Study Into Longitudinal Train Dynamics Based On Heavy Haul Freight Car’s Structure Feature

The heavy axle-load and long marshalling are the development trends of heavy haul railways. With the increase in number or weight of wagons, it is more difficult to handle train traction or braking operation and more severe longitudinal impulses may occur. Based on application practices at home and abroad, not only structural damages to couplers, draft gears,bolsters and wagon body but also train derailment accidents are found to be caused by longitudinal impulses, which usually occur in vehicle shunting operation, train emergency braking operation and train curving operation. According to heavy haul wagons in service in China, a one-dimensional longitudinal train dynamics model and a three-dimensional train with short group dynamics model are established. This paper focuses on the research of longitudinal vibration characteristics and their effects under vehicle shunting and train braking conditions, and it reports on the wheel/rail safety under train curving condition. As an academic discussion, this paper provides some useful suggests to help designing wagon parameters and improving train running safty.Main research contents and conclusions are as follows.1. Based on the structural characteristic and working principle of friction daft gears, a general friction daft gear model is established by adopting mechanical analysis methods and introducing correction parameters, and its result also agrees with that in the test. To abtain and evaluate more results including coupler forces, bogie impact forces, draft gear deformations and wagon body structural deformations, a new longitudinal connection model of heavy haul trains is developed by introducing center bowl sub-model and wagon body structure sub-model.It is proved that the longitudinal connection model is helpful not only to improving simulation results but also to expanding the application scope of using the longitudinal train dynamics theory.2. Taking vehicle shunting operations in China as the research backguound, a vehicle shunting simulation model is established, and some simulation results are compared with test results. This paper reports that the factors including wagon weights, impact modes, draft gear charteristics, connection stiffness between wagon body and bogie, structural stiffness of wagon body, hysteresis effects of goods and running resistances have some influnces on vehicle longitudinal impact characteristics. The results indicate as follow: With wagon weight improving, coupler forces and center bowl forces both increase. Under the same marshalling conditions, the largest coupler forces occur during loaded cars to loaded cars while the largest center bowl forces occur during loaded cars to empty cars. As the connection stiffness between wagon body and bogie increases, coupler forces have little chage while center bowl forces show a non-linear increase trend. The greater structural stiffness a wagon has, the larger coupler forces it shows. Coupler forces and running speeds in tank car decay more quickly than those in other cars during vehicle shunting operations. The greater running resistances in test car means the larger coupler forces and center bowl forces.3. Taking China’s heavy haul trains as the research object, a study into longitudinal vibration characteristics of a train under emergency braking condition is done. This paper also discusses on the effects of wagon axle-load, coupler slack and braking parameters on train longitudinal impulse and braking efficiency. The results indicate as follow: With the same train weight, the longitudinal impulse caused in large axle-load train is lower than that in long group train. With the same braking equipment, the longitudinal impulse in general train is greater than that in coal special train. Braking wave propagating characteristics with first quick back slow trend, brake cylinder pressure characteristics with convex type, and brake cylinder pressure change law with convergent type can help to get a better longitudinal train dynamics performace. The modified Locotrol system can significantly improve longitudinal train dynamics performance. For a train with mixed formation of both empty and loaded wagons,the number of empty wagons should be controlled and the empty wagons should be placed at the end of the train as possible, and the position where the maximum coupler compression force occurs from loaded train should be avoided. For a train with mixed formation of wagons with different axle loads and loading capacities, heavier wagons should be placed at the front of the train while lighter wagons should be placed at the rear, and all the wagons should be marshaled in descending order by weight. For a train with mixed formation of different wagon types, it should be avoided to place the wagon with softer structural stiffness such as flatcar and tanker in the middle of the train.4. Based on the three-dimensional train dynamics model, a research into the effects of coupler rotation, coupler compression, coupler height difference, brake shoe pressure and continuous longitudinal impulse on train safety on curved track under braking condition. The results indicate as follow: When a train with three wagons marshaled into runs on the curved track of R370m, the wheel-rail lateral force, derailment coefficient and wheel unloading rate in the train model are 11%, 8% and 6% greater than those in the single car model. In the loaded train model, the wheel-rail lateral force, derailment coefficient and wheel unloading rate under the condition of coupler compression force of 1 OOOkN are 6%, 5% and 1% greater than those under the idling condition. In the mixed train model with two loaded cars at both ends and one empty car in the middle, the wheel-rail lateral force, derailment coefficient and wheel unloading rate under the condition of coupler compression force of 1 OOOkN are 12%, 19% and 51% greater than those under the idling condition. Since braking forces in the three-piece bogie is generally applied on the wheel tread, brake shoe pressure provides additional stretching and yawing limits while a train is braking on curved track, and wheelsets have to be pushed towards outside of bogie pivot. Guide wheelset cannot steer on the curve easily, so that the angle of attack always keeps in a high value, which increases the probability of train derailment and wheel-rail wear. When the train takes an emergency brake, the wheel-rail lateral force, derailment coefficient and wheel unloading rate in the train braking model are 4%,5% and 1% greater than those in the train idling model.