38 The importance of tightening fasteners to their required clamp load cannot be emphasized enough. If a fastener is not tightened properly, the fastener will not apply the required clamp load on the application it is being used for and may become susceptible to failure. Conversely, if a fastener is overtightened and stretched too much, it becomes susceptible to failure by exceeding the yield point. There are three generally accepted methods employed to determine how much tension is exerted on a fastener: A. Torque B. Stretch C. Torque angle Of these methods, measuring the amount of stretch of a fastener has been proven to be the most accurate. However, since stretch can only be measured with the use of specialty type gauges or expensive ultra-sonic measuring equipment, it is only practical for measuring the stretch on connecting rod bolts and other fasteners, where it is possible to monitor the overall length of a fastener, as it is being tightened. Since most fasteners are installed in blind holes and can’t be accessed from both ends to monitor stretch, one will most likely use a torque wrench or torque angle monitoring device for the majority of assembly work. The Stretch Factor It is important to note that in order for a fastener to function properly it must be stretched a specific amount. The material’s ability to “rebound” like a spring is what provides the clamping force. If you were to simply finger-tighten a bolt there would be no clamp load. However, when you apply torque or rotate a fastener a specific amount and stretch it, you will be applying clamping force. The amount of clamp load a fastener will generate depends on the thread diameter, fastener material and the material’s mechanical properties, such as tensile strength and ductility. When a fastener is torqued or stretched beyond its capability the fastener material will yield. The yield point or yield strength of a fastener is the point at which the fastener has been overtightened and stretched too much, and will not return to its original manufactured length. As a rule of thumb, if you measure a fastener and it is .001˝ or more, longer than its original length it has been compromised and must be replaced. Another factor that must be considered is heat! Heat, primarily in aluminum, is another problem area. Because the thermal expansion rate of aluminum is far greater than that of steel it is possible to stretch a fastener beyond yield as the aluminum expands under heat. Thermal expansion can be offset by designing a fastener that is more flexible and installing the fastener at a safe percentage of yield. To obtain the correct amount of clamping force a fastener should actually be stretched a measured amount. A properly used fastener works like a spring! This graph shows the direct relationship between stretch and clamp load on a typical 3/8˝ diameter 8740 chrome moly rod bolt. 16,000 14,000 12,000 10,000 8,000 6,000 4,000 2,000 0 Clamp Load (lbs) Stretch vs. Clamp Load 0 .001˝ .002˝ .003˝ .004˝ .005˝ .006˝ .007˝ .008˝ Stretch (in.) Proper Fastener Retention FASTENER TECH
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