Estimate the minimum thread engagement length needed so the screw fails before the threads strip, based on the relative strength of the two materials.
Engagement length = diameter x (external strength / internal strength), with a 1x diameter floor. This is a simplified rule-of-thumb estimate, not a substitute for a full thread shear calculation on critical or safety-related joints.
How it works
When two materials of equal strength are threaded together, roughly one diameter of engagement is enough for the screw to fail before the threads strip. If the internal thread material is weaker, such as a steel screw into an aluminum tapped hole, more engagement length is needed to make up for the lower shear strength of the threads themselves.
FAQ
Should I use this for aerospace or safety-critical joints? No, use this only as a quick shop estimate. Critical joints should be checked with a full thread shear area calculation per the applicable standard.
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