Research On Dynamic Characteristics Of Mechanism With Mixed Joint Clearances

There are always clearances between connecting joints in actual mechanisms due to the machining errors and component wear.The existence of clearances directly affects the dynamic characteristics of the mechanism,reducing its motion accuracy and intensifying the damage to the mechanism.The influences of joint clearances on the normal operation of the mechanism can not be neglected.The study on mechanism dynamics with consideration of joint clearances has become a hot issue in multibody system dynamics.In this paper,the dynamic characteristics of the mechanism with the mixture of revolute clearances and translational clearances are studied,which has certain guiding significance for the design of mechanism and dynamic behavior prediction in engineering practice.The composition of the contact forces at the clearance of the joint are analyzed firstly,and the contact forces at the contact areas are decomposed into normal impact forces and tangential friction forces.A nonlinear spring-damp model and an improved Coulomb friction force model are used to describe the components of the two directions respectively.The dynamic equation of multibody system with flexible components is established.By introducing the step function into the system,the contact forces at the clearance are embedded into the multibody system dynamic equation,then the multibody system dynamic equation with the consideration of clearances and flexibility of the components is established.It lays a theoretical foundation for the analysis of mechanism dynamics in the following chapters.Considering the radial and axial clearances in the revolute joint,the spatial clearance model of the revolute joint is established.The spatial contact forms between the pin and hole are summarized,and the minimum distance expressions of the two components in radial and axial directions are derived.The criterion of separation-contact state switching is deduced.Based on the model of spatial clearance of revolute joint and the dynamic model of the mechanism with clearance,a crank slider mechanism considering the clearance of the revolute joint is taken as the object and its dynamics analysis is carried out.The effects of clearance size,driving speed and connecting rod flexibility on the dynamic characteristics of the mechanism are studied.Considering the clearance between slider and guide in the translational joint,the clearance model of translational joint is established.Four possible contact forms between slider and guide surface are analyzed and summarized.The vector expressions of the contact between slider and guide are derived,and the vector discriminant formulas for the contact penetration between the two components are derived.Based on the established translational clearance model,the dynamic characteristics of a crank slider mechanism with translational clearance are studied.The influence of the clearance in translational joint on the dynamic characteristics of the mechanism is analyzed.Finally,the influence of mixed clearances on the dynamic characteristics of the mechanism is studied.A combination of theoretical analysis and experimental research is adopted,the dynamic characteristics of the mechanism under different working conditions are simulated,then the experimental study under the corresponding working conditions is carried out,and the response curve of the output of the mechanism is obtained.By comparing the experimental results with the simulation results,the data are in good agreement,which shows the validity of the simulation analysis,and verifies the correctness of the multibody system dynamics model with the consideration of joint clearances.