It is an attractive research topic to propose some novel damping methods for rotors, and the dry friction damping is probably an effective and stable method. In this paper, a control strategy is proposed based on the relationship between the rotor modal characteristics and rotor responses. Firstly, a rotor model with a new kind of dry friction damper is established and corresponding complex nonlinear modes are derived. Then, based on that, the rotor modal characteristics are analyzed, with the nonlinear damping ratio used to evaluate the damping performance. Considering the effective working region for traditional dry friction dampers is usually narrow, a control strategy suiting for a broader range of load conditions is proposed. The results show that the vibration amplitude of the rotor at critical speed can be effectively controlled by the proposed strategy, implying its prospects in engineering practice.

Kalman filter has emerged as a powerful tool for unbalance identification in rotating machinery. Recently, the augmented Kalman filter combined with the finite element model has grown up and projects its potential for complex rotor systems. This paper investigates the application of the augmented Kalman filter (AKF) to a practical turbofan engine. The current study reveals that using steady-state responses as measurements can cause fluctuation in the estimated results, even divergence for some cases, while the available signals in practice are steady-state responses generally. To the authors' knowledge, this practical problem is revealed for the first time. To address the problem, the convergence criterion is employed to improve the AKF and formulates the adaptive fading augmented Kalman filter (AFAKF) proposed in this paper. Results indicate that the increase of the amplification factor, the insufficient measurement points, and the complexity of the dynamic model can all lead to the deterioration of the estimated unbalance. The proposed AFAKF method shows favorable convergence and can achieve accurate estimation with less than 5% relative errors, and the superiority over AKF in computation cost is also observed.

The joint structures, such as flanges with bolts and curvic-couplings, are commonly applied in aero-engine rotors. Those joints tend to loosen when subjected to heavy load, and the local deformation of the joints can induce nonlinearities to the rotor and further affect its dynamic characteristics. However, much research focuses on the bolted joint loosening, while the studies of curvic-coupling loosening in rotor systems are seldom reported. In this paper, the modeling and dynamics analysis for rotors with curvic-coupling loosening are performed. First, an analytical model is proposed to describe the nonlinear stiffness caused by looseness at the joint interface. Then, a rotor with curvic-coupling looseness is modeled and the related dynamic equations are derived. Finally, the modal characteristics of the rotor with curvic-coupling looseness are analyzed based on the complex nonlinear modes method, and the rotor’s steady-state response is obtained using numerical methods. The results show that the softening behavior introduced by joint looseness can lead to eigen frequency reduction in modal analysis, amplitude jump in response, and bifurcation around the critical speed. The above dynamic characteristics can provide reference for fault diagnosis in the rotor system with curvic-coupling loosening.

The dynamic characteristics of gear systems are affected by the sliding friction and time-varying pressure angle. In the previous studies, the sliding friction is usually neglected for simplification, while the pressure angle is always considered as a constant. Therefore, the coupling effect between the sliding friction and pressure angle is not considered. In view of this, a new six degrees-of-freedom model for spur gear systems is proposed. The sliding friction and time-varying pressure angle are emphasized in this model. The differential equations of motion are derived using Lagrange’s equation. The Runge–Kutta numerical integration algorithm is applied to gain dynamic response. The proposed model is somewhat more realistic compared to previous models. Furthermore, parametric studies are carried out to reveal the effects of friction coefficient and equivalent shaft-bearing stiffness (directly related to translational motion and time-varying pressure angle). The coupling effect between the sliding friction and pressure angle is verified not only by the mathematical model but also by the dynamic response. The results reveal that both friction coefficient and equivalent shaft-bearing stiffness affect the contact ratio and pressure angle and lead to distinctive dynamic responses. This research can provide a foundation for further study and be used as a reference for gear system design and vibration prediction.