This experimental study investigates the effect of tightening speed and coating on both the torque – tension relationship and wear pattern in threaded fastener applications. The fastener torque – tension relationship is highly sensitive to normal variations in the coefficients of friction between threads and between the turning head and the surface of the joint. Hence, the initial level of the joint clamp load and the overall integrity and reliability of a bolted assembly is significantly influenced by the friction coefficients. The effect of repeated tightening and loosening is also investigated using M12, Class 8.8, fasteners with and without zinc coating. The torque – tension relationship is examined in terms of the non-dimensional nut factor K. The wear pattern is examined by monitoring the changes in surface roughness using a WYKO optical profiler and by using a LECO optical microscope. A Hitachi S-3200N Scanning Electron Microscope (SEM) is used to examine the contact surfaces, under the fastener head, after each tightening/loosening cycle. Experimental data on the effect of variables and the tightening speed, fastener coating and repeated tightening on the nut factor are presented and analyzed for M8 and M12, class 8.8, fasteners.

In this paper, 3D finite element analysis (FFA) is used to simulate and evaluate different process control methods that are commonly used for automating the assembly of bolted joints in a mass production environment. The finite element (FF) model takes into account the thread helix angle of a fastener along with parallel and nonparallel contact surfaces under the bolt head. Simulation includes the torque-only and the torque-turn process control methods for achieving a desired level of the bolt preload at initial assembly of the joint. The torque-only process control option is simulated by applying the target torque at which the tightening process is automatically stopped. On the other hand, a torque-turn or torque-angle method is simulated by first applying a low level (threshold) torque, to the bolt head, followed by turning the bolt head by a specified angle of turn in order to achieve the desired bolt tension. The effect of variables such as thread and underhead bearing friction coefficients and bolt hole clearance is investigated. The FEA simulation provided in this study would be helpful in developing a reliable tightening strategy for joints with nonparallel bearing surfaces.

In an effort to enhance the reliability of clamp load estimation in bolted joints, this experimental study investigates the effect of tightening speed and coating on both the torque-tension relationship and wear pattern in threaded fastener applications. The fastener torque-tension relationship is highly sensitive to normal variations in the coefficients of friction between threads and between the turning head and the surface of the joint. Hence, the initial level of the joint clamp load and the overall integrity and reliability of a bolted assembly are significantly influenced by the friction coefficients. The effect of repeated tightening and loosening is also investigated using M12, class 8.8 fasteners with and without zinc coating. The torque-tension relationship is examined in terms of the nondimensional nut factor K. The wear pattern is examined by monitoring the changes in surface roughness using a WYKO optical profiler and by using a LECO optical microscope. A Hitachi S-3200N scanning electron microscope is used to examine the contact surfaces under the fastener head after each tightening/loosening cycle. Experimental data on the effect of tightening speed, fastener coating, and repeated tightening are presented and analyzed.

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This study provides an experimental and analytical investigation of the behavior of a double bolted single lap shear composite joint. Various scenarios of bolt tightness are considered for composite-to-composite and composite-to-aluminum bolted joints. Progressive damage analysis is provided for the composite coupons in two regions; namely, the surface under bolt heads and near the contact with the shank of the bolt; the damage analysis is performed using an optical microscope. Four tightening configurations are used in the testing of each double bolted joint. These configurations permit each of the two bolts to be either tight or loose. The analytical part of the study utilizes a 3-D finite element model that simulates the bolt tightness and the multilayered composite coupons. The experimental and finite element results are correlated.

This paper investigates the effect of various tool speed combinations on the torque-tension relationship in a two-stage torque control process for the tightening of threaded fasteners. The tightening speed has a considerable affect on the thread and bearing friction coefficients. Hence, the amount of clamp load generated by a specified level of the tightening torque is significantly influenced by the tool speed combinations in many high volume applications. In two-stage processes, the fastener is first tightened to an intermediate torque level beyond which the tool speed is changed but the tightening is continued until the full target torque is reached. The effect of tightening speed combinations (for stages one and two) on the torque-tension relationship is investigated. Data on the coefficients of thread and bearing friction as well as the Nut Factor K is investigated. The findings of this study are helpful in predicting the clamp load generated in bolted joints, which enhance its reliability and safety. Additionally, the study provides helpful data that may be utilized for developing reliable assembly strategies in mass production applications such as the automotive industry.

This paper investigates the kinetic and static frictional torque components in threaded fasteners during the initial fastener tightening, subsequent torque audit, as well as during the loosening of previously tightened bolts. In less critical applications, the peak kinetic torque value is often used for predicting the bolt preload. The peak value of the tightening torque and its frictional components are mainly determined by the kinetic friction coefficients between the engaged threads and between the rotating nut (or head) and the joint surface. During subsequent quality inspection of the joint after its initial assembly, an audit residual torque value (in the tightening direction) is often used for predicting the residual fastener tension and joint clamp load, as well as for predicting the stability of the clamp load. In contrast with the peak kinetic torque, the audit torque and its thread and under head/nut frictional components are mainly determined by the static friction coefficients. In some cases, the careful application of a breakaway torque in the loosening direction (loosening torque) may be used as a measure for the residual clamp load; similar to the quality audit torque, the loosening torque is determined by the static friction coefficients of the bolted system. An experimental procedure and test set up are proposed to investigate the effect underhead contact radius, thread pitch, surface coating, and fastener head versus nut side tightening on the static and kinetic frictional torque components.

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