| The gear transmission is one of the core components of electric vehicle powertrains,impacting the reliability and safety of automobiles.Because of the superior acceleration capability and starting performance,the type of multistage speed reducer with fewer gear stages is widely utilized in electric vehicles.Thus,the power transmitting paths of electric vehicles are shorter than conventional fuel vehicles.However,simplified transmission structures result in a surge of fatigue loading cycles for mechanical components such as gears,bearings,shafts,and others.Moreover,transmission systems of electrical vehicles are loaded under both driving and braking stages.Ranges and randomness of the torque and speed are more extensive compared with conventional fuel vehicles.The increasing loading cycle numbers and extended working ranges for electric vehicle transmission systems lead to new challenges in anti-fatigue design.The effective evaluation of fatigue life and reliability and structural optimization plays a significant role in the enhancement of transmission performance of electric vehicles.In this thesis,the fatigue life and reliability of an electric vehicle transmission are studied.Then structural parameters towards high reliability and lightweight are optimized.A fatigue life prediction model for transmission components of an electric considering load spectrum is proposed.Fatigue lives of transmission components under the load spectrum are determined.A reliability evaluation method considering strength degradation and failure correlation is developed.The dynamic reliability under the load spectrum is carried out.Structural parameters towards high reliability and lightweight are determined by a novel multi-objective optimization algorithm.Results and the methodology proposed in this work provide a high scientific value and practice significance.The contents of the work are summarized as follows:(1)Fatigue lives of the electric vehicle transmission components considering load spectrum are predicted.According to an electric vehicle transmission structural parameters and the load spectrum,a fatigue life prediction model is proposed based on the component load-capacity evaluation,the Archard’s wear estimation formula,the gear fatigue base data,and the Palmgren-Miner fatigue damage accumulation theory.The failure hazards and lives of transmission components under the load spectrum are evaluated by taking gear contact,bending,wear,and rolling bearings fatigue failure modes into account.The weak links and adverse working conditions under the load spectrum are found.Results reveal that the rolling bearing is the weak link of the electric vehicle transmission system.In contrast to the motor driving the transmission system,the reverse drag is the adverse operating condition.(2)Fatigue reliability of the electric vehicle transmission system considering strength degradation and failure correlation is estimated.The electric vehicle transmission system fatigue reliability evaluation method is developed considering the loading and strength uncertainties and the interaction effects between components and failure modes based on the Stress-Strength interference theory,the Copula function,and the finite difference method.The fatigue reliability under the load spectrum of electric vehicle transmission system can be obtained.Subsequently,the reliability sensitivity is obtained.Results reveal that in the first period of loading,the fatigue reliabilities of both transmission component and system decrease slightly,but they decrease remarkably in the last loading period.The mean time between failure(MTBF)of the transmission system is 82.98 h,and it is reasonable compared with the bench test results.Excepting fixed design structural parameters of the electric vehicle transmission system,the teeth number,gear module,pressure angle,and tooth face width impact the system reliability significantly.(3)Structural parameters are optimized towards high reliability and lightweight.According to the electric vehicle system reliability sensitivity from design parameters,the design variables are determined.Considering the working principle and design requirements of the electric vehicle transmission system,the design constraints are established.The electric vehicle transmission system structural parameters orienting high reliability and lightweight are obtained by the Non-Dominated Sorting Genetic Algorithm.Results reveal that the mass of the optimized transmission system decreases by 2.6% with the MTBF increasing by 26.36% under the load spectrum.This provides new insights into the high reliability and lightweight system structural parameters design. |
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