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Distributor's Link Magazine Spring 2023 / Vol 46 No 2


36 THE DISTRIBUTOR’S LINK Guy Avellon Guy Avellon has been in MRO and Fastener Distribution for over 30 years, in such positions Sales Engineer, Chief Engineer, Manager of Product Marketing, Product Engineering & Quality and Director of Quality & Engineering. He founded GT Technical Consultants where he performs failure analysis, lectures on fastener safety, works for law firms and designs/audits Quality systems. He is a member of SAE, is Vice Chairman of the ASTM F16 Fastener Committee, Chairman of the F16.01 Test Methods Committee and received the ASTM Award of Merit in 2005. Guy can be contacted at 847- 477-5057, Email: or visit WHAT DISTRIBUTORS NEED TO KNOW ABOUT TORQUE So, what really is torque? Torque is frequently thought of as the critical force when tightening a joint together that will produce a desired clamp load onto the joint. However, there are many other factors to consider when using torque as a measurement. First, how is torque used? Torque may appear to be universal, but there are three distinct industries who apply torque differently; mass assembly, structural assembly and in maintenance and repair operations. Should a distributor provide torque information? Only if the customer understands that torque values for a given product should be used as a guide only. There are many variables associated with assemblies which will be discussed in another article. Before we go into details about these variables, the customer should understand the following which the distributor should relate to them. Torque is a combination of forces. In physics, torque is a vector that measures the tendency of a force to cause an object to rotate about an axis while applying a tension to the axis. With a fastener, we are rotating either a nut or the bolt head about the axis of the bolt. Torque is the force applied to a lever (wrench), multiplied by the length of the lever rotated perpendicular about a fulcrum: the fulcrum being the axis of the bolt. The force is measured in pounds, or Newtons. The lever is measured in inches, feet or meters. Therefore, there is the pound-inch (lb-in), pound-foot (lb-ft) or Newton-meter (Nm) units of measurement. Torsion is the amount of twisting performed due to CONTRIBUTOR ARTICLE an applied torque. When we stop turning the wrench, or twisting the bolt, the torsional forces dissipate. All forces are at rest. However, it should be noted that in many cases, joint relaxation will occur due to the ‘spring-back’ effect of the bolt as it equalizes the twist. Normal ‘springback’ of the bolt results in an average loss of clamping force of about 10%. Torsion occurs during the tightening of the bolt as it applies a twist to the bolt’s body while the bolt is being elongated by the advancement of the nut along the helical threads (tension). This twist occurs due to the friction created between the threads of the nut, or tapped hole, and the threads of the bolt as the flanks of the mating threads engage under the pressure of tension. Therefore, torque is a function of friction. In essence, we are only measuring friction, not clamp load, which sets us up to be influenced by many variables. Friction is that resistance we feel as we rotate the wrench during tightening, regardless if we use a torque wrench or a box wrench. We feel the joint becoming tighter with each degree of rotation as the nut is literally being crushed against the joint surface. This is the friction at the interface of the nut and joint surface. We have two sources of friction: There is friction at the nut’s bearing surface, or at the washer-face of the bolt’s head, while the mating threads grind against each other at the same time in an attempt to apply enough strain on the bolt to carry it into its proposed elastic region and produce a clamp load upon the joint. CONTINUED ON PAGE 114


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