Squeeze-film Damper Bearings Research Articles

Influence of the Radial Clearance of a Squeeze Film Damper on the Vibratory Behavior of a Single Spool Gas Turbine Designed for Unmanned Aerial Vehicle Applications

This study presents a numerical investigation using the finite element method on the vibratory behavior of a single spool gas turbine designed for unmanned aerial vehicle applications. The shaft of the rotor-bearing system is supported on a front bearing composed of a deep groove ball bearing with a vibration absorber element and a rear squeeze film damper bearing. Three radial clearances for the squeeze film damper were analyzed to determine the best geometric configuration for the rear bearing, considering the rotordynamic performance of the entire system. Whirl speeds and unbalanced system responses were carefully evaluated to determine the best radial clearance for the squeeze film damper. After defining the best radial clearance, a transient analysis was performed to simulate the transition of the system through resonance, and a spectral map is presented to illustrate the vibratory behavior of the system considering the influence of all related important frequencies. The rotordynamic behavior of the system is predicted, and vibration problems are avoided. Its mechanical drawings were released to manufacturing, and the first prototype is in the experimental test phase, thus indicating that the numerical results presented in this study are consistent.

Empirical identification of the inverse model of a squeeze-film damper bearing using neural networks and its application to a nonlinear inverse problem

The identification of nonlinear squeeze-film damper (SFD) bearings, typically used in aero-engines, has so far focused on their forward model (i.e. displacement input/force output). The contributions of this paper are the non-parametric identification of the inverse model of the SFD bearing (force input/displacement output) from empirical data, and its application to a nonlinear inverse rotor-bearing problem. This work is motivated by the need for a reliable substitute for internal instrumentation, to enable the identification of rotor unbalance using vibration data from externally mounted sensors, in applications where the rotor is inaccessible under operating conditions and there is no adequate linear connection between rotor and casing. The identification of the inverse model is fundamentally different from that of the forward model due to the need to account for system memory. A suitably trained Recurrent Neural network (RNN) is shown to be capable of identifying the inverse model of an actual SFD through two validation studies. In the first study, the RNN model satisfactorily predicted the SFD journal’s displacement time histories for given periodic time histories of the Cartesian SFD forces, although it could not predict the user-applied static offset in the SFD since it was not trained to do so. This was no limitation for the second study where, for both centred and non-centred SFD conditions, the RNN proved to be a reliable substitute for actual instrumentation as part of the inverse problem solution process for identifying the amplitudes and phases of the external excitation forces on a simple test rig.

A theoretical and experimental investigation of the dynamic response of a squeeze-film damped twin-shaft test rig

This article performs a theoretical and experimental analysis of a twin-shaft test rig in which the two unbalanced rotors are nonlinearly coupled through the flexibly mounted housing of a squeeze-film damper bearing, as in a real aircraft engine. The research aims are two-fold: to validate recently developed fast computation methods and to achieve an insight into the effect of the ratio between the rotor speeds on the overall vibration response. The experimental data correlated reasonably well with the theoretical analysis. The correlation was best when the speed ratio was equal to a ratio of large integers (in its most reduced form). When the speed ratio was a ratio of two low integers, the predicted response was found to be particularly sensitive to the arbitrary initial phase angle between the rotors.

Empirical identification of squeeze-film damper bearings using neural networks

To date empirically obtained SFD models have been based upon the determination of linearised force coefficients; such models are severely limited in their range of applicability since they are only valid for small perturbations from a mean position. The present research provides the introduction and validation of a nonlinear SFD identification technique that uses neural networks, trained from experimental data, to reproduce the input–output function over the full range of the SFD clearance. Details of the commissioning of a specially designed identification test rig and its associated data acquisition system are presented. The neural network's construction and training process is described and relevant testing is detailed. The empirically identified neural network is progressively validated, culminating in remarkably accurate nonlinear vibration response prediction of an SFD test rig subjected to external dual-frequency orthogonal excitation, as present in twin-spool engines (where the nonlinear vibrations are driven by the unbalance on the two rotors turning at different speeds). When used within the dynamic analysis of the test rig, the trained neural network is shown to be capable of predicting complex nonlinear phenomena with excellent accuracy. By comparison to an advanced theoretical model, the results show that the neural networks are able to capture the effects of features that are difficult to include in a hydrodynamic model or are particular to a given SFD.

