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Bolted joints are the major connection form to hold the aero-engine rotor system as a discontinuous entirety. What is noteworthy is that the bolt assembly process (BAP) directly relates the uniformity of preload and the symmetry of joint stiffness when the rotor comes to assembly, thereby introducing a heightened level of complexity and unpredictability to the dynamic response of the rotor system. In this paper, the effect of BAP on the stiffness softening of bolted flange joints is analyzed based on a finite element contact model. Then, a bolted joint rotor system finite element model is established to discuss the magnitude-frequency characteristic under three assembly sequences of bolts. The rotor vibration response is further compared between the three sequences considering the stiffness loss. Finally, the theoretical studies are verified by a rotor test rig. Results demonstrate that the stiffness softens to varying degrees with the increase of external load, and the softening mechanism is intricately associated with the contact state of the joint interfaces. Intriguingly, joint stiffness and its softening law are also affected by the BAP, leading to substantial discrepancies in rotor critical speed and vibration response among different assembly sequences. It is further observed that the tangential stiffness softening complicates the dynamic properties of the rotor system and enhances the nonlinear features. [ABSTRACT FROM AUTHOR]

[Display omitted] • This paper proposes a method to obtain scaling laws for rotor systems. • The power in scaling laws is represented by a function related to scaling factor. • The proposed method can predict the dynamic characteristic in partial similitude. • Testing for both prototype and model is carried out to verify the proposed method. • The proposed method can improve prediction accuracy compared with existing methods. Similitude theory is widely used to predict the structural responses of the full-size prototype through scaled models and is categorized into complete and partial similitude according to the way of scaling parameters. Partial similitude is more flexible in practical design engineering, but its accuracy still needs to be improved. This study presents an improved partial similitude approach, which can be used to establish the scaling laws and understand the scaling behaviors of rotor systems. The aim is to overcome the insufficient accuracy of the existing methods in predicting the dynamic characteristic of rotor systems. The power in scaling laws is represented by a function, which is related to the scaling factor of the prototype to be predicted. In this work, the power function is developed by incorporating the Levenberg–Marquardt method and sensitivity analysis. The accuracy and effectiveness of the proposed partial similitude method are validated through numerical and experimental case studies, where the distortion in geometrical dimension is considered. The dynamic characteristic of prototypes is predicted through the scaled model. Furthermore, the accuracy of the proposed method is evaluated by using the existing methods. Both the numerical and experimental case studies indicate that the proposed method achieves higher accuracy than the existing methods in predicting the dynamic characteristic of rotor systems. [ABSTRACT FROM AUTHOR]

• A unified approach for modeling of the rotor with a bolted-disk joint is proposed. • Effect of tangential stiffness and transition point of bending stiffness are evaluated. • The dynamic model of the bolted-disk joint is presented with two-nodes. • Nonlinear properties of the rotor-bearing system with bolted-disk joint are investigated. • Present work provided a method to detect the change of bolted-joint performance. In large rotating machines, the rotor system involves multistage disks, where bolted joints are the basic form to connect the adjacent disks. Those bolted joints, subjected to unbalanced forces and moments during normal operation, influencing the rotor dynamics and giving rise to different motion states, which are of interest for investigations focusing on vibration signatures and dynamic stability. However, little research has been done on the rotor vibration characteristics considering dynamic parameters of bolted-disk joints. Thus, the bolted-disk joint element with two nodes is proposed by characterizing the mechanical relationship between the adjacent disks in this paper. Then, in terms of the proposed joint element and lumped mass modeling method, the dynamic model for the rotor system with a bolted-disk joint is established. After that, the responses of the system are solved numerically using the Newmark-β method, and bifurcation diagram, three-dimensional spectral plots, Poincare maps as well as spectral plots are used to analyze the dynamic behaviors under different tangential stiffness and transition point of bending stiffness of the bolted joint. Additionally, the influences of the bending stiffness of bolted-disk joint on system response are also examined. Finally, utilizing the rotor dynamic responses experiment performed at a bolted-joint rotor-bearing test rig with an electric tightening wrench, some of the results from the simulation study are verified. The present work provides a theoretical basis for detecting the performance of the bolted joint of the rotor-bearing system, and prevent failure caused by changes in the stiffness of the bolted joint. [ABSTRACT FROM AUTHOR]

[Display omitted] • A bolted joint dual-rotor system was established by considering bolted joint bending stiffness and rubbing fault. • The effect of the HP rotor and LP rotor subjected to the rubbing fault on rotor dynamics are evaluated. • The relationship between the soften effect of bolted joint bending stiffness and system responses are investigated. • The dynamic features of the bolted joint dual-rotor with rubbing fault are discussed through experimental and numerical studies. Large gas turbines often utilize dual-rotor structures to improve efficiency. To facilitate manufacture and maintenance, the large rotor systems are typically constructed by connecting components made from different materials together through the bolted joint. In this work, a dynamic model of a dual-rotor system with bolted joint included in the high-pressure (HP) rotor is established based upon the finite element theory of Timoshenko beam element, as well as taking into account the bearing forces and fixed-point rubbing fault. To evaluate the influence of rubbing faults on the dynamic behavior of a dual-rotor system, this study analyzed the response characteristics of the rotor under the cases of the LP rotor and the HP rotor subjected to the rubbing fault, respectively. In order to further reveal the effect of bolted joint structure on the system response while rubbing fault occurs, piecewise linear bending stiffness is also considered in this study. The nonlinear vibration responses of the bolted joint dual rotor-bearing system are studied through numerical simulation. Effect of rotor–stator contact stiffness and the occurrence of rubbing faults at the low-pressure (LP) rotor and the HP rotor are investigated through frequency-amplitude curves, waterfall diagrams, time-domain responses, and bending stiffness of the bolted joint. The results indicate that the presence of rubbing faults, whether occurring at the LP rotor or the HP rotor, leads to a decrease in the critical speed. Furthermore, under a rubbing fault, the bending stiffness of the bolted joint enters the stiffness softening region earlier and exhibits a wider range, which is exacerbated with increased contact stiffness. This phenomenon is considered one of the main causes of system response instability. Finally, the experimental studies are carried out on a bolted joint dual-rotor test rig to validate the numerical simulation results. [ABSTRACT FROM AUTHOR]

