength?
Only up to a point. Load concentrates in the first engaged threads, so beyond roughly 1.5 d additional engagement adds little. The linear model here is a reasonable approximation for standard lengths.