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Bolted connections are widely used in the machine tool manufacturing and equipment. As it is well known, bolt preload will be attenuated by the vibration of machine tools, which will degenerate the working life of machine tools. In this paper, a new experimental design method is introduced into the study on bolt preload attenuation: the quadratic general rotary unitized design. The dynamic analysis of the bolted connection is carried out based on a finely fragmented numerical model to interpret the generation of fretting slip and the reason for the loosening of bolts. The alternating load cyclic vibration test, implemented on the bolted joints, can provide the vibration response under the influence of different working factors. Statistical test and analysis are performed on the quadratic regression mathematical model of the bolt preload attenuation, and the principal working factor that affecting the bolt preload attenuation under vibration can be obtained. Based on the analytical result, the combination of the working factors at the lowest bolt preload attenuation rate, which can effectively slow the bolt looseness, is investigated. The results of this study can provide an experimental basis for improving the machining stiffness of machine tools. [ABSTRACT FROM AUTHOR]

Rough surfaces have a positive and important influence on the mechanical properties of the joint surface. Bolted joints are one of the common connection modes, whose assembly stiffness directly determines the machining accuracy of the machine tool. In this work, considering the joint stiffness between connectors, a mechanical model of the joint surface considering the asperity interaction is established based on fractal theory. On this basis, combined with the equivalent form of the bolted joint system, a new stiffness model depended on strain energy method is proposed. Then, the correctness of the proposed model is verified by experiments. Finally, through the application of FEA, the research on bolt tightening with different friction coefficients is completed. Results show that when the stiffness effect of the bolted joint surface is neglected, the equivalent stiffness of this system is too large, exceeding the accurate value by more than 20%. [ABSTRACT FROM AUTHOR]

Due to its strength advantage, high strength bolts are able to provide larger clamping force. However, the changing behavior of friction coefficient during repeated tightening will adversely affect the tightening torque - clamping force relationship. To ensure reliable clamping force of high strength bolts, an easy implement experimental method to investigate the changing behavior of friction coefficient is put forward in this work. The constraint torque of bolt head, as well as the tightening torque and clamping force, was measured during tightening. Friction coefficient interval of bolt underhead is obtained according to the preset assumptions and experimental results. Based on the mechanical equilibrium relationship of the bolt during tightening, thread reaction torque can be obtained to calculate the friction coefficient of threads and bearing surface. Three cases are considered, namely without lubricant, lubrication at threads and nut surface. Statistical analysis was carried out through replicated experiments to investigate the effect of clamping force on the changing behavior of friction coefficient. Without lubrication, repeated tightening caused the increasing of friction coefficient, which resulted in a reduction of clamping force. When using a lubricant, it can protect the contact surface, and the obtained clamping force during repeated tightening was more stable. Experimental results showed that the proposed method can be used to investigate the changing behavior of friction coefficient during repeated tightening. • An easy implement experimental method to investigate the changing behavior of friction coefficient is put forward in this work. • Based on the mechanical equilibrium relationship of the bolt during tightening, thread reaction torque can be obtained to calculate the friction coefficient of threads and bearing surface. • Statistical analysis was carried out through replicated experiments to investigate the effect of clamping force on the changing behavior of friction coefficient. • The friction and wear of the threaded fasteners during repeated tightening was observed and analyzed. [ABSTRACT FROM AUTHOR]