Dynamic Characteristics Analysis Of Spatial Parallel Mechanisms Considering Clearance Effect

At present,mechanical equipment is developing in the direction of high speed and high precision,which requires higher stability,accuracy and efficiency of mechanical system.Each part of the mechanism is connected by the pair of motion.Due to the limitation the processing technology,machining accuracy and assembly requirements,the clearance will inevitably be exist between the pair of motion elements.The existence of clearance will have adverse effect on the dynamics of mechanical system.Furthermore,it will affect the accuracy and stability of the equipment.At the same time,clearance is easier to cause wear of the motion pairs and aggravate the aging of the equipment.Therefore,it is of great significance to study the mechanism dynamics with the clearance and wear of pairs.In this paper,the spatial 4UPS-UPU parallel mechanism that considered the spherical joint clearance is taken as the research object,and its dynamic response,nonlinear characteristics and wear characteristics are studied.Firstly,the kinematics model of the spherical joint clearance is established according to the geometric relationship of the clearance element.The contact force model at clearance is established according to Lankarani-Nikravesh(L-N)normal model and Coulomb friction model.The dynamic model of 4UPS-UPU spatial parallel mechanism with clearance is established by Newton Euler method.The dynamic equation is solved and the dynamic response of the mechanism is obtained.The validity of dynamic modeling with clearance is verified by virtual prototype simulation,which lays a foundation for further research.Then,based on the basic theory of nonlinear dynamics and the numerical solution of mechanism dynamics equation,the nonlinear characteristics of spatial parallel mechanism with spherical joint clearance are analyzed by means of phase trajectory diagram,Poincare map,power spectrum diagram and bifurcation diagram.The effects of clearance value,friction coefficient and drive velocity on chaotic motion characteristics of the moving platform and clearance are studied.The bifurcation diagrams of moving platform and clearance joint are obtained when different parameters change,which provides a theoretical basis for parameter selection and chaos suppression.Finally,the three-dimensional wear prediction model of spherical joint clearance is established by combining Archard wear model and dynamics model.According to the wear depth,based on the idea of finite element method,the worn surface of spherical joint was reconstructed and the geometric morphology was obtained.The influence of irregular clearance on the dynamic response was studied.The influence of clearance value,friction coefficient and drive velocity on wear depth and wear area of spherical joint is studied,which provides some guidance for practical engineering.