SPRING 2018

8 THE DISTRIBUTOR’S

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. SELECTING FASTENERS FOR HIGH WORK LOADS WITH HIGH SERVICE LEVELS In previous articles we have looked at the types of joints that can be described as “Low Work Load” attachments. If the purpose of a screwed/bolted joint is just to keep the parts in place and not to absorb any significant external forces, fasteners with relatively low strength can be used. But, it is important that we keep manufacturing costs in mind since even low-cost fasteners may still require high assembly expenses. The In-Place-Cost (IPC) should always be an important part of our decision making in selecting the best fastener for the job. The IPC approach is equally valid when we are dealing with bolted/screwed joints that are subjected to higher external forces. However, economical consequences of joint failures often overshadow assembly costs when a highly stressed joint fails in service causing personal injuries and/or property damages. In my engineering classes on Fastening Technology and Bolted/Screwed Joint Design (FTB) I recommend that the approach to decision making should be made in the following order: [1] Joint geometry [2] Magnitude and direction of forces [3] Materials in joint parts and fasteners [4] Temperatures at assembly and service [5] Tightening methods (initial and service) [6] Fastener selection [7] Accessories (sealing, prevailing torque, etc.) As you can see from this list, the actual picking of a fastener is way down on the list. The reason for this is that if the designer does not have a really good grasp on the items 1-5, he/she will choose the fasteners by using CONTRIBUTOR ARTICLE some level of guess-work. The two areas where I find the highest level of uncertainty among design engineers are items 2 and 5. We can’t always know the exact size or magnitude of an external force (item 2) hitting the bolted/ screwed joint, but we must at least make a reasonable well founded assumption if we can’t actually measure it. The other “weakness” area is how we assume that we tighten the joint (item 5) at the initial assembly and when performing maintenance work on the joint in service. If we assign a tension scatter range in our calculation that is different from what the actual tool can produce, we may break the fastener when tightening or getting the wrong preload. So, we need to do some serious homework before selecting our fasteners and the method of tightening we are going to use. What kinds of fasteners would be good candidates for the “High Work Load/High Service Level”? We must, of course, consider the fastener types that are typically manufactured in higher strength classes (grades) and having driving geometries suitable for applying high tightening torque levels. There must also be a balance between the force created by the clamping fastener and the capability of the joint material to handle the pressures under screw head and/or nut without getting permanent deformation (embedment). For example – if we use a Class 8.8 (Grade 5) screw to assemble two pieces of plastic, we can’t even use 10 % of the fastener strength without causing permanent damage to the plastic parts. I guess that we have already made up our minds as to the kinds of fasteners that would fit in this “high” strength level category. CONTINUED ON PAGE 100