| Due to design tolerances,manufacturing processes,assembly errors,and wear fatigue,clearances inevitably exist in multi-body systems.The influence of clearances in kinematic pairs on the dynamic characteristics of multi-body systems is significant and cannot be ignored.With the rapid development of modern industry,higher demands are placed on the response speed,working accuracy,and stability of mechanical equipment in aerospace,military,industrial robotics,and other fields.This also poses greater challenges for the design and research of multi-body systems.As one of the important factors affecting the dynamic characteristics of multi-body systems,the study of clearances in kinematic pairs has become a focus of research for many researchers.This dissertation aims to study the dynamic characteristics of multi-body systems considering clearances in both revolute and translational joints,establish mathematical models for clearances in revolute and translational joints,explore the dynamic behavior of systems with clearances through numerical simulations and experiments,and use the Poincaré mapping method and mathematical analysis methods to qualitatively and quantitatively analyze the motion of multi-body systems.The research results provide a theoretical basis for evaluating the influence of clearances in kinematic pairs on the dynamic performance of mechanisms.The main work of this dissertation is su mmarized as follows:(1)According to the direction of contact forces in kinematic pairs,they are divided into normal contact forces and tangential contact forces.Combined with traditional contact force models,the advantages and disadvantages of various contact force models are compared and analyzed.Considering factors such as energy dissipation of colliding objects,initial collision velocity,material properties,and contact deformation,based on the normal contact force model proposed by Flores and the modified Coulomb friction force model,the dynamic equations of multi-body systems with clearances are established.(2)For the 3D model of revolute pairs with clearances,12 possible contact forms of bearings and journal are analyzed,and a mathematical model for 3D revolute pair clearances is established.Based on the established mathematical model of the clearance,combined with the normal contact force model,tangential contact force model,and dynamic equation of a multi body system with clearance,the influence of the clearance size of the revolute pair on the internal contact force of the revolute pair and the path of the inner journal center of the revolute pair bearing was analyzed through numerical simulation.The effects of clearance size,different driving speeds,and different material combinations of revolute pairs on the dynamic characteristics of crank-slider mechanisms are also analyzed through numerical simulation.(3)Taking the classic translational pair of sliders and guide rails as the research object,the contact between the slider and the guide rail is classified into point contact,line contact,and surface contact.11 possible contact forms are su mmarized based on the geometric characteristics of the contact area between the slider and the guide rail.After considering the frontal contact and lateral contact of the contact area,the mathematical expression of the contact stiffness K is redefined.A contact force model between the slider and the guide rail in a 3D translational pair is established based on the new generalized contact stiffness model.Finally,taking the planar crank-slider mechanism and the 3D crank-slider mechanism as examples,the influence of different clearance sizes,different driving speeds,and different material combinations of the translational pair on the dynamic characteristics of multi-body systems is analyzed.(4)Considering the influence of clearances in revolute and translational pairs on the dynamic characteristics of multi-body systems,the chaos phenomena of the crank-slider mechanism with clearances are identified using the Poincaré mapping method.The qualitative and quantitative analysis of the influence of different clearance sizes,driving speeds,and material combinations of the crank-slider mechanism on the dynamic characteristics are carried out.The motion fluctuation evaluation index and motion error evaluation index are proposed based on the mean absolute error formula(MRE)to analyze the displacement fluctuation index(DFI),acceleration fluctuation index(AFI),and crank torque fluctuation index(TFI)of the slider,reflecting the main influencing factors of the motion jitter and impact of systems with clearances.The motion error evaluation index is proposed based on the mean square deviation formula(SD)to analyze the displacement mean square deviation index(DSD),acceleration mean square deviation index(ASD),and crank torque mean square deviation index(TSD),reflecting the influencing factors of the motion stability of multi-body systems with clearances.(5)Combining the influence of clearances in revolute and translational pairs on the dynamic characteristics of multi-body systems,the study focuses on the construction of the experimental platform.Alloy steel,aluminum alloy,copper alloy,and nodular cast iron are selected as processing materials.Based on the clearance of the revolute pair,different clearance sizes of the revolute pair were manufactured using base hole machining.Different sizes of sliders are processed based on the size of the translational pair guide.The displacement and acceleration data of the slider are collected using a linear displacement sensor and a piezoelectric acceleration sensor.The influence of clearance in the revolute pair,driving speed,and material combination on the motion jitter,impact,and motion stability of the crank-slider mechanism is evaluated using motion fluctuation evaluation index and motion error evaluation index,and the correctness of the theoretical analysis is verified by comparing the measured data with the theoretical simulation data. |
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