Asperity Contact Pressure Research Articles

A numerical procedure for calculating roughness parameters for the Greenwood-Tripp model of asperity contact based on 3D measurements

This study introduces the proposed deterministic methodology for calculating the roughness parameters involved in the Greenwood-Tripp equation for asperity contact pressure. A robust methodology that takes into consideration the maximum possible peak morphology and establishes more stringent calculation conditions is proposed. The stability of the proposed method is examined on various samples taken from different engine components using an optical interferometer. The roughness parameters obtained from different methodologies and using different optical interferometer resolutions are compared, and then, these parameters are introduced into simulation model that employs the Greenwood-Tripp equation. The results demonstrate the sensitivity of the simulation model to the roughness parameters used.

Gear contact fatigue prediction under mixed elastohydrodynamic lubrication with ellipsoidal asperity rough surface: Experimental and numerical investigation

A three-dimensional(3D) line-contact mixed lubrication model and contact fatigue life prediction model for gears are established, considering the elastoplastic contact of ellipsoidal asperities and their interactions. The localized lubrication states across the actual 3D rough surfaces are visually depicted. The direct asperity contact pressures calculation exhibits greater accuracy and universality. The proposed mixed lubrication model can be applied accurately and efficiently to heavy-load conditions. The synergistic mechanisms of multi-scale surface integrity parameters, such as surface morphology-lubrication coupling, hardness gradient, and residual stress, on gear contact fatigue has been revealed. The results reveal that surface roughness significantly affects the competitive failure mechanism between surface-initiated micropitting and subsurface-initiated macropitting. The increase in surface compressive residual stress is more sensitive to the enhancement of near-surface and subsurface fatigue lives, compared to the increase of peak residual stress and the depth of peak residual stress. The proposed physics-based gear contact fatigue life prediction model has been validated through gear contact fatigue experiments under various operating conditions. The maximum deviation between the predicted and experimental fatigue lives is within 10%. This paper presents theoretical methodologies and data-driven support for optimizing the design and manufacturing processes of gears, specifically focusing on enhancing anti-fatigue properties.

Numerical analysis on lubrication failure of main bearings with lemon-shaped bearing shells in high-power diesel engine

The lubrication status of crankshaft main bearings significantly affects the stability and reliability of diesel engine operation. This paper primarily investigates the severe failure of main bearings during developing an industrial high-power 6 V diesel engine and analyzes the causes of bearing failure. Main bearings using lemon-shaped bearing shells exhibited failure after a short period of operation. Based on the multi-body dynamics model, average Reynolds equation, and contact model, a lubrication model for the main bearings was established, and the lubrication characteristics of each main bearing were simulated. The research shows that the abrupt peaks of the lemon-shaped bearing shells on the first and second main bearings are located within the area where the bearings suffer from burst pressure, resulting in additional hydrodynamic pressure and asperity contact pressure on the bearing surfaces. This makes the bearing surface prone to wear and high temperature, which are the primary causes of bearing failure. Under the operating conditions, surface damage of the bearings leads to particle abrasion and high temperature. Subsequently, the lubricating oil gradually deteriorates with the combined action of high temperature and wear particles, causing the main bearings to blacken and adhesive wear.

Piston ring and cylinder liner scuffing analysis in dual-fuel low-speed engines considering liner deformation and tribofilm evolution

The piston ring and cylinder liner (PRCL) system in larger-bore low-speed marine engines frequently experiences scuffing failures, which significantly decrease the engine reliability. To understand this failure mechanism, a scuffing failure model subjected to the PRCL system was developed considering multidisciplinary coupling effects that integrate asperity contact, hydrodynamic lubrication, tribochemistry reactions, thermal effects, friction, and surface wear. The impacts of large-scale deformation and gas-combustion mode on the scuffing performances of the PRCL system were examined. The key findings indicate that the larger-scale liner deformation can markedly reduce oil film thickness and exacerbate local asperity contact, influencing the evolution of the tribofilm by increasing the removal process. Under gas-combustion mode, the oil film thickness is even lower, and the asperity contact pressure further increases due to a more starved lubrication state and higher combustion temperature. This leads to tribofilm breakdown and severe wear near the ring opening area, which are aligning with the full-scale experimental results with scuffing failure.

