Design Of Dual Mass Flywheel Based On Shape Constraint And Nonlinear Vibration Research

When the frequency of the excitation torque input from engine is close to or coincides with the natural frequency of vehicle powertrain transmission system, resonance vibration of the system will occur, which will directly affect the comfort of vehicle, the strength and service life of the transmission system components, and even threaten the vehicle driving safety. Large torque and high power is the trend of engine development and with the increasing of engine torque and power, the irregularity of the engine output torque augments also. As a result, the torsional vibration and noise of transmission system increase. With the improvement of living conditions, people's demand for the comfort of car is increasing. At the same time, the relevant laws and regulations on the requirements of automotive noise standards continue to improve. All of these factors increase the difficulty of the control of torsional vibration and makes the problem become more outstanding. Dual mass flywheel(DMF), which has an excellent vibration isolation performance and develops rapidly in foreign countries since 1980's, can effectively reduce torsional vibration of vehicle powertrain transmission system. Due to the confidentiality and monopoly of foreign technology, complete breakthrough in this field is still not made in our country. So far, most of the DMFs used in domestic intermediate and advanced vehicles are imported from abroad and the prices are expensive. Therefore, researches on DMF have important theoretical significance and engineering application value.In order to ensure the vibration reduction performance, the stiffness of circumferential short spring DMF with single-stage stiffness is small in small torsional angles, and due to structure limitation, the maximum torque that this type DMF can deliver is limited. In addition, there is a certain empty angle between primary flywheel and secondary flywheel for machining and assembly reasons, as a result, shock in the DMF may occur when the flywheel turns around the empty angle. Piecewise stiffness DMF meets the requirement of matching the high torque of powerful engines. But the torsional stiffness changes discontinuously with the torsional angle variation and the discontinuous stiffness switching may result in the impact load and noise when the gears in the gear box meshing.To improve the output torque of single-stage stiffness DMF and release the impact caused by empty angle, and solve the impact problem of three-stage stiffness DMF caused by the discontinuity of stiffness variation, structure creative design and structure implementation of DMF based on shape constraint are put forward in this thesis on the basis of the previous researches on DMF and joint development with a flywheel manufacturer, and the design theory is further illustrated. Torsional vibration models of vehicle powertrain transmission systems equipped with DMFs are established to analysis the nonlinear torsional vibration characteristics of the system. The methods of theoretical deduction, simulation analysis and experimental verification are adopted in this thesis to conduct researches on DMF, and the main works are summarized as the following aspects:(1) The structure of single-stage stiffness DMF is optimized to further improve the output torque of the DMF at large torsional angles. The design idea and the structure implement way of friction bearing based on shape constraint are proposed. Taking displacement components as basic variables and introducing an intermediate variable function, in plane polar coordinates, the elastic mechanics model of the elastic contact effect between the friction bearing block and the secondary flywheel is established. The elastic displacement method is applied to calculate the strain, stress and the distribution of the equivalent stress on the contact surface between the friction bearing block and the secondary flywheel by theoretical calculation, and compared with the analysis results obtained through finite element method. With the pressure distribution on the contact surface, the nonlinear variations of torque and stiffness characteristics produced by the contact action are analyzed. And the whole torque characteristics and dynamic characteristics of DMF are studied.(2) Considering nonlinear factors in single-stage stiffness DMF such as the friction torque produced by axial pre-tightening force, stiffness with gap variation characteristics, and the contact wedging effect between friction bearing block and secondary flywheel, vehicle powertrain transmission system equipped with single-stage stiffness DMF is simplified to establish torsional vibration model to analysis the nonlinear vibration characteristics of the system. By applying average method and Runge—Kutta numerical integration method, the approximate analytical solution of frequency characteristics and forced vibration response of the nonlinear torsional vibration system are analyzed, and the calculated results are discussed.(3) For a three-stage piecewise step stiffness DMF, abrupt changes i.e. discontinuous variation in the torsional stiffness of this DMF due to linear variation of each segment torque may cause impact load and noise upon the gear engagement of the gearbox. To solve these problems, a new type of DMF with continuously variable stiffness is proposed creatively based on compensation principle. Torque compensation devices are introduced into the initial structure, and the design theory of the primary flywheel profile line is deduced. The acting force between parts in torque compensation device and the primary flywheel as well as the torque and stiffness characteristics of DMF after torque compensation are studied.(4) For the proposed multi-stage stiffness DMF with continuous stiffness based on compensation principle, the inertia force characteristics of the moving parts in the torque compensation device and its influence on torque characteristics of the DMF are analyzed. An inertia force balance mechanism is added at the basis of the torque compensation device to eliminate the inertia force effects on the DMF. And the effect of the balance mechanism on eliminating the influence of torque compensation device inertia force is illustrated. In addition, the requirement of balancing inertia force and the influence of friction considering the friction factor between the relative moving parts in the inertia balance mechanism are studied.(5) To study the nonlinear torsional vibration characteristics of the DMF with multi-stage piecewise step stiffness, considering the nonlinear factor of piecewise changing stiffness, the torsional vibration models of vehicle powertrain transmission system equipped with the multi-stage stiffness DMF is established. By applying average method and Runge—Kutta numerical integration method, the approximate analytical solution of frequency characteristics and the forced vibration response of the system are analyzed and discussed.(6) The material objects of the single-stage stiffness DMF with introduced friction bearing and the multi-stage stiffness DMF based on compensation principle studied in this thesis are developed with a flywheel manufacturer, and the experiments of the DMFs are conducted. Experiments scheme is put forward according to test target and requirement. The experiments results are analyzed and discussed to evaluate the torque and stiffness characteristics of the DMFs studied in detail, and compared with the theoretical analysis results to verify the structure implementations, theoretical calculation models and analysis methods.