A STUDY OF LUBRICATION BEHAVIOUR OF NON-CONFORMAL EHL CONTACTS

Abstract

Elastohydrodynamic lubrication (EHL) is a special form of hydrodynamic lubrication that involves extremely high contact pressures leading to substantial increase in lubricant viscosity along with significant elastic deformation of contacting surfaces. The most common examples of machine elements operating in EHL regime are the ones involving nonconformal contacts occurring in gears, cams, roller bearings etc. Owing to the high contact loads, EHL films are extremely thin and the contact pressures are quite high. The high contact pressure and lubricant viscosity in the vicinity of the contact elastically deforms the contacting surfaces thereby enhancing the lubricant film thickness. This not only helps in enhancing the useful life of the contact but also helps in avoiding the pre-mature failure due to wear and tear. It is due to this reason that contact pressure and minimum film thickness are the key EHL parameters to be evaluated with high degree of accuracy for any given specific operating conditions. In practice, the contacting surfaces are never perfectly smooth because of inherent constraints of the machining process, impulsive damage, rust, abrasive wear, etc. The local lubricant film thickness contributes randomly to the fluid film pressure of rough tribo-contact due to the random nature of surface topography. In reference to this, the tribologists became tempted to produce regular micro-dimple patterns on the surface of the machine elements in order to enhance the performance of the contact significantly. The textured surfaces offer great versatility in improving the tribological characteristics of non-conformal EHL contacts. The numerical studies available in this area focus on transient EHL conjunctions involving movement of micro-textured surface within the contact zone. However, the effect of stationary surface feature located within the inlet zone is expected to be more significant in case of unidirectional pure sliding EHL contact. Therefore, an exhaustive study on the effect of size, shape and location of such micro-textured surface with due consideration of non-Newtonian fluid is very much required. Nowadays, the lubricant formulators quite often blend different types of external organic and/or inorganic additives in base oil to enhance the characteristics of lubricants. The presence of characteristic additives makes the lubricant behave as non-Newtonian. The classical theory of Newtonian lubricant neglect the size of fluid particles and fails to illustrate the behavior of non- Newtonian lubricant such as polymer thickened oils, synthetic lubricants, bio-lubricants and lubricants with various additives. Therefore, the size of long-chain polymer molecules (additives) has been considered in the couple stress fluid model. Similarly, the micro-rotation and microtranslation of suspended fluid particles are also taken care of in micropolar fluid model. In the vi modern era of machinery, lubrication using electrically conducting couple stress lubricant(ECCSL) has emerged as an effective/smart way to improve tribo-contact performance. Thus, in the present work, different types of non-Newtonian lubricants such as micropolar and ECCSL have been used to investigate the performance of non-textured/textured non-conformal EHL contacts. Energy efficiency and wear mitigation are the common challenges of modern day precision machinery. The stringent operating conditions even make the lubricated contacts vulnerable to damage. In this study, different engineering materials have been studied under various working conditions to investigate the performance behavior of elastohydrodynamic contact. The performance behavior has been analyzed based on the rheological properties of different lubricating oils. Experimental investigations have been performed on various tribo-contact simulators, utilizing point, line and elliptical contact geometries, to assess the lubricant's ability to prevent / delay failure criterion of the non-conformal EHL contacts and to evaluate different oils in terms of failure resistance. It has also been observed that no significant experimental investigation has been performed to study the non-conformal EHL contacts under realistic operating conditions. Therefore, it is essential to explore the individual/combined influence of material, lubricant behavior, starvation and surface characterization of non-conformal EHL contacts. The work undertaken in this thesis begins with an introduction of the subject followed by a brief historical review commencing from the earliest achievements in the field of non-conformal EHL contacts. From review it is concluded that, in spite of the considerable progress that has been made over the years, the actual and realistic prediction for the non-conformal EHL contact problems considering realistic input data has not yet been fully investigated. As a consequence, accurate prediction of contact pressure, lubricant film thickness and friction could not be achieved. Hence, for an accurate analysis, the practical condition along with suitable non- Newtonian fluid model should be taken into consideration. This also calls for a fast and stable numerical