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Distributor's Link Magazine Spring 2019 / Vol 42 No2


32 THE DISTRIBUTOR’S LINK Bruno Marbacher Bruno Marbacher earned his mechanical engineering degree in Switzerland, he also holds a business degree. He started out as a tool and die maker (poly-mechanic) and over the years he has held various management positions in quality and engineering. During his time in America he has developed and given numerous seminars on topics related to the proper use of mechanical fasteners and machine elements, and assists engineers in solving fastening/assembly issues. His has groomed and directed many young engineers in fastening/assembly technology. He now offers his 40 years of experience through writing and lecturing. TECHNICAL BRIEFING: NUTS Dear Reader, I have worked in the fastening technology field for over 40 years, I have learned a thing or two. I will share that experience and knowledge in tech briefs. In this issue I will cover a simple fastening element. It’s a simple hexagon shaped thing with an internal thread. Its importance is often under estimated, for instance if one drops one, one often doesn’t even bother picking it up, there probably are few lonely nuts laying somewhere on your warehouse floor. However, without them many assemblies could not be completed. What I am talking about are hex nuts, they are typically designed in such away so that their load bearing capacity is higher than the tensile strength of the bolt. If the bolt is stressed too high, the bolt ought to be the one breaking, rather than the nut thread stripping. Like the screw, the nut also has certain functional elements. They all must be in line with each other. Functional Elements ¤ Threads, ¤ Hardness, (proof load) ¤ Height, ¤ Width across flats Nut Height If the height of the nut is too low, internal thread will strip. If the width across the flats is too small, it will result in a thin wall thickness between thread and flats. Consequently, the nut will dilate under load, which then will lead to thread stripping. Based on research some years back, the nut thread strips out, if the nut height is about 0.7 x the thread diameter or less. The nut thread dilates if the width across the flats is 1.4 x diameter or less. These applies to standard nuts that have a proof strength that is 80% of the strength of the mating bolt. Calculations and experience have shown that the hardness (strength) of normal hex nuts are only required to be about 80% of the hardness of the mating screws. DIN standards (German Institute for Standards) specify the nut height for fine thread and coarse thread to be equal to 0.8 x thread diameter. The internal thread is the most important functional element, it must be able be paired with the external thread. The height of the nut and its wall strength must conform to certain parameters for the nut to meet loadbearing requirements. TECHNICAL ARTICLE CONTINUED ON PAGE 116



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