| The split design and integrated assembly are the salient features of the assembled camshaft which possess a variety of properties such as manufacturing accuracy, low cost of production, production efficiency, materials match, weight saving and emission reduction, and therefore assembled camshafts have more advantages over traditional cast and forging camshafts. The optimal matching between high performance material of different parts and the connection technology of cam and shaft are the key technologies of the assembled camshaft, which have already been the research hotspot of assembled camshafts industry and the important project for the automotive engine industry.There are many connection techniques of cam and shaft. The knurling technology has become the main stream technology in the field of connecting cam and shaft, because it have characters such as small location deformation, high reliability and accuracy, general device. The technology has already been applied, however, the complete and systematic research on it is still lacking. The iron-based powder metallurgy materials have become the most important new materials for the cam with their mechanical property of high hardness, wear-resisting, resistance to pitting corrosion and impact resistance. But the iron-base powder metallurgy cams are easy to crack because of high brittleness, which is a problem need solving.With the method of combining numerical simulation and experiment, the whole knurling connection was studied systematically. The influences of critical process parameters on the press-fit force and connection strength were researched. Double target process optimization about reducing press-fit force and keeping connection strength was done by taking knurling knife parameters and amount of feed as the influence factors. The composite layer powder metallurgy cam was introduced firstly. The microscopic structure analysis, mechanical properties test, assembly reliability researching were carried out, which laid the foundation for the new powder metallurgy cam connection.The major research work of this dissertation is summarized as follows.1. Mechanical analysis and finite element modeling of the radial knurling interference assemblyBased on the principles of radial knurling interference assembly, connection process was divided into thick wall cylinder interference connection and shaft teeth cutting. Analysis formula of press-fit force and torsional strength was deduced through mechanical analysis. The tooth height, tooth angle, amount of feed of cutters and mechanical properties of cam material were determined to be the important impact factors in press-fit force, torsional strength and cam profile accuracy.The finite element models of radial knurling, interference assembly and torsion process were established. Because the material on the surface of shafts deformed greatly, the combined Euler-Lagrange finite element description method was introduced; The Johnson-Cook constitutive and failure models were used for shaft material with the hardening effect and failure behaviors.2. Numerical simulation and research of the processes of radial knurling, interference connection and static torsionThe assembly process would transmit the effects of cutter and processing parameters. At the same time, the hardening of shaft teeth after being knurled affected the material flow and press fitting stress during the assembly process. There was the change of the transformation and elastic press fitting in these two processes, which would influence bearing torsional performance of cams. So it is essential to consider the relevance among the three processes synthetically for camshaft assembly and static torsion analysis. The processes of radial knurling, interference connection and static torsion were systematically numerical simulated in this paper. Characters of distribution of stress and strain for cam and shaft, forming process of shaft teeth, force conditions of cutters, deformation and force conditions of cam and shaft were deeply explored. In addition, the deformation of simulation and experiment were compared to verify the simulation reliability. The results revealed that the plastic deformation only existed on the surface of shaft in the knurling process. Furthermore, the residual stress as large as 211 MPa existed on the surface of shaft after knurling process. And Comparison of the deformation of material between numerical simulation and experiment were highly consistent.3. The influence of critical process parameters on the quality of assemblyThe assembly and bearing torsional performance of composite layer powder metallurgy cam was researched for the first time. The effects of material match of connecting pieces, feed amount and teeth parameters of cutters on press-fit force, bearing torsional performance of cam and the precision of cam profile were analyzed systematically. The Comparisons between numerical simulation and experiment about curves of press-fit and static strength were implemented to verify the validity of simulation results. The results showed that:(1) Materials of cam and tube had prominent impacts on press-fit load and torsional strength, especially to material of tube.(2) There was positive correlation between tooth height and press-fit load and torsional strength. In contrast, negative correlation between tooth angle and the two ones.