Gregory Bregion Daniel

Assessment of Numerical Model to Determination of Parameters to Stability Experimental Tests

Rotating machines have to meet rigorous requirements in order to prevent instability during its operation. In the context of stability analysis, experimental tests such as stepped sine are widely used to determine the modal parameters of rotating systems: forward and backward natural frequencies and damping factors. Nevertheless, classical modal analysis techniques require prior knowledge of the system behavior, so that rotational speed and external excitation frequencies can be defined for the experimental tests. This work aims the assessment of model based numerical calculations to reduce or even stave off the preliminary tests. Validation starts with the evaluation of lubricated bearings model by shaft center locus. Afterwards, mass unbalance response is evaluated, and then, the stability analysis is conducted based on the logarithmic decrement. Finally, the numerical evaluation is compared to an experimental procedure regarding the precision of predicted critical frequencies for the tests and the evaluation of stability threshold.

Thermal Effects in Hydrodynamic Cylindrical Bearings

It’s known that a rotating machine, when in operation, is susceptible to vibrations, which occurs due to external excitations or to vibrations inherent from the machine operation, as the residual mass unbalanced excitations. If the rotating machine is supported by bearings with hydrodynamic lubrication, those are, consequently, susceptible to sub-syncronous vibrations due to a fluid-induced instability. The sub-syncronous vibrations, known as oil whirl/whip, can cause critical failures in the system, and consequent sudden stops and irreversible damages in the bearings. Through characterization of the oil film, by linearized stiffness and damping coefficients, it is possible to obtain an approximation to the threshold of instability. The hydrodynamic lubrication’s classical theory applies the constant viscosity condition to calculate the dynamic coefficients. Nevertheless, when the bearing is under operation, viscous fluid shear occurs, resulting in the increasing of the lubricant temperature, influencing the dynamic behavior of the entire rotational system. This paper presents a comparative analysis of the dynamic behavior, regarding the threshold of instability, considering the Lund critical mass and the logarithmic decrement theories for the classical hydrodynamic model and the thermohydrodynamic model.

Dynamic Analysis of Rotating Systems Considering Uncertainties in the Bearings’ Parameters

Industrial applications, like steam turbines and pumps, require an adequate rotor and bearing modeling to predict its dynamic behavior. The use of robust models allows simulating the dynamic condition in order to prevent critical operation and possible faults. In general, the dynamic analyses of rotating systems are performed with deterministic models, however, they do not take into account uncertainties that could affect the system response. Thus, uncertainties analysis related to the components that compose the rotating machines are important to better estimate the response and guarantee the adequate operational conditions of the rotor system. This paper aims to analyze the dynamic behavior of rotors considering journal bearing parametric uncertainties. The shaft is modeled with the Timoshenko beam and a computational model is constructed by means of the Finite Element Method (FEM), where the rotor is supported by short fluid film bearings (Orcvik model). Uncertainties in the bearings’ parameters are taken into account. The probability theory is used for the uncertainty modeling, and Monte Carlo simulations are applied to approximate the statistics of the response. The analyses performed in this paper evaluate mainly the influence of uncertainties in the first natural frequency and the vibration orbit of the shaft inside the bearing. It has been observed that the response of the system, for instance, the first natural frequency and the rotor’s orbit, might change due to uncertain bearing coefficients, and the radial clearance is the most influent parameter in the simulations delevoped.

Thermo-Hydrodynamic Model Influence on First Order Coefficients in Turbocharger Thrust Bearings

High rotation turbochargers, to automotive applications, are continually subjected to axial forces due to gas flows in the turbine and the compressor. These axial forces are supported by lubricated thrust bearings, and their effect is introduced in the dynamic system through its equivalent stiffness and damping coefficients. These coefficients are estimated utilizing a thermo-hydrodynamic model of the bearing, which is composed by the Generalized Reynolds Equation and Energy Equation, to estimate pressure and temperature distribution in the oil film. This work analyzes the influence of geometric and operational parameters of the fixed-geometry thrust bearings in pressure and temperature distributions along the fluid film, solving the governing equations by Finite Volume Method. Along with the pressure distribution, the supported axial load is evaluated and, after that, the equivalent coefficients are estimated. In this work, the Energy equation is solved utilizing 3D model and 2D model (neglecting the radial heat exchange), to check the difference in these results in a computationally less expensive model, and other simplifications, disregarding the conduction heat exchange in the circumferential direction and the convection heat exchange in the axial direction. The load capacity and the equivalent coefficients are compared with a purely hydrodynamic model, disregarding the viscosity variation through the oil film. In lower rotational speeds, the heat generated by fluid shear is small, so a HD model can be utilized considering a constant mean temperature of the oil film. This last approach can reduce the cost to solve the pressure distribution that govern the oil flow in the bearing clearance.

Numerical model proposed for a temporomandibular joint prosthesis based on the recovery of the healthy movement

Abstract The temporomandibular joint (TMJ) is an anatomical set of the buco-maxillary system that allows the movement of the mandible in most varied ways. Several factors can influence the malfunctioning of the joint and lead to the use of a total prosthesis. However, current prostheses do not supply the maximum amplitude of movement during protrusion and opening, due to mainly the anatomical differences between patients. For this reason, this article aims to study the patient’s kinematic characteristics for a better comprehension of the problem and, consequently, to develop a numerical model for TMJ prostheses able to recover the healthy movement. The numerical model is based on the development of a mechanical joint whose profile is able to reproduce the movement of the health system. The results obtained through the developed model showed a good agreement with the experimental results, representing, therefore, a promising alternative to approach the problems related to TMJ.

Experimental Estimation of Equivalent Damping Coefficient of Thrust Bearings

A specific class of rotary machines is the high rotation turbochargers, to automotive application, wherein the shaft is continually subjected to axial forces of different magnitudes due to gas flows in the turbine and the compressor. These forces are supported by axial lubricated thrust bearings. The thrust bearings are modeled through equivalent stiffness and damping coefficients and the objective of the work is to get good estimates of these coefficients, comparing simulated results with experimental results. The stiffness coefficient is first obtained by small perturbation around the equilibrium position and used in a finite element model of the system at specific rotational speeds, and this value is compared to experimental results. Then, the damping coefficient is estimated, by running an optimization problem on this parameter, to approximate the simulated dynamic response of the system to experimental results of the turbocharger excited by an impact hammer, where both the displacement and force were measured.

Evaluation of hydrodynamic conditions for journal bearings under static load

This project presents the results obtained from the numerical solution of Reynolds equation in static condition. A computational code was developed in FORTRAN in order to obtain the pressure distribution and, consequently, the hydrodynamic forces in the journal bearing. Finally, the locus of the shaft inside the bearing can be obtained from the balance of force for each rotational speed.

Investigation of the Thermal Effects of Tilting Pad Bearing Interacting with Fluid Seals on the Stability of Rotating Systems

This work aims to evaluate the influence of the thermal effects of tilting pad bearings in the stability conditions, through the simulation of a rotating system with cylindrical fluid seal and tilting pad journal bearing. Mathematical models obtained by Finite Element Method are used to represent this rotating system, in which the seals and bearings are considered through dynamic coefficients. The dynamic coefficients of the tilting pad journal bearing are obtained through the thermohydrodynamic and hydrodynamic analysis. Two different approaches are also considered when evaluating the mathematical models of the rotating system: the first considering the full matrices of the tilting pad journal bearing, while the second considers the synchronously reduced model of the coefficients. The stability analysis is carried considering the log-dec of the complete rotating system, allowing the discussion of the influence of the thermal effects on the rotating machines’ dynamic behavior.