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Distributor's Link Magazine Summer 2018 / Vol 41 No3

36 THE DISTRIBUTOR’S

36 THE DISTRIBUTOR’S LINK Michael Mowins Michael Mowins is the President-Global Licensing for Phillips Screw Company. He is the author of numerous articles on innovation, assembly, and quality. He has served as Associate Chairman for the National Fastener Distributors Association, Chairman for the Industrial Fastener Institute’s Associate Supplier Division, and Chairman of the Aerospace Fastener Standards Advisory Committee. He serves on the SAE E-25 Engine Bolt, EG-1B Hand Tool, and G-21H Counterfeit Hardware Committees. He holds 4 U.S. Patents and is a graduate of the U.S. Naval Academy (BS) and University of Rhode Island (MBA). FASTENER HEAD STRENGTH WHITE PAPER Fastener Head Strength - Defining a Common Metric and Why It is a Critical Characteristic High strength threaded fasteners are critical components in the assembly of virtually every form of transportation and industrial machinery. Industry accepted standards for the measurement and performance of these critical parts have existed for decades, but recent failures of threaded fasteners in service and qualification testing are bringing new focus to the critical area between the head and shank of the fastener. This head-shank juncture has been highly analyzed in large diameter (≥ ½”/12mm) high strength externally wrenched bolts (Hex Head, 12 Point, etc.) used in automotive and aerospace applications, but less attention has been paid to the many bolts and screws below this threshold. Recent qualification failures of M5 (0.197”) diameter fasteners made to a European standard have brought new focus on this issue. The aerospace industry has always been concerned with the weight of the airframe structure and the thin materials used in airframe construction necessitated the use of 100° countersunk flush head fasteners. These thin profile heads as well as other low profile designs intended to reduce weight in aerospace, automotive and industrial applications present unique challenges for the design of an effective torque transfer mechanism (internal recess or external head shape) while still assuring head to shank integrity. Fastener Standards Development Organizations CONTRIBUTOR ARTICLE (SDO’s) and company fastener standards engineers focus their attention on developing part standards that provide attributes (length, diameter, head diameter, head height, etc.) that can easily and accurately be measured to confirm conformance to form and fit requirements. Unfortunately, it is difficult to accurately measure the critical stress area between the top of the shank of the fastener and the bottom of the internal recess or lightening hole in an externally wrenched head without destroying the fastener to perform the measurement. A consensus must be reached on how to calculate the Head Strength Ratio (HSR) to achieve the minimum acceptable tensile strength for the head to shank juncture and what method and measureable data should be used in the calculation. This white paper sets forth the current design limitations and a practical strategy to effectively assess the Head Strength Ratio (HSR) on current and future fastener designs. CONTINUED ON PAGE 114

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