| A connecting column is an important subjected part in the pre-stressed frame of a hydraulic press.The complicated load it suffered often lead to an early crack and tended to fatigue fracture in the tooth-root portion under high-frequency working conditions.The fatigue life of the connecting column is one of the key factors that determine the service life of the hydraulic press,which should be seriously considered in the frame design stage.The bolted join of the connecting column in a hydraulic press pre-stressed frame was studied in this paper by using the elastic-plastic contact finite element method.The distributions of stress,deformations and tooth suffering loads of the original design scheme of the thread are all obtained for further analysis.After that,the parametric model of the bolted join was established by Python script language for the automatic finite element calculation and the simulation results post process.According to the classical theory of fatigue,the fatigue crack will be initiated in the region where the maximum shear stress amplitude is occurred,and the crack will be expanded with the action of principal tensile stresses.Furthermore,the non-uniform load distributions between the threads teeth,which resulted from the inconsistent deformation,also affect the amplitude of local concentrated stress in the tooth-root portion.Therefore,besides the maximum of equivalent Mises stress and the maximum of principal stress,the non-uniform coefficient of load distribution between the screwed teeth are also taken as one of the optimization objectives in this paper.After that,the principle of orthogonal experimental design method was used to choose the main design variables for the parameterized model of the bolted connection structure.The visual analysis method and the variance analysis method were performed in the orthogonal experiment results,and ultimately,six design variables were selected,which were thought to be have an significant effect on the above three optimization objective functions,they are the namely fillet radius of thread root,the pitch of thread,the major diameter of the thread in the connecting column,the bearing side angle of the thread,the thread form angle and the number of screwed teeth.For obtaining the optimal design of the bolted join,an approximate modelling method based on the neural network of radial basis function was used to setup the multi-objective optimization model,where the above six main parameters were set as design variables and theupper and lower boundary of those parameters and the mechanical properties of the material of the connecting column and the nut were taken as the design constraints,the maximum value of Mises stress,the maximum value of the first principal stress and the non-uniform coefficient of load distribution between teeth were configured as optimization objectives.The second generation non-dominated sorting genetic algorithm was used to solving the optimization problems and thus a set of Pareto optimal solutions was obtained.An optimization design scheme was chosen among the optimization results and verified by the finite element analysis.The numerical results indicate that the maximum value of Mises stress in the optimized design is reduced by 10.59%,and the maximum first principal stress is dropped by 12.48%,and the load distribution non-uniform coefficient between teeth is cut down by 43.11%with comparing to the original design.Subsequently,the key factors that affecting the mechanical performance of the connecting structure were determined.It is shown that the mechanical performance of the connecting structure will be perfect when the namely fillet radius of thread root is set to 2.2mm,and the pitch of thread is 10mm,and the bearing side angle is equal to 15 degrees.The fatigue strength of the connecting structure is also studied later in this paper.The fatigue life of the connecting column and the nut were estimated.The influence mechanism of the fatigue life in the bolted joint was revealed by analyzing the effects of the root transition radius,the notch coefficient,the mean stress correction method,the survival rate and the tooth angle.The results shown that the peak value of equivalent stress is not the only factor affecting the fatigue life of the connection structure.Under certain conditions,the smaller the stress peak in the structure does not directly mean that the higher fatigue life.This conclusion needs to be further confirmed by using the fracture mechanics method under the sufficient material fatigue test data conditions. |
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