28 FASTENER TECH 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 sus- ceptible 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. Using a torque wrench B. Measuring the amount of stretch C. Torque angle: rotating the fastener a predetermined amount Of these methods, measuring the amount of stretch of a fas- tener 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 practi- cal 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 fasten- er to function properly it must be stretched a specif- ic 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, primar- ily 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 design- ing a fastener that is more flexible and installing the fastener at a safe percentage of yield. The Stretch Gauge We highly recommend using a stretch gauge when installing rod bolts and other fasteners, where it is possible to measure the length of the fastener. It is the most accurate way of measuring clamp load of any bolt. Simply follow manufacturer’s instruc- tions, or use the chart on pages 25-27 of this catalog for ARP rod bolts. When using a stretch gauge it’s best to measure the fastener prior to starting and monitor the fastener overall length during installation. When the bolt has stretched the specified amount, the cor- rect clamping force has been a c h i e v e d . We recom- mend that you maintain a chart of all rod bolts and make a note of the fastener length prior to installation and after any disassem- bly. If there is a permanent increase of .001˝ or more in length, there is a deformation and the bolt should be replaced. A sample stretch monitoring chart is located on page 30. Using A Torque Wrench There are a number of things to consider when using a torque wrench. The “friction factor” changes from one cycle to the next. That is, friction is at its highest value when the fastener is first tightened. Each subsequent time the fastener is torqued and loos- ened, the amount of friction reduces. Eventually the friction levels out and becomes fairly consistent for all following repetitions. Three basic elements that contribute to the friction factor: 1. Most importantly - the fastener assembly lubricant 2. The condition of the threads 3. The surface finish of the fastener Because of these variables, a phenomenon known as clamp load scatter occurs. Clamp load scatter is the variation in clamp load a fastener generates due to fluctuating levels of friction from one torque cycle to subsequent cycles. It’s not uncommon to see clamp load scatter in the range of 4,000-8,000 pounds between the first and tenth pull on a new fastener depending on the lubricant used. PROPER FASTENER RETENTION 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.) A stretch gauge is the best way to accurately determine the clamp load of a rod bolt.