Research On Dynamic Characteristics Of Mechanical Joint Based On Asperity Interaction

Machine tools and most of the complex mechanical equipment are composed of a large number of mechanical parts with different functions,which constitute a variety of mechanical joint surfaces through different connection forms.The study of the contact characteristics of the joint surface is an important theoretical basis for analyzing and optimizing the dynamic characteristics of the whole machine.At the same time,considering the different design concept,processing technology and mechanical conditions,the influence factors of the dynamic characteristics of the joint surface and its mechanism are various and complex.These factors also influence each other,which leads to the indeterminism of the dynamic characteristics and the unclear mechanism of the contact characteristics of the joint.Therefore,it is of great significance to improve the dynamic characteristics and performance of the whole machine by theoretically establishing an accurate contact model and revealing the mechanism of the contact characteristics and its influencing factors.In this paper,the fractal geometry theory is combined with Hertz contact theory.A contact model of loaded joint was established based on classical contact model,which considers the effect of asperity interaction on dynamic characteristics of the joint.The expression of normal contact stiffness of the joint surface was also derived.The influencing factors of the normal contact stiffness and the corresponding mechanism were studied through the simulation and experimental research.Also,the contact characteristics of the oil-bearing joints were theoretically investigated,and the improvement effect of the liquid medium on the dynamic characteristics was discussed according to the simulation and experimental results.The experiments show that the proposed fractal contact model considering the asperity interaction is more in line with the actual stiffness load relationship.The main contents of the research are as follows:(1)In view of the unpredictability of the contact characteristics of mechanical joints under medium and high loads,a fractal contact model considering the effect of asperities interaction was established,and the relationship between the normal load and the contact area of asperities was solved.The model of contact stiffness was also established from the relationship between load and deformation.Based on the structure function method,the influence of the surface morphology on the dynamic characteristics of the joint was studied.(2)Given the dynamic characteristics of the joint surface cannot be directly verified and tested,a rail slider bench was designed to test the dynamic characteristics of typical mechanical joints.The frequency response function of each substructure was tested by modal tests,and the dynamic parameters of the joint were identified according to the method of equivalent single degree of freedom system.Also,the prediction accuracy of proposed model and the causes of the error were finally analyzed.(3)The contact characteristics of the oil-bearing joint were investigated.The dynamic characteristics of the oil-bearing joint were theoretically and experimentally analyzed from the point of solid contact and liquid contact.Considering the effect of the interaction between asperities,the expression of contact stiffness and contact damping of the oil-bearing joint were obtained by solving the generalized Reynolds equation of mean flow.Based on the results of simulation,the influencing factors of the dynamic characteristics of oil-bearing joints were also discussed.(4)Since the effect of the interaction between asperities on rough surface was not obvious under lower load,the concept of domain expansion factor was introduced to distinguish the actual contact area and the micro contact area.A contact model considering the asperity interaction and the domain expansion factor was established.The results of theoretical prediction and experimental test were also further compared and analyzed.