Thermal and viscoelastic coupled influences analysis on mixed lubrication in dynamic journal bearings

The interaction of temperature variations and viscoelastic deformation is critical in dynamically loaded journal bearings, especially under fluctuating heavy load conditions. Previous studies have not fully explored the coupling of these two factors. Addressing this gap, this research introduces a novel transient mixed lubrication model that simultaneously considers thermal and viscoelastic effects. This model reveals intricate dynamics influencing the lubrication performance of journal bearings. The findings show that the combined impact of thermal and viscoelastic factors significantly alters the amplitude and phase of lubrication performance parameters. Notably, thermal effects tend to decrease oil film pressure while increasing asperity contact pressure, whereas viscoelastic effects enhance the oil film pressure peak and adjust its phase, concurrently reducing asperity contact pressure. Moreover, varying the eccentric speed profoundly influences the interplay between bearing thermal and viscoelastic effects. This study contributes to a deeper understanding of mixed lubrication in journal bearings and provides technical support for more accurate and predictive reliability improvement in complex mechanical systems.

A fully coupled finite element solution for the mixed elastohydrodynamic lubrication of finite line contacts

A fully coupled FE model for mixed EHL of finite line contacts is proposed, integrating all of the governing equations into a monolithic system. The solution reveals the fully coupled model achieves 5% higher accuracy and 50% greater efficiency compared to the weakly coupled one. Additionally, there is evidence to support the assertion that profiling contributes to the mitigation of surface roughness contact. Logarithmic profiling can increase the minimum film thickness by a factor of 30, resulting in a reduction of up to 64.6% in the maximum asperity contact pressure, and thus, the boundary friction force is decreased by more than 19%, under heavy-load low-velocity condition. This contributes to the reduction of abrasive wear.

Effect of surface roughness on lubrication performance of water-lubricated bearing for energy recovery turbocharger

In this study, the lubrication performance of the water-lubricated bearing of the energy recovery turbocharger was studied considering the surface roughness. By simulating the rough surface of the bearing with the Weierstrass-Mandelbrot function and combining the water film thickness equation, Reynolds average equation, Greenwood-Tripp contact model and motion equation, the theoretical model of water-lubricated bearing is established and its solution is developed. The variational rules for the hydrodynamic pressure and force, asperity contact pressure and force, and other performance parameters of the water lubricated bearings during start-up and stable operation were obtained by solving this model, and the effect of different surface roughness on the start-up performance and steady operation lubrication performance is studied. The results show that the hydrodynamic force increases rapidly and the asperity contact force sharp decreases during the early start-up stage, and the smaller the surface roughness, the faster the hydrodynamic increase and asperity contact force decrease. The thickness distribution and pressure distribution of water film with different surface roughness have slight differences in stable operation. Therefore, the surface roughness has an important effect on the lubrication performance of the water lubricated bearing in the start-up stage, but has little effect on the stable operation.

Influence of Cavitation and Shaft Deformation in the Analysis of Lubrication of the Stern Bearing

The cavitation phenomenon and shaft deformation have a significant impact on the tribological performance of the journal bearing. A mixed lubrication model is developed that takes into account surface roughness and asperity contact, as well as the effects of cavitation and deflection. The fluid–solid coupling effect in bearing deformation, asperity contact, and film pressure are investigated. The effect of boundary conditions on the lubrication regimes is discussed. The results of simulations with and without cavitation are compared under steady-state conditions. The results show that when cavitation is considered by the mixed lubrication model under a given load, the eccentricity is reduced, and the maximum oil film pressure is also reduced. The speed range of the bearing simulated with the mixed lubrication model increases after considering deflection deformation. The mixed lubrication model proposed in this paper is able to provide accurate results of pressure distribution and coefficient of friction and can be applied in the design and analysis of journal bearings.

