Study On Friction And Lubrication Mechanism Of Bionic Non-smooth Interface In Piston-linear System

Piston-linear is a complicated system, suffering from to-and-fro motion, impact action, thermal effect, etc. In order to gain the best reliability, economical performance and proper service life, the theory of friction, wear and lubricate is must be introduced to analyze the influences on friction and wear of Piston-linear system. Traditional methods have reached extremity to improve the capability of piston-linear frictional pairs, such as improvement of the material character, using laser, quenching, chromeplating, improving the machining precision, reducing surface roughness and so on. In recent years, researchers from the Key Laboratory for Terrain-Machine Bionics of Engineering belonging to Ministry of Education have designed bionic non-smooth piston, which has better wear resistance and lubrication effect in practice, using bionic non-smooth theory and technology.Five kinds of bionic unsmooth experiment sample are designed based on bionic principle. For sample test, dry friction experiment, part lubrication experiment and full lubrication experiment are conducted on five unsmooth and plat structure comparatively. The effect of temperature is also considered. The relation between frictional coefficient and wear ration is researched. The different wear and loading ability of all kinds of structure are analyzed. From above results, the distribution law of bionic non-smooth structure is optimized.From the sample test, it is concluded:On dry friction condition, the frictional ability of smooth and unsmooth structure is almost the same. With the loading increasing, friction coefficient decreases and wear ration increases. With the loading and frictional distance increasing, the status of friction changes from slightly scratch to agglutination.On mixed lubrication condition, all kinds of unsmooth structure have super anti-wear ability than smooth structure. The anti-agglutination distance of the best structures is almost 4 times than the smooth structure, and 77% friction energy is saved. From the experiment, type C, D and F are selected as the best frictional ability structure.On full lubrication condition, all kinds of unsmooth structure have better advantage on loading ability than smooth structure. The anti-agglutination ability of the best structure has improved 85% compared with smooth structure. The type F is selected as the best frictional ability structure.With the loading inceasing, friction coefficient decreases and wear ratio increases.With temperature increasing friction coefficient keeps steady, but wear ratio increases.Mathematical models of dry friction, mixed lubrication and full lubrication are developed on the base of experiment study. LS-DYNA solver is applied to solve the nonlinear finite element model.For dry friction test, the proper contact interface conditions are selected as non-injectivity, normal contact pressure and tangential friction contact condition. Nonlinear contact finite element model is established by adding restraint conditions to functional by penalty function method. In order to reduce the calculation, the computational domain is just twentieth of the piston-liner contact area in the circumferential direction. The result shows that stress gradient distribution is obvious on skirt of the smooth piston. Contrarily, stress distribution is disorderly on skirt of the nonsmooth piston. Accordingly, wear partially appears on the smooth piston skirt. Stress distribution on the nonsmooth piston skirt is much more uniform, so partial wear is avoided and the service life of piston is prolonged.For the mixed lubrication test, a mathematical model is built up in order to describe large-amplitude sloshing of the lubricating oil with the up-down motion of the piston. This model is solved by an ALE (Arbitrary Lagrange-Euler) finite element method. The results of simulation actually show the sloshing behavior of the lubricant in the non-smooth pits, and also get the sloshing wave height and pressure distribution. The oil storage lubrication physical phenomena of non-smooth pits are explained. Excessive distortion and winding of the finite element mesh is avoided and the mesh-updating scheme is better. The oil storage capacity of the three different pits is calculated: The lubricating effect of wedge-shaped pits is best. Hemispherical pits is secondary; and the conical pits is worst. This conclusion is in good agreement with experimental results.For the full lubrication test, this paper firstly surveys the lubricating theory, model and research situation. CEL (Coupling Euler Lagrange) fluids and structure coupling method is introduced to solve this problem. Lubricating oil kinetics equation is described with N-S equation in Euler coordinate system. Isotropic linear elastic equation in Lagrange coordinate system describes the piston-linear, which is constrained. Penalty function constrained Method Is used to transfer the correlative parameters of structure to the fluid element, thus it can be realised that structure couples fluids directly. It can be seen from deformation and flowing status of lubricating oil that the non-smooth piston skirt extrudes lubricating oil into non-smooth pits, but smooth piston skirt extrudes lubricating oil out of the gap between the piston skirt and cylinder can not store the lubricating oil. In summary, effects of non-smooth piston skirt on lubricating oil have two processes: extrusion and storage, but mooth piston skirt only has extrusion process. Views on contact stresses contour from non-smooth piston surface, contact stresses increases as the lubricating oil drains during extrusion process. Especially contact stresses on the surface of smooth piston skirt increase severely, because the smooth piston skirt has no ability to store the lubricating oil and finally the contact surface becomes dry friction status. Compared with the dry friction in chapter 4, contact stresses on the surface of the smooth and non-smooth piston skirt decrease one order. It is obvious that contact stresses on the surface of the non-smooth piston skirt decrease greatly.Finally, the friction heat process of piston-cylinder is analytical. Firstly analyze the contact problem and then obtain the normal contact force of friction interface, thus get the work from friction force. This work is transfered into thermal energy as a surface heat flux influx which is the boundary condition of temperature field governing equations, so the coupling between stress field and temperature field is achieved. Galerkin weighted residual method is used to solve the heat conduction governing equations. The results show that the maximum temperature of smooth piston surface is lower than that of non-smooth piston surface. Although the temperature of local non-smooth piston surface is higher, overall temperature distribution is uniform. One of the main reasons for local wear of smooth piston skirt is resulted from the temperature focus which can cause poor heat conduction. It does not easily cause wear and high-temperature adhesive phenomenon because the frictional heat disperses on the non-smooth piston skirt. Therefore, non-smooth pits can improve the temperature distribution of frictional pair, enhance the wear resistance of the piston skirt surface, and prolong the service life of piston.The wear resistance and lubrication specification of bionic non-smooth structure is verified with exploratory study on the wear and lubrication technology of piston-linear wear pairs based on bionic non-smooth theory. This method is not only dependent on material improvement and surface enhancement technique, but also reaches to the original research intention using physical experiment and numerical simulation technology which are low-cost and short development life cycle. A new way is provided to solve the wear and lubrication problem of piston-linear wear pairs in automobile industry.