Double shank bolt

Claim 1

A fastener system comprising in combination: an external member and an internal member arranged to lockingly interengage within an aligned concentric hole passing through an overlapping pair of structural members; wherein said external member is provided with a cone shaped solid head-end portion having an included cone angle of approximately 70* adapted to seat in a corresponding countersink of said hole to provide a flush installation on one side of the structural joint; said head-end portion having a convex end surface with a sufficient curvature to redistribute driving forces applied by conventional driving tools; said head-end portion being integrally connected to a shank portion having an essentially cylindrical exterior surface by a radiused surface having a radius of curvature greater than approximately .030 inches; said shank portion having an interior recess defined by a longitudinally aligned concentric conical smooth inner wall which tapers uniformly from a maximum diameter at one end of said external member to a minimum diameter near an interior bottom wall of said recess; wherein said internal member is provided with a head-end portion integrally connected to a conically shaped stud-end portion tapered to match the taper of the inner wall of said recess, said stud-end portion including a lead-in section having a smooth conical surface and a serrated section having at least six closely-spaced notches for engagement with the smooth inner wall of the recess of said external member, the diameters of said stud-end portion being greater than the corresponding diameters of said recess by an amount such that when the fastener system is fully installed within said hole, said internal member will force said external member to expand radially outwardly to a prestressed condition and said serrated section lockingly engages the smooth inner wall of said shank portion to prevent disassembly Of said fastener system; wherein the head-end portion of each of said members is provided with laterally extending surfaces for engagement with laterally extending surfaces of said pair of structural members to establish a limit for the interferring engagement of said stud with said inner wall of said shank portion; and wherein when said fastener system is fully installed in said hole, the exterior shape of said external member has been diametrically expanded to cause an interference fit with the walls of said hole ranging from .001 inches to .006 inches to generate hoop tension stresses in the surrounding material for the purpose of increasing the effective fatigue life of the joint.