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 LOGICAL APPROACH TO A BOLTED JOINT DESIGN “Let’s drill a few holes here and put some bolts through and torque them up.” Sounds somewhat familiar, but it is not unheard of, even in today’s bolted joint designs. Unfortunately, the bolted/screwed joints holding our designs together in a safe and economical way are sometimes afterthoughts and not always properly planned. The lack of proper education in engineering classes is creating a starting point for bad behavior later in our professional lives. Failing joints are way too often blamed on substandard fasteners and not on poor designs. This can be a problem for the fastener supplier, who does not know the details about the forces, assembly methods and other relevant items, but is often blamed for broken fasteners in failed joints. Before spending time and money replacing failed fasteners (most often very likely OK before being abused) we should start by taking a good look at the design. Chances are that this is where the problems can be found (or eliminated). But for a supplier with limited knowledge in engineering matters it can be difficult to ask the right questions. Let me make a walk-through design of a relatively simple bolted joint to illustrate where “hidden” problems may be found and where good engineering start. For this I will be using some materials from my 2-day FTB (Fastening Technology and Bolted/Screwed Joint Design) for illustrations. With all good joint designs we should always start by establishing the GEOMETRY of the assembly. If the clamping length, LC, is four (4) times the diameter of the fastener we should have a good beginning. More than four CONTRIBUTOR ARTICLE times is always better than less. LC is the clamping length of the bolt between the parts, or from under the bolt head to the nut face. In this case we have five (5) times the diameter, a good start. Secondly, we need to find the external FORCE (static or dynamic) acting on the joint. In this case the maximum dynamic load is 20 kN shared by 4 bolts taking 5 kN each. The load is applied eccentrically with a small bending moment added. From the work sheet, Figure 2 on the following page, we can make an estimation of bolt size by first finding the next larger force level (column 1) = 6300 N (6.3 kN), as a starting point. Looking at section B we can find that if the force is dynamic and eccentric we need to move 2 more steps from the starting point or up to 16 kN. Since the tightening is made by a calibrated torque wrench we need to look at section C telling us to move another step for this. FIGURE 1 CONTINUED ON PAGE 94
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