(3) In a certain range, the more amount of feed, the higher press-fit force and higher connection strength. If the amount of feed was too small, the interference connection couldn’t form, and if it was too large, the shaft teeth would be destroyed.(4) In the same assembled conditions, the curves of press-fit force and static torsion between simulation and experiment were similar. The results of simulation were reasonable.4. Processing parameter optimization of radial knurling connection for steel camsIt is favorable to decrease press-fit force for decreasing equipment load, increasing production efficiency and avoiding the deformation of shaft and low precision of cam profile. The press-fit force is proportional to the static torsional strength, so increasing the torsional strength would raise the press-fit force at the same time. The Central Composite Design and Response Surface Methodology method were introduced to establish the second response surface model. The independent variables included tooth height of cutters, tooth angle and feed amount for cutters. The optimal objectives included press-fit load and static torque strength. The results indicated that there was only one group of optimal parameter combination when the tooth height was 0.73 mm, tooth angle was 70 degree and amount of feed was 90%. The response surface equation had high prediction accuracy and the optimal value which could be used to design knurling cutters and formulate processing parameters of knurling.5. Damage parameters and mechanics performance test of new material and construction camsAim at integrated and new composite double layers powder metallurgy cams, the wear ability,hardness,porosity and microstructure were researched. The tensile Strength ζ, elasticity modulus E and fracture energy Gf were obtained by the way of flattened Brazilian disc experiment and numerical simulation calibration method, which could provide the material parameters for assembly and fracture numerical simulation. The results revealed that the Brazilian disc experiment have high accuracy in testing the mechanical properties of brittle powder metallurgy materials. The experiment results could provide high precision material parameters for assembly and failure simulation of powder metallurgy cams. The cast iron constitutive model was chosen for the powder metallurgy materials because of its low tensile compressive strength ratio.6. Research on fracture of the new material and construction cams in assembly processFrom continuum mechanics point of view, the comparitions of press-fit load, torque and cam precision between GCr15 steel, integrated powder metallurgy cam and composite layer powder metallurgy cam were executed. There might be some micro crack and damage under the tensile stress in actual assembly process because of the high carbon content, high porosity and stress concentration on inner bore and key groove. The micro crack and damage would result in the failure of assembly. Based on the fracture mechanics theory, considering the circumferential tensile stress on cams in pressure fitting process, the Maximum Tensile-Stress Criterion and fracture energy based G Criterion were selected to assess the initiation and extension of fracture. The fracture energy based cohesive zone models of powder metallurgy cams for analyzing assembly damage were established. The exponential type cohesive fracture model was verified to have high precision when it was used for fracture analysis of brittle powder metallurgy material.The results revealed that:(1) The press-fit force and connection strength of GCr15 steel, integrated powder metallurgy cam and composite layer powder metallurgy cam were obvious different and all satisfied with technical requirements. The press-fit force and connection strength of GCr15 steel cam were 9.7% and 10.3% higher than composite layer powder metallurgy cam, respectively.(2) By analyzing the distribution law of circumferential tensile stress of cams during the pressure fitting process, the region of assembly stress concentration was determined and cohesive elements were set in this region. The location of initiation and propagation characteristics of fracture crack of one-layer and composite layer type cams were studied; The maximum bearable press-fit load of two kinds of cams was compared; The effects of assembly centering on the stress of cams and damage were researched. The results revealed that under the same condition, the inner layer of composite cams were subject to high circumferential tensile stress, while the press-fit force on outer layer of composite cams was much lower than that on integrate cams, which could decrease the possibility of crack initiation of outer brittle material and then increase the maximum bearable press-fit force of powder metallurgy cams.(3) Further more the tooth height of cutters for integrate cams should be lower than 0.82 mm. The poor centering would lead to the uneven loading of cam and finally result in the damage at preliminary stage of assembly.As the guidances for manufacturing and assembled technologies of cam, the research could avoid damage in assembly effectively. In addition, it provides the basement of pertinence optimization for powder metallurgy cam assembled process. |
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