The Analysis Of Dynamics Characteristics Of Impact System With Dry Friction

The research of dry friction impact systems provides strong theoretical support to improve the reliability,stability and design improvement of non-smooth mechanical systems for various types of railway trains.In this dissertation,the mechanical model of the dry friction impact system is established and its dynamic characteristics are explored,taking into account the nonsmooth characteristics of various railway trains and mechanical systems.In general,dry friction impact systems are multi-dimensional and multi-parameter systems,in which small changes in system parameters may lead to qualitative changes in system dynamics.Therefore,the technical-mechanical model for both systems containing dry friction and symmetric clearance is developed in this thesis,and the numerical simulation is used to investigate the dynamics of these two types of systems in the region of a given excitation frequency.Then the values of the system parameters are varied one by one to investigate their effects on the distribution areas,distribution characteristics and transformation rules between the various types of periodic impact motions of the systems,and it offers a specialized theoretical foundation for the comprehensive optimization of the design of impact system parameters containing dry friction finally.Firstly,the mechanical model of the single-degree-of-freedom impact system with dry friction and symmetric clearance is developed,and a Stribeck friction model is used to characterise the friction forces that exist in the actual system.The 1―1―1 motion of the system is transformed into the 2―2―2 motion by the N-S Bifurcation(Neimark-Sacker Bifurcation)as the excitation frequency decreases,and the 2―2―2 motion is transformed into a 2―2―2motion by the inverse cycle.The 2―2―2 motion is transformed into 1―2―1 motion by the Inverse Period Doubling Bifurcation and the Saddle Node Bifurcation,and then 1―2―1motion is bifurcated by multiple Grazing Bifurcation.When the number of collisions p and q between the left and right constraint surfaces is large enough,the system enters a state of chattering motion.Because of the existence of dry friction,the system will appear part of the friction-viscous properties of the periodic impact motion and chaotic motion window,so that the system will show a special slip-sticking-impact motion state.As the excitation frequency increases,the chattering motion goes through Rising Bifurcation,Saddle Node Bifurcation,Inverse Period Doubling Bifurcation,etc.,and the number of collisions decreases to 1―1―1motion.Secondly,a comparative analysis of the effect of different system parameters on the dynamic characteristics of the system reveals that the change in the value of each parameter has a more significant effect on the complexity of the dynamic characteristics of the system and the distribution area and distribution characteristics of the periodic collision motion.The clearance value b has a significant effect on the dynamic characteristics of the system,which is mainly reflected in the distribution area,distribution characteristics and transformation rules of various impact motion types.When the clearance increases gradually,the impact motion type of the system in the given frequency region decreases gradually,and the impact motion range shrinks to the low frequency region,while the 1-0-0 motion without impact appears in the high frequency region.Similarly,with the increase of the clearance value,the friction viscosity window increases.It shows that the clearance value has a great influence on the friction and viscosity characteristics of the system.And the impact velocity of the system also increases with the increase of the clearance b value.Finally,the distribution characteristics of the periodic impact motions of two-degree-offreedom impact system containing dry friction and symmetric clearence impact,the distribution regions,and the role of each system parameter on the interconversion rules between its various types of impact motions are studied.It is found that the two-degree-of-freedom impact system with dry friction have very similar distribution areas and the transformation rules between the various types of impactmotions are not very different from those of the single-degree-offreedom impact system with dry friction.However,since in a two-degree-of-freedom impact system with dry friction,the motions of the two masses affect each other,the two-degree-offreedom system is more prone to chaotic motions with complex dynamics in some parameter regions.