Elastohydrodynamic Analysis Research Articles

Experimental and numerical investigations of the wear behavior and sealing performance of PTFE rotary lip seals based on the elasto-hydrodynamic analysis with considerations of the asperity contact

Sealing performances of Polytetrafluoroethylene (PTFE) rotary lip seals could be deteriorated due to wear behaviors, objective investigations of the potential leakage are therefore demanded in the early design stage. Numerical assessments based on the elasto-hydrodynamic analysis proposed in the authors’ recent work in conjunction with the Greenwood and Williamson contact model are presented. The asperity contact pressure is calculated and implemented into the modified Archard wear model to estimate the wear depth of the seal lip under the lubricated operation. Current analysis procedures can fairly estimate evolutions of the seal lip geometry under both the dry and lubricated operation, and a sign of the reverse pumping rate is further verified as an effective index of the leakage.

Elastohydrodynamic analysis of a journal bearing with different grade oils considering thermal and cavitation effects using CFD-FSI

1. Nour AM. An experimental investigation of hydrodynamic journal bearing with different oil grades. Port-Said Engineering Research Journal 2019; 23(2):46-54. http://dx.doi.org/10.21608/pse.... CrossRef Google Scholar

Optimum boundaries for maximum load-carrying capacity in water-lubricated composite journal bearings incorporating turbulences and inertial effects based on elastohydrodynamic analysis

Abstract The application of polymer-based composite materials as bearing liner materials in eco-friendly water lubrication has received considerable attention owing to their superior tribological behaviors, corrosion resistance, and high damping characteristics, and their design flexibility can improve the bearing performances in response to the distribution of lubricant film pressure based on the regulation of elastic constants. However, the low viscosity and high density of water essentially cause thin-filmed lubrication accompanied by a low load-carrying capacity. Particularly, a high rotational speed enhancing the wedge effect induces turbulence and considerable inertial effect. Moreover, substantial elastic deformation of the composite bearing liners alters the formation of the lubricant film. In this study, we analyze a water-lubricated composite journal bearing system incorporating the turbulence, inertial effect, and elastic deformation of the bearing liner. Reynolds equation was modified considering the turbulence and inertial effect. The elastic deformation of the composite bearing liner was determined by solving the constitutive equation. The Reynolds equation and the constitutive equation were solved via the finite difference method and finite element method, respectively. In addition, the analytical relation for the elastic deformation was derived that suitably eliminated the requirement of solving the constitutive equation. With the introduction of the primary parameters, Sommerfeld number, Reynolds number, and deformation coefficient, the relation of the normalized minimum film thickness with respect to the parameters was modeled based on the Gaussian regression model. Accordingly, we proposed the new optimal maximum load-carrying capacity boundaries that narrowed down the operating region compared to conventional boundaries.

Sealing performance assessments of PTFE rotary lip seals based on the elasto-hydrodynamic analysis with the modified archard wear model

Numerical assessments based on the elasto-hydrodynamic analysis (EHA) in conjunction with the modified Archard wear model are conducted to efficiently appraise the potential leakage of polytetrafluoroethylene (PTFE) rotary lip seals after high cycle operations. The modified Archard model with the introduction of exponential relationships between the wear coefficient and film thickness obtained via the EHA is implemented into the commercial finite element software to estimate the wear amount on the seal. The EHA is then periodically activated to evaluate the variation of wear coefficient and reverse pumping rate for the updated geometry of the seal lip. Simulation results reveal that the design initially having large positive reverse pumping rate does not necessarily bear longer service durations before the leakage.