Mixed lubrication performances of misaligned stern bearing considering turbulence and elastic deformation

PurposeThis study aims to investigate mixed lubrication performances of stern bearing in a misaligned state considering turbulence and bearing deformation impacts.Design/methodology/approachA mixed lubrication model of stern bearing is established. The generalized average Reynolds equation governing the turbulent flow of lubricant is analyzed by considering the interaction of bearing elastic deformation, asperity contact pressure and film pressure. The bearing behaviors including minimum film thickness, hydrodynamic pressure, asperity friction force and frictional coefficient are studied under different models. The correctness of this model is verified by comparing it with that of the published data.FindingsNumerical results indicate that elastic deformation noticeably decreases the maximum film pressure, the asperity contact force and the friction coefficient in the mixed lubrication stage. The effect of elastic deformation and turbulence reduces the transition speed from mixed to liquid lubrication.Originality/valueThis model includes both turbulence and bearing deformation impacts on journal bearing performances. It is expected that the numerical results can provide useful information to establish a stern bearing exposed to mixed lubrication conditions.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-11-2022-0352/

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.

Finite element modeling and simulation of mixed elastohydrodynamic lubrication for the finite line contact under tilting loads

Misalignment is a prevalent problem in rotor machinery. This paper proposes a numerical analysis for the mixed EHL of finite-line contacts considering tilting loads based on FEM. The Reynolds and linear elasticity equations, and Greenwood-Tripp contact model are coupled in an iterative process to solve the fluid-solid/solid-solid interaction under the compound external force and moment. Lubrication characteristics at both ends of the cylinder are analyzed and an asymmetric profile design scheme is presented to improve the lubrication performance at the cylinder's lower end. It is found that proper profile parameters can reduce EHL pressure by 40%, increasing the minimum oil film thickness by up to 100%, and thus reduce asperity contact pressure by up to 50%.

Lubrication failure analysis of camshaft bearings in V6 diesel engine

This contribution investigates the lubrication characteristics and failure causes of camshaft bearings belong to a V6 diesel engine. Numerical simulation based on the theory of elastohydrodynamic lubrication and flexible multibody dynamics is performed to analyse the lubrication characteristics of camshaft bearings. The results reveal that lubrication performance of different bearings of camshaft shows great differences. For a certain bearing, the mixed lubrication state and elastohydrodynamic lubrication state alternately occur in one engine cycle. The duration of mixed lubrication varies greatly in different bearings. For the bearing closest to the timing drive gear, the total pressure is dominated by the asperity contact pressure, and the predicted safety factors of the asperity contact pressure, hydrodynamic pressure and oil film thickness are less than 1, which means that this bearing will have lubrication failure.

Mixed thermo-elasto-hydrodynamic lubrication and wear coupling simulation analysis for dynamical load journal bearings

In the crankshaft-connecting rod system of the diesel engine, the complicated lubrication and wear mechanisms of journal bearings are comprehensively affected by the elastic deformation effect, the asperity contact effect, the thermal effect as well as the lubricating oil viscosity-temperature effect. This paper proposes an adaptive iterative method based on a mixed thermo-elasto-hydrodynamic (TEHD) lubrication and wear coupling model to investigate the lubrication and wear behaviors of journal bearings in the mixed lubrication. The evolution laws of the oil film thickness, the hydrodynamic pressure, the asperity contact pressure, the friction power loss, and the wear distribution are discussed and compared under different running times, oil temperatures, external loads, and shaft rotational speeds. The three-dimensional surface models based on the asperity contact power loss and the maximum wear depth with the variation of the running time and the oil temperature are fitted under specific external loads and shaft rotational speeds. The simulation results demonstrate that the bearing wear process includes the initial wear stage and the stable wear stage, which has a significant influence on the changing trend and distribution of lubricating parameters. The three-dimensional surface fitting models can provide theoretical guidance for the future performance prediction of journal bearings under time-varying working conditions.

