ABSTRACT
Although insert manufacturers sometimes state pullout loads, the loads frequently have to be calculated. Pullout values for specific inserts are given in MIL-1-45914. Pullout allowables can be calculated by using the formula
7rdJsL p = __:.::...._::_ 3
where
P = pullout load (lb) dm =mean diameter of threaded hole (approximately equal to pitch diameter) (in.)
(2)
Fs =material ultimate or yield shear stress allowable (psi) (Note that aerospace design practices normally require both yield and ultimate margins of safety.)
L = length of full thread engagement (The last two or three threads at the bottom of the tapped hole are only partially formed.)
Note that the 1/3 factor is empirical. If the threads were perfectly mated, this factor would be 1/2, since the total cylindrical shell area of the hole would be split equally between the bolt threads and the tapped hole threads. The 1/3 factor is used to allow for mismatch between threads.
