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Document Type

Original Study

Subject Areas

Mechanical Design

Keywords

Finite element (FE) model, linear & Non-Linear analysis, Rotor dynamic Model, Non-Linear bearing coefficients, Hydrodynamic bearing forces.

Abstract

Abstract BACKGROUND: Analyzing the vibrations for rotor-bearing systems is a critical issue in the field of rotor dynamics. It is crucial to identify rotary machine vibrations, behaviors, and stability conditions. The main causes of vibration in rotating machinery are unbalanced masses, misalignment, mechanical looseness, shaft cracks, and other defects. METHODS: This paper investigates the experimental verification of a theoretical model, using a steel shaft with a disc set at the midpoint, supported by two symmetric fluid film bearings. The study examines the effect of unbalance on the dynamic behavior of the rotor and its vibration characteristics. The experimental investigation involved setting up a test rig, installing the journal bearing and rotor, and measuring relevant parameters. The theoretical analysis employed the solution of the Reynolds equation to determine the bearing coefficients, which were then modeled as a function of the Sommerfeld number using a polynomial fit. A finite element model with a consistent matrix formulation was used to simulate the shaft, including the external load and four degrees of freedom per node.. RESULTS: The theoretical model was validated against experimental results in both the time and frequency domains, considering the effect of unbalanced masses. The results are presented using orbit plots, system responses, and FFT spectra. The vibration analysis results show the whirl phenomena before it becomes uncontrollable and leads to self-excited vibration. CONCLUSIONS: The theoretical model based on nonlinear analysis was in agreement with the experimental analysis for all rotational speed ranges. The resonant speeds of 5107 rpm and 5850 rpm were observed in both the theoretical and experimental studies, respectively. However, a noticeable discrepancy was observed when the speed exceeded the threshold speed in both the first and second-order theoretical analyses.

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