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Abstract Mechanical power loss in hydraulic piston pumps comes from the friction between parts in relative motion, and wear is among the top pump failure mechanisms. Reducing friction and preventing wear require lubrication enhancement, which may be accomplished by proper surface texture design. Cylinder length is an important factor affecting the lubrication performance of the piston-cylinder system in a swash-plate pump. Our previous analyses revealed that under the same load, increasing the cylinder length could decrease the maximum pressure and increase the minimum film thickness, favorable for the pump operation with reduced wear; however, friction would become higher. The work reported in this paper intends to lower friction by textures in the piston surface while increasing, or at least maintaining, the load capacity of the piston system. Several textures are designed and their lubrication performances investigated, aiming at reducing the piston friction and improving the load-carrying ability of the system in a wide range of operation conditions. The effects of texture type, application location, and texture shape are studied, and their relationships with the friction and film thickness characteristics of the piston-cylinder interface are explored. Two optimal surface-texture designs are suggested, which are named the Step-Multiple-Grooves design and the Combine design. Highlights • This paper is aimed at the development of a texture performance simulation methodology for the design of friction-reduction textures for the pistons of a swash plate pump in a wide range of operation conditions. • The designed textures should not lower the load capacity and minimum film thickness of the original design. • In this paper, the effects of texture type, location, and width and depth are investigated, and their relationships on friction and film thickness characteristics of the piston-cylinder interface are explored. • Two optimal surface texture designs are suggested, which are the Step-Multiple-Grooves design and the Combine design. [ABSTRACT FROM AUTHOR]

Elastohydrodynamic lubrication (EHL) is an important branch of the lubrication theory, describing lubrication mechanisms in nonconformal contacts widely found in many mechanical components such as various gears, rolling bearings, cams and followers, metal-rolling tools, traction drives, and continuous variable transmissions. These components often transmit substantial power under heavy loading conditions. Also, the roughness of machined surfaces is usually of the same order of magnitude as, or greater than, the estimated average EHL film thickness. Consequently, most components operate in mixed lubrication regime with significant asperity contacts. Due to very high pressure concentrated in small areas, resulted from either heavy external loading or severe asperity contacts, or often a combination of both, subsurface stresses may exceed the material yield limit, causing considerable plastic deformation, which may not only permanently change the surface profiles and contact geometry but also alter material properties through work hardening as well. In the present study, a three-dimensional plastoelastohydrodynamic lubrication (PEHL) model has been developed by taking into account plastic deformation and material work-hardening. The effects of surface/subsurface plastic deformation on lubricant film thickness, surface pressure distribution, and subsurface stress field have been investigated. This paper briefly describes the newly developed PEHL model and presents preliminary results and observed basic behavior of the PEHL in smooth-surface point contacts, in comparison with those from corresponding EHL solutions under the same conditions. The results indicate that plastic deformation may greatly affect contact and lubrication characteristics, resulting in significant reductions in lubricant film thickness, peak surface pressure and maximum subsurface stresses. [ABSTRACT FROM AUTHOR]

The average flow model proposed by Patir and Cheng offers a great convenience for the analysis of rough surfaces in lubrication. The contact factor introduced by Wu and Zheng helps to solve a difficulty in local film evaluation using the average flow model This paper reports a simple method to calculate the contact factor Method validation is demonstrated by the comparison of the contact factors for Gaussian surfaces obtained with the present method and the fitting formula of Wu and Zheng. The proposed method cannot only easily compute the contact factor values for Gaussian surfaces; it can also be used for those of non-Gaussian and measured surfaces, especially those with unknown probability density distribution of the roughness height. [ABSTRACT FROM AUTHOR]

Damping systems using granular particles as the damping medium are promising for application in extreme temperature conditions. In particle-based damping systems, mechanical energy is dissipated through inelastic collisions and friction between particles. In this work, we use particle dynamics simulations to investigate and compare the damping mechanisms of a piston-type thrust damper and a box-type oscillation damper. The mechanisms of energy transfer and energy dissipation are investigated. The roles of friction and inelastic collisions, as well as the wall effects in energy dissipation, are examined. The simulation results provide better understanding of the particle damping mechanisms, which may help in the design of next generation particle damping devices. [ABSTRACT FROM AUTHOR]

This paper reports the development of a novel three-dimensional (3D) deterministic model (3D L-EHL) for rough surface line-contact mixed-elastohydrodynamic lubrication (EHL) problems. This model is highly demanded because line contacts are found between many mechanical components, such as various gears, roller and needle bearings, cams and followers, and work rolls and backup rolls in metal-forming equipment. The macro aspects of a line-contact problem can be simplified into a two-dimensional (2D) model; however, the topography of contacting rough surfaces, microasperity contacts, and lubricant flows around asperities are often three-dimensional. The present model is based on Hu and Zhu's unified 3D mixed-EHL model (Hu and Zhu, 2000, "Full Numerical Solution to the Mixed Lubrication in Point Contacts," ASME J. Tribol., 122(1), pp. 1-9) originally developed for point contacts and the mixed fast Fourier transform (FFT)-based approach for deformation calculation formulated by Chen et al. (2008, "Fast Fourier Transform Based Numerical Methods for Elasto-Plastic Contacts With Normally Flat Surface," ASME J. Appl. Mech., 75(1), 011022-1-11). It is numerically verified through comparisons with results from the line-contact Hertzian theory and the conventional 2D line-contact smooth-surface EHL formulas. Numerical examples involving 3D sinusoidal and digitized machined surfaces are also analyzed. Sample cases indicate that transverse roughness may yield greater film thickness than longitudinal roughness. This observation is qualitatively in agreement with the trend predicted by Patir and Cheng's stochastic model (1978, "Effect of Surface Roughness on the Central Film Thickness in EHL Contacts," Proceedings of the Fifth Leeds-Lyon Symposium on Tribology, London, pp. 15-21). However, the roughness orientation effect does not appear to be quantitatively as great as that shown in the work of Patir and Cheng for the same range of λ ratio. [ABSTRACT FROM AUTHOR]

