8 THE DISTRIBUTOR’S LINK Bengt Blendulf Bengt Blendulf was educated in Sweden and moved to the US in 1974 to start a subsidiary for a Swedish fastener manufacturer. After working as a technical consultant on the faculty of the College of Engineering and Science at Clemson University, he established EduPro US in 1997 to teach highly rated courses in Fastening Technology in the US, Canada, Mexico, Europe and Asia. Being one of the founders, Bengt served as the chairman of ASTM F16.96 Bolting Technology from 1996 to 2006. In 2006 he received the Fred F. Weingruber award from ASTM for “his efforts to promote and develop standards for the fastener industry.” In 2013 he also received IFI’s Soaring Eagle Award for “significant contributions to the technological advancement of the fastener industry”. Bengt is the author of an extensive lecture book, well over 110 articles and “Mechanical Fastening and Joining”, a book published in 2013 by the Industrial Fasteners Institute. He can be contacted through www.edupro.us or by email bblendulf@yahoo.com. JOINT OR FASTENER FAILURE When a bolt or screw fails in service or at assembly, the first action from the user is usually to make an angry call to the fastener supplier to complain about the poor fastener quality. In the vast majority of these cases, however, it is nothing wrong with the fastener. When problems like this end up in courts due to accidents or property damage, typically only one of six cases will eventually lay the blame on fastener quality, but five of six blame poor design or sloppy assembly. The only “pure” fastener failure I am aware of, where joint design or assembly are not involved, is when we test the fasteners after manufacturing. Normally, we would first apply a specified proof load to a sample lot to make sure there are no permanent length changes in the fasteners. Once we have verified the lot that way we can then take another, smaller sample from the test lot and take it to tension failure (wedge test) to make sure it also meets tensile strength requirements. All those testing procedures are performed by slowly applying a pure axial load to the fastener and record the results. If we have chosen a supplier/manufacturer performing all the required testing and also certifying that the fasteners meet all mechanical requirements, it is very likely that broken fasteners failed because of misuse rather than poor fastener quality. But, if we only select suppliers based on their prices, chances are that we will end up in the one of six cases mentioned above. So, why do fasteners fail when being used in bolted/screwed joints Let’s start with the joint design. Most of the engineers I have in my fastener technology/design classes are not well prepared in joint design from engineering schools. Old text books, professors with no (or lukewarm) interest in the subject, too short study time, absence of lab work, all add CONTRIBUTOR ARTICLE up to lack of understanding of the bolted/screwed joint. If we are approaching high duty joint designs with some “popular” numbers like 75 % of yield, nut factors of 0.2, friction coefficients based on guess work (dry, oiled, as received, etc.) we are simply asking for trouble. We must design and build our joints in a systematic way in order to function as intended and to prevent failures both in fasteners and other joint parts. The seven steps listed below are the recommendation I give to my students to follow when designing joints with threaded fasteners. The steps must be applied in the order they are listed here: [1] Joint geometry [2] External forces (magnitude, directions) [3] Materials in all joint members [4] Working temperatures [5] Tightening methods [6] Fastener selections [7] Accessories (if any) From this it is obvious that we can’t start by picking a fastener if we don’t know what it is up against. If we give serious attention to this approach we can minimize fastener failures and build much better performing joints. Let’s now put some “meat on the 7 bones.” Joint Geometry The best performing joints have the clamped part behaving stiff and the clamping fasteners act springy. If we can design so that the clamping length, LC, is 4 times the nominal diameter (or more) of the fastener we will have a good start. This way we have developed a large joint volume (often barrel shaped) in which we can “stack up” compressed energy from the tightened fastener. CONTINUED ON PAGE 108
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