Inter-shaft Bearing Research Articles

Application of the HB–AFT method to the primary resonance analysis of a dual-rotor system

This paper focuses on the primary resonance analysis of a dual-rotor system having two rotor unbalance excitations of different rotating speeds and being connected by an inter-shaft ball bearing. Due to the complex excitation condition and the complicated nonlinear bearing forces of the inter-shaft bearing, the general analytical methods, e.g., the multiple scales method or the harmonic balance method, are failed to give the theoretical solutions. Thus, the harmonic balance–alternating frequency/time domain (HB–AFT) method is formulated to deal with this problem. The basic idea of the method is using the inverse discrete Fourier transform and the discrete Fourier transform, instead of the direct analytical relationship between the supposed solutions of the system and the nonlinear forces, to construct the harmonic expressions of the nonlinear forces, which is the so-called alternating frequency/time domain technique. By using the HB–AFT method, therefore, a Newton– Raphson iteration procedure can be performed to demonstrate the approximate solutions of the system. Accordingly, the frequency responses of the system affected by some critical parameters, such as rotating speed ratio, unbalances of both the inner and outer rotors, and clearance of the inter-shaft bearing, are analyzed, respectively. A variety of phenomena including double resonance peaks, biperiodic and quasi-periodic behaviors, and resonance hysteresis phenomenon are obtained, which are discussed in detail through diagrams for separated frequency responses with different frequency components and rotors’ orbits for different combinations of system parameters. Most prominently, for a relatively small unbalance of rotor as well as a relatively large clearance of the inter-shaft bearing, the resonance hysteresis phenomena are more obvious. The results obtained are also compared with the direct numerical simulation results, and the comparisons show good agreements. In addition, the methodology formulated in this paper is a general approach, which can be applied to other engineering systems with multi-frequency excitations.

Failure Analysis of an Inter-shaft Bearing of an Aero Gas Turbine Engine

Failure of an inter-shaft bearing in an aero gas turbine engine is investigated to determine its root cause. Teardown inspections followed by metallurgical investigations are carried out on the parts of the failed bearing. None of the components of the bearing has shown any discolouration, wear, or overheating marks. But severe flaking is observed on one side of the outer raceway. It indicates progressive fatigue due to overload. Insufficient clearance due to differential expansion of inner ring and contraction of outer ring due to heating and cooling, respectively, can also result in this type of phenomena. Maintaining oil discipline and using correct grade of oil at proper frequency can address these issues. A comprehensive engine-bearing prognostic approach may be followed utilizing available sensor information on-board the aircraft such as rotor speed, vibration, lube system information, and aircraft maneuvers to predict bearing life and incipient bearing failure.

Nonlinear dynamics of a bladed dual-shaft

In the industrial context of performance improvement of dual-shaft aircraft engines, experimental results demonstrate how important it is to consider the influence of the dynamics of the high pressure (HP) shaft on the response of the bladed disk located on the low pressure (LP) shaft. Indeed, this coupling seems to play an important role in the design purposes in rotating machinery industry as it can have a significant impact on the dynamic behaviour of turbomachines. The model developed here consists of a HP shaft and a LP bladed shaft connected by an intershaft bearing. Nonlinear features of this intershaft bearing require the development of specific nonlinear algorithms. Thus, this paper aims at coupling the two modelling levels in order to grasp the nonlinear vibratory phenomena of a bladed dual-shaft under unbalances.

