Study On Dynamic Contact Characteristics Of Joint Surfaces In Mixed Lubrication

The joint surface is a key component of the machine tool,which plays a crucial role in transmitting load and energy during the operation of the machine.Meanwhile,the continuity of the system is destroyed,which greatly affects the performance of the whole machine.The current research mainly focuses on the contact of dry friction surface,but there are few reports on the research of mixed lubrication joint surface which is more suitable to the actual working condition.In fact,most surfaces of the machine tool are in a mixed lubrication regime,and the lubricant film directly affect the dynamic behavior of the whole machine.Therefore,it is of great significance to study the dynamic characteristic(stiffness and damping)of the mixed lubrication joint surface for the prediction and optimization of the machine performance.In this paper,a theoretical contact model of the mixed lubrication joint surface is established,and the contact stiffness and damping characteristics were analyzed,which can provide a theoretical basis and experimental reference for improving the dynamic performance of the whole machine.The specific works are as follows:(1)A fractal contact model of the solid and a fluid squeeze lubrication model are proposed respectively.Firstly,the three-dimensional Weierstrass-Mandelbrot function was employed to characterize the fractal characteristics of the rough surface.Based on the micro-contact mechanism of rough surface,a fractal contact model of single asperity was proposed based on the deformation mechanism of the asperity.Then,the single asperity system is extended to the entire rough surface based on the fractal theory,and the normal contact fractal model of the joint surface solid is established,and the real contact area and normal contact load were investigated.Finally,based on the generalized Reynolds equation of average flow considering the influence of surface roughness,a squeezing and lubrication model was proposed,which helped to establish the following liquid lubricant contact stiffness and damping model.(2)The normal contact stiffness model of the mixed lubrication joint surface was developed.Firstly,based on the load sharing concept,the contact stiffness of the joint surface is equivalent to a parallel stiffness model,which includes the solid stiffness and lubricant film stiffness.Then,solid stiffness and liquid film stiffness models were developed respectively.The stiffness model of the single asperity is extended to the joint surface based on the fractal theory,and the solid stiffness model was presented.On the other hand,the lubricant film stiffness is established based on the generalized Reynolds equation of average flow.Two components stiffness are associated with each other via the equivalent lubricant film thickness.This model also avoids the problem of measuring the lubricant thickness,and achieves the separation and characterization of the contact stiffness of the two parts.Finally,the solid stiffness,lubricant film thickness and stiffness,the stiffness of joint surface and its distribution were analyzed,respectively.(3)The normal contact damping model of the mixed lubrication joint surface was developed.Similarly,the parallel damping model of the joint surface is first established.Then,the energy storage and energy dissipation model are established based on the contact mechanics model of the single asperity,and it is extended to the entire rough surface,the fractal contact damping model of solid of the joint surface is established in combination with the fractal theory.On the other hand,the viscous contact damping model of the lubricant film was also established based on the generalized Reynolds equation.Finally,the elastic strain energy,plastic dissipative energy,damping loss factor and the damping of solid,lubricant damping,and the damping of the joint surface are simulated,respectively.