Analysis of a Linear Model for Non-Synchronous Vibrations Near Stall

Non-synchronous vibrations arising near the stall boundary of compressors are a recurring and potentially safety-critical problem in modern aero-engines. Recent numerical and experimental investigations have shown that these vibrations are caused by the lock-in of circumferentially convected aerodynamic disturbances and structural vibration modes, and that it is possible to predict unstable vibration modes using coupled linear models. This paper aims to further investigate non-synchronous vibrations by casting a reduced model for NSV in the frequency domain and analysing stability for a range of parameters. It is shown how, and why, under certain conditions linear models are able to capture a phenomenon, which has traditionally been associated with aerodynamic non-linearities. The formulation clearly highlights the differences between convective non-synchronous vibrations and flutter and identifies the modifications necessary to make quantitative predictions.

Study on the Different Effects of Power and Trailer Wheelsets on Wheel Polygonal Wear

The wheels of power and trailer wheelset show different polygonal characteristics since their structures are obviously different. Therefore, the frictional self-excited vibration models of wheelset-track systems are established based on the viewpoint of the frictional self-excited vibration in reducing the wheel polygonal wear. Then, the motion stability of wheelset-track systems is studied by using the complex eigenvalue method. The results show that when the creep force between the wheel and rail is saturated, the unstable vibration frequency of the power wheelset is prone to induce 19-20th-order polygonal wear of the wheel, and the trailer wheelset is prone to induce 20-21th-order polygonal wear of the wheel. Meanwhile, the wheel polygonal wear can be effectively alleviated through changing the gearbox position of the power wheelset. And avoiding disc braking at high speeds can suppress the occurrence of wheel polygonal wear. In addition, the development tendency of wheel polygonal wear can be reduced by increasing the Young’s modulus of the brake pad, but Poisson’s ratio has little effect on the development tendency.

Influence of Two External Excitations for Brake Stick-Slip Behavior Using New Numerical Calculation Method

In this paper, a dynamic brake model has been constructed by incorporating the brake rotor's speed and the brake normal force as excitation sources. By introducing two permissible errors (ε1,2), a novel computation algorithm is proposed to reduce the ill-conditioning, arising from the nonlinear friction. Its validation illustrates that the proposed method, using double-changed time-steps and smarter adaptive time-step reduced method, is more reliable than other integral equation solvers with a higher accuracy as well as less computation time. Moreover, the influences of external excitations on the dynamic characteristic of the brake system are also analyzed, and an estimation for the occurrence of unstable vibration is investigated. The results demonstrate the different contributions of the two external excitations on the dynamic characteristic. The brake system has more unstable vibration at a higher brake normal force and a lower brake rotor's speed with small fluctuation. Furthermore, the higher brake rotor's speed could generate more positive damping effect, which could reduce and suppress the occurrence of the sick-slip vibrations. In practice, these instabilities can be minimized by appropriate selection of the two external, which can be adjusted according to the advanced working requirements.

Automatic analysis methods of vibration parameters for diagnosis of marine rotating machines

Автоматическое диагностирование ответственных роторных машин по вибрации является одним из основных способов обеспечения их надежности и безопасности эксплуатации. Известные методы автоматической обработки вибрационных параметров в диагностических целях обладают ограниченной эффективностью в судовых условиях из-за нестабильной виброактивности машин в установившихся режимах их работы, вынуждающей завышать пороги опасности, и существенного взаимовлияния близко расположенных узлов и агрегатов, приводящего к ошибочным диагнозам. Для решения первой указанной проблемы предложен метод адаптации пороговых значений, позволяющий своевременно обнаружить и прогнозировать ухудшение технического состояния судовых роторных машин. Для решения второй проблемы предложен инвариантный к типу объекта контроля метод автоматического определения причин ухудшения технического состояния судовых роторных машин, который позволяет конфигурировать диагностические правила в табличном виде с возможностью учета влияния дефектов на вибрацию разнесенных в пространстве точек. Рассмотренные методы успешно используются в системах диагностирования роторного оборудования по вибрации. Automatic diagnosis of important rotating equipment using vibration signal is one of the main ways to ensure their reliability and operational safety. Known methods for automatically processing vibration parameters for machinery diagnostics have insufficient effectiveness on shipboard. The reason for this is unstable vibration activity in steady operating modes, which requires increasing thresholds, and the mutual influence of neighboring mechanical components and machines, which leads to erroneous diagnoses. The article provides methods to solve these problems. The first threshold adaptation method allows timely detection and reasonable prediction of marine machinery condition deterioration. The second automatic diagnosis method allows determining causes of this condition deterioration. The diagnosis method does not depend on the type of machine and uses the configuration of diagnostic rules in table form. In addition, this method allows to use defects influence on vibration at spaced control points. Declared methods are successfully applied in diagnostics systems of rotating machines.

