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The bolted connections are widely used in high-end equipment such as aerospace and energy fields. However, when the equipment is assembled, the assembly tightness of bolted connection structure is easily affected by friction and manual experience, resulting in inconsistent preload. It will affect the dynamic performance of equipment, resulting in abnormal vibration or failure of equipment. Currently, the existing assembly tightness testing methods mainly focus on whether the bolts are loose, ignoring the impact of assembly tightness on the vibration of the whole machine. This paper proposes an assembly tightness recognition method for bolted connection states based on modified ensemble empirical mode decomposition (MEEMD) and genetic algorithm least squares support vector machine (GALSSVM). Firstly, an ultrasonic pre-tightening force test system is used for multi-bolt pre-tightening force calibration and precise control, and subsequently, accurate vibration signal samples are collected. The feature extraction of multi-bolt connection states is carried out based on MEEMD. Secondly, a quantitative evaluation index of assembly tightness, singular-value entropy, is proposed. Finally, experimental verification is performed. Multiple sets of different preloads case experiments are designed to verify the identification accuracy of the proposed algorithm. The results show that with the bolted connection structure from loose to tight, the singular-value entropy of the quantitative index decreases monotonically. It provides some theoretical references for improving the reliability of high-end equipment and the evaluation of assembly performance. [ABSTRACT FROM AUTHOR]

The bolted rotor is a typical rotor structure with light weight, high strength, good rigidity, flexible selection of all kinds of wheel materials, etc., which is formed by pressing and combining several sections of rotors or discs and is widely used in aero-engines and gas turbines. Aiming at blind assembly, trail assembly and inconsistent assembly tightness of bolted rotor, and the degradation and loosening of bolt tightness in service, a quantitative detection method of bolt assembly quality based on GHM multiwavelet was proposed. Using the GHM multiwavelet with excellent characteristics such as symmetry, compactness, orthogonality, vanishing moment and high order vanishing moment, feature extraction of the bolted rotor assembly tightness states was performed. The quantitative detection index of assembly quality was defined by the relative energy band distribution based on multiwavelet. The experimental verification shows that the proposed method can accurately identify six kinds of bolt states from tightness to looseness.

• Daubechies wavelet combined with the relative energy decomposes the vibration response signals. • Wavelet energy entropy identifies the tightness degree of bolted joints. • Wavelet energy entropy monotonously increases with the decrease of assembly tightness. • Accelerometer installation position or striking position of exciter has little effect on the assembly tightness detection. The jointed rotor is a representative rotor structure widely used in gas turbine and aero-engine, in which two or more adjacent parts are pressed together by bolted joints with sufficient preload force. However, the assembly tightness states of bolted joints are prone to be affected by frictional factor and workers' experience when they are assembled by axial elongation control method, leading to preload inconsistency in the whole rotor and having a significant effect on rotor dynamics. At present, the current assembly tightness detection method mainly focuses on whether bolt looseness fault occurs but ignores the effect of assembly tightness degree on rotor vibration. Therefore, an assembly tightness detection method is proposed based on wavelet energy entropy to detect different assembly tightness degree of bolted rotor with six different assembly tightness states. By means of vibration test on jointed rotor, various vibration response signals are acquired with bolted rotor from tightness to looseness. The vibration response signals of rotor are decomposed and reconstructed by db wavelet and finally a quantitative index based on wavelet energy entropy is defined to realize the identification of tightness degree of bolted joints. The result shows that the quantitative index tends to monotonously increase with bolt joints assembled from tightness to looseness, which is also proved to be stable, applicable and efficient in comparative experiment. [ABSTRACT FROM AUTHOR]