STUDY OF GEOMETRICALLY IMPERFECT MULTIRECESS FLUID FILM HYBRID JOURNAL BEARINGS

Abstract

Bearings are the key machine elements used extensively in all rotating machinery to support loads and constrain relative motion between moving parts for the desired motion. In the present age of rapid and continuing technological development, it will not be wrong to say that a bearing plays the same role in machinery as a heart in living being. 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 of vital importance. For heavier load applications such as power plant, crusher, turbine etc. fluid film bearing 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 number of researcher focused their research efforts in the area of multirecess hybrid journal bearing due to their wide engineering applications such as high speed turbo-machinery, machine tool spindles, reactor coolant pumps, liquid rocket engines, etc. Furthermore, hydrostatic/hybrid journal bearing is classified as (i) Cylindrical journal bearing, (ii)Non-Cylindrical journal bearing: Elliptical (Lobed) bearing, offset bearing, Conical journal bearing, Spherical journal bearing. Modern manufacturing techniques provided more flexibility to the bearing designer to deal with the geometry of the journal bearing system. In recent years the application of conical journal bearing system becomes popular in various industrial applications due its advantageous feature over other bearings. Conical journal bearing can support radial as well as axial loads simultaneously. Besides this, a conical journal bearing also has the advantage of clearance adjustability as per the desired performance. The conical journal bearings are generally used in high speed turbine, precision machine tools such as lathes, grinding machines etc. Reliable, efficient, robust and satisfactory operation of journal bearing under the desired condition is of utmost consideration concern for an efficient, satisfactory and viii durable operation of rotating machineries. This puts a great constraint on bearing designer to design the bearing system for very realistic operating conditions. In general, a machine element is bound to have different forms of geometric imperfections during manufacturing processes although availability of highly précised, accurate and versatile manufacturing techniques. Likewise manufacturing processes of journal and bearing may cause several geometric imperfections. Geometric imperfections are also inevitable in the manufacturing of journal and bearing as there may be error of environmental/instrumental/gross error. The geometric imperfections of journal exhibit dynamic conditions while geometric imperfections of bearing exhibit static conditions as journal is rotating and bearing is fixed during the operation of journal bearing system. Hence, for realistic performance of journal bearing system, geometric imperfections of journal are more important than geometric imperfections of bearing. That is why many researchers focused their attention to examine the effect of geometric imperfections of journal on the bearing performance. The most common types of geometric imperfections of journal are barrel shape, bellmouth shape and circumferential undulations type. These geometric imperfections are of the order of fluid film thickness. As a consequence of this, the resultant fluid film profile in bearing system gets changed due to these geometric imperfections of journal. Hence, these geometric imperfections significantly contribute in the performance of a journal bearing system. Recent studies indicate that the performance of a journal bearing generally gets deteriorated due to the presence of different geometric imperfections. It is therefore imperative to consider these geometric imperfections of journal in the analysis of bearing to generate the more realistic, accurate and précised design data. A thorough review of the available literature pertaining to the influence of geometric imperfections of journal on bearing performance reveals that the presence of various forms of geometric imperfections of journal generally deteriorates the performance of the bearing system. As discussed earlier, the consideration of geometric imperfections of journal in the analysis of hybrid journal 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 transient operations (start/stops) and continuous supporting of shafts for a long period of time are the cause of significant wear of the bearing surface. Thus, wear in journal bearings may be attributed as a defect which is developed due to long term use of rotor bearing ix systems. Therefore, the operating radial clearance gets changed due to wear and this ultimately affects the bearing performance characteristics significantly. It has been revealed that the performance of a bearing system in general significantly deteriorates due to influence of geometric imperfections of journal and as well as wear defect in bearing. Further, the review of the studies related to the micropolar lubrication of the bearings indicates that the use of micropolar lubricant enhances the bearing performance vis-à-vis Newtonian lubricant. Hence, it is expected that a micropolar lubricant may partially overcome the deterioration in the bearing performance due to the combined influence of geometric imperfections of journal and wear defect. Fluid film journal bearings in real sense quite often operate in a misaligned condition that can substantially alter the bearing performance. Therefore, the influence of misalignment of journal on the bearing performance cannot be overlooked. Generally, misalignment can have a considerable and usually detrimental effect on the performance of a hybrid journal bearing. Thus, to generate an accurate bearing characteristic data inclusion of journal misalignment in 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 rectangular, square, circular, elliptical and triangular geometries of pockets and investigated the comparative performance of the corresponding bearing configurations. Besides this, the orientation of pockets also plays an important role on the performance of bearing. Recent studies in literature indicate that the performance of the bearing system also gets affected significantly due to the orientation of pocket. In view of the above, it was felt that there is a need to carry out a study to investigate the influence of geometric imperfections journal on the performance of multirecess hybrid cylindrical and conical multirecess journal bearing in conjunction with other issues such as wear defect, micropolar lubrication etc. in the analysis. Therefore, the present work is an attempt to bridge this gap in literature and evaluate the performance of multirecess cylindrical and conical journal bearing with the following objectives: • To study the influence of geometric imperfections of journal viz. barrel shape, bellmouth shape and undulated shape on the performance of micropolar lubricated multirecess cylindrical journal bearing system. x • To study the combined influence of wear and geometric imperfections of journal on the performance of a micropolar lubricated multirecess cylindrical journal bearing system. • To evaluate the effect of journal misalignment, effect of geometric shapes of pocket and orientation of pocket in conjunction with the influence of geometric imperfections of journal on multirecess cylindrical journal bearing system. • To study the influence of circumferential undulations of journal on the performance of multirecess conical journal bearing system. The results in the present study have been numerically simulated by using the developed computer program in Fortran 77 and have been presented for the generally used values of bearing operating and geometric parameters i.e. = 1.0, = 0.2 − 1.0, ∗ = 0.5, Ω = 0.5,1.0, = 0.14,0.25, for multirecess cylindrical/conical journal bearing system in order to highlight the influences of different geometric imperfections of journal in conjunctions with above discussed aspects on the performance of the bearings. This study make possible to have reliable information about the static and dynamic performance characteristics of multirecess cylindrical and conical journal bearing system. A theoretical model is used to account and analyze the influence of geometric imperfections of journal in conjunction with the influence of wear defect, micropolar lubrication, effect of pocket geometries etc. on the performance of cylindrical hybrid journal bearing. Furthermore, a mathematical model to examine the influence of circumferential undulations of journal along with the effects of wear and micropolar lubrication in the performance of conical hybrid journal bearing is developed. To govern the flow of micropolar lubricant through the clearance space between journal and bearing, the desired Reynolds equations have been derived in Cartesian/Cylindrical coordinate system for cylindrical/conical journal bearing system respectively. Dufrane’s wear model has been used to tackle the wear defect in the analysis of worn bearing system. A pair of non-dimensional parameters ,Ψ have been defined and used to account for the journal misalignment effect. In the present work, the non-linear equations for orifice and membrane restrictors are been solved 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. xi The results presented in this study indicates that bearing performance characteristics parameters like minimum fluid-film thickness, frictional torque, stiffness coefficient and stability margin are significantly influenced due to the influence of geometric imperfections, influence of wear, micropolar lubrication, geometric shape of pockets, journal misalignment, orientations of pockets, semi cone angle for the chosen bearing configurations. 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 bearing designer.