SPRING 2011

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52 THE DISTRIBUTOR’S LINK TIGHTENING STRATEGIES FOR BOLTED JOINTS - Methods for Controlling and Analyzing Tightening continued from page 46 The first zone is the rundown or prevailing torque zone that occurs before the fastener or nut contacts the bearing surface. Prevailing torque, due to thread locking features such as nylon inserts or deformed threads, will show up in the rundown zone. Frictional drag on the shank or threads due to misalignment of parts, chips or foreign material in the threads as well as unintended interference due to out of tolerance threads are additional causes of prevailing torque in the rundown zone. The second zone is the alignment or snugging zone, wherein the fastener and joint mating surfaces are drawn into alignment, or a stable, clamped condition. The nonlinear alignment zone is a complex function of the process of drawing together the mating parts, and bending of the fastener as a result of nonparallelism of the bearing surface to the fastener underhead surface. In addition to the macro effects related to alignment of parts, there are micro effects within the alignment zone. The micro effects include contact stress-induced deformations of plating and coatings as well as local surface roughness and thread deformations. These macro and micro effects are illustrated in Figure 2. to the point where the torque is removed can be multiplied by the angle-tension coefficient to estimate the tension that has been created by the tightening process. To further illustrate the concept of the elastic origin, the torque-angle signatures in Figure 3 show the increased slope, induced by increased friction, in the elastic-tightening zone. Note that as friction increases, the torque required to bring the bolt to yield is also increased. The curves in Figure 4 show that, as friction increases, the clamp force at the yield point is reduced, while the torque that is required to reach the yield point increases. This illustrates the fact that for a given fastener size, the torque requiredto yield the bolt is a function of the material yield strength and the thread friction coefficient. The third zone is the elastic clamping range, wherein the slope of the torque-angle curve is essentially constant. The elastic clamping zone torque-angle slope is a very important characteristic of each bolted joint. This slope can be projected backward to zero torque to locate the elastic origin. For joints with prevailing torque in the rundown zone, the elastic origin is located at the intersection of the prevailing torque level and the backward projection of the tangent to the elastic clamping zone. If the angle-of-turn is measured from the elastic origin to the point where torquing was stopped in the elastic clamping zone, the tension in the fastener is directly proportional to that angle-of-turn. In this elastic zone the compression of the parts and the stretching of the fastener are occurring in a linear fashion from the projected elastic origin. Even if friction between threads or in the underhead region of the fastener is varied, it still will be found that within the elastic zone, the tension generated is always proportional to the angle-of-turn from the elastic origin. The angle-of-turn from the elastic origin The fourth zone is the post-yield zone, which begins with an inflection point at the end of the elastic clamping range. Yielding can occur in the bolt or in the joint assembly as a result of underhead embedment or as thread strip in the bolt or mating threads. This fourth zone can be due to yielding in the joint or gasket, or due to yield of the threads in the nut or clamped components or nut rather than to yield of the fastener. The yield point of the bolt can be used to approximate the angle-tension coefficient for the tightening process. Note the yield clamp load of a torqued fastener is less than the tensile yield due to the combined tension and thread torque. Since the thread friction coefficient is unknown an initial assumption could be that the clamp load at yield torque is about 90 percent of the tensile yield load. This would be approximately correct for an average friction coefficient, ges, of 0.1. please turn to page 62
THE DISTRIBUTOR’S LINK 53 DROP FORGED PRODUCTS Turnbuckles Shackles Wire Rope Clips (also malleable) Machinery Eye Bolts Eye Bolts regular & shoulder Eye Nuts Rod Ends: blank & machined Ring Bolts Hooks Swivels Pad Eyes Yoke Ends WIRE PRODUCTS "S" Hooks Turned Eye Bolts (threaded) lag, mach. or welded U-Bolts (galv. & zinc) rd., sq. & long tangent Threaded Rod SPECIAL PRODUCTS Manufactured to Specifications OTHER PRODUCTS Alloy Steel Hoist Rings Wire Rope Thimbles Bevel Washers Clevis Pins Coupling Nuts Plated Steel Shapes Brass - rounds, flats & angles STAINLESS & METRIC (check availability) Eye Bolts & Eye Nuts U-Bolts “S” Hooks Wire Rope Clips Blank Rod Ends Threaded Rods HEADQUARTERS: Franklin Park, Illinois (847) 455-6609 WAREHOUSE LOCATIONS: Chino, CA (909) 591-1099 Monroe, GA (770) 266-5600 Houston, TX (713) 664-7722
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SPRING 2011

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