Assembly - January 2009 - (Page 12) Fastening THREADS Dissecting the Nut Factor I ■ By David Archer President Archetype Joint LLC Orion, MI Archetype Joint specializes in joint design, testing and validation. Dave Archer can be contacted at darcher@ archetypejoint.com n a previous column, I explained why calculating bolt tension is more difficult than measuring torque. The relationship between the torque applied to a fastener and the tension created from the resulting bolt elongation is usually described by T=KDF. T is torque, K is the nut (or friction) factor, D is the bolt diameter and F is the bolt tension. Because this single variable K determines the critical torque-tension relationship, the factors that cause its value to vary about 300 percent aren’t clear. The nut factor, as defined in the equation (often called the short-form relationship), is not derived from engineering principles. Instead, it is arrived at experimentally to make the equation valid. Published test procedures call for tensioning a threaded fastener in a controlled manner, while monitoring both torque and tension. At the prescribed torque or tension value, the nut factor is calculated by inserting T, D and F in the short-form equation and solving for K. Given the unsophisticated nature of the nut factor, one may be surprised to know that there are published torque-tension relationships derived from engineering principles. They produce similar results to one another and take the general form T=XF, where X is a placeholder for a series of terms containing dimensional variables and friction coefficients. So, if there are more “intelligent” ways of relating torque to tension, why do we insist on using fudge factors? Well, the best answer is that the fudge factor K gives the most accurate result. As with any decision of whether physical testing can be eliminated by numerical analysis, all derived equations are only as good as the assumptions going into the derivations. Accurate results require that all the variables are known and can be measured accurately. When competent testing of the actual components is practical, the short-form equation will provide the most accurate torque-tension relationship. However, when testing is not in 12 ASSEMBLY / January 2009 www.assemblymag.com http://www.gagebilt.com http://www.assemblymag.com
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