A 5-DOF Model for Aeroengine Spindle Dual-rotor System Analysis

This paper develops a five degrees of freedom (5-DOF) model for aeroengine spindle dual-rotor system dynamic analysis. In this system, the dual rotors are supported on two angular contact ball bearings and two deep groove ball bearings, one of the latter-mentioned bearings works as the inter-shaft bearing. Driven by respective motors, the dual rotors have different co-rotating speeds. The proposed model mathematically formulates the nonlinear displacements, elastic deflections and contact forces of bearings with consideration of 5-DOF and coupling of dual rotors. The nonlinear equations of motions of dual rotors with 5-DOF are solved using Runge-Kutta-Fehlberg algorithm. In order to investigate the effect of the introduced 5-DOF and nonlinear dynamic bearing model, we compare the proposed model with two models: the 3-DOF model of this system only considering three translational degrees of freedom (Gupta,1993, rotational freedom is neglected); the 5-DOF model where the deep groove ball bearings are simplified as linear elastic spring (Guskov, 2007). The simulation results verify Gupta's prediction (1993) and show that the rotational freedom of rotors and nonlinear dynamic model of bearings have great effect on the system dynamic simulation. The quantitative results are given as well.

Dynamic Behavior of a Double Spool Rotor Integrating an Oil-Damped Intershaft Bearing

In order to reduce the embarked mass, the new designs of turbojet engines use softened rotors working with higher speeds. Following this trend, the use of intershaft bearings in dual-spool turbojets is then recommended for reducing weight. However, due to residual unbalances present in both rotors, the intershaft bearing is submitted to hard working conditions. A supplementary damping might then be necessary in order to enable working to rotation speeds higher than the critical ones. The adopted solution consists of introducing a thin fluid film between the intershaft bearing and one of the two rotors in order to provide the necessary damping. However, due to the rotation of the inner and of the outer rings, fluid film instabilities similar to those met in classical hydrodynamic bearings can appear. This paper is focused on the experimental investigation of several designs of the intershaft bearing damper and of their influence on the dynamic behavior of a double spool rotor. First, a double spool rotor test rig integrating an intershaft bearing is presented. Subsequently, the experimental results are discussed for two dynamic configurations of the double spool rotor and for three intershaft bearing damper designs. For co- and counterrotating shafts, the tests show that the integration of a fluid film on the intershaft bearing makes it possible to attenuate the vibrations on both rotors. Furthermore, the tests confirm that instabilities appear under certain conditions depending on the type of the crossed mode and on the design of the intershaft bearing damper.

Experimental and Numerical Investigations of a Dual-Shaft Test Rig with Intershaft Bearing

This paper deals with an experimental study of a dual rotor test rig. This machine, which was developed and built at the Laboratoire de Tribologie et Dynamique des Systèmes, Ecole Centrale de Lyon, will be first presented. It is composed of two coaxial shafts that are connected by an intershaft bearing and rotate independently, each one driven by its own motor. Their lateral vibrations and whirling motion are coupled by the intershaft bearing. The experimental tests consisting in run-ups and the associated measured unbalance response of the dual rotor will be investigated. The influence of the rotation of each rotor on the critical speeds and the associated amplitudes will be discussed. Moreover, this paper presents a numerical model of the dual rotor. Correlations between the experimental and numerical tests will be investigated. The objective is to be able to predict phenomena observed in experiments, starting from a rather fine numerical model.

Some Unusual Cases of Rotor Instability

In the paper presented here three peculiar cases of rotor instability are presented. The first is a tilting-pad bearing used as an intershaft bearing in a two-spool gas turbine. It was initially chosen for its good stability properties. However, in a deeper study it was found that those benefits are greatly reduced when both inner shaft and outer bearing are rotating. The second case comprises a turbine which showed a very strange performance with vibrations alternating between heavy and mild amplitudes. The periods were a couple of minutes long. After a more elaborate instrumentation was installed it was found that vibrations were synchronous and that the rotor experienced continuous changes of vibrations interrupted by sudden changes of amplitude. This revealed that nonlinearity and thermal effects coupled to the rotor vibrations must be incorporated to understand the behavior. Many hypotheses were tested and the one coming closest to explaining the case was one incorporating thermal bending caused by heat conduction through an annulus partly filled with oil. The third case is a simple one of more pure curiosity, where a rotor was provided with a substantial amount of inner damping, and just according to the school book showed dynamic instability.