Elastohydrodynamic lubrication analysis of a radial oil seal with hydrodynamic features

Radial oil seals prevent leakage in mechanical systems where a rotary shaft passes a housing interface. Hydrodynamic features are added to augment the self-induced back-pumping effect and extend the lifetime of the seal. A numerical approach for the soft elastohydrodynamic analysis of radial oil seals with pumping structures is introduced, which comprises state of the art sub-models for the elastic deformation of the contacting surfaces and the pressure buildup in the fluid-filled sealing gap. The back-pumping flow in the sealing contact is resolved in conjunction with the surface deformation due to hydrodynamic pressure buildup. From the hydrodynamic pressure and film thickness, characteristic performance parameters such as the pumping rate and the friction torque are presented and discussed in greater detail.

Mixed elastohydrodynamic analysis of a coupled journal-thrust bearing system in a rotary compressor under high ambient pressure

The hermetic rotary compressor's bearings, including the thrust bearing, the main and sub bearings, work together under high ambient pressure. The thrust bearing works under light axial loads, but sever wear is found, which may be related to the crankshaft's misalignment and deformation and the high ambient pressure. To investigate this phenomenon, a complete coupled journal-thrust bearing model is constructed. The bearing forces are calculated with the mixed elastohydrodynamic model under high boundary pressure. The effects of the deformations and ambient pressure on the bearing performance are investigated. Results show that ignoring the crankshaft deformation makes the simulation underestimate the thrust bearing wear, and the high ambient pressure is the leading cause of the thrust bearing wear.

Transient Analysis of Isothermal Elastohydrodynamic Point Contacts under Complex Kinematics of Combined Rolling, Spinning and Normal Approach

This paper presents a brief review of elastohydrodynamic analysis in commemoration of the immense contributions of Duncan Dowson. This paper also presents an elastohydrodynamic analysis of the elliptical point contact problem under steady state as well as transient conditions. The overall methodology is validated against numerical predictions and experimental observations of acknowledged historical sources. The validated methodology is used to make original contributions in the elastohydrodynamics of elliptical point contact subjected to complex combined contact kinematics, including rolling/sliding, mutual convergence and separation (squeeze film motion) of contacting pairs, when subjected to reciprocating and spinning motions. This combined complex contact kinematics under transient conditions has not hitherto been reported in the literature. This paper shows the critical role of squeeze film motion upon lubricant film thickness. The results also show that the influence of spin motion is only significant at fairly high values of angular velocity and in the absence of a rolling/sliding motion.

Elasto-hydrodynamic analysis of journal bearing operating with nanolubricants

In this study, performance parameters of a rigid and flexible journal bearing using non-Newtonian nanolubricants have been investigated and compared. Krieger-Dougherty viscosity model has been implemented with a non-Newtonian model (power law) to evaluate the effective viscosity of nanolubricants. In order to attain the pressure field of nanolubricants, a numerical solution of the modified Reynolds equation has been derived with initial assumptions and boundary conditions using finite difference method. The static performance parameters for both the bearings have been examined at different values of power law index and eccentricity ratio. The results demonstrate that the load-carrying capacity, maximum pressure and friction force increase significantly using nanolubricant in comparison to base lubricant. Also, the values of these parameters increase with increase of power law index. An insignificant effect of elastic deformation is observed on performance parameters except attitude angle at a particular power law index value.

Subsurface stress-field analysis of generating ground helical gear based on finite line-contact mixed elastohydrodynamic lubrication

Tooth surface roughness and lubrication status have significant influence on the contact performance and fatigue life of helical gear pair. Yet, despite the development in elastohydrodynamic lubrication-based contact analysis and solution of subsurface stress field, researches in subsurface stress field of helical gears considering both lubrication and surface roughness are not quite comprehensive. In this study, three-dimensional surface roughness of generating ground gear is measured, a finite line-contact mixed elastohydrodynamic lubrication model is established to perform the contact analysis, and, on this basis, the influence of tooth surface roughness on the subsurface stress field is studied. Results show that compared with the smooth surface, the overall level of subsurface stress is raised; maximum stress values and plastic zones occur in the close vicinity of tooth surface, which adds to the risk of surface failure; within sections in the valley regions of roughness, locations of maximum stresses are generally similar to the smooth surface situation, i.e. in relatively deep zones, while within sections in the peak regions, the majority of locations with maximum stresses shift much closer to the surface; contact pressure and stress status see only mild undulation between different sections distributed along the contact line, but intense changes between sections distributed along the entraining direction.