Visual experimental investigation on the stability of pressure regulating poppet valve

The unstable vibration of a poppet valve drastically fluctuates pressure which directly affects the stability, reliability and safety of a hydraulic system. The unstable vibration of the poppet and cavitation is studied using visual experiments. Experimental results show that for a poppet valve, an orifice at the front of the valve cavity, the compressibility of oil in the sensitive cavity will lead poppet to unstable vibrations. For poppet valve without orifice, the instable vibration appears as three states: impact valve seat, transition (impact or non-impact valve seat occurs randomly) and does not impact valve seat. When poppet impacts valve seat, there will be severe cavitation phenomenon at the valve port. Although poppet does not impact valve seat, the cavitation come up is affected by the pressure difference at the port or vibration amplitude of the poppet. In addition, both of the flow pulsation of the hydraulic pump and the coupling between the poppet and pipeline system will lead the poppet to be unstable.

Study on Thermal Unstable Vibration of Rotor under Journal Whirl with Large Amplitude in Journal Bearing

To investigate the thermal unstable vibration caused by journal whirls with large amplitude in journal bearing, an analysis model of lubricant film thickness is established. The journal surface temperature distribution is solved, and the reason for journal surface temperature difference appearance and its influence on rotor vibration are analyzed. Taking a turbogenerator as an example, the journal surface temperature difference and the induced rotor thermal bending under synchronous whirl in the bearing are calculated. Meanwhile, an engineering vibration fault with its treatment is presented. Results show that, the journal surface circumferential temperature difference is caused by viscous shearing within lubricant film under journal whirls with large amplitude in journal bearing. The direction of temperature difference is related to the direction of unbalanced force acting on journal. The temperature difference causes rotor thermal bending, which can be converted to a thermal unbalance on the rotor. The rotor vibration is caused by both thermal and initial unbalance. When the rotor is running below or at the critical speed, the vibration is on the increase until it leads to instability of the rotor eventually. When the rotor is running above the critical speed, the rotor vibration fluctuates periodically. Reducing the initial (mechanical) unbalances decreases the rotor vibration and the journal surface circumferential temperature difference.

Stability Analysis of Rotating Systems Supported by Textured Journal Bearings

Fluid-induced instability in rotating systems due to the presence of hydrodynamic journal bearings consists of an undesirable phenomenon with a considerable destructive potential. Surface texturing of journal bearings is currently investigated as a possible approach to improve the stability characteristics of rotating systems. Thereby, this work aims to evaluate the influence of textured journal bearings in the stability threshold and unstable vibration mode of rotating systems. The classical Reynolds equation is used to model the pressure distribution inside the bearing, being solved by the finite volume method (FVM). The rotating system evaluated in this work is a steam turbine that is modeled using the finite element method (FEM). Numerical results show that textured geometric parameters, i.e., shape, area density, and maximum depth, are capable of changing the stability threshold (for worse or better) as well as the corresponding unstable vibration mode. Moreover, the present study also indicates that a full texturing of journal bearings is desirable to achieve a better improvement in the stability threshold when compared with partial texturing. Based on the results obtained in this work, the textured journal bearings represent a promising and feasible tool to improve the stability conditions of rotating systems in industrial applications.

