The Dynamic Tightening of a Bolted Joint
In an assembly that contains threaded fasteners, the nut or bolt needs to be physically tightened to a specific torque. This is usually performed with a torque gun operating at a specific speed to drive the fastener to the final torque target. This type of tightening process is considered “dynamic” in the fastener community. The act of dynamically tightening a fastener creates tension in a bolt, clamp-load in the jointed members, and a complex set of shear stresses in the under-head region as well as in the engaged portion of the threads of the fastener. FEA models of fasteners are usually created without these internal and external threads (without a helical thread path). The clamp-load is then created in the FEA joint with the aide of pre-tension elements and not through the application of a physical torque. Analysis conducted with the aide of these pre-tension elements thus needs to be considered “static”. These elements not only fail to produce the actual shear stresses, but also do not accurately depict the deflection and plastic deformation in both the bearing surface and engaged threads of the fastener when non-linear material properties are used. A new type of bolted joint model has been developed which allows for the dynamic tightening of a bolt into a threaded through-hole using non-linear material models and a helical thread path. This model was patterned after an actual joint created in a test laboratory. The thread and under-head coefficients of friction were measured from the actual joint and were then used as inputs for the new model. Comparison of the clampload results from the model and the actual joint showed a 0.15% difference when 50 deg of rotation was applied to both the actual and FEA bolts. The average plastic deformation on the bearing surface of the joint from the model matched that of the actual joint, 0.003 mm. The pretension model showed no evidence of plastic deformation on the bearing surface however.
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