The effect of aviation anti-wear additives on tribofilm formation and micropitting propensity

This article studies the effect of anti-wear additives on micropitting and tribofilm formation. In particular, anti-wear additives used in aviation applications have been compared against a commonly used automotive anti-wear additive (zinc dialkyldithiophosphate ((ZDDP)) that is known to promote micropitting. All tests were performed under test conditions representative of the Power GearBox within Rolls-Royce's new engine, the UltraFan©. Tests using a micropitting rig found that ZDDP formed micropits the fastest, which then propagated onto the largest amount of wear showing the catastrophic effects of micropitting. Whereas other aviation anti-wear additives, such as tricresyl phosphate and additive X, formed micropits more slowly but faster than the formulation without additives containing only base oil. Focus variation microscopy was used to characterise the micropits and found that ZDDP formed smaller and shallower micropits than the other anti-wear additives, and as a result, the wear track of the ZDDP roller had the lowest roughness. The unadditised base oil, whilst it generated micropits more slowly, propagated quickly to a more severe failure mode, showing the harmful effects of having no anti-wear additive present. The profilometer results showed the counterface ring roughness for ZDDP remained the highest, confirming that ZDDP forms a tribofilm quickly. This protects asperities, which consequently promotes micropitting, as high localised asperity contact pressures are maintained. This was further confirmed by evaluating the tribofilms using a mini traction machine with spacer layer imaging to form tribofilms under representative conditions. This demonstrated that ZDDP forms a thicker tribofilm than the aviation anti-additives and at a faster rate. Overall, the study showed that the aviation anti-wear additives promote micropitting less than ZDDP but more than base oil alone. The slower action of the tribofilm formation allows the initial running-in of surfaces, which slows down the initiation of micropitting.

Multiscale Wear Simulation in Textured, Lubricated Contacts

Specific surface textures may reduce the friction and increase the lifting forces in lubricated contacts. For the detrimental operating condition of mixed friction, wear is induced by the solid contact. In this study, a methodology for wear calculation in textured, lubricated contacts is presented that considers the wear-induced surface topography evolution. Based on the Reynolds differential equation, the mass-conserving cavitation model according to Jakobsson, Floberg, and Olsson (JFO), a wear-dependent asperity contact pressure curve and the wear equation according to Archard, wear in a wedge-shaped, textured lubrication gap was calculated. The results show the wear behavior of textured lubrication gaps. Based on the wear simulations, the tribological behavior of the textured surfaces compared to smooth surfaces is discussed. It is evident that textures, which improve the tribological performance in the hydrodynamic lubrication regime, are not necessarily associated with low wear values in a lubrication condition in the mixed friction regime. The analysis of the wear-dependent parameters initially showed a ‘recovery’ of the tribological system with increasing wear until the performance decreased again after a specific reversal point. This behavior is attributed to the relative position of the surface textures in the lubrication gap.

Wear simulation in lubricated contacts considering wear-dependent surface topography changes

Along with friction, wear in lubricated contacts is one of the most essential design criteria. The use of low-viscosity lubricants, as well as changing operating conditions, may lead to the wear-causing operating condition of mixed friction. For wear assessment, in the last decades numerical simulations have been confirmed as a powerful tool. The level of detail of the simulation model has a significant influence on the simulation results. The local wear heights for a hydrodynamic journal bearing under constant operating conditions are calculated numerically. A variant study is used to determine the influence of elastic deformation, cavitation model and the consideration of the wear-dependent surface topography. The surface topography, which is subject to change especially during running-in, is taken into account in the form of wear-dependent asperity contact pressure curves, respectively wear-dependent flow factors. To validate the simulation results, tests were carried out on a rotational tribometer. The results show that the consideration of the wear-dependent surface topography has a significant influence on the wear development and thus the trend evaluated from the experiment can be better assessed.

