Athanasios Chasalevris

A Journal Bearing With Variable Geometry for the Reduction of the Maximum Amplitude During Passage Through Resonance

A concept for a journal bearing with variable stiffness and damping properties is developed in order to decrease the vibration amplitude of a rotor-journal bearing system during passage through resonance. The introduction of an additional fluid film thickness in the bearing is proposed in this work in order to alter the dynamic properties in the bearing. The bearing ring is divided into two parts with the upper part being fixed with the housing and the lower part being flexibly mounted by a preloaded spring in parallel with a viscous damper. This allows relative motion between the two parts of the bearing ring. The relative motion introduces an additional fluid film zone in the bearing under the passive displacement of the lower part due to increased impedance forces that are developed in the lubricant film at resonance operation. The general concept is to change the system’s damping and stiffness coefficients using this extra fluid film thickness only when the system passes through its critical speed in order to quench the vibration amplitude. For rotational speeds outside of the resonant regions, the bearing is considered to be fixed in order to behave as it was designed under the nominal loading operational conditions. [DOI: 10.1115/1.4007242]

On the analytical evaluation of the lubricant pressure in the finite journal bearing

The Reynolds equation for the pressure distribution of the lubricant in a journal bearing with finite length is solved analytically. Using the method of the separation of variables in an additive and in a multiplicative form, a set of particular solutions of the Reynolds equation is added in the general solution of the homogenous Reynolds equation and a closed form expression for the definition of the lubricant pressure is presented. The Reynolds equation is split in four linear ordinary differential equations of second order with non constant coefficients and together with the boundary conditions they form four Sturm-Liouville problems with the three of them to have direct forms of solution and one of them to be confronted using the method of power series. The mathematical procedure is presented up to the point that the application of the boundaries for the pressure distribution yields the final definition of the solution with the calculation of the constants. The current work gives in detail the mathematical path with which the analytical solution is derived, and it ends with the pressure evaluation and a comparison with past numerical solutions and an approximate analytical solution for a finite bearing. Also the parameters of primary interest to the bearing designer, such as load capacity, attitude angle, and stiffness and damping coefficients are evaluated and compared with numerical results.Copyright

Experimental detection of an early developed crack in rotor-bearing systems using an AMB

Purpose – The purpose of this paper is to present a method for early crack detection in rotating shafts. A rotor-bearing system, consisting of an elastic rotor mounted on fluid film bearings, is used to detect the presence of the crack at a depth of around 5 percent of shaft radius. The fluid film bearings, the shaft and the crack introduce coupled bending vibrations both in the horizontal and vertical plane. Experimental time series of the rotor composite response under normal steady-state operation are uncoupled, to develop a signal processing procedure able to reveal the presence of the crack. Design/methodology/approach – The variation of the coupling property that a crack (breathing or not) or a cut (always open) introduces into the system and the localization of the coupling in the time domain is a concept proposed as a means to detect transverse surface cracks in rotating shafts. This consideration is combined with the concept of external excitation for the development of an additional crack-sensit...

Symptoms of Misaligned Worn Journal Bearings in Rotor Response Under External Excitation by a Magnetic Bearing

The defect of wear in journal bearings appears usually after long-time operation of rotating systems and the main symptom is the decrease in the bearing load carrying capacity. Wear in aligned or misaligned pattern can result in higher eccentricities of whirling and changes the properties of stiffness and damping of the fluid film. In this work the operation of worn journal bearings is investigated by incorporating a radial active magnetic bearing (AMB) on the same shaft which acts as a collocated sensor/actuator unit. This combination offers a possibility to identify and observe the characteristic symptoms that the wear provokes under external excitation and without disturbance from the nominal operation conditions of the system. Under specific electromagnetic excitation the harmonic components that are introduced due to wear are detected. A simple rotor mounted in two worn journal bearings is used and the response at steady operational speed and at transient electromagnetic external excitation with linearly varying frequency (chirp) is analyzed in the time and frequency domains. The occurrence of additional sub- and super-harmonic components is observed when the external frequency lies in the region of the first natural frequency of the operating system. The time-frequency analysis discloses the onset of additional frequency components, especially in the sub-harmonic domain. This offers a distinct detection of wear if wear depth becomes higher than 20% of the radial clearance. An experimental test rig is outlined for future studies in order to benchmark the observations from the present numerical calculations.Copyright