• A 3D clearance fit model between bearing rings and bearing pedestal is proposed. • A general analytical stiffness model of ball bearings in rotor systems is proposed. • A multi-supported rotor system is established and verified by the experiment. • The vibration and contact characteristics of bearings in the system are analyzed. Clearance fit is a common assembly method for bearings in rotor systems, which leads to a certain clearance between the outer ring of the bearing and the bearing pedestal, that is, the fit clearance. The nonlinear factors caused by the fit clearance may affect the mechanical properties of the bearing and rotor system. Considering the fillet and six-degree-of-freedom (6-DOF) motion of the outer ring, a three-dimensional (3D) clearance fit model is proposed. In addition, considering the interference fit between the inner ring and spindle, an analytical stiffness model of ball bearings is developed. By coupling the clearance fit model and the analytical bearing stiffness model with a finite element (FE) model of a spindle, a dynamic model of a multi-supported rotor system is established and verified by the experiment, which fully considers the nonlinear factors of ball-raceway contact and outer ring-bearing pedestal contact. The effects of fit clearance, radial force, and axial force on the vibration behaviors and contact characteristics of the ball bearing in the system are analyzed. The results indicate that the larger fit clearance of the unilateral bearing may lead to misalignment of the bearing and reduce the bearing force. Under radial force, the bearing clearance and fit clearance are eliminated, which improves the stability of the system. At this time, further application of axial force causes the load-bearing area to become larger and weakens the variable compliance (VC) characteristics of the bearing. This investigation can provide technical support for the assembly and design of the bearing-rotor system. [ABSTRACT FROM AUTHOR]

In the design of nonlinear complex systems, the output responses of a complex system subject to demanding loading conditions need to be assessed before the system can be used in practice. Current assessment approaches (including physical model based analysis, Finite Element (FE) simulation and similitude experiments) are often time-consuming and problematic. The objective of this study is to develop a novel data-driven approach to predict the output responses, as well as their uncertainties, of a complex system for the aim of design assessment. The results can be used to decide whether redesign of the complex system is required to avoid damage for actual use. To achieve this goal, a novel design-oriented data driven modelling approach has been developed based on the FROLS (Forward Regression Orthogonal Least Squares) algorithm and K -fold cross validation. In this approach, the NARX (Nonlinear AutoRegressive with eXogenous input) model of the system is identified and validated. After that, the Monte Carlo Simulation (MCS) method is applied to quantify the uncertainties of the NARX model coefficients. Subject to specified inputs, the model coefficients' uncertainties are propagated to the model predicted output responses for design assessments. Finally, the proposed approach is verified by a case study on the design assessment of a Chinese Space Station Scientific Experiment Rack (CSS-SER). The results indicate the efficiency of the proposed approach and its potential to be applied to solve many engineering design problems. [ABSTRACT FROM AUTHOR]

• A 5-DOF bearing force model considering misalignment and cage fracture is proposed. • The effect of cage fracture on radial and axial vibration of the rotor is studied. • The effect of bearing clearance and load on cage fracture vibration is analyzed. • The experimental study on the cage fracture of rolling bearing is carried out. The rolling bearing misalignment may lead to fracture failure of the cage. However, the research on the vibration response of the rotor system caused by bearing misalignment and cage fracture has not attracted enough attention. In this paper, the vibration characteristics of cage fracture fault are studied from two aspects of simulation and experiment. Firstly, a five-degree of freedom nonlinear bearing force model with the bearing misalignment and cage fracture fault is given. Secondly, the bearing restoring force model is introduced into the rotor finite element model. Then, the dynamic model of the deep groove ball bearing-rotor system is established. The effects of key parameters such as cage fracture degree, radial load, and bearing clearance on the vibration characteristics of the system are simulated and analyzed. Thirdly, the bearing test pieces with real cage fatigue fracture fault are obtained by carrying out the bearing life cycle test. On this basis, the vibration signals of bearing with cage fracture under different loads are collected. The simulation and test results show that the system amplitude increases when cage fracture occurs. The obvious impact vibration characteristics appear in the time domain waveform. The vibration energy gradually concentrates to low-frequency vibration. The frequency components related to cage frequency, such as nf c and nf r ± mf c can be more obviously observed in the envelope spectrum. The results can provide a useful reference for the identification and diagnosis of rolling bearing misalignment and cage fracture. [ABSTRACT FROM AUTHOR]