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Screw Thread Strength Calculations FED-STD-H28/2B, 1991
and Machinery's Handbook twenty fourth Edition.
These calculations strictly apply to UN thread series however, may be used for metric series threads. In practice when the values are calculated the value for the screw shear strength is similar to the very convenient formula provided above.
Length of Thread
Where:
Le = Fastener thread engagement
At = Tensile Stress Area
Knmax = Maximum minor diameter of internal thread.
Esmin = Minimum pitch diameter of external thread.
n = Number of threads per inch
If material in which the female thread is tapped is significantly weaker that the screw material then J must be evaluated.
Where;
As = Shear area of the external thread (screw or fastener)
An = Shear area of the internal thread ( hole )
If the value of J is greater than 1 then the length of engagement must be increased to at least
Where:
Le = Fastener Thread Engagement
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