Elasto-hydrodynamic analysis of reciprocating piston seals with micro-asperities on cylinder surface

ABSTRACT The reciprocating piston seals are crucial parts in the hydraulic system, which are widely used in aerospace and military industry. A direct fluid–structure coupling method with high efficiency is proposed for solving the transient elaso-hydrodynamic-lubrication problem in the hydropneumatic suspension reciprocating piston seal system. A detailed three-dimensional fluid–structure coupling model is built using finite element discretization. Material tests are carried out to obtain the parameters of the third-order Ogden constitutive model of the rubber O-ring. The sealing performance and friction force of the sealing system are analysed for different piston speed. The critical speed from mixed lubrication to full-film lubrication is obtained. Three different micro-asperity geometries on cylinder surface are researched for their influence on piston sealing and lubrication performance.

Multi-objective optimization of hypoid gears to improve operating characteristics

In this paper a multi-objective optimization method of hypoid gears correlating to the operating characteristics is presented. Optimal design of hypoid gears demands that multiple objectives be simultaneously achieved. Four objectives considered in this study are the minimization of the maximum tooth contact pressure, transmission error and the average temperature in the gear mesh, and the maximization of the mechanical efficiency of the gear pair. The goals of the optimization are achieved by the optimal modification of meshing teeth surfaces. In practice, these modifications are introduced by applying the appropriate machine tool setting for the manufacture of the pinion and the gear, and/or by using a tool with an optimized profile. The proposed optimization procedure relies heavily on the loaded tooth contact analysis for the prediction of tooth contact pressure distribution and transmission errors, and on the mixed elastohydrodynamic analysis of lubrication to determine temperature and efficiency. A fast elitist nondominated sorting genetic algorithm (NSGA-II) is applied to solve the model. The effectiveness of the method is demonstrated by using hypoid gear examples. The obtained results have shown that by the optimization considerable improvements in the operating characteristics of the gear pair are achieved.

Coupled simulation of elastohydrodynamics and multi-flexible body dynamics in piston-lubrication system

In this work, we propose a robust modeling and analysis technique of the piston-lubrication system considering fluid–structure interaction. The proposed schemes are based on combining the elastohydrodynamic analysis and multi-flexible body dynamics. In particular, multi-flexible body dynamics analysis can offer highly precise numerical results regarding nonlinear deformation of the piston skirt and cylinder bore, which can lead to more accurate results of film thickness for gaps filled with lubricant and of relative velocity of facing surfaces between the piston skirt and the cylinder block. These dynamic analysis results are also used in the elastohydrodynamic analysis to compute the oil film pressure and asperity contact pressure that are used as external forces to evaluate the dynamic motions of the flexible bodies. A series of processes are repeated to accurately predict the lubrication characteristics such as the clearance and oil film pressure. In addition, the Craig–Bampton modal reduction, which is a standard type of component mode synthesis, is employed to accelerate the computational speed. The performance of the proposed modeling schemes implemented in the RecurDyn™ multi-flexible body dynamics environment is demonstrated using a well-established numerical example, and the proposed simulation methods are also verified with the experimental results in a motor cycle engine (gasoline) which has a four cycle, single cylinder, overhead camshaft (OHC), air cooled.

