Torque and Angle Controlled Tightening of Bolted Joints

Doctoral thesis, 2006

Reduced weight and fuel consumption, improved reliability and engine performance are the main requirements in the automotive industry of today. These requirements demand robust and optimized bolted joints, where a high amount of preload obtained by effective utilisation of the bolt is essential. This work presents a method, based on probabilistic design engineering, of tightening a bolt over its yield point. The bolt is utilised to its maximum in a controlled fashion. The proposed torque and angle control method can be used in modern assembly plants as well as in small workshops with less sophisticated tightening tools. This study determines an appropriate combination of snug torque and tightening angle to exceed the yield point of a bolt. Previously underestimated, or in some cases undetermined, deformations resulting in a non-elongating effect on the bolt are here covered and studied in detail. These deformations include the plastic deformation of the bearing surfaces of the fasteners and clamped parts, plastic and elastic twisting of the bolt, and plastic deformation of the nut threads. The total sum of these non-elongating parameters constitutes 16 – 35% of the total deformation of the studied bolted joint, depending on how far into the plastic range the bolt is tightened. Two theoretical models for torque and angle controlled tightening, the elastic ideal-plastic and elastic-plastic models, respectively, are developed and studied. The main difference between the models is the bolted-joint behaviour after reaching the plastic region of the bolt. Both models determine the momentary and permanent elongations, final preload and torque, etc., with an excellent agreement with experimental findings. This important information on the joint behaviour has been neglected in previous research work. It is proposed that a split of the bolted joint into groups of sections exposed to tensile and compressive forces during tightening is necessary when measuring bolt elongation by means of having the measurement points at the unengaged and tension free sections of the bolt. This leads to a new definition of the spring constants and characteristics of the bolted joint, which is presented in this thesis. In addition, the spring characteristic of an M10 bolt with external hexalobular drive (Torx) with a conical shaped flange is also covered. Additionally, a retightening model is also presented, where the bolted joint behaviour is described for reusing the fasteners.