Vibration quenching in a large scale rotor-bearing system using journal bearings with variable geometry
Introduction
The current journal bearing with variable geometry is a very recent idea of the authors that comes under the growing up topic of adjustable journal bearings. The results of the simulation of the application of this bearing in a simple rotor bearing system [1] were very encouraging for developing further the current concept of rotor mounting with detailed mechanical drawings that were submitted as a patent [2]. The principle of operation is inspired by the variation of spring characteristics [3] for vibration quenching. An additional fluid film is introduced in the journal bearing under the passive displacement of the half bearing due to the development of the higher forces during the passage through a critical speed. The comments during the review process of the patent [2] and of the papers [1] and [2] acted as a motivation for the construction of this prototype bearing and the application of it in a real experimental rotor bearing system. The up to now feedback from the industry people during the commercial promotion (see further links) of this patented bearing is actually the prompt for a survey in the effect of the current bearing in the mounting of large scale rotating systems such as those met in industrial gas turbines and in marine propulsion. Thus the current paper is considered to be a theoretical study in the application of this bearing in a medium-speed large scale system in order to notify the ability of it in the vibration quenching at the critical speed. Such systems experience the critical speed with quite sensible way for their structural environment of foundation due to their considerably high amount of rotational mass.
In the current paper, the principle of operation for the current bearing is briefly described together with the detailed constructive drawings that are presented in correspondence to [1]. A simulation of a multi step continuous rotor mounted in four of those bearings is presented according to the simulating method of nonlinear-dynamic rotor bearing systems developed recently in [4]. The fluid film bearings of infinitely short or long length are introduced with the well known formulas for the forces. A normal start up is performed for the system and the journal responses are used for the further processing of the results considering the amplitude of each of them during the passage through the critical speed. The use of the bearings with variable geometry are very promising in vibration quenching of 60–70% of the rotor's critical speed amplitude giving to the system and its structural environment much less loading stresses than in the case of mounting with normal bearings. The operational parameters of the journal bearings are presented in detail and the dissipation of the energy in the additional fluid film and in the external damping mechanism is an issue that is commented very thoroughly since the principle for the vibration quenching using the current bearing is not to apply external damping but to alter the spring and damping properties of the journal bearing fluid film during the resonance.
The methods that have been proposed in the literature for this wide concept of vibration quenching are quite many. The simplest method is to add damping by supporting the bearings elastically with rubber elements [5] or leaf spring elements [6]. Kirk and Gunter [7] and Ota and Kanbe [8] applied the dynamic vibration absorber theory to an elastically supported rotor. Iwada and Nonami [9] adopted a self-optimizing theory for the foundation. In aircraft gas-turbines, squeeze-film damper bearings are widely used [10], [11]. In the past, vibration suppressions by various kinds of active vibration control theories were investigated, for example, the optimal regulator theory [12], the disturbance observer theory [13], or the variable velocity feedback method [14], [15]. Ishida and Liu proposed a discontinuous spring characteristic [3] as a simple and effective concept that can suppress the rotor vibrations.
The concept of a discontinuous stiffness characteristic [3] in combination with observations on the variation of the stiffness and damping coefficients in worn journal bearings [16], [17], [18] forms the base of the present work. The proposed concept of increasing the fluid film thickness in a journal bearing only during operation close to a critical speed provides variable stiffness and damping properties at the same time. This variation results in a quenching of the vibration level during passage through resonance. During the start up process, the fluid film force acting on the rotor can be calculated by solving the Reynold's equation for every discrete time step at the corresponding journal response. Since the fluid film forces reach higher values during passage through resonance (due to higher eccentricity of the journal), the moving part of the bearing (that is pre-stressed by an external spring and attached to an external damper) experience a displacement which leads to the introduction of an additional fluid film zone. The input (fluid film vertical force) and the output (bearing moving part displacement) of the moving ‘spring-damper’ system are related by the convolution integral since the input is always a function of the displacement of the bearing moving part. By introducing such a self-adaptable bearing the resonance amplitude can be decreased down to 70% without loss in the designed bearing load carrying capacity at nominal speed and without the introduction of any additional external components along the shaft.
The novelty of the current work could be described as the application of discontinuous spring and damping properties of a fluid film bearing in the mounting of rotors. According to the literature the majority of vibration quenching techniques are based on the introduction of additional damping in to the system. The current fluid film bearing does not add damping into the system but it results in vibration quenching through the alteration of the fluid film properties in the specific time duration during the passage through resonance. The alteration of spring and damping properties is an already known concept for vibration quenching but in the current work this happen under completely different mechanism from this of attaching springs into a rotor.
Section snippets
Description of the operational principle for the bearing with variable geometry
The proposed concept for vibration reduction during passage through resonance establish a journal bearing that possesses the ability to introduce an additional fluid film zone by displacing its lower semi-bearing part if, and only if, the fluid film forces exceed a well defined pre-load of an external spring, see Fig. 1.
The main aim is to change the effective damping and stiffness of the system only in when the rotational speed is close to a critical speed. The important feature of this concept
Theoretical application of the journal bearing with variable geometry in a medium speed rotor bearing system with complex geometry
A rotor bearing system parted from a multi step rotor and four journal bearings is used in this section in order to study the efficiency of the journal bearing with variable geometry in general kind of rotating systems, where the vibration amplitudes during the passage through the critical speed provoke considerable influence in the foundation and the surroundings of the system regarding the strains and the stresses.
The system operates at the medium speed domain 500–5000 rpm. The simplified
Conclusions
The proposed journal bearing design enables to change the system's effective damping and stiffness by a selective activation of an additional fluid film during critical operation. A journal bearing of variable geometry (VGJB) is able to decrease the resonance vibration amplitude up to 60% compared to a conventional journal bearing in the current simulation of a large scale rotor bearing system. The amplitude is decreased by almost the same extent in both, the vertical and horizontal directions.
Acknowledgments
The current work was performed under the joint financial support from the German Federal Ministry of Economics and Technology (BMWi) and the Institute of Structural Dynamics for developing and building a prototype bearing.
The authors would like to thank Transmit GmbH for the support towards commercial promotion of the corresponding patent EP12153239.4 and Prof. R. Markert (Technical University of Darmstadt, Department of Mechanical Engineering) for his valuable contribution in the procedure of
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