The performance of non-circular bearings, such as multi-lobe bearings is influenced by many bearing parameters such as the number of lobes, the preload factor, and the tilt angle. In this study, the effects of bearing preload factor on the static performance characteristics of micropolar lubricated two-, three-, and four-lobe bearings are investigated. For this purpose, modified Reynolds equation for micropolar fluids is solved by FEM, to find the fluid film pressure. Then, in turn, bearing performance characteristics, namely, load-carrying capacity, attitude angle, frictional force, friction coefficient, and side leakage flow are calculated. Results show that, in general, micropolarity of the lubricant improves the performance of the bearings; however, non-circularity parameter of the lobe of the bearings, preload, has a favourable effect on certain performance characteristics of the bearings, such as the attitude angle which decreases as non-circularity of the bearing increases. The results further show that the rate of influence of preload factor on the performance characteristics of different lobed bearings is not the same.
In this paper, steady-state and stability performance characteristics of circular and noncircular two-, three-, and four-lobe journal bearings with micropolar fluids are presented. For this purpose, lubricating oil containing additives and contaminants is modeled as micropolar fluid. The modified Reynolds equation in dynamical state is obtained using the micropolar lubrication theory, and it is solved by finite element method. The bearing performance characteristics in terms of load carrying capacity, whirl frequency ratio, and critical mass parameter of journal are determined for different values of design parameters such as eccentricity and aspect ratio, preload factor, and micropolarity characteristics of lubricant in terms of material characteristic length and coupling number. Results show that in the case of noncircular bearings, the critical mass to load carrying capacity ratio decreases with increasing of preload factor, so for a constant vertical external load, the stability performance of rotating system can be improved by replacing the circular journal bearing with similar noncircular types. The results compared with Newtonian fluids indicate that micropolar lubricant exhibits better steady-state and dynamic performance. Also, results reveal that upgrading the micropolarity characteristics of lubricant causes increase in critical mass, load carrying capacity but decrease in whirl frequency ratio. It is also observed that the effect of micropolar fluids is more pronounced at high coupling numbers.
In this work, the effects of preload factor on the stability performance of noncircular two and three lobe journal bearings with incompressible micropolar lubricant are presented based on the linear and nonlinear dynamic models. Assuming that the rotor is rigid, the governing Reynolds equation for the hydrodynamic lubrication of finite length lobed journal bearings has been modified using micropolar theory. Then, the linear and nonlinear dynamic models which include a certain harmonic disturbance and a time dependent trajectory of rotor center are applied to analyze the stability performance of the considered bearings. The generalized differential quadrature method and fourth-order Runge–Kutta technique have been used to solve the governing Reynolds equation and time-dependent dynamic equations of rotor motion, respectively. Finally, the numerical results for the critical mass parameter and the whirl frequency ratio of the rotor as the stability characteristics of the lobed bearings are obtained for different values of preload factor and are compared together. Results show that the dynamic stability of the noncircular bearings is enhanced by decreasing the preload factor i.e. increasing the amount of bearing noncircularity. These enhancements are in terms of increase of the critical mass parameter and decrease for the whirl frequency ratio. Also, by comparing the two dynamic analysis methods, it is seen that the results of linear model are more conservative in different investigated cases. The results of nonlinear analysis reveal that by changing the preload factor the dynamic responses of the lobed bearings appear in different manners. From the responses, it is observed that the type of dynamic trajectory of rotor center varies from stable fixed point to limit cycle periodic motions and also contact between rotor and bearing’s shell.
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