Screw Thread Stress Calculator
Calculates the shear stress on screw threads given applied force, thread geometry, and engagement length. Use it when verifying that a threaded joint won't strip under tensile or torsional loading.
Last updated: September 2026
Formula below · 2 sources (asme.org, Wikipedia) · Updated Sep 2026
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About this calculator
When a bolt or stud is loaded in tension, the engaged threads must transfer that force through shear. For the external (bolt) thread stripping at the nut's minor diameter, the shear area of a 60° ISO metric or UN thread is approximately A_s = 0.75 × π × D₁ × L, where D₁ = d − 1.0825 × p is the basic minor diameter, d is the nominal diameter, p the pitch and L the engagement length (this is the simplified form of the FED-STD-H28 / Machinery's Handbook stripping-area formula: per thread the shear width is about 0.75 p, and there are L/p threads). The shear stress is τ = F / A_s. The formula assumes uniform load sharing across all turns; in practice the first few engaged threads carry more of the load. Compare the result against the allowable shear strength (typically about 0.577 × yield strength by von Mises, divided by a safety factor), and check the internal (nut or tapped-hole) thread too, which strips first when it is the softer material.
How to use
Suppose an M16 bolt (nominal diameter 16 mm, pitch 2 mm) in a steel nut has an engagement length of 20 mm and carries an axial force of 30,000 N. Minor diameter D₁ = 16 − 1.0825 × 2 = 13.835 mm. Shear area = 0.75 × π × 13.835 × 20 ≈ 651.9 mm². Shear stress τ = 30,000 / 651.9 ≈ 46.0 MPa. If the bolt material has a yield strength of 640 MPa, the allowable shear stress is 0.577 × 640 ≈ 369 MPa, a safety factor of about 8. Reducing engagement length would reduce that margin proportionally. The defaults (M10 × 1.5, 15 mm, 5,000 N) give about 16.9 MPa.
Frequently asked questions
How does thread engagement length affect the risk of thread stripping?
Thread engagement length directly determines the total shear area available to resist the applied axial load. Doubling the engagement length doubles the shear area and halves the shear stress, making stripping much less likely. A common rule of thumb for steel-on-steel joints is an engagement length equal to one times the nominal bolt diameter, but softer materials like aluminium typically require 1.5 to 2 times the diameter to achieve equivalent strength. Always verify the engagement length against the calculated shear stress and the material's allowable shear strength rather than relying solely on rules of thumb.
What is the difference between thread shear stress and bolt tensile stress?
Bolt tensile stress acts along the bolt axis and is resisted by the cross-sectional tensile stress area of the bolt shank, which is calculated at the minor (root) diameter. Thread shear stress acts perpendicular to the bolt axis across the thread flanks and is resisted by the shear area of the engaged threads. Both failure modes must be checked independently. In a properly designed joint with adequate engagement length, the bolt body should fail in tension before the threads strip, because tensile failure is more predictable and gives visible warning signs such as elongation.
Why does the thread pitch affect screw thread shear stress calculations?
Thread pitch affects both the minor diameter (finer pitch leaves more material, so D₁ is larger) and the shear width per thread. In the simplified stripping area A_s ≈ 0.75 × π × D₁ × L, the number of threads (L/p) and the width per thread (about 0.75p) cancel, so for a given engagement length finer threads have slightly more stripping area mainly through the larger minor diameter.