The purpose of this paper is to establish a three-dimensional model of sliding friction and to study the influence of surface topography fractal parameters on the model. Firstly, the analysis of the contact between two asperities is completed, for according to the classical molecular-mechanical friction theory, the sliding friction among rough surfaces should be the sum of mechanical force and molecular adhesion. Then based on the fractal theory, the three-dimensional fractal model of sliding friction is deduced. Finally, the influence of the maximum contact area of asperity al, the fractal roughness G and the fractal dimension D on the sliding friction is analyzed by a simulation example, and the analysis results show that the sliding friction F has positive correlation with al and G, and there is an optimal fractal dimension D which minimizes F. The study of the paper can be used to explain the existing experimental results and the friction theory reasonably.
According to the influence of the normal contact damping of joint surfaces on the dynamic characteristics of highprecision machinery (machine tool, robot, etc.), in this article, a three-dimensional fractal model of normal contact damping of dry-friction rough joint surfaces based on Hertz theory and fractal theory is established. The threedimensional surface topography is constructed, according to the modified double variable Weierstrass-Mandelbrot function. The fractal model of strain energy E e , dissipated energy E p , damping loss factor D, and normal contact damping C n are deduced in detail, and the influence of fractal parameters and dynamic friction coefficient m on them are simulated. The simulation curves show that strain energy E e , dissipated energy E p , damping loss factor D, and normal damping C n increase with the increase in the fractal roughness G; the influence of fractal dimension D on E e is more changeable, first E e decreases with the increase in D and then increases. E p , D, and C n increase with the increase in D and m, respectively; the effect of m on E e is not obvious, so simple change in m has no significant change in E e ; a comparative analysis of the theoretical calculation of normal damping and experimental results show that their general trend is consistent, and they increase with the increase in total normal contact load P, the relative error is 5%-25%. The theoretical model can provide reference for the design of normal contact damping of the joint surfaces.
Purpose
The purpose of this paper is to provide a static friction coefficient prediction model of rough contact surfaces based on the contact mechanics analysis of elastic-plastic fractal surfaces.
Design/methodology/approach
In this paper, the continuous deformation stage of the multi-scale asperity is considered, i.e. asperities on joint surfaces go through three deformation stages in succession, the elastic deformation, the elastic-plastic deformation (the first elastic-plastic region and the second elastic-plastic region) and the plastic deformation, rather than the direct transition from the elastic deformation to the plastic deformation. In addition, the contact between rough metal surfaces should be the contact of three-dimensional topography, which corresponds to the fractal dimension D (2 < D < 3), not two-dimensional curves. So, in consideration of the elastic-plastic deformation mechanism of asperities and the three-dimensional topography, the contact mechanics of the elastic-plastic fractal surface is analyzed, and the static friction coefficient nonlinear prediction model of the surface is further established.
Findings
There is a boundary value between the normal load and the fractal dimension. In the range smaller than the boundary value, the normal load decreases with fractal dimension; in the range larger than the boundary value, the normal load increases with fractal dimension. Considering the elastic-plastic deformation of the asperity on the contact surface, the total normal contact load is larger than that of ignoring the elastic-plastic deformation of the asperity. There is a proper fractal dimension, which can make the static friction of the contact surface maximum; there is a negative correlation between the static friction coefficient and the fractal scale coefficient.
Originality/value
In the mechanical structure, the research and prediction of the static friction coefficient characteristics of the interface will lay a foundation for the understanding of the mechanism of friction and wear and the interaction relationship between contact surfaces from the micro asperity-scale level, which has an important engineering application value.
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