Thread Stripping Strength Calculator
Calculate thread stripping shear area and maximum pull-out load for UNC and Metric threads by engagement length.
Introduction
When a bolt is torqued into a tapped hole, two failure modes compete: the bolt breaking in tension, and the threads shearing — stripping — out of the bolt or the tapped material. The Thread Stripping Calculator evaluates the second mode, computing the thread shear area for your engagement length and the load at which the engaged threads strip. This is the calculation that answers the eternal shop questions: how many threads of engagement are enough, and will a steel bolt strip an aluminum hole before the bolt itself fails? Designing so the bolt breaks first matters because a broken bolt is replaceable, while stripped parent threads often scrap the part.
How it Works
The shear area depends on which side strips: for external (bolt) threads, As = 0.5 × π × d_minor × Le, using the bolt's minor diameter (approximated as d − 1.299/n for Unified threads); for internal (nut or tapped-hole) threads, As = 0.5 × π × d × Le on the nominal diameter. The 0.5 factor reflects that only about half the cylindrical section at the shear plane is thread material. Strip load is then F = As × 0.577 × Sy, where 0.577 is the von Mises ratio of shear yield to tensile yield strength.
Usage Scenarios
- Sizing thread engagement for a steel screw in a 6061 aluminum housing — finding the engagement length where the soft internal threads outlast the bolt's tensile capacity.
- Deciding whether a thin tapped plate needs a nut, an insert, or more thickness, by comparing strip load at the available engagement against the required preload.
- Specifying minimum thread depth on a drawing with numbers instead of the generic 1.5×D folklore.
- Evaluating a stripped-thread field failure: back-calculating whether the reported installation torque could plausibly have exceeded the computed strip load.
- Choosing between a helicoil repair and a thicker boss after comparing internal-thread shear capacity in the parent material.
FAQ
Why do internal and external threads use different diameters?
The shear cylinder forms where the threads interlock: bolt threads shear near the bolt's minor diameter, while nut or hole threads shear near the bolt's nominal (major) diameter — so each side gets its own area formula.
Where does the 0.577 factor come from?
Von Mises yield theory gives shear yield strength as tensile yield divided by the square root of 3, i.e. 0.577 × Sy. It converts the material's tensile datasheet value into usable shear strength.
How much engagement makes the bolt break before threads strip?
In equal-strength steel, roughly one nominal diameter. In softer parent material — aluminum or cast iron — solve for the engagement length with the parent's yield strength; 1.5–2×D is often required.
Does the calculation account for the first threads carrying most load?
It assumes uniform load sharing, as the standard method does. Real joints load the first engaged threads hardest, which is one reason designs keep healthy margins on the computed strip load.