Non-linear dynamic analysis of hybrid squeeze-film damper-mounted gear-bearing system and hydraulic active control

The hybrid squeeze-film damper bearing is proposed in this article. The pressure distribution and the dynamics of a gear pair supported by such bearing are studied. Numerical results show that, due to the non-linear factors of lubricant film force, the trajectory of the gear demonstrates a complex dynamics with rotational speed ratio s. Poincaré maps, and bifurcation diagrams are used to analyse the behaviour of the gear trajectory in the horizontal and vertical directions at s = 6.0. The maximum Lyapunov exponent is used to determine if the system is in a state of chaotic motion. Numerical results show that the maximum Lyapunov exponent of this system is positive at the non-dimensional speed ratio s = 6.0, which indicate that the gear trajectory is chaotic under such operation condition. In order to avoid the non-synchronous chaotic vibrations, an increased proportional gain k p = 0.1 is applied to control this system. It is shown that the gear trajectory leaves chaotic motion to periodic motion in the steady state under control action.

Computational Studies of the Unbalance Response of a Whole Aero-Engine Model With Squeeze-Film Bearings

The computation of the unbalance vibration response of aero-engine assemblies fitted with nonlinear bearings requires the retention of a very large number of modes for reliable results. This renders most previously proposed nonlinear solvers unsuitable for this application. This paper presents three methods for the efficient solution of the problem. The first method is the recently developed impulsive receptance method (IRM). The second method is a reformulation of the Newmark-beta method. In addition to these two time-domain methods, a whole-engine receptance harmonic balance method (RHBM) is introduced that allows, for the first time, the frequency domain calculation of the periodic vibration response of a real engine. All three methods use modal data calculated from a one-off analysis of the linear part of the engine at zero speed. Simulations on a realistically-sized representative twin-spool engine model with squeeze-film damper bearings provide evidence that the popular Newmark-beta method can be unreliable for large-order nonlinear systems. The excellent correlation between the IRM and RHBM results demonstrates the efficacy of these two complementary tools in the computational analysis of realistic whole-engine models.

An impulsive receptance technique for the time domain computation of the vibration of a whole aero-engine model with nonlinear bearings

The direct study of the vibration of real engine structures with nonlinear bearings, particularly aero-engines, has been severely limited by the fact that current nonlinear computational techniques are not well-suited for complex large-order systems. This paper introduces a novel implicit “impulsive receptance method” (IRM) for the time domain analysis of such structures. The IRM's computational efficiency is largely immune to the number of modes used and dependent only on the number of nonlinear elements. This means that, apart from retaining numerical accuracy, a much more physically accurate solution is achievable within a short timeframe. Simulation tests on a realistically sized representative twin-spool aero-engine showed that the new method was around 40 times faster than a conventional implicit integration scheme. Preliminary results for a given rotor unbalance distribution revealed the varying degree of journal lift, orbit size and shape at the example engine's squeeze-film damper bearings, and the effect of end-sealing at these bearings.

Inertia Effects in Squeeze-Film Damper Bearings Generated by Circumferential Oil Supply Groove

The dynamic properties of an industrial Squeeze-Film Damper (SFD) bearing design are described using the well-known perturbation approach, where the reaction forces induced by small movements away from the position of equilibrium are expanded into a Taylor series in terms of displacement, velocity, and acceleration. Although generally negligible, the acceleration term can become significant in SFD bearings when inertia effects in the damper lands are enhanced by the flow in a central circumferential oil supply groove. By using a bulk flow approximation in the oil supply groove an explicit expression is derived for the acceleration term. Experimental results confirm the significance of the oil supply groove geometry and appear to validate the bulk flow approximation.

