SPRING 2022

10 THE DISTRIBUTOR’S

10 THE DISTRIBUTOR’S LINK Rob LaPointe AIM TESTING LABORATORY Rob LaPointe is a noted authority in materials and fastener technology. With extensive experience in the management and science of materials testing laboratories combined with master’s degrees in physics and education, he excels at bringing solutions to the client. Working specifically in the fastener testing industry, he has developed expertise in mechanical, nondestructive, metallurgical and chemical testing. With a background of 20 years in physics education, Rob is effective at communicating complex ideas in a simple and understandable manner, communicating well with clients enabling them to make informed decisions about their products and business. AIM is located at 1920 Cordell Court #101, El Cajon, CA, 92020. Tel: 909-254-1278, email: sales@aimtestlab.com or online at www.aimtestlab.com FASTENER SCIENCE: HOW TIGHT IS RIGHT TIGHT? If you’ve spent a large amount of time while growing up, or as an adult, wrenching on things like engines, automobiles, motorcycles, equipment, or other various mechanical devices, you’ve come to know a bit about how tight is right-tight. For fasteners from 6-32 up to about 3/4-10, I have an internal torque-sense that keeps me in the sweet spot of fastener tightness most of the time. I’ve gained this torque-sense through wrenching many fasteners to the point of “Uh-oh…. I turned this one too far.” By either taking the bolt or screw into yield (a point where the fastener stretches and deforms permanently) or all the way to ultimate tensile failure (the point where the fastener breaks into two pieces), you get a good sense of how tight a fastener should be, or more particularly, how much torque (rotational force) should be applied to the fastener to get to a tension (elastic force caused by stretching) between 50 – 80 % of its ultimate tensile value. Bear in mind that I’m not often measuring to confirm this tension for applications where a specified torque value is not required, but that it’s a feel that has been calibrated by many failures and occasionally by comparing to actual data acquired through measuring torque-tension relationships in the laboratory. Acquiring this internal sense of right-tightness can also lead to other helpful “by feel” sensations. One can gain a sense of the performance of a particular metal or of a hardened material’s characteristics as it progresses from yield to ultimate tensile. For example, stainless TECHNICAL ARTICLE FIGURE 1. TIGHTENING A BOLT IS OFTEN A MATTER OF “FEEL.” steel has a great deal of stretch (yield) before it ultimately breaks, and high hardness alloy steels have only a small amount of stretch between yield and breaking. For me, this experience has been calibrated by seeing hundreds of tensile tests with a variety of metals in the laboratory. It’s amazing to see the stress-strain curve of a tensile test and gain a sense of comparison to what you feel when you tighten a nut and bolt made form that material. It is being sensitive to a very real feeling and comparing that to knowledge about how metals behave under stress that enables right-tight by feel. For most installations, where the correct tension is critical to the application or installations where “tension by torque feel” is neither acceptable nor legitimate, we need another more quantitative method for relating torque to tension. Typically, the correct tension for fasteners is somewhere between 50-80% of the fastener’s ultimate tensile strength. CONTINUED ON PAGE 98