Research On Nonlinear Contact Forces And Impact Dynamics Of Mechanism With Clearance Joints

With the development of mechanical engineering,aerospace engineering and robotics industry,a series of design and application requirements have been put formard for mechanical systems,such as high speed,high precision,high efficiency,high reliability and long life.As an important part of mechanical system,the accurate design,modeling and simulation of mechanism play an important role in the realization of movement and power transmission.For the commen phonomenon that clearance joint and contact are always existant,which lead to the oscillation of dynamic output,and the decrease of the movement precision,the reliability as well as the service life of the mechanism,and even the failure of the mechanism,the dissetation makes further reseach on modelding of contact force model,impact dynamics simulation of mechanism with clearance joint and experimental research.Then the contact model and simulation results were verified by the experimental results under different conditions,such as different clearance sizes,crank speed and material dimensions.The study results in this paper provide theoretical support for effective identification the influence fators and effects of mechanism with clearance joints,accurately predict the dynamic characteristics of mechanism has clearance joints,further active suppression,or even eliminate the effects caused by clearance joint,such as vibration,shock,friction and wear.The works in the paper also laid a theoretical fundation for optimization design and reliability analysis of mechanism.The important results in this paper as follow:1.A vector and mathmatical model for clearance joint are presented,which are based on assume of positive impact between the journal and bearing,and the bearing/shaft axial size is also introduced.The model is suitable for dry contact situition and the condition with lubtication.Then based on this model the dynamic simulation model was presented,which laid a foundation for follow-up study.2.A detailed comparative analysis about the traditional contact force model of non damping and with damping is made based on an example of bearing and journal,the contact force models without damping contain the Hertz contact model,Perrson contact model and improved Winkler elastic fundation model,and the models with damping include hunt-Crossley model and it's improved model,and the limitations of the traditional models are pointed out,then three different nonlinear contact force models were introduced,which are based on the clearance model presented above,and an improved Coulomb friction model also introduced to describe the friction effects in clearance joint.A number of simulations were presented based on a journal-bearing model and a planar slider-crank mechansim,which verified the correctness of the model presented in the paper.From the simulation results,it also can be concluded that under different clearance size,ilitial impact velocity and coefficient restitution,the third improved nonliear contact model always has higher accuracy than classical contact model presented by Lankarani and Nikravesh.3.Based on the nonlinear contact force model presented in the paper,and consider the lubrication in clearance joint,the paper introduces an improved transfer force model,which includes dry contact force and hydrodynamic forces,accounts for not only squeeze-film effects,but also wedge-film effects,and is applicable to both long and short bearings.Then the dynamic behaviors of a planar slider-crank mechanism with lubricated clearance joint are presented based on this transition force model,in order to better analysis the effects of lubrication and transition mode on clearance joints,the results of lubricated clearance joints also compared with those of dry contact clearance joints.From the simulation results,it can be concluded that the dry joint clearances cause obvious peaks on the dynamic behaviors of multibody system,whereas the lubricated joint can effectively reduce the vibration caused by impact force,and lead to a better dynamic response of the system.4.The impact dynamic of mechanism with clearance joints has been investigatd using the nonlinear contact firce model and improved transfer foece model presented in the paper.A planar slider-crank mechanism is used as an example,and a number of numerical simulations are shown for different clearance size,crank speed,restitution coefficients,materials dimensions and dynamic viscosity of oil,it can be concluded from simulation results that(1)with the effect of clearance joint,the dynamic characteristics has significant vibration,and the larger clearance size,crank speed and restitution coefficients,the more obvious vibration and larger amplitude.(2)the vibration frequency with different material dimension are almost same,but with larger Young's modulus produces more obvious influence on dynamic characteristics of mechanism,and the results based on material with smaller Yong's modulus have a delay phenomenon.(3)when consider the effect of lubrication in clearance joint,the mechanism has a better dynamic characteristics,but clearance size,crank speed and dynamic viscosity have obvious influence on lubrication force,with the increase of clearance size and crank speed,and the decrease of dynamic viscosity,the minimum film thickness shows a clear downward trend near the dead position of mechanism,even to be less than safe film thickness.5.A test experimental system of mechanism with clearance joint is designed,and a number of tests have been presented with different clearance size,crank speed and material,and the results of experimental also compared with those of numerical,which further verified the correctness of the nonlinear contact model and impact dynamic simulation results presented in the paper.Finally,some conclusions and observations are given and the developmental prospect of mechanism with clearance joints are discussed and outlined.