Fixture Optimization in Turning Thin-Wall Components

The turning of thin-walled components is a challenging process due to the flexibility of the parts. On one hand, static deflection due to the cutting forces causes geometrical and dimensional errors, while unstable vibration (i.e., chatter) could compromise surface quality. In this work, a method for fixturing optimization for thin-walled components in turning is proposed. Starting from workpiece geometry and toolpath, workpiece deflections and system dynamics are predicted by means of an efficient finite element modeling approach. By analyzing the different clamping configurations, a method to find the most effective solution to guarantee the required tolerances and stable cutting conditions is developed. The proposed method was tested as a case study, showing its application and achievable results.

Transfer Function Modeling and Experimental Variation of Rotor System Considering Morton Effect Caused in Journal Bearing

In recent years, a rotating machinery are required to operate at high rotational speed for high efficiency. However, the rotating machinery may become unstable due to the increase of rotational speed. One cause of unstable vibration is the Morton Effect generated in a journal bearing. To avoid unstable vibration due to the Morton Effect, construction of a mathematical model for predicting it becomes an important subject. Many researches on the Morton Effect have been conducted previously. Conventional researches are mostly divided into two types. The first one is a study based on detailed numerical simulation using computational fluid dynamics (CFD), thermoshydrodynamics (THD) and so on. It tries to find solution of a differential equation which indicates the Morton Effect induced vibration for a specific machine or a test rig. Therefore, this approach has led not comprehensive model. The other one is a study expressed by a simple mathematical formula. However, modeling in the time domain has been mainly focused and modeling in the frequency domain has not been investigated in detail. In this research, a model based on the frequency response that can quantitatively evaluate the Morton Effect induced vibration in the rotating machinery supported by the journal bearing is developed. First, experimental data was collected for modeling by using an experimental rig. Using these experimental data of journal position in the journal bearing and temperature of journal, a model of the Morton Effect was constructed based on frequency responses. In the proposed method, the characteristic of the journal bearing was considered as a proportional differential controller from control engineering point of view. In addition, the proposed model considers the Morton Effect induced vibration as a new bending mode of a rotating shaft caused by thermal difference. Then, the developed model of the Morton Effect was evaluated in the frequency domain. The characteristics of vibration calculated by the proposed model indicated good correlation with that of the experimental data. Finally, the behavior of the rotating shaft at another rotational speed was predicted by using the proposed model. It was confirmed that the experimental data well agreed with the predicted results. These results show the usefulness of the proposed method of this research for predicting the Morton Effect.

Study on Fatigue Degradation Behavior of a Cracked Rotor Subjected to Lateral Vibration

Fatigue crack in a rotary shaft is a common failure observed in rotor systems. Since vibration of the shaft causes alternating fatigue loads, the crack propagates slowly. Meanwhile, the propagating crack may cause nonlinear or unstable vibration of the rotor system. In fact, growth of the crack and vibration of the shaft are coupled with each other. Hence, it is necessary to study the fatigue degradation behavior of the cracked rotor accounting for this coupling effect. In this paper, a coupling model of rotor vibration and crack growth is established through dynamic theory and fracture mechanics, and a sequential iterative procedure is proposed to solve the coupling model. Then, the competing degradation failure mode of the cracked rotor is analyzed with regard to the rapid crack growth failure and the unstable vibration failure. And degradation measures are proposed based on the competing degradation failure criterion. At last, degradation behaviors with the coupling effect of nonlinear vibration behavior and multiple parameters including rotation speed, unbalance eccentricity and orientation angle, and damping are investigated by numerical simulation. The results indicate that nonlinear vibration behavior and multiple parameters have considerable influence on the degradation behaviors, which present complex regularity. The findings are of significance to guide the safety design of the rotor system for long time operation and help to the further research on prognostics and lifetime prediction.