Modeling And Application Of Directional Friction On Regular Asperity Surface

The development of new mechanical processing technology has made the fabrication of regular asperities on a friction surface possible. Regular asperities refer to a series of micro structures on a surface with similar shape, same size and determined distribution. The surface with regular asperities is called regular asperity surface (RAS). The emergence of RAS means that people can precisely control the shape of the surface at micro or even lower level. It is then possible to realize required friction behavior through design and construction of regular asperities with special shape, proper size and distribution on a surface. For example, surface with regular anisotropic asperities could realize directional friction, a kind of friction behavior where the magnitude of the friction force changes along with the friction direction. It might be possible that the magnitude of the friction force on a RAS can be precisely controlled as well. Therefore, the tribological study of the RAS will be a meaningful and important work in the area.A detailed review of current literatures has found that the tribological society did not yet conducted sufficient experimental or theoretical research on RAS. Particularly, due to the limitation of the current technology, the fabrication of regular anisotropic asperity surface (RAAS) with satisfied regularity is not yet possible, which leads to the following problems: the knowledge of the friction behavior on RAAS in different lubrication state is still missing; there is not a theoretical model that can be used in tribological study of RAAS, theoretical model refers particularly to friction models in different lubrication state; there is not a research that is devoted to the application of RAAS and the friction behavior it can produce. In the thinking of solving all these three problems, so that a better understanding of the RAAS friction behavior can be obtained and a foundation for tribological study and application of RAAS can be provided, the following research has been conducted.(1) The high resolution pocketing based molding plus casting method was used for the first time in fabricating RAS. Both regular isotropic asperity surface (RIAS) and RAAS were fabricated through this method. These fabricated RASs were put into a series of friction test, under the purpose of exploring the friction behaviors that can be realized on RAAS as well as RIAS. Through carefully conducted analysis, the experimental result proves that the directional friction can definitely be realized on RAAS. The experimental results also shows that the degree of directional friction will be affected by the height of the asperity, the direction of friction and the friction speed. The affect of asperity stucture and pressure on the magnitude and fluctuation of the friction force were also discussed by the experimental result.(2) A detailed review has been devoted to different friction models, which shows that these models have their own disadvantages in friction force modeling. Among them, LuGre model as well as the other bristle deflection models cannot model the directional friction behavior on RAAS due to the assumption of constant parameter values. An amendment was proposed based on mixed switch theory. The amendment was aimed to enable those bristle deflection models the modeling of directional friction. A new friction model called mixed switch model was developed based on LuGre model and the amendment. Mixed switch model can model both directional friction and many other friction behaviors. The model was mathematically analyzed and its result was compared with experimental result, both the analysis and the comparison proved the reliability of the new model. The feasibility of the amendment was therefore proved as well.(3) Many current fluid lubrication models (FLM) were carefully examined, which found that the Reynolds equation based FLM cannot be applied in the fluid lubrication simulation on RAS, and NS equation based model cannot deal with the asperity caused cavitation properly. A new cavitation model was established based on new density-pressure and viscosity-pressure relations. Along with the NS equation, a new FLM was developed. The new FLM can calculate the pressure distribution, cavitation area distribution and the forces on the solid surfaces. The experimental result from a RIAS and a normal journal bearing proves the reliability of the new FLM.(4) The new FLM was used to study the friction behavior on fully lubricated RAASs. The friction force on different RAASs was calculated under two different situations constant gap and constant load. The result from both of those two situations proves that the fully lubricated RAAS can also produce directional friction behavior, and the direction behavior are opposite under those two different conditions. The result also shows that the friction speed can increase the degree of directional friction, so as the shape of the asperity, which will change the degree of directional friction.(5) The mixed switch model was applied in the modeling of piezo-electrical friction inertia actuators (PFIA) with RAAS as its driving surface, and a PFIA model was established. Based on this model, the displacement of the end-effector in a PFIA was calculated when the PFIA was stimulated by different kinds of voltages. The calculated results were compared with the experimental results, which shows that the PFIA model with the application of mixed switch model can accurately simulate the actuation process. The reliability of the mixed switch model was therefore proved in the application of PFIA modeling.