Coupled torsional-flexural vibration of shaft systems in mechanical engineering—II. FE-TM impedance coupling method

A new approach is developed for calculating the dynamic response of multi-spool rotor systems with several intershaft bearings using the combined methodologies of the finite element method (FEM), transfer matrices (TM) and impedance methods. The approach makes use of FEM to model shaft elements and then transforms the system properties to transfer matrix mode. The intershaft bearings are treated by the impedance method. It combines the advantages of these three methods. Some numerical examples are considered to check the viability of the method.

Stability analysis of intershaft squeeze film dampers

Intershaft squeeze film dampers have been investigated for damping of dual rotor aircraft jet engines. Initial investigations indicated that the intershaft dampers would attenuate the amplitude of the engine vibration and decrease the force transmitted through the intershaft bearing, thereby increasing its life. Also it was thought that the intershaft damper would enhance the stability of the rotor-bearing system. Unfortunately, it was determined both theoretically and experimentally that the intershaft squeeze film damper was unstable above the engine's first critical speed. In this paper, a stability analysis of rotors incorporating intershaft squeeze film dampers is performed. A rotor model consisting of two Jeffcott rotors with two intershaft squeeze film dampers is investigated. Examining the system characteristic equation for the conditions at which the roots indicate an ever growing unstable motion results in the stability conditions. The cause of the instability is identified as the rotation of the oil in the damper clearance. The oil rotation adds energy to the forward whirl of the rotor system above the critical speed and thus causes the instability. Below the critical speed the oil film removes energy from the forward rotor whirl. It is also shown that the backward whirl of the rotor system is always stable. Several proposed configurations of intershaft squeeze film dampers are discussed, and it is shown that the intershaft dampers are stable supercritically only with a configuration in which the oil film does not rotate.

The Transfer Matrix Impedance Coupling Method for the Eigensolutions of Multi-Spool Rotor Systems

In this paper, the transfer matrix impedance coupling method for calculating the eigensolutions of multi-spool rotor systems with several intershaft bearings is developed. This method can also be used for the eigensolutions of the damped rotor systems. The numerical examples of dual and tri-spool rotor systems with intershaft bearings show that the results are in good agreement with the exact solutions.

Investigation of the Steady-State Response of a Dual-Rotor System With Intershaft Squeeze Film Damper

This paper presents an analysis of the steady-state unbalance response of a dual-rotor gas turbine engine with a flexible intershaft squeeze film damper using a simplified transfer matrix method. The simplified transfer matrix method is convenient for the evaluation of the critical speed and response of the rotor system with various supports, shaft coupling, intershaft bearing, etc. The steady-state unbalance response of the rotor system is calculated for different shaft rotation speeds. The damping effects of an intershaft squeeze film damper with different radial clearances under various levels of rotor unbalance are investigated.

Dynamic simulator for advanced gas turbine engine component tests

A rotor-bearing system has been designed and manufactured to be dynamically representative of an entire class of gas turbine engines: the two-spool front-drive, power-turbine engine, which is widely used in helicopter applications. The test rig configuration, with the addition of low-pressure compressor and fan simulator disks, would be dynamically representative of the turbojet and turbofan classes of engines, respectively. The simulator has been designed to permit developmental testing and evaluation of a number of advanced components and procedures. Among these are advanced intershaft bearings, rolling-element bearing dampers, and advanced balancing procedures. The apparatus has been utilized to date for initial evaluation of air-lubricated intershaft foil bearings, and for demonstration of an advanced balancing procedure. For these tests, the shaft was operated on elastically-mounted ball bearings without support damping over a speed range encompassing two bending critical speeds. Rotor balancing via the recently-developed multiplane-multispeed balancing procedure was used to suppress both the first bending critical speed and a pedestal resonance, and to leave a distinct vibration peak at the second bending critical speed. An air-lubricated intershaft foil bearing of the hydrodynamic, resiliently-supported type proved to be capable of raising the second critical speed above the normal operating speed range of the power turbine shaft.