Research On Assembly Performance Of Precision Mechanical Structure Based On Fractal Joint Surface

Precision mechanical structure is an important part of sophisticated equipment and assembly is the final integration cycle.The joint surface between the parts is the real contact region and has a great influence on the function and performance of the structure.The assembly joint surface with fractal characteristics is called the fractal joint surface.And the asperities on the joint surface will produce the spring damping effect which becomes the key factor affecting the dynamic performance of the whole machine.In order to find out the influence rule of assembly process on the dynamic performance of precision mechanical structure theoretically,the fractal joint surface is taken as the research entry point to establish the contact model.An optimization algorithm for machined surface fractal parameters is put forward and a method for the oblique asperity contact parameters is proposed.A reliable modeling method is studied to transform the micro-scale joint surface to macro-scale.The dynamic model of the whole machine considering the fractal surface is constructed to achieve the prediction of the dynamic performance.Therefore,the research method of assembly process-contact model of fractal joint surface-machine dynamics model-dynamics performance of precision mechanical structure is provided theoretically,which which provides a theoretical basis for assembly process design and assembly simulation.The main research contents are as follows:1.An extended WK(Wang-Komvopoulos)fractal contact model is established.The contact state of the asperity is extended from peak contact to oblique contact.Contact angle ci and its actual contact area ratio Ai/Ar are analyzed from the geometric perspective.And the normal contact stiffness and damping model of the joint surface are established.The load-deformation experiment is used to calculate the joint stiffness of the specimen.Compared with the numerical simulation results,it shows that,the extended WK model had a higher accuracy than the original WK model for light load.And the deviation fractal dimension D and fractal roughness G will affect the accuracy of the stiffness model.2.The fractal dimension algorithm based on Discrete Wavelet Transform(DWT)is optimized.Inverse power spectrum density transform method,WM(Weierstrass-Mandelbrot)function method,fractal Brownian motion method and inverse wavelet transform method are used to generate machind profiles.Box counting method,power spectral density method,roughness length method and structural function method are treated as the reference algorithm.The precision of DWT with different parameter combinations are discriminated and the optimal combination is selected.Based on the root mean square deviation Rq of the surface profile,the relationship between D and G is established.Thus,G can be calculated by the optimal D.The profiles of surface roughness comparison specimens are applied to analyzed the fractal characteristics of different machined specimens.3.A calculation method is proposed to calculate ci and Ai/Ar based on surface point cloud.The ICHA(Interacting and Coalescing Hertzian Asperities)model considering the elastic coupling and coalescence between asperities is extended to m-ICHA model by introducing the oblique contact.Contact angle ?,equivalent radius of curvature R(r)and actual contact area Ar are calculated based on the geometric characteristics of the point cloud.According to the iterative numerical simulation of surface point cloud,the distribution of ? and R(r)along with Ar is obtained.Then,the optimized DWT algorithm and numerical simulation based on surface point cloud are utilized to correct the stiffness calculation results.Compare with the stiffness experimental results,it is proved that the WK expansion model could improve the accuracy by more than 5 times.4.The Equivalent Material(EM)parameter modeling method is put forward based on oblique asperity contact of fractal joint surface.The bolted plate with different pre-tightening force and different process is tested in the finite element simulation and modal experiment.Compared with the simulations of spring-damping method,direct simulation of rough surface and scanning laser vibration measurement experiment,,it was verified that EM can acquire the highest resonance frequency accuracy.5.For a certain type of seeker servo mechanism,a prototype of two-axis and two-frame servo mechanism was developed.Adjustable bearing support hole and its assembly process were designed.The fit tolerance between bearing outer ring and supported hole,the pre-tightening torque of bearing end cap are under assembly control.Based on the measured data of bearing outer ring and supported hole,the contact model and surface parameters are calculated and EM is used to transform the micro-scale joint surface to macro-scale.In the ADAMS software,the dynamics simulation is performed according to the orthogonal table.The modal vibration experiment of the prototype and scanning frequency experiment of the motor are carried out.Simulation and experimental analysis show that the accuracy of simulation with EM is higher than that without EM.And the simulation results have a good agreement with the experiment results.Finally,the fitting model between the assembly process parameters and resonant frequency are obtained.The influence from the fractal surface on resonant frequency is analyzed.It is verified that it is feasible to resreach the dynamic performance based on fractal surface.