Numerical modelling of hydrostatic lift pockets in hydrodynamic journal bearings – Application to low speed working conditions of highly loaded tilting pad journal bearings

Abstract

-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. Key words: Hydrostatic lift pockets / hydrostatic lubrication / hydrodynamic lubrication / tilting pad journal bearingsRésumé -Prise en compte numérique de la contribution des poches de soulèvement dans les paliers hydrodynamiques -Application aux cas de fonctionnementà basse vitesse de paliers a patins oscillants fortement chargés. 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 encoreétablie. Les poches de soulèvement hydrostatique présentes dans ces paliers permettent d'assurer cette fonction qui préserve l'intégrité du coussinet en assurant uneépaisseur de film mince suffisante dès les premiers tours de la machine. Cetteétude présente tout d'abord les modifications apportéesà 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 limiteà des vitesses de rotation faibles, pour lesquelles les sauts de pressionà l'interface film mince/poche (discontinuité géométrique) dus aux effets d'inertie peuvent encoreêtre négligés dans l'allure globale du champ de pression. Puis, des résultats de calculs concernant plusieurs vitesses de rotation allant de 1à 200 tr.min −1 avec poches de soulèvement seront présentés et comparés aux mêmes cas sans poches pour des vitesses allant de 75à 200 tr.min −1 .

Measured and Predicted Operating Characteristics of a Tilting-Pad Journal Bearing with Jacking-Oil Device at Hydrostatic, Hybrid, and Hydrodynamic Operation

“…The latter result is in accordance with the one from Ref. [7]. The increase of film thickness from 2.0 to 2.5 MPa in Figure 13b was confirmed by repeating the measurement several times.…”

“…A self-aligning coupling (6) connects the drive train with the test shaft (1). The test bearing is virtually unconstrained adjustable in the radial plane and it is aligned by six transversely flexible chains (7). Rods (9) connect the test bearing housing to a double-acting pneumatic bellow compensator (8) that applies static loads on the test bearing.…”

“…Elwell [6] presents an empirical equation for the breakaway pressure, which depends on geometry, load and additional test data. Raud et al [7] study the impact of jacking-oil pockets on steady state conditions of a large tilting-pad bearing with a special emphasis on misalignment between journal and bearing. Beside their nominal applications, lift-oil devices can be used as a short-term measure if field issues occur.…”

“…In their case, the jacking-oil pockets are machined into the white metal layer, however, without a bore communicating to an outer high pressure supply system or a check valve. Raud et al [7] compare predictions with and without jacking-oil pockets and obtain significant influences of the concrete configuration on the journal displacement, shaft friction torque and minimum film thickness.…”

Characteristic Analysis and Simulated Test of Hybrid Bearing with the Introduction of Piezoelectric Controller

“…Then the technology of high-pressure injection pockets is used for tilting pad journal bearing. But the hydrodynamic pressure only works in the startup and stop phase to lift the rotor [11]. When the rotation speed raises up, the high-pressure oil will be shut down.…”