Strength = Cross sectional area of the screw times the shear strength of
the screw material
Shear strength is often taken to be 60% of the ultimate tensile strength
of the screw
Cross sectional area is usually given as "stress area" in tables
listing strengths of hardware (e.g., MIL-HDB-5E (1987) available online,
I believe.
but that assumes that the shear is across the unthreaded shoulder, not
across the threads. If across the threads then, strictly speaking, you
should take the minor diameter and use that in the calculation.
So...for example, a grade 5, 8-32 machine screw is listed in the
MIL-HDBK as having a stress area of 0.021 sq inches, 75,000 psi shear
strength (material) for a screw shear strength of 1575 lbs force.
Don't take the calculated results too seriously. They depend on all
sorts of conditions being exact, which they never are in real life
(i.e., all screws resisting the force equally). I can't recall being
"off" by more than one screw, however.
--Steve
On 3/1/19 10:49 AM, Daniel Kirk wrote:
Looking at Richard Nakka's A-100M motor, I see the bulkhead screws are
designed to shear at approximately 2000 psi. I understand that sheer
strength = 2000 psi / (pi * r^2 * 2 screws)
but how do you find the shear strength of the screws?
