Effect of presence of lifting pocket on the THD performance of a large tilting-pad thrust bearing

Hydrostatic assistance is a commonly used method to improve limited load carrying ability of tilting-pad 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.