An investigation into the non-linear dynamics of an unbalanced flexible rotor running in an unsupported squeeze-film damper bearing

The non-linear dynamics of a multimodal flexible rotor running in an unsupported squeeze-film damper (SFD) bearing are investigated analytically and experimentally. The main aim is to assess the ability to predict and explain the non-linear performance using an integrated analytical technique and a standard model for the SFD. A fast harmonic balance method that uses receptance functions is used to determine periodic solutions. A modal-based approach is used for the analysis of the stability and bifurcation of these solutions, as well as the analysis of aperiodic motion. Non-synchronous motion with combination frequencies and subharmonic motions are correctly predicted. It is also shown that such an SFD introduces subcritical superharmonic resonance when it is apparently inactive. It is concluded that, despite the economy in design, the benefits of using an unsupported SFD in a flexible rotor-rigid bearing housing system are dubious.

Fluid forces in short squeeze-film damper bearings

This paper has studied influences of fluid inertia on fluid velocity profiles and provided an axial inertia velocity profile for short squeeze film damper bearings (SFDs) using as a starting point a simplified Navier–Stokes equation. The velocity profile reasonably represents influences of fluid inertia on fluid flow. From the inertia velocity profile, present work develops new theoretical models for fluid forces in cylindrical short SFDs. Published experimental work confirms that the new models for fluid forces are better than traditional short-SFD theory, especially for tangential force. The new models show that the damping coefficient for 2 π film is related to fluid inertia.

Fundamental dynamic performance of fluid-pivot and squeeze-film damper bearings

Due to the increase in rotational speeds and performance of modern turbomachinery the rotordynamic stability of such machinery has reduced. In order to improve rotordynamic stability, fluid-pivot and squeeze-film damper bearings have been developed. By using a simple analytical model to predict the unbalance response of a single mass rotor and by modelling the above bearings as a two-mass-spring and damper system the fundamental performance of these types of bearings has been determined. The results are that these bearings can significantly improve rotordynamic performance over a certain frequency range. This is mainly due to the ability of these bearings to reduce the effective bearing stiffness and hence improve the damping efficiency of the bearings. These auxiliary damped bearings increase the bearing damping efficiency rather than increase the bearing damping.

Development of Porous Squeeze Film Damper Bearings for Improving the Blade Loss Dynamics of Rotor-Support Systems

An efficient oil film damper known as porous squeeze film damper (PSFD) is developed based on conventional squeeze film damper (SFD) for more effective and reliable rotor vibration control and especially for improving the blade loss dynamics for rotor support system. The permeability of the outer race of PSFD could remarkably improve the squeeze film damping properties. The transient response of a simple rigid rotor and flexible Jeffcott’s rotor supported on PSFD and SFD subjected to sudden unbalance of blade loss are investigated. Time transient simulation show that PSFD could operate effectively under much greater unbalance as compared with SFD, especially under relative large impact loading of blade loss. Furthermore, the effective eccentricities of PSFD with small transmissibilities (T<1.0) extend to a range of ε<0.9, and optimum film stiffness and damping distribution within the whole film clearance could be achieved.

The effect of fluid inertia in hydrodynamic lubrication studied by a linearization method: Two useful applications

In several recent articles a linearization method was presented to study the effect of fluid inertia in fluid film bearings. Very accurate results are obtained in closed form which were previously amenable only to numerical analysis or asymptotic analysis (e.g. very weak inertia effects). Two applications presently studied are: (a) the one-dimensional journal bearing; and (b) the squeeze film damper bearing. The solutions are applicable at arbitrary modified Reynolds number and eccentricity ratio, provided the flow is laminar and the film is thin.

An Experimental and Theoretical Investigation of an Uncentralized Squeeze-Film Damper Bearing and the Test Results on a Jet Engine

An Experimental and Theoretical Investigation of an Uncentralized Squeeze-Film Damper Bearing and the Test Results on a Jet Engine

The Damping Capacity of a Sealed Squeeze Film Bearing

The purpose of this paper is to establish a theoretical model to represent a sealed squeeze-film damper bearing and to assess it against results from a test rig, simulating the essential features of a medium-sized gas turbine aero engine.