Experimental Measurement of Roughness Data and Evaluation of Greenwood/Tripp Parameters for the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

<div class="section abstract"><div class="htmlview paragraph">For the investigation of the tribological behavior of lubricated contacts, the choice and the calibration of the adopted asperity contact model is fundamental, in order to properly mimic the mixed lubrication conditions. The Greenwood/Tripp model is extensively adopted by the commercial software commonly employed to simulate lubricated contacts. This model, based on a statistic evaluation of the number of asperities in contact and on the Hertzian contact theory, has the advantage of introducing a simple relationship between oil film thickness and asperity contact pressure, considerably reducing the simulation time. However, in order to calibrate the model, some non-standard roughness parameters are required, that are not available from commercial roughness measuring equipment. Standard values, based on some limited experiences, are typically used, and a limited literature can be found focusing on how to evaluate them, thus reducing the predictivity of the model. In this contribution, the roughness profile of the surfaces of the conrod small-end and the piston pin of a high performance motorbike engine are measured, adopting an optical measurement equipment. An algorithm is developed in order to properly evaluate the Greenwood/Tripp non-standard roughness parameters and set an elastohydrodynamic analysis of the lubricated coupling. A complementarity form of the Reynolds equation capable of handling the cavitation problem is coupled with both Greenwood/Tripp theory and with an alternative complementarity formulation of the asperity contact problem. The results obtained adopting the two different models are critically discussed, comparing them with empirical evidences provided by small-end/piston pin coupling failures, occurred in a severe running condition test at an early stage of the design process of the engine.</div></div>

Elastohydrodynamic analysis of couple stress lubricated cylindrical journal bearing

The elastohydrodynamic analysis of plain cylindrical rotor bearing system lubricated with couple stress fluid is performed in this paper. The hydrodynamic pressure is generated entirely by the motion of the journal and depends on the viscosity of the lubricated fluid. In this analysis the static and dynamic characteristics are analyzed against the flexibility of bearing liner. The modified Reynold equation and three dimensional elasticity equations are solved using an iterative approach. A non-dimensional parameter ‘l’ has been used for couple stress fluid. An increase in couple stress parameter with flexibility causes the enhancement of load carrying capacity. The static and dynamic characteristics of journal bearing system with flexible liner are improved with use of couple stress fluid as lubricant.

Development of a new test rig for the analysis of hydrodynamic bearings for rotors of microGT

The aim of the present work is to design a test rig suited to investigate the dynamic interaction between rotor and hydrodynamic journal bearings in micro gas turbines (microGT), i.e. with reference to small bearings (diameter in the order of ten millimeters). Particularly, the device is capable of measuring the journal location. Therefore, the journal motion due to rotor vibrations can be displayed, in order to assess performance as well as stiffness and damping of the bearings. The new test rig is based on Bently Nevada Rotor Kit (RK), but substantial modifications are carried out. Indeed, the relative radial clearance of the original RK bearings is about 2/100, while it is in the order of 1/1000 in industrial bearings. Therefore, the same RK bearings are employed in the new test rig, but a new shaft has been designed in order to reduce the original clearance. The new shaft enables us to study the bearing behaviour for different clearances, as it is equipped with interchangeable journals. The experimental data yielded by the new test rig are compared with numerical results. These are obtained by means of a suitable finite element (FEM) code developed by our research group. It allows the Thermo Elasto-HydroDynamic (TEHD) analysis of the bearing in static and dynamic conditions. In the present paper, bearing static performances are analysed in order to assess the reliability of the journal location predictions by comparing numerical and experimental results. Such comparisons are presented for both large and small clearance bearings of original and modified RK, respectively. Good agreement is found only for the modified RK equipped with small clearance bearings (relative radial clearance equal to 8/1000). Nevertheless, rotor alignment is quite difficult with small clearance bearings and a completely new test rig is designed for future experiments.

