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. A FEW COMMON MISTAKES ABOUT BOLTED JOINTS A bolted joint would typically consist of four different types of components: 1. Parts to be joined 2. Bolts or screws 3. Nuts (or threaded blind holes) 4. Accessories (washers, gaskets, locking elements) Of these four, most fastener people and designers focus most of their attention on #2 = the external fastener. From a design point of view, the primary focus should actually be #1 = the parts to be joined. This is really where we build most of our successes and failures. I have discussed this in earlier Link articles, but let me just summarize it this way. The Joint Materials The basic joint geometry is extremely important for a good joint. If we can design in such a way that the clamping length, LC, is at least 4 times the nominal diameter of the fastener we are off to a good start. LC is the distance between the bolt head and the nut face. For threaded blind hole applications LC is between the screw head to the interface where the tapped hole starts. Well, this would be a screwed joint, not a bolted joint. Through bolting (bolt and nut) have some definite advantages over the tapped screw joint primarily because we have more material in compression (higher joint stiffness) and automatically a longer clamping length. To clamp together a couple of 1/4” (6mm) parts with a 1/2” (M12) bolt/nut is to be asking for trouble. Another benefit of using longer fasteners is that the assembly is less likely to vibrate loose compared to using shorter bolts/screws. The Bolt Or Screw As we have discussed in earlier Link installments this part (the threaded fastener) of the bolted joint should CONTRIBUTOR ARTICLE behave like a “glorified rubber band” to function properly. Maybe it is hard to imagine a high strength fastener as a rubber band, but it is necessary to have some predicable elastic properties for the bolts and screws. It does not matter if we use a Grade 2 (4.8) or Grade 8 (10.9) from an elasticity point of view, the elastic modulus (aka Young’s modulus) is the same for all steel fasteners. Of course, with a lower grade bolt we get a lower yield (from elastic to plastic deformation) and tensile strengths compared to stronger fasteners. We should try to avoid using stronger bolts/screw than necessary for the function or the joint. Over-design is not only more expensive, but can also lead to increased risk of hydrogen assisted cracking or stress corrosion failures due to harder fasteners. A fastener starting to yield a little in the joint can also serve as a preferred failure mode by causing a detectable loss of clamp load in the joint. The Nut (Blind Hole) We have extensive standards (inch and metric) for both external fasteners and nuts. It is important that we follow the “rules of engagement” when matching up externally and internally threaded parts. For metric fasteners the rules are very simple: 8.8 screws/bolts need class 8 nuts, 10.9 fasteners need class 10 nuts and so on. The same rules should also be applied to inch fasteners, but I see a lot of mis-matches in the industries where I teach or consult. Let us first establish the bolt/nut strength principle: The stronger component is the NUT! This is easy to remember if we consider a bolt/nut assembly as a “marriage” between the two components and compare that to a human marriage. I don’t like the terms “male” and “female” for describing fasteners, but let’s just for this case consider bolts/male and nuts/female. In a traditional human marriage the female is, no doubt, the stronger part. CONTINUED ON PAGE 114
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