Micro-scale deterministic asperity contact FEM simulation

There are numerous stochastic approaches to indirectly couple solid asperity contact with the fluid hydrodynamics in the region of boundary or mixed lubrication. In contrast, deterministic approaches for calculating solid contact pressure curves offer advantages in terms of flexibility and accuracy. This contribution aims at providing a publicly available, automated method to derive solid asperity contact pressure curves for given surfaces, implemented in commercial software based upon Finite Element Method (FEM). Solid asperity pressure curves were calculated and compared to various established stochastic models for artificially generated surfaces and surfaces measured via laser scanning microscopy. Thereby, it was shown that the usage of artificially generated surfaces based on stochastic parameters only allowed an approximate representation of real measured surfaces as well as to lower calculated pressures, so that 3D measurement data is preferred to calculate the contact pressure. Moreover, the values of the FEM model were in a similar region but slightly below the stochastic models over a wide range of gap distances and the asperity pressure graphs were more curved/convex. At very small gap height values, the pressure in the FEM model reached values similar to the stochastic models. This was attributed to the fact that real surface topographies were considered, also allowing for peak-to-valley pairings instead of merely asperity-to-asperity contacts, as well as to the fact that the roughness and mean planes were re-calculated in each simulation step, while most stochastic approaches neglect the elastic deformation of asperities when determining the distances between the rough surfaces.

Modelling Approaches of Wear-Based Surface Development and Their Experimental Validation

Surface topography has a significant influence on the friction behaviour in lubricated contacts. During running-in, the surface topography is continuously changed. The surface structure influences the contact stiffness (asperity contact pressure) as well as the microhydrodynamics (flow factors). In this study, different models for wear simulation of real rough surfaces were created in Matlab© (MathWorks, Natick, MA) and Abaqus© (ABAQUS Inc., Palo Alto, CA, USA) using the Usersubroutine Umeshmotion. The arithmetic mean height Sa(wh), the maximum height Sz(wh), as well as the asperity contact pressure pasp(h,wh) as a function of the wear height (wh) are used to characterise the surface for the respective wear state. The surface characteristics obtained from the simulations are validated with parameters from experiments. The aim of this study was to create a simulation methodology for mapping surface development during the running-in process. The results show, that the qualitative course of the surface parameters can be reproduced with the applied simulation methodology. Compared to the experiments, the rough surfaces are flattened faster. By adapting the simulation results in postprocessing, good agreements with the experiments can be achieved.

Surface topography effect on tribological performance of water-lubricated journal bearing under mixed-EHL regime

Water-lubricated journal bearings (WLJB) are one of the important components of the stern tube shaft. Surface topography of WLJB is an important aspect in the design of stern tube shaft, and significantly affects the lubrication characteristics of WLJB. This work aims to investigate the effect of non-Gaussian surface topography on tribological performance of water-lubricated journal bearing (WLJB) under mixed-elastohydrodynamic lubrication (mixed-EHL) regime. The non-Gaussian flow factors, and a normalized Weibull probability density function is used to tackle the non-Gaussian roughness in hydrodynamic and asperity contact models respectively. From results, it is observed that skewness (S sk ) significantly affects both asperity contact pressure and hydrodynamic pressure along circumferential direction of bearing. It is concluded that for skewness, S sk = −1 to 1, lambda ratio falls between 1 to 3. The relation between altitude angle and skewness is established. The effect of bearing length to diameter (L/D) ratio on lubrication performance of WLJB is also discussed in detail.

Mixed EHL Problems: An Efficient Solution to the Fluid–Solid Coupling Problem with Consideration of Elastic Deformation and Cavitation

For transient mixed Elastohydrodynamic lubrication (EHL) problems, a novel solution is required to predict friction loss and wear in sliding or rolling parts. However, existing solutions have numerous limitations. In general, the lower the oil film thickness is, the more serious the non-linear problem is. This paper presents an efficient solution to tackle the non-linearity of the mixed EHL problem. The elastic deformation in the fluid–solid iteration coupling problem is divided into two parts: One is induced by the hydrodynamic pressure. This part of the deformation is obtained by the unsteady EHL-FBNS (Fischer–Burmeister–Newton–Schur) solver by considering both mass-conserving cavitation and elastic deformation. The other part of the deformation is introduced by the asperity contact pressure. It can be obtained by the Newton–Raphson method. After some limited iterations, the mixed EHL problems can be solved by evaluating the residual total pressure (including hydrodynamic pressure and asperity contact pressure). The proposed methodology was validated against the results from the published literature and applied to characterize the tribological performance of point contact with moving texturing. It appears that the developed method can be effectively used for tracking the tribological behavior of friction pairs.