A STUDY ON THE PERFORMANCE OF HYBRID SPHERICAL JOURNAL BEARINGS

Abstract

Bearings play an important role on the overall performance of the machines. Turbines, pumps, generators, electric motors, and other revolving equipment are essential components of modern day industries. Most of the rotating devices possess an interface through the bearings. A bearing is a machine element that permits the wanted relative motion, restrains the unwanted motion between the contacting surfaces, and supports the externally applied load. The proper functioning of the bearing is essential for an accurate and smooth operation of the machines. During the last few decades, technological developments across the globe necessitated that the machine elements be designed for precision, accuracy, improved performance, increased life span, etc. As a result, in recent times, various research efforts have been undertaken by several researchers globally to obtain better performance of the bearings. Advances in fluid film bearing technology led to the development of newer bearing configurations to harness better performance. Spherical hybrid journal bearing configuration is a type of fluid film bearings having a spherical tribo-contact that generates less noise and less vibration and provides an extended life span. Spherical hybrid journal bearing configuration supports the rotating tribo pairs by virtue of fluid film pressure generated due to the wedge action of lubricating fluid in clearance space between spherical geometrically conforming surfaces. In comparison to the conventional circular journal bearing configuration, the spherical hybrid journal bearing configuration has specific advantages, such as self-aligning property, which allows them to have tolerance for shaft misalignment and the ability to support both radial as well as axial loads. Spherical bearings are widely applied in many applications, such as gyroscopes, telescopes, machine tools, and radar tracking units. Besides this, spherical bearings also find use in biomedical applications such as artificial hip joints and knee joints. Recently, the use of textured surfaces has gained a lot of momentum to harness the better performance of tribo-pairs. The common theme of all these studies is to obtain improved performance of lubricated contacts. The application of textured bearing surfaces to improve the performance of tribo-pairs is basically inspired by biomimetics; for example, a special surface pattern enables the shark movement more efficiently in the water, and frogs walk quite easily on wet surfaces. Surface roughness is a random process, whereas surface texturing is well organized or deterministic in nature. Micro-texture is fabricated on the bearing surface as continuous (micro-groves) and discrete patterns (micro-dimples). Micro-dimples also act as innumerable micro-fluid film bearings, which generate additional hydrodynamic action. Thus, these micro texturing enhances the load carrying capacity of the bearing. Micro-dimples act as reservoirs of the lubricant and supply oil to smear at the tribo-contact interface. Further, these reservoirs entrap wear debris/foreign particles and prevent damage to the surface as wear debris may abrade the rest of the bearing surface. In recent years, unprecedented technological developments require machines to have better performance, service life, and less friction and wear. The new generation of bearings must cope with these requirements efficiently and, therefore, needs to be designed based on more realistic and accurate design data. In actual reality, the realization of perfectly smooth surfaces is rather difficult. Even a highly polished surface, when examined microscopically or with a profilometer, has an irregular surface or contains micro-asperities. The surface topography significantly affects the lubricating performance of the tribological system depending on the lubrication regime and film thickness to roughness ratio. Lubricants are required to lubricate the machine elements in order to ensure smooth relative motion of tribo-pairs. The performance of the fluid film journal bearings greatly depends on the lubricating performance of the lubricants. Nowadays, a wide variety of lubricants are being developed for the tribological system to obtain the desired lubricating performance. The behavior of the lubricants significantly depends on the rheology of the lubricants. The operating conditions, i.e., load, speed, temperature range, the material of the surfaces, etc., are the key elements for selecting a particular type of lubricant for a specific application. During the last few decades, tribologists and lubrication engineers focused their research efforts on formulating a newer class of lubricants whose rheological behavior can be adjusted or varied as per the desired operating conditions in nearly real-time. Such classes of lubricants are known as smart lubricants. A tribological system based on smart fluid technology is the new generation design of tribo systems