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Distributor's Link Magazine Summer 2018 / Vol 41 No3


182 THE DISTRIBUTOR’S LINK MICHAEL MOWINS FASTENER HEAD STRENGTH WHITE PAPER from page 172 RANGE OF TOTAL RECESS DEPTH: 0.093 MINIMUM TO 0.116 MAXIMUM Influence of Recess Geometry on Head Strength Ratio The last factor to consider when analyzing the head strength in the head shank juncture area is the geometry of the recess drive system itself. Cruciform drive systems like Type 1 (PHILLIPS), NAS33781 (TORQ-SET), Type 1A (POZIDRIV), etc. have a defined cruciform shape for most of the recess depth from the top of the head to the bottom of the recess. In calculating the head strength ratio we typically will ignore this shape and use a basic circular cross section based on the outer wing diameter at the closest intersection to the under head transition radius for ease of calculation. Straight walled drive systems like Six Lobe (TORX) and others have a constant outer shape from the top of the fastener head to the tapered bottom of the recess. In these cases the distance from the head transition radius to the closest intersection with the recess shape is still used except that the nearest intersection is often a circular area in the bottom of the recess inside below the plane where the recess outer form has transitioned to a circular cross section. Fasteners with unthreaded shanks are typically used in higher stress structural applications and in these situations an HSR of 1 or greater is recommended. Ideally the maximum recess depth will be shallow enough to provide a Head Strength Ratio greater than 1 especially if the fastener is in an application that is deemed critical or semi-critical. While the configurations shown above show an HSR below or near 1 at maximum recess depth, if the parts are manufactured with the recess depth closer to the minimum allowed in the part standard the Head Strength Ratio will increase as the recess depth approaches this minimum limit. COMPARISON OF HSR FOR BOLTS WITH UNTHREADED SHANK BENEATH THE HEAD CONTINUED ON PAGE 186


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