solver. The solver should take into consideration the characteristic properties of new generation lubricants and their constitutive relations governing the non-Newtonian behavior micro-texture of the surfaces, operating conditions and the characteristics of present generation engineering materials. In non-conformal EHL contact problems the amount of lubricant used continues to decrease and therefore so-called starved lubrication condition may arise. Hence, this also has been accounted for in the present work. In particular, this study is directed towards the theoretical solution of lubricated line contact problem. The numerical procedure involves simultaneous analysis of the coupled system of vii generalized Reynolds equation, elasticity equation and load equilibrium equation along with the calculation of pressure dependent lubricant viscosity and density subjected to appropriate boundary conditions. Discretization of Reynolds equation is done employing finite element method and solved along with the force equilibrium condition using Newton-Raphson technique. In the first of this thesis, the theoretical models have been developed to analyze the individual and combined effects of textured surfaces, non-Newtonian lubricants (micropolar and ECCSL) on the characteristic parameters of non-conformal EHL contacts. Further, the enhancement in steady-state EHL characteristics pertaining to unidirectional pure sliding contacts due to an artificially produced inlet zone surface micro textures (IZMT) has been investigated numerically. The theoretically simulated results in this study reveal that the performance characteristics of nonconformal EHL contacts are significantly enhanced by the rectangular textured surface when lubricated with various types of non-Newtonian lubricants (micropolar and ECCSL). The second part of this thesis basically deals with the findings obtained from the experimental investigations. The test facilities used and the procedure followed for the experimental evaluation of different lubricants has been discussed. The work presented in this thesis mainly focuses on the experimental investigation of EHL contacts as those occurring in between spur gears, roller bearings, cam-followers etc. Further, experimental investigations have been performed on different tribological test-rigs using contact set-up such as four ball tribo-tester, block-on-roller, ball-on-disk and pin-on-roller to investigate the tribo-performance characteristics of EHL contacts. The experimental study undertaken in the present work examines the load carrying capacity of various material pairs (AISI E52100, Brass and EN-31), lubricated with different oils such as SERVO SYSTEM HLP (N), SERVO HYDREX, SERVO GEAR HP SERVO MESH SP and SERVO GEAR OIL75W80 under various working conditions. Experiments were performed on different tribological test-rigs for the measurement of friction and wear. Wear scar obtained on the surface of used test specimens indicates the extent of anti-wear performance characteristics of lubricants. Experimental studies carried out in this work show how friction varies over a range of working conditions with the change of material, type of oil and temperature of lubricant. The main objective behind the experimental study was to investigate the effect of operating parameters on the surface characteristics of the elastohydrodynamic lubricated contacts and to establish an analogy between them by employing the response surface methodology (RSM). For this a series of experiments were scientifically planned using Taguchi design of experiments concept. The roughness of the surface is a very important property, which affects the performance of pairing surfaces in contact. Hence, a correlation between the contact friction and parameters of surface roughness has been undertaken. Prior to the tribological tests, the 3-D topography and viii corresponding roughness parameters were measured using Taylor Hobson stylus profilometer. The roughness parameter 𝑆𝑘𝑢 and 𝑆𝑠𝑘 may be used to analyze the topography of surface in lubricated contacts to achieve less friction. 𝑆𝑘𝑢, 𝑆𝑠𝑘 and 𝑆𝑘𝑣 have been observed to show a real similarity with the tribological attributes of the contacting surfaces. The contact friction is lower at roughness parameters having lesser values of arithmetic average height (𝑆𝑎) and root mean square height ( 𝑆𝑞) but not prevailing at average roughness. On the basis of Tribological evaluation and surface characterization, a model was proposed for low wear and low friction under different test conditions. Tribological test-rigs, produce accurate results and have long-life in assessing critical properties of lubricants such as oil film thickness and shearing strength of the lubricant. The theoretical and experimental studies reported led to some interesting new insights both in general as well as specific with respect to the lubricant and materials in non-conformal EHL contacts. The outcomes presented in this thesis are expected to be beneficial for the practicing lubrication engineer and bearing designers as well as academic community for designing an efficient tribological system. Finally, this thesis is concluded with some key recommendations for future research.