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Noncircular lobed journal bearing performance, in comparison with circular types, depends on various design parameters such as tilt and mount angles. Mounting orientation of this kind of bearings with respect to machine frame (mount angle) and also the way of setting their lobes with respect to each other (tilt angle), can change the bearings configuration and as the result their performances. In present study the thermo-hydrodynamic performance of noncircular two, three and four lobed journal bearings for different values of tilt and mount angles, using generalized differential quadrature (GDQ) method, are investigated. The results show that the thermal effects on these bearings performance are considerable and that the thermal consideration makes the results closer to real performance situations. The results of bearings performances due to rise in temperature in rotor, lubricant fluid and bearing shell, when compared to their isothermal conditions, show that viscosity of lubricant as well as load carrying capacity of bearings are decreased, depending on tilt and mount angles especially in case of two lobed bearings. The results also show that the effects of tilt and mount angles on bearing performance are periodic and so it is possible to select these angles suitably for bearings to be optimum.

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Poor lubrication is known as an important factor in the bearings failure. Therefore, it is very important to detect the lubrication condition. Hydrodynamic lubrication, mixed lubrication and boundary lubrication are the basic regimes of the fluid film lubrication. In a proper condition, development of hydrodynamic pressure is adequate to support the load and the bearings operate under hydrodynamic lubrication condition. However, in most situations, they operate in mixed lubrication or boundary lubrication regime and have metal-to-metal contact. To establish these regimes, using the so-called Stribeck curve is a useful method. In this curve, the oil film thickness is proportional to the lubricant viscosity and sliding velocity and inversely proportional to the applied load. However, distinguish of the exact range of hydrodynamic lubrication regime from mixed and boundary regime using this curve and relation related to the sliding bearings, due to high number of affecting design factors and operating parameters is difficult. The present study focused on the acoustic emission measuring method in order to monitoring the lubrication conditions in a type of journal bearings. Thus, condition monitoring of the journal bearing lubrication is provided and the numerical value of operating variables of the bearing for lubrication regime change from hydrodynamic to mixed is achieved. Using wavelet method, frequency features for each regime is identified. Then, for each lubrication regime, metal-to-metal contact detection is performed.

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The lubricant's ability to maintain the dynamic stability of rotor particularly in special conditions such as operating at critical speeds and instantaneous turbulences in loading or lubricant properties is always one of the most prominent characteristics of the journal bearings. Aspect or length to diameter ratio of bearing is an important factor that in different loading conditions will have an obvious effect on the performance of the trapped lubricant film between the rotor surface and bearings shell. So, the effects of aspect ratio on the damping of rotor disturbances with linear and nonlinear dynamic analysis approaches are studied in this research. Initially, the static equilibrium point of the rotor center in noncircular two, three and four lobe bearings space is obtained using the governing Reynolds equation of micropolar lubrication for different values of aspect ratio. Later, assuming the rotor perturbation as the limit cycle oscillations around the equilibrium point, critical mass and whirl frequency ratio are determined as the linear dynamic stability indexes for recognizing the converging disturbances. In nonlinear analysis model, the simultaneous solving of the lubrication and the rotor motion equations in successive time steps with Runge-Kutta method is done to differentiate the converging or diverging rotor perturbations. Results show that decreasing the aspect ratio improves the stability and the chance of controlling disturbances and returning the rotor center to static equilibrium position. Comparison of linear and nonlinear dynamic analysis results also indicates more cautious behavior and limited stability range of linear model in most of investigated cases.