Research On Thermal Load Of Tread Braking Of Freight Car Running On Ultra-long Continuous Large Ramp

The Sichuan-Tibet railway is an important project for speeding up the construction of“the Belt and Road” interoperability.The terrain along the Sichuan-Tibet railway is very steep and has a significant elevation difference.Therefore,only when the slope increases and the slope length extends can the line requirements be satisfied.However,the extreme line conditions make the braking safety of trains face a great challenge.Tread braking is a common braking method for freight trains.When freight trains brake uniformly or urgently downhill on the ultra-long continuous ramp of the Sichuan-Tibet railway,the kinetic energy and gravitational potential energy of trains need to be dissipated by the thermal energy generated by the friction between the wheel tread and the brake shoe.The ultra-long continuous large ramp leads to the increase of vehicle braking time,which makes the thermal load of wheel braking worse.Therefore,in this thesis,the transient temperature field and thermal stress field of the wheel are numerically simulated when braking down the ultra-long continuous large slope of the Sichuan-Tibet railway.And the wheel web shape is fatigue based optimized using thermal-mechanical coupling method.The research methods and results can provide theoretical references for the designs of ramp parameter,driving control and wheel optimization of the Sichuan-Tibet railway line.The research work of this thesis is as follows.(1)A three-dimensional finite element transient thermal analysis model for freight car wheel is established,in which the influence of temperature on the material parameters is taken into account.According to the interim regulations of survey and design of the Sichuan-Tibet railway and the characteristics of the railway line,freight locomotives and vehicles are selected to determine the basic calculation parameters.The energy conservation theory and heat transfer theory are used to calculate the heat flux and convective heat transfer coefficient of the wheel under uniform braking and emergency braking.(2)The gradient,slope length,vehicle speed and ambient temperature of the ultra-long continuous large ramp of the Sichuan-Tibet railway are selected as the research factors.The influence of each factor on the temperature field of the wheel is investigated under uniform braking and emergency braking on downhill.The maximum temperature of the wheel,as well as the change rule of the transverse temperature of the tread and the radial temperature of the rim area are obtained.The results show that the gradient factor has the greatest influence on the temperature rise of the wheel when braking at constant speed.The higher the gradient,the higher the temperature rise of the wheel.When the gradient increases from20‰ to 30‰,the maximum wheel temperature increases from 247 ℃ to 368 ℃.For every2‰ increase in gradient,the maximum wheel temperature increases by about 10%.Therefore,the gradient selection design should be focused on,and the slope length design can be appropriately extended.The initial braking speed has the greatest effect on the temperature rise of the emergency braking wheel.When the initial braking speed is increased from 90 km/h to 120 km/h,the wheel temperature increases by 15%.Therefore,when the train is running the initial speed of emergency braking should be mainly controlled,and the factor of ambient temperature can be neglected.(3)After the transient thermal analysis of the wheel,the indirect thermal-mechanical coupling method is used to carry out the structural stress analysis.The influences of various factors on the wheel thermal stress field under uniform braking and emergency braking of the car downhill are studied.The equivalent thermal stress,the radial stress of the web fillet and the lateral equivalent thermal stress of the tread are analyzed.From the results of the equivalent thermal stress of the uniformly braking wheel,it can be found that the gradient has the greatest influence on the equivalent thermal stress o f the wheel.When the gradient increases from 20‰ to 30‰,the equivalent thermal stress of the wheel increases by 9.28%on average.That indicates that the gradient should be carefully considered,and the slope length can be extended appropriately when designing the line.When the gradient is selected as 24‰,the initial braking speed of the vehicle should not be higher than 100 km/h.In addition,the effect of ambient temperature is neglected.(4)Using the orthogonal test method,the structure of the wheel web is optimized,and three optimization schemes are obtained under the optimization objectives.The three optimization schemes and the prototype wheel are respectively checked for static strength and fatigue strength.The results show that the four wheel webs satisfy the requirements of static strength and fatigue strength.The static strength optimization scheme of wheel web can effectively improve the equivalent stress of the web,and the fatigue strength optimization scheme can effectively improve the fatigue strength of web under the condition of thermal-mechanical coupling.