Effect of Crankshaft and Crankcase Material Stiffness on Load Distribution in Main Bearings

In multi-cylinder diesel engines, hydrodynamic pressure distribution of main bearings has significant effect on the load distribution applied on engine crankcase, therefore, has an effect on the stress distribution and deflection of crankcase. Flexibility of crankshaft and crankcase assembly has significant effect on load distribution in engine main bearings. In this paper, the effect of elastic deformations of both crankshaft and crankcase on the load distribution in the main bearings is studied. Considering deformations of crankcase and crankshaft together with the elasto-hydrodynamic (EHD) analysis of main bearing is vital to obtain the realistic load distribution and accurate main bearing performance. At the beginning, the results of flexible body dynamic analysis of engine crank train with considering EHD joint for main bearings are presented and the main bearing force and moment are compared with rigid body dynamic analysis. Then, the effects of crankshaft and crankcase flexibilities on main bearing force and moment are studied. Several models with different flexibilities and stiffness of crankcase and crankshaft were prepared using Elasto-hydrodynamic simulation of diesel engine main bearings. According to the results, the load distribution on the engine main bearings and consequently the bearing shell deformation are affected by crankcase and crankshaft elasticities. A fast and time-saving method to obtain reasonably accurate results is proposed in the present paper.

Investigation of the Influence of Different Asperity Contact Models on the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

Investigation of the Influence of Different Asperity Contact Models on the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

A simplified approach for thermal elastohydrodynamic lubrication analysis of circular contacts using realistic lubricant properties

Thermal effect in elastohydrodynamic lubrication has been the subject of extensive research for several decades. The focus of this study was primarily on the development of an efficient numerical scheme to deal with the computational challenges involved in the solution of thermal elastohydrodynamic lubrication model; however, some important aspects related to the accurate description of lubricant properties such as viscosity, rheology, and thermal conductivity in elastohydrodynamic lubrication point contact analysis remain largely neglected. A few studies available in this regard are based upon highly complex mathematical models difficult to formulate and execute. The end-users may not have the specialized skill, knowledge, and time required for the development of computational codes pertaining to these models. Therefore, this paper offers a very simple approach to determine the distribution of mean fluid temperature within an elastohydrodynamic lubrication film. While it is an approximate method, it yields reasonably accurate results with only a little increase in computation time with respect to the isothermal case. Moreover, it can be added as a small module to any existing isothermal algorithm. Using this simplified thermal elastohydrodynamic lubrication model for point contacts, this work sheds some light on the importance of accurate characterization of the lubricant properties and demonstrates that the computed thermal elastohydrodynamic lubrication characteristics are highly sensitive to lubricant properties. It also emphasizes the use of appropriate mathematical models with experimentally determined parameters to account for the correct lubricant behavior.

Microgeometrical tooth profile modification influencing efficiency of planetary hub gears

Planetary hub systems offer desired speed and torque variation with a lighter, compact and coaxial construction than the traditional gear trains. Generated friction between the mating teeth flanks of vehicular planetary hubs under varying load-speed conditions is one of the main sources of power loss. Modification of gear tooth geometry as well as controlling the contacting surface topography is the remedial action. The paper studies the effect of tooth crowning and tip relief upon system efficiency. It includes an analytical elastohydrodynamic analysis of elliptical point contact of crowned spur gear teeth. The analysis also includes the effect of direct contact of asperities on the opposing meshing surfaces. Tooth contact analysis (TCA) is used to obtain the contact footprint shape as well as contact kinematics and load distribution. A parametric study is carried out to observe the effect of gear teeth crowning and tip relief with different levels of surface finish upon the planetary hubs' power loss.

Numerical Modeling of Piston Lubrication with Body Deformation through Modal Reduction Method

An analysis for operating characteristics of piston lubrication system is performed based on the numerical model in this study. Dynamic piston lubrication characteristics such as oil film pressure and thickness distribution can be analyzed through a numerical model with an integration of elastohydrodynamics and multi-flexible-body dynamics (MFBD). In particular, the oil film thickness variation by elastic deformation is considered in the elastohydrodynamic analysis by using the modal reduction method in MFBD system. And this effect is reflected on the fluid governing equations to evaluate the oil film pressure in the lubrication region. A series of process proposed in this study is available for the analysis of realistic elastohydrodynamic lubrication phenomenon. A numerical example for the piston lubrication system is also demonstrated.