Investigation On Hydraulic Expanding Performance Of Assembled Camshafts With Isometric-trilateral Profile

With the rapid development of society,people’s requirements for engine performance continue to increase,especially in the aspects of lightweight structure,smooth operation,connection strength,service life and overall performance of camshafts.The traditional integral camshafts have disadvantages such as heavy weight,low manufacturing accuracy,multiple processing procedures,high consumption of materials and energy,and low level of automation.Therefore,it is imminent to improve,optimize and reform the manufacturing technology of camshafts to obtain lightweight,low-cost and high-performance camshafts.Assembled camshafts adopt a new combined design and manufacturing mode,with novel ideas,and significant advantages in reducing weight,reducing costs,and optimizing materials.It has become the development direction of camshafts manufacturing technology.Assembled camshafts are mainly composed of cam,mandrel,journal and other parts,which are processed and manufactured according to the different performance of parts.As a kind of assembled camshaft connection technology,hydraulic expanding technology can realize effective connection by filling high-pressure liquid to make the mandrel produce elastic-plastic deformation and produce interference fit with the inner hole of the cam.This dissertation intends to use an isometric-trilateral inner profile cam and hollow mandrel for hydraulic expanding,carry out the mechanical analysis of the hydraulic expanding process of the assembled camshafts,reveal the frictional characteristics of the expanding mating surfaces,and study the connection strength and failure modes during service,which will provide certain theoretical basis and guiding significance for the actual production and manufacture of assembled camshafts by the hydraulic expanding technology.In this dissertation,the connection strength and expanding performance of assembled camshafts by hydraulic expanding technology are studied by the combination of theoretical analysis,experimental research and numerical simulation.The main work and innovative achievements of this dissertation are as follows:(1)Mechanical analysis of hydraulic expanding process of assembled camshafts is developed.According to the expanding characteristics of mandrel and the variation of hydraulic pressure,the hydraulic expanding process is divided into free expanding stage of mandrel,constrained expanding stage of mandrel and unloading springback stage of mandrel.The stress variations of mandrel and cam in each stage are analyzed,and the expression of limit expanding hydraulic pressure is determined,which lays a foundation for subsequent hydraulic expanding experiments.The mechanical matching of yield strength and elastic modulus of cam and mandrel are analyzed,and the mechanical conditions for hydraulic expanding of dissimilar materials are established,that is,hydraulic expansion can be realized only when the ratio of elastic modulus of mandrel and cam is greater than the ratio of flow stress after strain hardening of mandrel to yield stress of cam.Through finite element simulation,the variations of stress-strain field and displacement field of mandrel and cam in camshafts with different mating surface structures are discussed,and the relevant information such as the strongest point of stress concentration and the weakest point of connection are obtained,which indirectly reflects the connection strength and hydraulic expanding performance of assembled camshafts.(2)The frictional characteristics of the expanding mating surfaces of the assembled camshafts are revealed.By constructing the observation models of the surface morphology of the mandrel at different positions,the variations of the surface morphology of the mandrel before and after the torsion are compared.The distributions and variations of surface roughness of the mandrel before and after torsion are analyzed.The effects of surface roughness of the mandrel on contact forms such as friction,contact pressure and contact area between the mandrel and cam are revealed.The relationship between surface roughness and the connection strength of assembled camshafts with an isometric-trilateral inner profile is established.The results show that the surface roughness of the mandrel decreases after torsion.With the increase of surface roughness of mandrel,the friction and contact pressure between mandrel and cam increase,while the contact area decreases in the last stage of torsion,and the connection strength of assembled camshafts increases.(3)The connection strength of assembled camshafts by hydraulic expanding technology is analyzed and predicted.The contact states between cam and mandrel(mandrel filling,contact clearance,elastic recovery and residual contact pressure)are analyzed to indirectly reflect the connection strength of the assembled camshafts.The connection strength of the assembled camshafts made of three kinds of mandrel materials(304 stainless steel mandrel,TP2 copper mandrel and 6063 aluminum mandrel)and with different mating surface structures(isometric-trilateral profile structure,circular profile structure and logarithmic spiral profile structure)is respectively compared.The expanding process(hydraulic pressure,loading path and annealing temperature)on the connection strength of assembled camshaft are revealed.Aiming at the shortcomings of the existing technology,a method for predicting the connection strength of assembled camshafts by hydraulic expanding technology based on the viewpoint of energy balance is proposed.The results show that the connection strength of assembled camshafts with three kinds of mandrel materials increases with the increase of maximum hydraulic pressure,but the rate of connection strength shows a trend of first increasing and then decreasing.The assembled camshaft with isometric-trilateral inner profile structure has the best connection strength and hydraulic expanding performance.The connection strength of assembled camshafts under non-pulsating hydraulic pressure is slightly lower than that under pulsating hydraulic pressure.With the increase of annealing temperature of mandrel,the connection strength of assembled camshafts show a downward trend,but the decline rate of connection strength decreases gradually.The prediction method can efficiently and accurately predict the connection strength of the hydraulic expanding assembled camshafts with different mating surface structures.(4)The failure modes of assembled camshaft after hydraulic expanding in service are analyzed.A BP neural network method for predicting the main failure factors of hydraulic expanding assembled camshaft is proposed.The correlation between the training results of the neural network model and the target value is analyzed by using the regression analysis method.The research shows that the BP neural network model has good accuracy,and the prediction accuracy of all results is more than 91%,of which the highest prediction accuracy is 99.78%.According to the microscopic observation experiments and main failure factors,the failure modes of hydraulic expanding assembled camshafts are determined,mainly including wear,scratch,damage,hole,pit and distortion.The failure manifestations of three kinds of mandrel materials after service are compared.The causes of assembled camshafts failure are qualitatively analyzed by using the failure tree theory,and the measures to reduce and prevent assembled camshafts failure are put forward from a macro point of view.The above-mentioned research results will lay a theoretical foundation and provide a certain scientific basis for the rational design of products,formulation of assembly process,expanding performance evaluation,connection strength analysis and prediction,failure modes exploration and prevention of assembled camshafts by hydraulic expanding technology,which is of great significance and practical application value.