A Numerical Model for the Dry Sliding Contact of Layered Elastic Bodies with Rough Surfaces

A numerical model for the two-dimensional dry sliding contact of two elastic bodies with real rough surfaces has been developed, where an elastic body contacts with a multi-layer surface under both normal and tangential forces. The model uses surface profile data directly recorded with a stylus measuring instrument and it is suitable for use on a microcomputer. Green's function for a unit normal load and a unit tangential load for the generalized plane strain problem are derived. Verification of the accuracy of the model by reproduction of test case results is presented. Contact pressure distribution for layers of varying coefficient of friction, thickness and elastic modulus is analyzed.     

Analyzing Elastic-Plastic Real Rough Surface Contact in Running-in

A numerical method is presented for evaluating the elastic-elastic contact of real rough surface contacts during running-in. For the surface contact, an elastic-plastic model based on the variational method is applied to analyze the pressure distribution and contact area of worn surfaces during running-in. In conjunction with the classical statistic model of Greenwood and Williamson, the numerical result showed that the plasticity index Ψ was decreased to one in the elastic range as running-in proceeded. In comparison with the Hertzian solution, the influence of the asperities is very significant on the pressure distribution, thereafter causing a higher peak value of contact pressure. For the subsurface, the interior stress from the von Mises criterion was calculated to evaluate the subsurface stress field subject to both normal and tangential forces. In the calculated of the interior stress, the total stress is decomposed into a fluctuating component and a smooth component. The fluctuating part is solved by using FFT from the concept of the convolution theorem while the smooth part is obtained directly by analytical solution. Calculations of contact area and subsurface stress on experimentally produced surfaces whose topography has been determined using an atomic force microscope and friction coefficient front sliding have been carried out. The results showed that asperities and friction coefficient gave rise to stress increase in the near-surface stress field and produced a high stress zone towards the surface. As a result, transverse asperity cracking was produced. The calculations and supporting experimental evidence clearly confirmed that the reduction of peak pressure during running-in decreased the plastic deformation of contact.     

Effects of Friction on the Contact and Deformation Behavior in Sliding Asperity Contacts

Finite-element analyses are carried out to study the effects of friction on the contact and deformation behavior of sliding asperity contacts. In the analysis, on elastic-perfectly-plastic asperity is brought in contact with a rigid flat at a given normal approach. Two critical values of the normal approach are used to describe the asperity deformation. One is the approach corresponding to the point of initial plastic yielding, and the other at the point of full plastic flow. Additional variables used to characterize the deformation behavior include the shape and size of the plastic zone and the asperity contact size, pressure, and load capacity. Results from the finite-element analysis show that the two values of critical normal approach decrease significantly as the friction in the contact increases, particularly the approach that causes plastic flow of the asperity. The size of the plastically deformed zone is reduced by the friction when the contact becomes fully plastic. The reduction is very considerable with a high friction coefficient, and the plastic deformation is largely confined to a small thin surface layer. For a low friction coefficient, the contact size, pressure and load capacity of the asperity are not very sensitive to the friction coefficient. For a moderate friction coefficient, the contact pressure is reduced and the junction size increased; the load capacity of the asperity is not significantly affected due to the compensating effects of the pressure reduction and the junction growth. For a high friction coefficient, the pressure-junction compensation is not longer sufficient and the asperity load capacity is reduced. The degree of the friction effects on these contact variables depends on the applied force or the normal approach. Although the analyses are conducted using a line-contact model, the authors believe that the effects of friction in sliding asperity contacts of three-dimensional geometry are essentially the same and the same conclusions would have been reached. These results may provide some guidance to the modeling of rough surfaces in boundary lubrication, in which the asperity friction coefficient can be high and vary significantly both in time and from one micro-contact to another.     

The Effect of Scale-Dependent Hardness on Elasto-Plastic Asperity Contact between Rough Surfaces

Statistical methods are used to model elasto-plastic contact between two rough surfaces using a recent finite element model of elasto-plastic hemispherical contact and also recent advances in strain gradient modeling. The elasto-plastic hemispherical contact model used to model individual asperities accounts for a varying hardness effect due to deformation of the contact geometry that has been documented by other works. The strain gradient model accounts for changes in hardness due to scaling effects. The contact between surfaces with hypothetical material and surface properties, such as the elastic modulus, yield strength, and roughness are modeled. A model is also constructed to consider a variable asperity contact radius to evaluate if the strain gradient model will affect it differently. The models produce predictions for contact area, contact force, and surface separation. The strain gradient effects decrease the real area of contact and increase the average contact load in comparison to the model without these effects. The strain gradient model seems to have a larger influence on the predictions of contact load and area than does considering a variable asperity contact radius for the cases considered in this work.     

A Two-Wavelength Model of Surface Flattening in Cold Metal Rolling with Mixed Lubrication

A new model of surface flattening is developed for cold metal rolling in the mixed regime. Longitudinal surface roughness is modeled by two separate wavelengths. The new model follows the asperity crushing analysis of Sutcliffe (1999) for unlubricated rolling but additionally includes a hydrodynamic model to account for the effect of the lubricant. The effect of various parameters including speed, reduction in strip thickness, roughness wavelength and lubricant properties is examined. The results show similar behavior to previous models of mixed lubrication, with a speed parameter A_s having the most influence, and confirm the results for unlubricated rolling that the short wavelength components of the surface roughness persist more than the long wavelength components. The predicted changes in roughness are in good agreement with experiments.     

