A STUDY ON THE LUBRICATION OF HYDROSTATIC THRUST PAD BEARINGS

Abstract

Bearings play a key role in all rotating machinery to support loads and constrain wanted motion between moving parts for the motion in desired direction. Without the proper functioning of bearing system, operation of machinery cannot be feasible. That is why there is a great need to pay attention on the research and development of this machine element with vital importance. For heavier axial and radial load applications such as large hydropower plant, large hydro turbine etc. fluid film bearings are very useful. During last few decades these classes of bearings have witnessed many noteworthy developments and many researchers focused their research in this direction. In recent years a number of researchers focused their research efforts in the area of fluid film hydrostatic thrust bearing due to its wide engineering applications such as heavily loaded turbo-machinery, precision equipment, liquid rocket engines, etc. Fluid film thrust bearings in real sense quite often operate in a tilt condition that can substantially alter the bearing performance. Therefore, the influence of tilt of thrust pad on the bearing performance cannot be neglected. Generally, tilt can have a considerable and usually detrimental effect on the performance of a hydrostatic/hybrid thrust bearing. It decrease the load carrying capacity and increases fluid film damping coefficient. Therefore, to generate an accurate bearing characteristic data inclusion tilt of thrust pad within the analysis becomes more imperative. Moreover, modern manufacturing techniques provide more flexibility to the bearing designer to make proper use of pocket geometry. In recent years, several investigators used sectorial, rectangular, square, circular, elliptical and triangular geometries of pockets and investigated the comparative performance of the corresponding bearing configurations. The most common types of surface texture are spherical and conical cap type. These surface textures are of the order of fluid film thickness. The surface texture on bearing surface itself acts as microbearing. As a consequence of this, the resultant fluid film profile in bearing system gets altered. Recent studies indicate that the performance of a thrust bearing generally gets enhanced due to surface texture. It is therefore iv necessary to analyze the effect these surface textures in the analysis of bearing to generate the more realistic data. A thorough review of the available literature pertaining to the influence of surface texture on bearing performance reveals that the presence of various forms of surface texture on thrust surface generally enhanced the performance of the bearing system. The consideration of temperature of thrust in the analysis of hybrid thrust bearing system is quite important for the generation of more realistic design data so that the bearing can operate satisfactorily under more realistic and stringent operating conditions. Further, the lubricant behavior in the bearing machinery is an important parameter. Generally polymer based additives such as poly isobutylene are blended with lubricating base oil to enhance the performance of the lubricants. Due to addition of the additives in the lubricant, it makes the behavior of lubricant non-Newtonian. Hence, they do not follow the Newton's law of viscosity. Therefore, the consideration of non-Newtonian behavior changed the bearing performance characteristics. Further, the review of the studies related to the non-Newtonian lubrication of the bearings indicates that the use of non-Newtonian lubricant enhances the bearing performance vis-à-vis Newtonian lubricant. Hence, it is expected that a non-Newtonian lubricant may partially overcome the effect of tilt in the bearing performance due to the influence of tilt of thrust pad. In view of the above, it was felt that there is a need to carry out a study to investigate the influence of surface texture and tilt on the performance of hydrostatic/hybrid thrust bearing in conjunction with other issues such as temperature, non-Newtonian lubrication, flexibility of thrust pad etc. in the analysis. Therefore, the present work is an attempt to bridge this gap in literature and evaluate the performance of thrust bearing with the following objectives:  To develop analytical expression for fluid film pressure, load carrying capacity, fluid film stiffness and fluid film damping of hydrostatic thrust bearing  To evaluate the effect of tilt, effect of geometric shapes of recess in conjunction with power law lubricant on hydrostatic/hybrid thrust bearing system.  The influence of surface texture viz. conical and spherical cap on the performance of non-Newtonian lubricated thrust bearing system v  To study the influence of surface texture pattern on the performance of a non-Newtonian lubricated thrust bearing system of various recess shapes.  To study the influence of tilt of thrust pad by considering effect of temperature variation and effect of bearing pad deformation on the performance of thrust bearing system. The results in the present study have been numerically simulated by using the developed computer program in MATLAB 2013a and have been presented for the generally used values of bearing operating and geometric parameters in conjunctions with above discussed aspects on the performance of the thrust bearings. This study makes possible to have reliable information about the static and dynamic performance characteristics of hybrid/hydrostatic thrust bearing system. A theoretical model is used to account and analyze the influence of surface texture on thrust bearing surface in conjunction with the surface texture pattern, non-Newtonian lubricant, bearing pad deformation and effect of lubricant temperature variation in oil film etc. on the performance of hydrostatic/hybrid thrust bearing. Furthermore, a mathematical model to examine the influence of temperature variation of oil film along with the effects of surface texture in the performance of hydrostatic/hybrid thrust bearing is developed. To govern the flow of non-Newtonian lubricant through the clearance space between shaft and bearing surface, the desired Reynolds equations have been derived in Cartesian coordinate system for thrust bearing system. A pair of non-dimensional non-Newtonian parameter have been defined and used to account for the thrust bearing. In the present work, the non-linear equations for orifice and non-Newtonian lubricant have been solved simultaneously with energy equation and Heat conduction equation by using the Newton-Raphson iterative method. The variation in static and dynamic performance characteristics are presented and discussed for the judiciously selected wide range of geometric and operating parameters. The results presented in this study indicates that thrust bearing performance characteristics parameters like pocket pressure, frictional torque, fluid-film stiffness coefficient and fluid-film damping coefficient are significantly influenced due to the influence of recess shape, influence of surface texture, influence of restrictor design vi parameter, restrictor type and type of lubricant behaviour. Thus, to generate more realistic bearing characteristics design data, these effects are essential to be considered in the analysis. The results presented in the thesis are expected to be quite useful to the thrust bearing designer.