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SUMMER 2018

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Distributor's Link Magazine Summer 2018 / Vol 41 No3

186 THE DISTRIBUTOR’S

186 THE DISTRIBUTOR’S LINK MICHAEL MOWINS FASTENER HEAD STRENGTH WHITE PAPER from page 182 While we have used 100° countersunk aerospace fasteners for our discussion the Head Strength Ratio should be evaluated for any other head configuration (pan head, fillister head, button head, etc.) during the initial design evaluation. While protruding heads like these will often have a higher HSR the move to lighter weight and tighter tolerances has begun to require lower protruding head height designs and the recess depth may not always be considered when head height reductions are done to accommodate tighter clearances. Higher head strength is achieved with protruding head configurations where the bottom of the recess has significant distance from the head to shank juncture as in the Fillister head shown above. Increases in head strength can also be achieved in thin head designs if a shallow recess can be used (AS6305 MORTORQ spiral drive, NAS33750 Dovetail slot, etc.), however in a bolt application where significant torque is required the drive system should have a very high contact area between the driver and recess to accommodate the high torque required to achieve the needed clamp load. Conclusions Recent failures of some fastener designs in tensile testing have highlighted the need for a better understanding of the dynamics of the head to shank juncture area. By developing a set of defined characteristics that can be evaluated to judge the Head Strength Ratio the design engineer can assure that there is sufficient strength in the head to shank area to at least equal the tensile capability of the threaded portion of the fastener. MICHAEL MOWINS

SFA SPING CONFERENCE & GOLF OUTING DALLAS, TX - MAY 2-4, 2018

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