Influence Of Draft Gear Modeling On Longitudinal Train Dynamics Simulation

Draft gear is an important part to reduce the longitudinal impulse of the train,the heavy haul train draft gears are classified into friction type and cement type,all of which have the characteristics of large loading capacity and strong nonlinearity,among them,the MT-2 friction draft gear is the most commonly used in China.For friction draft gear,the commonly used modeling methods include look-up table method and wedge-spring model method and different modeling methods largely affect the accuracy of longitudinal dynamics simulation.In this paper,the MT-2 heavy haul train draft gear is taken as the research object.Firstly,the composition of the friction-type draft gear is introduced,and its mechanical properties are analyzed in combination with the working principle.Secondly,the longitudinal dynamics modeling method of heavy haul train is introduced.A complete longitudinal dynamics modeling of heavy haul train involves some important systems' modeling methods such as locomotive traction or braking force,train running resistance,train air brake and draft gear.The two main modeling methods used in the current friction draft gear are introduced,namely,the look-up table method and the wedge-spring model method.Two common draft gear mathematical models are established based on the vehicle impact test data.Then,through the impact test,the drop hammer test and the bench test simulation,the two modeling methods are validated.The differences between the two simulation results are compared and analyzed.Using the TDEAS train longitudinal dynamics software compiled by the National Key Laboratory of Traction Power of Southwest Jiaotong University,the ‘1+1' grouped 20,000-ton combined heavy haul train model is established.For the braking condition of the straight track,the simulation results of the look-up table model and the wedge-spring model are compared with the test data.The line operation simulation is carried out with a section of a heavy haul railway as the line data,and the coupler force and longitudinal acceleration of different model simulation results are compared and analyzed.Finally,the influence of the draft gear model on the fatigue damage calculation of the coupler is compared by using the rainflow counting method.The following conclusions were obtained:1.During the operation of the draft gear,the energy of the train impact is mainly absorbed by the friction member and the spring member.The working process of buffer loading and unloading from the longitudinal force of the plate is divided into four stages,which are composed of two loading stages and two unloading stages.They are mainly related to parameters such as displacement,friction surface inclination angle and friction coefficient.The force-displacement curve at the stage is linearly continuous,but there is discontinuity from one working phase to another.2.The look-up table model obtains the characteristic curve of loading and unloading generally according to the fitting of the buffer-related experimental results,and the two curves can simultaneously consider the gap element,the initial pressure,and the rigid contact.The wedge-spring model simplifies the actual working principle of the draft gear,which better reflects the fact that the impedance depends on the stroke of the draft gear and the relative speed between the vehicles.The characteristics of the gap element,the initial pressure,and the rigid contact are presented by nonlinear springs.For the discontinuity of the loading characteristic curve and the unloading characteristic curve,both models adopt a reasonable transition method.3.For the look-up table model,under various experimental conditions,regardless of the change rate of the draft gear stroke,the simulated draft gear characteristic curve is along a fixed trajectory.While the simulation result of the wedge-spring model depends on the change rate of the draft gear stroke.It embodies the dynamic characteristics of the draft gear and conforms to the law that the friction coefficient is affected by the speed.The research shows that the simulation results of the wedge-spring model in each working condition are more consistent with the experimental results.And the model has better adaptability and wider application range.4.Both models can simulate the braking condition of straight track accurately.For the simulation of common full braking conditions,it is more accurate to calculate the longitudinal dynamic performance using the wedge-spring model.The variation trend of the coupler force calculated by different models is basically the same,but the amplitude and frequency of the longitudinal impulse are different.In the heavy haul line running simulation,the longitudinal relative speed between vehicles is much smaller than the initial impact test speed and the drop hammer test.At this time,the performance of the draft gear is closer to its static characteristics.The maximum acceleration of the wedge-spring model simulation is larger than the look-up table model,and the fatigue damage result of each coupler calculated by the look-up table model is higher,and the total fatigue damage of the train coupler is larger.It is safer to use the look-up table model to calculate the fatigue damage of the coupler.