An Investigation Into the Effect of Side-Plate Clearance in an Uncentralized Squeeze Film Damper

An experimental investigation has been carried out into the influence of side-plate flow restrictors on the performance of a squeeze film damper bearing. The experimental rig used was a flexible rotor with a disk positioned midway between two squeeze film damper bearings. One of the squeeze film dampers was fitted with side plates that could be adjusted and accurately located with respect to the squeeze film damper journal. It has been found that the influence of the side-plate clearance on the ability of the squeeze film damper to reduce the amplitude of the central disk can be considerable if the side-plate clearance is less than the radial clearance. As the side-plate clearance reduces towards zero, the effectiveness of the squeeze film damper diminishes until the amplitudes obtained are the same as those measured when the rolling-contact bearing is rigidly supported. An interesting type of precessing elliptical orbit was discovered for conditions where the “jump” phenomenon was operating.

The Effect of Journal Misalignment on the Oil-Film Forces Generated in a Squeeze Film Damper

Squeeze film damper performance is usually assessed on the assumption that the axis of the journal is parallel to that of the bearing housing. For many practical cases, for example that of the overhung fan shaft in an aero gas turbine, these two elements are unlikely to be parallel, even when self-aligning bearings are used. In this theoretical study an attempt has been made to evaluate the effect of misalignment on the magnitude of the oil-film forces produced in a squeeze film damper bearing, and to this end a computational procedure has been established. From the results reported in this paper, it has been clearly shown that the effect of misalignment in a two-land, squeeze film damper can lead to a significant increase in the transmission of unbalance force through the oil film, As an example, data from a previously reported investigation into the performance of a simple two-bearing model with a single centrally supported disk have been used in a typical calculation. The results from this computation indicated that the oil-film forces generated, could have been several times greater than those calculated on the assumption that the journal and bearing housing were parallel. Unfortunately, there do not appear to be any clear guidelines to lay down to the designers of squeeze film dampers at this moment, in relation to journal misalignment. In general, the effect of misalignment is strongest when the ratio of land-length to radial clearance is greatest, when large unbalance is being accommodated, and when the orbit size is large. In our own analytical studies, the effect of misalignment is allowed for whenever the angle of misalignment is greater than 0.0005 radians.

A Theoretical Investigation of an Overhung Flexible Rotor Mounted on Uncentralized Squeeze-Film Damper Bearings and Flexible Supports

Previous investigations have shown that the uncentralized type of squeeze-film damper is an effective means of reducing the transmission of unbalance forces into the supporting structure. In this theoretical study a more complex model, which includes an overhung “fan” disk and and a noncentral “turbine” disk has been employed. This model represents the conventional gas turbine somewhat closer than does the previously studied single disk system. This investigation has shown that it is possible to minimize the force transmitted into the surrounding structure by a careful selection of squeeze-film damper characteristics, although it may be found that some larger amplitudes of motion accompany the minimized transmissibility.

The Thermal Behavior of Oscillating Squeeze Films

An analytical solution to the energy equation is provided for oscillating squeeze film flow, such as that which occurs in squeeze film damper bearings. Assumptions are: (a) the usual lubrication conditions for two-dimensional flow, (b) small oscillations, and (c) constant viscosity. Specific conditions are established for the latter assumption, which may be in force for stably operated squeeze film bearings. Relatively simple expressions are presented for the thermal performance variables (temperature field, average temperature, Nusselt number) for a variety of cases. The results are interesting from a heat transfer point of view in that the proper energy balance consists of dissipation, conduction, and storage terms, rather than convection terms as is usual in lubrication studies. Thus inferences drawn from other thermal analyses may be in error. Both average cross-film temperature rise and heat transfers are seen to increase from zero Pe´cle`t number and asympotically approach constant values. Due to the negligible convection conditions, the conventional definitions of bulk temperature, Nusselt number, and Pe´cle`t number are not appropriate. As a result of the thermal storage effect, interesting temporal phase-shift and temperature swing effects are exhibited.

Investigation of a Rotor Bearing Assembly Incorporating a Squeeze-Film Damper Bearing

Squeeze-film damper bearings, when properly designed, are a simple means of curbing instabilities and reducing vibration in rotor-bearing assemblies. This paper describes vibration tests carried out on a rigid rotor — flexible bearing test rig incorporating a squeeze-film damper; comparisons are made with theoretical predictions.