Part of the heat generated by the shearing of the lubricating film during operation of a hydrodynamic bearing is transferred to the bearing components. In the case of the pad, which is usually fully submerged in the lubricating oil, heat is further transferred at the pad free walls to the oil by convection. This mechanism causes a thermal gradient in a pad and, consequently, its thermal deflection. In large hydrodynamic thrust bearings, thermal deflection of the pads is an important phenomenon influencing bearing performance. For such bearings, pad distortion can reach the level of hydrodynamic film thickness and can significantly change the bearing's properties. In this paper, the study of the influence of the heat convection coefficient on the predicted performance of a large hydrodynamic thrust bearing is presented. Two sets of convection coefficients at the pad free surfaces are investigated with the use of thermo-elasto-hydrodynamic (TEHD) calculations. An analysis is carried out for the Itaipu hydro turbine thrust bearing with the outer diameter equal to 5.2 m, which is one of the biggest hydro power plants in the world. The results of the theoretical predictions are compared to the measured data collected during bearing operation.

Hydrostatic assistance is a commonly used method to improve limited load carrying ability of tiltingpad thrust bearings during transient states of operation of vertical shaft hydro-generators. Despite of special hydraulic equipment (as pumps, valves, etc.), it also requires manufacturing of special recesses/pockets at pad sliding surfaces, into which oil is injected under high pressure. It allows to lift the rotor before start-up of the machine and form a hydrostatic film between pads and collar. There is a quite wide variety of geometry of recesses (shape, depth, and size) met in practical large bearing applications. The presence of a hydrostatic pocket (usually located in the sliding surface above the pivot area, where thin film, high oil pressure and temperature are observed) affects bearing performance under hydrodynamic operation. In theoretical researches, there is an almost common practice not to include hydrostatic recess in thermohydrodynamic (THD) or thermoelastohydrodynamic (TEHD) analysis. This is probably due to the problems with obtaining solution for oil film geometry with pocket, the order of magnitude of the pocket depths being larger than gap thickness. In this paper, an attempt was taken to study the effect of lifting pocket on THD performance of a large tilting-pad thrust bearing of Itaipu power plant. Bearing performance was evaluated including recess shape for several cases of its depth. The results show that hydrostatic recess changes calculated bearing properties quite significantly, especially in vicinity of the pocket.

Mechanics & Industry, An International Journal on Mechanical Sciences and Engineering Applications

Using the experience acquired within our lab in terms of both thermohydrodynamic (THD) and thermoelastohydrodynamic (TEHD) numerical simulations, a new THD code has been improved by adding the possibility of taking into account geometrical defects, and particularly scratches, which are often discovered by turbine users during maintenance operations. To examine this issue, two numerical codes were coupled to provide the TEHD analysis presented in this work. To validate the numerical results, experimental tests were conducted using the Pprime Institute bearing test rig. The performance of the same two-lobe journal bearing (preload 0.524) as used in a previous study, lubricated with ISO VG 46 oil, was evaluated. Scratches of different depths (varying as a function of the radial clearance) were directly machined onto the shaft. TEHD solutions and experimental data were compared for various rotational speeds and applied loads. Pressure and temperature comparisons for the three scratch depths show good correlation, and give the expected results for cases with a scratch. It was also found that the asymmetry in the pressure field created by the presence of a scratch led to a slight misalignment. The comparisons were improved by taking into account this misalignment, using the balance of momentum.

During the start-up period, if the journals bearing bushing and shaft materials have different heat conductivities, the thermal expansion of the shaft might be more rapid than that of the bushing. This reduces the radial clearance between the bearing components and generates higher shear stresses in the lubricating fluid, which can lead to bearing seizure, resulting in the deterioration of the journal bearing. (1) First, we present an experimental case study of the seizure of a central groove journal bearing with a carbon-filled polytetrafluoroethylene (PTFE) bushing and a steel shaft; (2) Then, a static thermoelastohydrodynamic (TEHD) simulation is compared to Cristea’s experiments (Cristea, A.-F., 2012); (3) Then, a pseudo-transient TEHD simulation is performed in order to numerically predict the observed phenomena on the test rig; (4) Finally, the evolution of the oil flow rate and the bushing temperatures during the start-up period determined with the pseudo-transient TEHD modelling are compared to the experimental data of the test rig. The pressure field is determined with the thermohydrodynamic (THD) Elrod’s modelling, with Vijayaraghvan’s robust solver algorithm. The pseudo-transient temperature field in the bearing components as well as the thermal and mechanical deformations are calculated with a commercial software.

Hydrodynamic journal bearings supporting high mass rotating machinery are subjected to low rotational speeds during start-up or idling phases. These working conditions need a hydrostatic pressure contribution to ensure a correct load capacity when the hydrodynamic pressure is not established yet. This is possible by means of hydrostatic lift pockets. These pockets avoid mechanical damages via sufficient film thickness from start-up to hydrodynamic working conditions. In a first part, it is showed how an existing isothermal 3D hydrodynamic numerical modelling of tilting pad bearing has been adapted to take into account the lift pocket impact on static bearing performance. This approach is only valid for low rotational speeds, when inertia step pressure effects (rises or drops) at the film-pocket edge can be neglected compared to the total bearing pressure pattern. Then, numerical simulations have been performed, from 1 to 200 rpm with pockets and from 75 to 200 rpm without pockets. The results have been compared and discussed in terms of pressure fields and bearing static performance such minimum film thickness, friction torque or rotor center position. Therefore, the interest of such lift pockets has been highlighted. Finally, calculations with and without pockets have been carried out in case of a strong bearing misalignment. As a result, the benefits of the lift pockets in this configuration are also significant in terms of static performance. Les paliers hydrodynamiques présents sur les lignes d’arbre de forte masse nécessitent, pour les phases de démarrage et basse vitesse, l’aide d’une pression hydrostatique afin d’assurer leur mission de supportage lorsque la pression hydrodynamique n’est pas encore établie. Les poches de soulèvement hydrostatique présentes dans ces paliers permettent d’assurer cette fonction qui préserve l’intégrité du coussinet en assurant une épaisseur de film mince suffisante dès les premiers tours de la machine. Cette étude présente tout d’abord les modifications apportées à une modélisation 3D hydrodynamique en régime isotherme existante pour prendre en compte l’impact de telles poches sur les caractéristiques statiques du palier. Cette approche se limite à des vitesses de rotation faibles, pour lesquelles les sauts de pression à l’interface film mince/poche (discontinuité géométrique) dus aux effets d’inertie peuvent encore être négligés dans l’allure globale du champ de pression. Puis, des résultats de calculs concernant plusieurs vitesses de rotation allant de 1 à 200 tr.min-1 avec poches de soulèvement seront présentés et comparés aux mêmes cas sans poches pour des vitesses allant de 75 à 200 tr.min-1. Les champs de pressions et les différentes caractéristiques statiques comme l’épaisseur minimale de film, le couple de frottement, la position du rotor ainsi obtenus, seront analysés dans chaque cas et l’intérêt de ces poches de soulèvement sera mis en avant. Enfin, des calculs avec et sans poches seront également présentés dans le cas d’un fort mésalignement du palier et l’impact favorable des poches sur les caractéristiques statiques sera illustré.