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.
About this calculator
When a bolt or stud is loaded in tension, the threads must transfer that force through shear across their root area. The shear stress τ is given by τ = F / (π × (d − 0.6495 × p) × L / p), where F is the applied axial force, d is the nominal (outer) diameter, p is the thread pitch, L is the thread engagement length, and 0.6495 × p is the standard thread height correction for a 60° profile (ISO metric or UN threads). The term (d − 0.6495 × p) approximates the minor (root) diameter, and L/p gives the number of engaged thread turns. Multiplying these gives the total shear area. This formula assumes uniform load sharing across all turns, which overestimates strength slightly since the first few engaged threads carry disproportionately more load in practice. The result should always be compared against the material's allowable shear strength (typically 0.577 × yield strength per von Mises criterion).
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. Calculate: shear area = π × (16 − 0.6495 × 2) × (20 / 2) = π × (16 − 1.299) × 10 = π × 14.701 × 10 ≈ 461.8 mm². Shear stress τ = 30,000 / 461.8 ≈ 64.9 MPa. If the bolt material has a yield strength of 640 MPa, the allowable shear stress is 0.577 × 640 ≈ 369 MPa, giving a safety factor of 369 / 64.9 ≈ 5.7. The joint is well within limits. Increasing the applied force or reducing engagement length would reduce that margin.
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 determines two things simultaneously: the height of each individual thread (and therefore the depth of engagement per turn) and the number of thread turns within a given engagement length. A finer pitch creates more turns per millimetre of engagement, increasing the total shear area. However, finer threads also have shallower flanks, which slightly reduces per-turn shear area. The formula balances these effects through the (d − 0.6495 × p) minor-diameter term and the L/p turn-count term. In practice, fine-pitch threads generally provide higher resistance to stripping for a given engagement length but are more sensitive to corrosion and cross-threading.