Topographic Investigation of the Wear Resistance of PTFE Seals in Sliding Contact with Ceramic Materials

This paper is concerned with the wear of PTFE seals used in connection with reciprocating ceramic-coated rods. An analysis of the relationship between the surface topography of ceramics and wear of PTFE seals was undertaken, formulating three hypotheses which have been investigated experimentally using a seal test rig and a system for three-dimensional surface roughness analysis. It was observed that no running-in of the rod surface lakes place and, consequently, the tribological situation never stabilizes. It was shown that seal wear rate is dependent on the number of asperities penetrating the lubricant film thickness, the wear rate being correlated to a functional parameter (S_{pk_0}) which was especially developed to describe the peak height above the mean plane. Furthermore, it was illustrated how the structure of ceramics allows the lubricant to flow unhindered between isolated asperities in contrast to the traditionally polished structure of steel which restricts the lubricant flow.     

The Effects of Cylinder Wall Surface Roughness and Bore Distortion on Blow-by in Automotive Engines

A laboratory test device, was developed to study the effects of cylinder wall, surface roughness and bore, distortion on blow-by in automotive engines. The test device was constructed of components from a four-cylinder engine with a test box welded into the engine block to take the place of one of the cylinders. Removable liners used in the lest box were honed to varying surface roughnesses and bore distortion levels, Blow-by, used as a measure of conformance between the piston rings and cylinder walls, was found to increase at the end of the test with increasing initial distortion and initial surface roughness.     

On the Relation of Load to Average Gap in the Contact Between Surfaces with Longitudinal Roughness

A computer simulation model for the contact between longitudinally-oriented rough surfaces has been formulated. This model closely duplicates the actual surf ace contact deformation behavior by taking into account the elastic interactions between the asperities. There were no assumptions made about the shapes, or any deformation behavior of the asperities, except for their obeying the laws of elasticity. The plastic deformations on the high asperity peaks were taken into account by setting a ceiling on their contact pressures at the material hardness value. The simulations used real surface profiles which were digitized from unworn circumferentially ground steel surfaces. Each pair of these profiles was mathematically combined to form an equivalent rough profile pressing against an infinitely rigid flat and having the appropriately adjusted elastic modulus. A total of 28 different pairs of profiles were used in the simulations. Each contacting pair was subjected to 30 different load levels and the local contact pressures and deformations were calculated. The contact simulations yielded some important mathematical relationships between parameters, such as the real area of contact, average gap, and average asperity load through statistical curve fitting. Two analytical functions were generated to relate the average load to average gap and the real area of contact to load.     

Prediction of Real Rough Surface Deformation in Pure Rolling EHL Contact: Comparison with Experiment

The ability to predict the in-contact deformation of surface topography is very important for the design of machine components with respect to minimizing the friction and wear of rubbing surfaces. In this study the amplitude attenuation principle is verified as a simple tool for this purpose. Measured lubricant film profiles are compared with prediction based on this principle. From the results obtained it appears that the amplitude attenuation principle provides reasonable estimation of the deformation of rubbing surfaces that can be used for the prediction of in-contact behavior of surface roughness. Good agreement was obtained under pure rolling conditions, which provides a good initial point for the other studies under rolling/sliding conditions where the wear of rubbing surfaces is of key importance.     

Frictional Behaviors of Some Nitrogen Ceramics in Conformal Contact with Tin Coated Al-Si Alloy,Steel and MMC

The frictional behavior of certain nitrogen-containing ceramics, such as silicon nitride, alpha sialons, and beta sialons as journal materials were studied in conformal contact with a tin-coated Al-Si alloy (Al-Si/Sn), forged 1141 steel and a cast aluminum matrix composite with silicon carbide reinforcement (cast MMC) as bearing materials while lubricated with SAE WW30. A case-hardened 1016 steel was also tested with the Al-Si/Sn and cast MMC bearings under the same conditions. The friction values of the ceramic and the steel journal wear pairs were compared and their frictional behaviors were evaluated.

Silicon nitride and one of the beta sialons exhibited higher load-supporting capacities than the others when they were in contact with the 1141 steel bearings. The journal surface roughness was found to be very important when the journals were in contact with the Al-Si/Sn bearings. The frictional behavior of the ceramics and cast MMC pairs and the steel and cast MMC pairs were controlled by different wear mechanisms, namely for the former, hard particle pull-out and matrix plowing, and for the latter, iron transfer from the journal to the cast MMC bearing surface.     

对心滚子移动从动件盘形凸轮机构的接触应力

当工作载荷较大时,凸轮表面和从动件端部的接触疲劳破坏就成为凸轮机构失效的主要形式之一。但在有关凸轮的教科书中,对接触应力的计算过程论述较少。本文详细分析了对心滚子移动从动件盘形凸轮机构接触应力的计算方法,并给出具体计算例题。     

弧齿锥齿轮三维接触应力的有限元——线性规划法求解及分析

采用有限元—线性规划法对弧齿锥齿轮的三维接触应力进行了计算和分析,在对载荷分布不加任何假设的条件下,先粗划轮齿网格,然后再在可能的接触点域细划网格,线性规划法求出了接触区域的载荷分布,有限元法求出了接触区内的接触应力分布.     

行星减速器支撑盘孔销式输出机构的应力和变形计算

少齿差行星减速器的支撑盘孔销式输出机构是制约减速器承载能力的主要环节之一，主要影响因素是销轴、销孔间接触强度、刚度及销轴的弯曲强度、刚度，而所涉及到的力学计算属于超静定问题。本文采用材料力学的方法和弹性力学有限元软件COSMOSDesignSTAR对销轴及其相关零件的强度和刚度进行分析，目的是找出提高减速器承载能力的途径。