where dynamic performance and accuracy are the important aspects. Electro-rheological (ER) and magneto-rheological (MR) lubricants are some examples of smart lubricants. These lubricants possess quick responses, and their rheological properties can be easily controlled by externally applied electric or magnetic fields. On the basis of the literature review, it may be observed that the reported work concerning the spherical journal bearings is rather very limited. Further, the majority of the available studies reported in the literature regarding spherical journal bearings are mainly limited to hydrodynamic bearings. A thorough scan of the available literature also reveals that the performance of fluid film bearings improved considerably by the application of the textured surfaces. Further, the available literature also indicates the availability of several studies in published literature dealing with surface texturing. However, they are mainly restricted to circular journal bearings configuration. No study has yet been reported in the open literature that deals with the influence of textured surfaces on hybrid spherical journal bearings. From the available literature, it may be realized that surface topography plays a vital role on the performance of tribo-pairs and should be considered. However, existing studies related to the lubrication of hybrid spherical journal bearings are mainly confined to smooth surfaces, thus ignoring the effects of surface roughness. The review of available literature related to spherical journal bearings indicates that the studies of these classes of bearings are limited to Newtonian lubricant. The work reported in this thesis was planned to study the influence of textured surfaces (micro-dimples and micro-grooves) on the performance of hybrid spherical journal bearing systems considering the combined influence of various aspects such as surface roughness effects, types of non-Newtonian lubricants (i.e., cubic stress law lubricant, and piezo-viscous shear-thinning lubricants and MR lubricants, ER lubricants) and different geometric shapes of recess. A numerical model has been developed to analyze the individual and/or combined influence of textured surfaces (micro-dimples and micro-grooves), surface roughness effects, geometric shapes of recess, and behavior of non-Newtonian lubricants (i.e., cubic stress law lubricant, piezo-viscous and shear-thinning lubricant, ER and MR Lubricants) on the performance of hybrid spherical journal bearings. The model presented in this work involves high mathematical complexity owing to the spherical geometry of the bearing. The simultaneous solution of the modified Reynold equation, along with the restrictor flow equation with suitable boundary conditions, is required for such cases. The finite element method has been used to obtain the numerical solution of the modified Reynolds equation of hybrid spherical journal bearing. A MATLAB code is developed to obtain the value of fluid film pressure generated in the lubricant domain. After obtaining the nodal fluid film pressure field, the static and dynamic performance characteristic parameters of the hybrid spherical journal bearing system have been computed. The bearing static performance characteristics parameters such as minimum fluid film thickness (ℎ𝑚𝑖𝑛), maximum fluid film pressure (𝑝𝑚𝑎𝑥), lubricant flow rate (𝑄̄), frictional torque (𝑇 ̄𝑓) have been computed. The rotor dynamic performance characteristics of the bearing system, such as fluid film stiffness and damping coefficients, have been computed. The bearing stability threshold speed (𝜔𝑡ℎ) has also been computed to determine the stable operation of the bearing system. The numerically simulated results presented in this work indicate that bearing performance characteristics parameters of hybrid spherical journal bearing get altered due to the consideration of various aspects like the presence of textured surfaces on bearing surfaces, use of various types of non-Newtonian lubricants (cubic stress law lubricant, piezo-viscous and shear-thinning lubricant, ER and MR Lubricants), use of different geometric shapes of recess/pockets. The numerically simulated results show that the performance of hybrid spherical journal bearing gets improved by the application of textured surfaces (micro-dimples and micro-grooves). The partially texturing provides a better bearing performance as compared to full texturing. The longitudinal roughness pattern offers the enhanced value of fluid film stiffness and damping coefficients. The performance characteristics of hybrid spherical journal bearing get affected due to the geometric shapes of recess and non-Newtonian behavior of lubricant. Thus, the careful selection of these parameters is essential for a bearing designer to obtain better performance. The numerically simulated results presented in the thesis are expected to be quite useful to the bearing designers as well as to the academic community to develop an efficient spherical journal bearing system.