Coatings are widely used for interface performance enhancement and component life improvement, as well as for corrosion prevention and surface decoration. More and more mechanical components, especially those working under severe conditions, are coated with stiff (hard) thin coatings. However, the effects of coatings on lubrication characteristics, such as film thickness and friction, have not been well understood, and designing coating for optimal tribological performance is a grand challenge. In this paper, the influences of coating material properties and coating thickness on lubricant film thickness are investigated based on a point-contact isothermal elastohydrodynamic lubrication (EHL) model developed recently by the authors. The results present the trend of minimum film thickness variation as a function of coating thickness and elastic modulus under a wide range of working conditions. Curve fitting of numerical results indicates that the maximum increase in minimum film thickness, Imax, and the corresponding optimal dimensionless coating thickness, H2max, can be expressed in the following forms: Imax =0.769M0.0238R20.0297L0.1376 exp(-0.0243 ln² L) and H2max =0.049M0.4557R2-0.1722L0.7611 exp(-0.0504 ln² M -0.0921 ln² L). These formulas can be used to estimate the effect of coatings on film thickness for EHL applications. [ABSTRACT FROM AUTHOR]

The mathematical expression of surface geometry and topography is the first step towards modeling the performance of tribological elements with rough surfaces. The recent trend of tribological simulations, as well as failure prediction, demands the description of surface changes in a wear process. Reported in this paper is a method developed to simulate surfaces in a wear process through connecting wear tests with statistical and artificial-intelligent analyses. The simulation method consists of four components: (1) wear tests for the pivot information of surface variation in wear, (2) statistical analyses for surface feature extraction, (3) artificial neural network (ANN) processing for test result generalization, and (4) surface synthesis for worn surface regeneration. Selected wear tests are performed to lay the ground for the simulation. The simulation method is used to generate the worn surfaces, based on the original statistical parameters and the prediction of their variations due to wear, for a group of engineered surfaces of the same nature and subjected to the same wear process. [Copyright &y& Elsevier]

Abstract: Surface texturing, which fabricates micro dimples or micro channels on the surface of parts, is a growing technique for improving tribological characteristics of materials. Currently, electrical discharge texturing (EDT) technique, one surface texturing method suitable for mass production, is being used to texture aluminum sheets for the applications in automotive industry. It has been widely accepted in industry that EDT improves the forming behavior of aluminum sheets due to better friction behavior. However, how the textures on the surface of sheet metal change the friction behavior has not been investigated. In this paper, the influence of EDT on the friction behavior of aluminum automotive sheet at different contact pressures and sliding speeds is investigated based on both experimental studies and numerical simulations. To fully investigate the tribological behaviors, a flat-on-flat friction test device was built and a numerical code based on mixed lubrication theory was developed. It was found that EDT texturing can reduce the friction coefficient of contacting pair at high contact pressure, however, increase friction coefficient at low contact pressure. Numerical simulations confirmed this finding. Furthermore, the model provides valuable information for the prediction of friction behavior of EDT sheets and helps to optimize processing parameters for various forming processes using EDT aluminum sheets. [Copyright &y& Elsevier]

Abstract: The fast Fourier transform (FFT) technique is becoming an important tool in tribological modeling because an FFT-based algorithm can be extremely accurate and efficient. However, certain subtle issues should be properly dealt with so that efficient and accurate FFT implementation can be achieved. This paper discusses the use of three typical convolutions, two convolution theorems, influence coefficients, and shape functions, as well as the influence of domain size. This paper also explores the applications of FFT to inverse problems and periodic contacts and summarizes available FFT-based algorithms for solving various contact problems. [Copyright &y& Elsevier]

Piston skirt/cylinder bore scuffing is one of the main failure mechanisms affecting the life of automotive engines. A piston scuffing apparatus that simulates the relative motion between the piston and the cylinder liner of a typical engine has been designed and constructed to investigate scuffing mechanisms and determine the conditions under which scuffing happens. With this apparatus, the scuffing performance of an aluminum–silicon piston material with a variety of skirt coatings is studied and scuffing mechanisms investigated. The research also explored the influence of the piston skirt surface texture and cylinder bore surface roughness on the cylinder bore–piston skirt contact, as well as the microstructures and morphological features of the surface and the near-surface materials in scuffing. [Copyright &y& Elsevier]