Stress coining bolt

Claim 1

A fastener for increasing the fatigue strength of metallic members having apertures defined by aperture walls therethrough and for retaining said metallic members together; said fastener including: a substantially rigid elongate generally cylindrical shaft of a predetermined diameter having first and second opposite ends; a fastener head connected to said first end of said shaft; and stress coining means connected to said second end of said shaft, said stress coining means including at least two annular protuberances, each of said protuberances having a front generally toroidal surface for contact and expanding the aperture wall as said fastener is driven second end first through the apertures, a cylindrical surface smoothly faired into said front generally toroidal surface and a back generally toroidal surface smoothly faired into said cylindrical surface, said annular protuberances having predeterminately graduated maximum diameters, the protuberance with the largest maximum diameter being closest to said shaft, said largest maximum diameter being larger than said predetermined diameter, said back generally toroidal surface of said annular protuberance closest to said shaft being faired into the surface of said shaft by a smooth curved surface, whereby said smooth curved surface is for stressing the end of the aperture through which the fastener is driven to improve the fatigue strength of the metallic member, said annular protuberance located furthest from said shaft having a back generally toroidal surface with a cross-sectional radius which is smaller than the crosssectional radius of said front generally toroidal surface thereof, said stress coining means further including at least one ridge spaced from said protuberances and said shaft, whereby said ridge and said back generally toroidal surface of small cross-sectional radius enable an anchoring member to gain purchase thereon.